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2003 Arterial and venous adaptations to short-term handgrip exercise training Mahmoud Awad Alomari Louisiana State University and Agricultural and Mechanical College, [email protected]
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Recommended Citation Alomari, Mahmoud Awad, "Arterial and venous adaptations to short-term handgrip exercise training" (2003). LSU Doctoral Dissertations. 188. https://digitalcommons.lsu.edu/gradschool_dissertations/188
This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please [email protected]. ARTERIAL AND VENOUS ADAPTATIONS TO SHORT-TERM HANDGRIP EXERCISE TRAINING
A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Kinesiology
By Mahmoud Alomari B.S., Yarmouk University, Irbid, Jordan, 1990 M.S. Minnesota State University, Mankato, MN, 1995 December, 2003
© Copyright 2003 Mahmoud A. Alomari All right reserved
ii DEDICATION
I dedicate all of my work to my parents, the love of my life. They feel as though they took every exam with me and were as anxious as I was for each defense. Their confidence in me never wavered and helped me to accomplish the dream of my life. Their motivation made me a better person and they continue to show me what service to others really is. Abuwael and Umwael thank you for the endless and exceptional support, sacrifices, inspiration and love.
iii ACKNOWLEDGMENTS
All gratitude goes to “Allah” (God) SW ….. without him my dissertation would not make
it to reality. I must express my deep gratefulness to my committee members for their help and guidance particularly Dr. Wood for his constant positive and encouraging remarks.
Rhonda and Rafael (good luck ……. your next!!!)…… we have always toned on the
same brainwave …….. I really appreciate the bright suggestions, creative ideas, and
fruitful discussions. I also wish to thank Muaz and Nicki for being instrumental in this
work ….. and the rest of my colleagues Arturo, Rania, Andrea, Zeki, and JoeyLynn I will
always remember you. Deep thanks also goes to all my friends and family around the
world for their special encouragement and support.
His teaching, guidance, scholarship, and service to students as well as to the
academic world are models of professional and personal investment. Despite the bad,
ugly, ugly and ugly we have had many many goods…….. I feel extremely lucky to work
under the supervision of Dr. Welsch for the past 6 years ….. Mike has made a change in
my life not just in academia but also has crafted a different person out of me. Words
certainly fail to completely express my appreciation …………….. but I sincerely would
like to say THANK YOU Mike.
iv TABLE OF CONTENTS
DEDICATION...... iii
ACKNOWLEDGMENTS ...... iv
LIST OF TABLES...... vii
LIST OF FIGURES ...... viii
ABSTRACT...... x
CHAPTER 1. INTRODUCTION ...... 1 1.1. REFRENCES ------4
CHAPTER 2: MEASUREMENTS OF VASCULAR FUNCTION USING STRAIN- GAUGE PLETHYSMOGRAPHY: TECHNICAL CONSIDERATIONS, STANDARDIZATION AND PHYSIOLOGICAL FINDINGS ...... 7 2.1. INTRODUCTION------8 2.2. METHODOLOGY------8 2.3. RESULTS ------13 2.4. DISCUSSION------18 2.5. CONCLUSION------26 2.6. REFERENCES ------27
CHAPTER 3. INFLUENCE OF VENOUS FUNCTION ON EXERCISE TOLERANCE IN CHRONIC HEART FAILURE ...... 31 3.1. INTRODUCTION------32 3.2. METHODS------33 3.3. RESULTS ------35 3.4. DISCUSSION------38 3.5. CONCLUSION------42 3.6. REFERENCES ------42
CHAPTER 4. MODIFICATION OF FOREARM VASCULAR FUNCTION FOLLOWING SHORT-TERM HANDGRIP EXERCISE TRAINING ...... 46 4.1. INTRODUCTION------47 4.2. METHODS------48 4.3. RESULTS ------51 4.4. DISCUSSION------54 4.5. REFERENCES ------59
CHAPTER 5. ARTERIAL AND VENOUS ADAPTATIONS TO SHORT-TERM HANDGRIP EXERCISE TRAINING ...... 61 5.1. INTRODUCTION------62 5.2. HYPOTHESES ------63
v 5.3. METHODS------63 5.4. RESULTS ------68 5.5. DISCUSSION------76 5.6. CONCLUSION------86 5.7. REFERENCES ------86
CHAPTER 6. SUMMARY...... 93 6.1. REFERENCES ------95
APPENDIX A: LITERATURE REVIEW...... 97
APPENDIX B: PROPOSAL ...... 280
APPENDIX C: LETTERS OF PERMISSIONS...... 310
VITA...... 315
vi LIST OF TABLES Table 2.1. Subject characteristics...... 13
Table 2.2. Resting vascular indices following varying occlusion pressures...... 14
Table 2.3. Between-days ICCC for forearm arterial and venous indices ...... 17
Table 2.4. Pearson product moment correlation between arterial, venous and physical fitness measures ...... 20
Table 2.5. Differences between the cardiorespiratory groups in forearm arterial and venous indices...... 21
Table 2.6. Differences between the handgrip strength groups in forearm arterial and venous indices...... 21
Table 3.1. Participant characteristics ...... 36
Table 3.2. Independent t-test comparisons between CHF and control groups...... 37
Table 4.1. Subject characteristics...... 52
Table 4.2. Dominant and non-dominant FVR changes in the LO and HI groups at rest, following OCC and following ROCC...... 55
Table 4.3. Literature overview...... 56
Table 5.1. Subject characteristics...... 68
Table 5.2. Cardiovascular hemodynamics and HRV before and after 4-week handgrip exercise training...... 68
Table 5.3. Arm ergometer exercise test, forearm skinfolds, and sum of three skinfolds before and after 4-week handgrip exercise training...... 69
Table 5.4. Resting arterial and venous indices ...... 70
Table 5.5. Post occlusion arterial and venous indices ...... 70
Table 5.6. Pearson product moment correlation between arterial, venous and physical fitness measures ...... 75
vii LIST OF FIGURES Figure 2.1a. Venous capacitance using different pressures...... 15
Figure 2.1b. Venous outflow using different pressures...... 15
Figure 2.2. Venous capacitance and outflow following 5 & 10 minutes of venous occlusion...... 16
Figure 2.3. Venous capacitance over a 10 minute venous occlusion period...... 16
Figure 2.4a. Relationship between estimated VO2 & post arterial occlusion blood inflow...... 17
Figure 2.4b. Relationship between estimated VO2 & resting venous outflow...... 18
Figure 2.5a. Relationship between handgrip strength & resting venous capacitance...... 19
Figure 2.5b. Relationship between handgrip strength & resting venous outflow...... 19
Figure 3.1. Relationship between forearm venous capacitance and maximum walking distance...... 38
Figure 3.2. Relationship between forearm venous outflow and maximum walking distance...... 39
Figure 4.1. FBF changes in the LO group following HG exercise training...... 53
Figure 4.2. FBF changes in the HI group following HG exercise training...... 54
Figure 5.1. Changes in peak forearm blood inflow...... 71
Figure 5.2. Changes in peak forearm vascular resistance...... 71
Figure 5.3. Changes in resting forearm venous compliance...... 72
Figure 5.4. Changes in post occlusion forearm venous capacitance...... 73
Figure 5.5. Changes in post occlusion forearm venous outflow...... 74
Figure 5.6. Relationship between handgrip strength & resting venous capacitance ...... 74
Figure 5.7. Relationship between handgrip strength & resting venous outflow...... 83
Figure 5.8. Relationship between post occlusion blood inflow & venous compliance .... 84
Figure 5.9. Relationship between post occlusion blood inflow & venous outflow ...... 85
viii
Figure 5.10. Relationship between changes in peak blood inflow & resting venous compliance ...... 85
ix ABSTRACT
Four studies on vascular and exercise physiology are presented in this document. The 1st study examined the relationships between measures of fitness and FVF in 55 young [22.6
± 3.5 years] adults. Estimated VO2peak correlated with arterial inflow (Ainf) [r=0.54; p=0.012] and resting venous outflow (Vout) [r=0.56; p=0.016]. Lastly, HG strength was associated with Vcap [r=0.57; p=0.007] and Vout [r=0.67; p=0.001].
The 2nd study examined the relationship between FVF and exercise tolerance
(ExT) in 20 patients with HF [age: 59 ± 13 years] and 10 age-matched controls [age: 51 ±
16 years. The ExT was measured as the maximum walking distance (MWD) in 6 minutes. FVF [Ainf: HF 15.3 ± 6; controls 22 ±6.7; Vcap: HF 1.4 ± 0.5; controls 2.0 ±
0.4; Vout: HF 24.5 ± 9.4; controls 33 ± 10 mL • 100 mL tissue-1 • min –1; and forearm vascular resistance: HF 7.8 ± 3; controls 4.6 ± 1.4U] indices and MWD [HF: 178 ± 65 m; controls: 562 ± 136m, P = .0001] were different between groups. Correlation analysis revealed significant associations between FVF indices and MWD.
The 3rd study examined the effect of 25% (LO) and 75% (HI) of MVC short-term
HG exercise training on FVF in 28 healthy men [Age:23+4.3]. The 4-week program consisted of non-dominant HG exercise performed 5 d/wk for 20-min. Training resulted in increased Ainf in the non-dominant arm in the LO and HI groups by 16.51% and
20.72%, respectively.
The final study examined the time-course FVF adaptations to HG exercise training in 17 men [Age: 22.6 ± 3.5]. The HG exercise was performed in the non- dominant arm 5 d/wk for 20-min at 60% of MVC. The 2 X 5 ANOVA revealed arms X visits interaction for Ainf [p=0.02], while the LSD post-hoc demonstrated unilateral
x increase in Ainf following the 1st week. Additional 2 X 5 split-plot ANOVA tests revealed arms X visits interaction [p=0.04] for venous compliance (Vcomp) with LSD post-hoc demonstrating a decrease in trained arm Vcomp in visit 2 followed by an increase in visit 4 and return to baseline level at visit 5.
xi
CHAPTER 1. INTRODUCTION
The original understanding of vascular physiology has evolved from a passive conduit,
transporting blood to and from body tissues, to a multifaceted system that is interlinked
with many other body systems, regulated by a variety of mechanisms, and adjusts to
different conditions. Our daily poor lifestyle choices, such as high fat meals, cigarette
smoking, inactivity, and work-related stress exposes the vascular system to numerous
assaults. These assaults result in a variety of deteriorating diseases including
atherosclerosis, coronary artery diseases and hypertension. Many studies have
demonstrated that exercise training can improve sympathetic vascular tone, insulin
sensitivity, and lipid and haemostatic profiles as well as lower blood pressure, obesity,
and stress hormones. Therefore, hand-in-hand with dietary modifications, exercise training appears to be an effective tool to minimize the influence our daily choices and
may results in numerous health benefits.
The current document presents four research projects, designed to examine the acute
and chronic effects of exercise training on vascular function and control in health and
disease. These research projects are introduced in the subsequent four chapters and aimed
at:
1. Addressing technical concerns and determining the reproducibility of vascular
outcomes using strain gauge plethysmography,
2. Examining the ability of vascular measures to differentiate between various
populations, AND
3. Evaluation of the influence of interventions on changing measures of vascular
function.
1 Chapter 2 addresses methodological considerations for mercury strain-gauge
plethysmography (MSGP) and the reliability of measuring arterial and venous function
indices as well as examined the relationships between fitness and vascular indices.
Standard recommendations for venous occlusion pressure and duration have been 50
mmHg and 2 minutes, respectively (7, 11). Others have used, venous pressure “just
below” diastolic blood pressure (22). However, the rationale for these recommendations
are not clear. Additionally, few studies have examined the influence of physical fitness
(cardiovascular and/or muscular) on indices of venous function. The majority of these studies report improvements in venous hemodynamics secondary to changes in blood
volume following endurance training (6, 13, 15). More recently, Wecht et al. attributed
venous modifications, in part, to enhanced vasomotor tone, vessel compliance and vessel
responses to mechanical and neural stimulation (22). Therefore, the major aim of this
study was to examine relationships between measures of physical fitness and arterial and
venous function. It was hypothesized that individuals with the highest levels of fitness
would have greater arterial inflow and venous outflow patterns. However, considering the
lack of standardization and reliability of the protocols used to examine the venous
function, a secondary aim of this study was to examine the effects of different venous
occlusion times and pressures on venous capacitance and outflow as well as the reliability
of MSGP.
Chapter 3 focuses on the effect of vascular function on exercise tolerance in heart
failure (HF). Chronic HF is a complex clinical syndrome characterized by an abnormality
of the heart and by a recognizable pattern of hemodynamic, renal, neural and hormonal
responses (17). Despite of the research describing the effect of HF on blood inflow and
2 the subsequent treatments aimed at improving arterial function, many patients continue to
experience activity-related symptoms (4), possibly suggesting the involvement of other
factors. Interestingly, the role of the venous system on exercise tolerance in patients with
HF has not received much attention, despite evidence of changes in venous structure and
function in patients with HF (12, 18) and in patients with a variety of vascular diseases
(16, 20). Accordingly, the purpose of this study was to examine the relationships between
indices of arterial and venous function and exercise tolerance in chronic HF patients and
age-matched controls. Given the complexity of the HF syndrome, it was hypothesized a
direct relationship between indices of vascular function and exercise tolerance.
Chapter 4 examines the effect of low and high intensity localized exercise training on
arterial function. Exercise training induces arterial adaptations characterized by increased
working muscle perfusion and reduced vascular resistance. The precise mechanisms for
these adaptations are not known however they appear to be exercise training modality
dependent (3, 8, 9). Interestingly, vascular adaptations following small muscle group
exercise training programs using different intensities have not been adequately explored.
A study by Yasuda et al. (23) was the only study where subjects performed 6-weeks of
HG exercise training at intensities of 33 or 50% of maximum voluntary contraction
(MVC). Peak forearm blood flow increased similarly in both groups. However, the
difference between training intensities was only 17% and may have been insufficient to
determine whether the adaptations are indeed intensity-dependent. Accordingly, the
purpose of this study was to determine the effects of low and high intensity short-term
HG exercise training programs on localized vascular function in healthy young men. It
3 was hypothesized that a training exercise intensity of 75% of MVC would promote
greater vasodilatory capacity compared to a training exercise intensity of 25% of MVC.
Finally in chapter 5, the time course changes to 4 week HG exercise training program
on arterial and venous function indices were examined. Structural and functional changes
in the arterial system following exercise training are well documented (2, 5). Recent
studies report training-induced adaptations may be observed following one week of training in animal models (14, 19), and in human single large conduit arteries (i.e.
brachial) (1). Interestingly, the effect of exercise training on venous function has received
considerably less attention despite compelling evidence demonstrating its dynamic role
beyond a typical role as a “passive volume reservoir” (10, 21). Therefore, the purpose of
this study was to examine the influence of 4-weeks of handgrip exercise training on
arterial and venous function. A secondary aim of this study was to determine the time
course of change for both arterial and venous indices associated with the handgrip
exercise training program. It has been hypothesized that handgrip exercise training will
result in changes in arterial and venous function indices following the 1st week.
Appendix A includes an extensive literature review of vascular function and control
at rest and during exercise in health and disease. Appendix B is the proposal of the final
study followed by the author’s vita.
1.1. REFRENCES
1.Allen JD, Geaghan JP, Greenway F and Welsch MA. Time Course of Improved Flow- Mediated Dilation after Short-Term Exercise Training. Med Sci Sports Exerc 35: 847- 853, 2003.
2.Alomari MA, Welsch MA, Prisby RD, Lee CM and Wood RH. Modification of forearm vascular function following short-term handgrip exercise training. Int J Sports Med 22: 361-365., 2001.
4 3.Bond V, Jr., Wang P, Adams RG, Johnson AT, Vaccaro P, Tearney RJ, Millis RM, Franks BD and Bassett DR, Jr. Lower leg high-intensity resistance training and peripheral hemodynamic adaptations. Can J Appl Physiol 21: 209-217, 1996.
4.Clark AL, Poole-Wilson PA and Coats AJ. Exercise limitation in chronic heart failure: central role of the periphery. J Am Coll Cardiol 28: 1092-1102, 1996.
5.Clarkson P, Montgomery HE, Mullen MJ, Donald AE, Powe AJ, Bull T, Jubb M, World M and Deanfield JE. Exercise training enhances endothelial function in young men. J Am Coll Cardiol 33: 1379-1385, 1999.
6.Convertino VA, Mack GW and Nadel ER. Elevated central venous pressure: a consequence of exercise training-induced hypervolemia? Am J Physiol 260: R273-277, 1991.
7.Cramer M, Langlois Y, Beach K, Martin D and Strandness Jr DE. Standardization of venous flow measurements by strain gauge plethysmography in normal subjects. Bruit VII: 33-39, 1983.
8.Delp MD and Laughlin MH. Time course of enhanced endothelium-mediated dilation in aorta of trained rats. Med Sci Sports Exerc 29: 1454-1461, 1997.
9.Green DJ, Cable NT, Fox C, Rankin JM and Taylor RR. Modification of forearm resistance vessels by exercise training in young men. J Appl Physiol 77: 1829-1833, 1994.
10.Hester RL and Hammer LW. Venular-arteriolar communication in the regulation of blood flow. Am J Physiol Regul Integr Comp Physiol 282: R1280-1285, 2002.
11.Hokanson DE, Sumner DS and Strandness Jr. DE. An electrically calibrated plethysmograph for direct measurement of limb blood flow. IEEE transactions on bio- medical engineering 22: 21-25, 1975.
12.Ikenouchi H, Iizuka M, Sato H, Momomura S, Serizawa T and Sugimoto T. Forearm venous distensibility in relation to severity of symptoms and hemodynamic data in patients with congestive heart failure. Jpn Heart J 32: 17-34, 1991.
13.Kenney WL and Armstrong CG. The effect of aerobic conditioning on venous pooling in the foot. Med Sci Sports Exerc 19: 474-479, 1987.
14.Laughlin MH. Endothelium-mediated control of coronary vascular tone after chronic exercise training. Med Sci Sports Exerc 27: 1135-1144, 1995.
15.Louisy F, Jouanin JC and Guezennec CY. Filling and emptying characteristics of lower limb venous network in athletes. Study by postural plethysmography. Int J Sports Med 18: 26-29, 1997.
5
16.Monos E, Berczi V and Nadasy G. Local control of veins: biomechanical, metabolic, and humoral aspects. Physiol Rev 75: 611-666, 1995.
17.Poole-Wilson PA, Buller NP and Lipkin DP. Regional blood flow, muscle strength and skeletal muscle histology in severe congestive heart failure. Am J Cardiol 62: 49E- 52E, 1988.
18.Sato H, Ikenouchi H, Aoyagi T, Matsui H, Mochizuki T, Momomura S, Serizawa T, Iizuka M and Sugimoto T. Pathophysiology and evaluation of severity of congestive heart failure on the basis of venous characteristics. Jpn Circ J 53: 141-145, 1989.
19.Shen W, Zhang X, Zhao G, Wolin MS, Sessa W and Hintze TH. Nitric oxide production and NO synthase gene expression contribute to vascular regulation during exercise. Med Sci Sports Exerc 27: 1125-1134, 1995.
20.Tretbar LL. Chronic venous insufficiency of the legs: pathogenesis of venous ulcers. J Enterostomal Ther 14: 105-108, 1987.
21.Tschakovsky ME, Shoemaker JK and Hughson RL. Vasodilation and muscle pump contribution to immediate exercise hyperemia. Am J Physiol 271: H1697-1701, 1996.
22.Wecht JM, de Meersman RE, Weir JP, Bauman WA and Grimm DR. Effects of autonomic disruption and inactivity on venous vascular function. Am J Physiol Heart Circ Physiol 278: H515-520, 2000.
23.Yasuda Y and Miyamura M. Cross transfer effects of muscular training on blood flow in the ipsilateral and contralateral forearms. Eur J Appl Physiol Occup Physiol 51: 321- 329, 1983.
6 CHAPTER 2: MEASUREMENTS OF VASCULAR FUNCTION USING STRAIN- GAUGE PLETHYSMOGRAPHY: TECHNICAL CONSIDERATIONS, STANDARDIZATION AND PHYSIOLOGICAL FINDINGS
7
2.1. INTRODUCTION
The relationship between exercise performance and arterial inflow is well documented
(25). Surprisingly, the contribution of the venous system to overall cardiovascular function and skeletal muscle activities has received considerably less attention, despite compelling evidence demonstrating its importance beyond the typical role as a “passive volume reservoir” (3, 11, 31)
Additionally, few studies have examined the influence of physical fitness
(cardiovascular and/or muscular) on indices of venous function. The majority of these studies report improvements in venous hemodynamics secondary to changes in blood
volume following endurance training (5, 16, 19). More recently, Wecht et al. attributed
venous modifications, in part, to enhanced vasomotor tone, vessel compliance and vessel
responses to mechanical and neural stimulation (33). Therefore, the major aim of this
study was to examine relationships between measures of physical fitness and arterial and
venous function. It was hypothesized that individuals with the highest levels of fitness
would have greater arterial inflow and venous outflow patterns.
However, considering the lack of standardization and reliability of the protocols
used to examine the venous function, a secondary aim of this study was to consider
several methodological and technical aspects regarding the examination of the venous
system.
2.2. METHODOLOGY
2.2.1. Study Design and Inclusion, Exclusion Criteria
This study consisted of 2 distinct phases. In phase I, 2 separate experiments were conducted to address methodological aspects (Experiment 1) concerning the examination
8 of venous function and to determine the reliability (Experiment 2) of vascular function using mercury in-silastic strain gauge plethysmography (MSGP). In phase II, 2 additional experiments were designed to examine the relationships of vascular function and cardiovascular fitness (Experiment 3) and handgrip strength (Experiment 4). Men and women between the age of 18 and 35 years were recruited to participate. Smokers and individuals with acute medical conditions (e.g. orthopedic injury), active infection and/or on pharmacotherapy with known vascular effects (e.g. anti-inflammatory therapy, cardiovascular medications) were excluded. Following comprehensive explanation of the study, its benefits, inherent risks and expected commitments with regard to time, all participants signed an informed consent approved by the Institutional Review Board of
Louisiana State University.
2.2.2. Vascular Assessments
Instrumentations and measurements
Forearm arterial inflow, vascular resistance, venous capacitance, and outflow for all experiments were obtained using MSGP (model EC5R, D. E. Hokanson Inc, Bellevue,
WA, 99). Upon arrival, blood pressure cuffs were positioned around the participant’s upper arm and wrist, and a mercury-in-silastic strain gauge (13) placed around the forearm approximately 10 cm distal to the olecranon process while the subject in a supine position. The forearm was extended and slightly supinated and supported by a styrofoam block. Immediately before the blood flow measurements, hand circulation was occluded for 1 min by inflating the cuff at the wrist to 240mmHg. Forearm blood inflow was estimated at rest and following 5 min of upper arm arterial occlusion. Subsequently forearm venous capacitance and outflow were obtained following specific venous
9 occlusion pressures and periods as will be described below in each experiment. Arterial occlusion was achieved by inflating the cuff on the upper arm to 240 mmHg (1, 29).
Throughout the testing procedures, heart rate and blood pressure were obtained at rest, and during and immediately following Occ.
Experiment 1: Effect of varying occlusion pressure and time on vascular indices.
This experiment was conducted on two separate days in eight apparently healthy young men and women (Age: 22.1 ± 1.4). Forearm blood inflow measures were obtained as described above in both days. Day 1: In order to examine the influence of varying upper arm venous occlusion pressures on venous capacitance and outflow, resting forearm venous indices were evaluated following four minutes of upper arm venous occlusion pressure at 7, 14, 21, 28, and 35 mmHg below diastolic blood pressure obtained in the opposite arm. Day 2: In order to examine the influence of upper arm venous occlusion durations, venous capacitance measures were taken every minute for a period of ten minutes, and venous outflow evaluated following 5 and 10 minutes of upper arm venous occlusion pressure using 50 mmHg.
Experiment 2: Reliability of arterial and venous function measurements.
Forearm blood inflow, and venous capacitance and outflow were obtained in 8 apparently healthy young men (Age: 24.0 ± 5.0) on three different occasions within 10 days to determine between-days reliability of MSGP. Vascular indices were evaluated at rest and following 5 min of upper arm arterial occlusion. Venous capacitance and outflow for the two conditions (rest & post occlusion) were evaluated following 6 minutes of upper arm venous occlusion at 7 mmHg below diastolic blood pressure.
10 Experiment 3 & 4: Associations between physical fitness measures and arterial and venous function.
These experiments were designed to examine relationships between forearm arterial and
venous function indices and measures of physical fitness (estimated VO2peak and handgrip strength). Cardiorespiratory fitness (Estimated VO2peak) was determined using the YMCA’s cycle exercise test (2) in twenty-one apparently healthy young (Age: 21.4 ±
1.4) women and expressed in mL•kg-1•min-1. Maximum handgrip strength was
determined, in twenty-two apparently healthy young (Age: 24.5 ± 5.0) men, as the
average of 3 maximum consecutive contraction trials using a hand-dynamometer (model
78010, Lafayette Instrument, Lafayette, IN, 02) and expressed in pound. Vascular indices
were evaluated at rest and following 5 min of upper arm arterial occlusion. Venous
capacitance and outflow for the two conditions (rest & post occlusion) were evaluated following 6 minutes of upper arm venous occlusion at 7 mmHg below diastolic blood pressure.
2.2.3. Data Analysis
Arterial indices
Resting forearm blood inflow was recorded at a paper speed of 5 mm/second and values
were derived from the slope drawn at a best-fit tangent using the first 3 pulses.
Calculations were made as a function of 60 seconds divided by the horizontal distance
(mm) needed for the slope to rise vertically from baseline to the top of the recording paper and multiplied by the full chart range. Peak arterial in-flow following occlusion
was recorded at a paper speed of 25 cm/ second. Analyses were performed using a slope
drawn at a best-fit tangent to the curves of the first 2 pulses of the flow curve post cuff
release. The blood flows were then calculated from 60 sec. multiplied by the paper speed
11 (25 cm/sec) divided by the horizontal distance (mm) needed for the volume slope to increase by 20 mm vertically. Mean arterial pressure (MAP) was calculated using the standard equation: MAP = Diastolic Pressure+[(Systolic Pressure-Diastolic Pressure)/3].
Forearm vascular resistance (FVR) was calculated by dividing mean arterial pressure by forearm blood inflow (FVR = MAP/ forearm blood inflow).
Venous indices
Forearm venous capacitance was measured as the vertical distance (mm) representing the increase in forearm-volume graph after the designated period (as described for each experiment) for venous filling. Analysis of forearm venous outflow was derived from a tangent line that represents the vertical drop in volume-graph from the excursion line and drawn at 0.5 second and 2 seconds after the release of the venous occlusion pressure (7,
12).
2.2.4. Statistical Analysis
All statistical analyses were performed using SPSS statistical software package for windows (version 11.0, Chicago, Ill). Group data were expressed as mean ± sd and alpha was set a priori at P < 0.05.
Experiment 1: Effect of varying occlusion pressure and time on vascular indices.
In order to evaluate the difference in forearm inflow, and venous capacitance and outflow following each venous occlusion pressure, repeated measure ANOVA and Bonferroni pairwise comparison were used. Dependent t-tests were used to compare vascular function measures at 5 and 10 minutes.
12 Experiment 2: Reliability of arterial and venous function measurements.
Interclass correlation coefficient (ICCC) was used to determine the reliability of MSGP.
Measures of the arterial and venous function indices at rest and following occlusion were included.
Experiment 3 & 4: Relationship between physical fitness measures and arterial and venous function indices
Pearson product moment correlation was used to examine the relationship between physical fitness measures (estimated VO2peak & handgrip strength) and arterial and venous function indices as well as the relationship between arterial and venous function indices. Additionally, with participants subdivided into above (HI) and below (LO) average fitness groups according to the YMCA and the Canadian Society of Exercise
Physiology (22), and independent t tests were used to compare fitness measures and vascular indices.
2.3. RESULTS
2.3.1. Participants Characteristics
Table 2.1. Subject characteristics Mean ± sd Range Age (yrs) 22.6 ± 3.5 18-34 Height (cm) 167.5 ± 8.5 150-185 Weight (kg) 69.1 ± 15.2 43-114 Arm length (cm) 25.5 ± 2.2 20-30 Arm circumference (cm) 25.17 ± 3.0 21-35 HR (bpm) 59.0 ± 10.0 45-93 SBP (mmHg) 107.0 ± 10.0 87-142 DBP (mmHg) 62.0 ± 8.0 46-86 MAP (mmHg) 76.0 ± 8.0 60-105
13 A total of 55 individuals (28 men and 27 women) volunteered to participate in these experiments. The participants’ characteristics are presented in Table 2.1.
Experiment 1: Effect of varying venous occlusion pressure and time on vascular indices
The results for experiment 1 are depicted in Table 2.2 as well as Figures 2.1, 2.2 and 2.3.
There was no significant affect in altering the venous occlusion pressure and time on forearm resting or reactive hyperemic blood flow (Table 2.2). The data indicate a stepwise increase in measures of forearm venous capacitance (Figure 2.1a) and outflow
(Figure 2.1b) with increasing venous occlusion pressures. In contrast, forearm inflow was not affected by the pressure maneuvers. Furthermore, forearm venous capacitance and outflow were consistently greater following 10 minutes compared to 5 minutes of venous occlusion (Figure 2.2). Finally, venous capacitance showed a stepwise increase as the venous occlusion time increased (Figure 2.3).
Table 2.2. Resting vascular indices following varying occlusion pressures Press 1 Press 2 Press 3 Press 4 Press 5 (7
Experiment 2: Reliability of MSGP to measure arterial and venous function indices.
Between-day reliability values using the ICCC are presented in Table 2.3. The ICCC values for the peak blood inflow and vascular resistance were 0.81 and 0.75, respectively.
14 Additionally, resting and following upper arm arterial occlusion venous capacitance and outflow ICCC values ranged between 0.70 and 0.94.
6 1 = 07mmHg < DBP 2 = 14mmHg < DBP 5 3 = 21mmHg < DBP 4 = 28mmHg < DBP 5 = 35mmHg < DBP 4 ‡ ‡ ‡ * § 3 * Venous capacitance
(ml·100ml tissue-1·min-1 2
1 12345 Venous occlusion pressures (mmHg)
Figure 2.1a. Venous capacitance using different pressures. Values are in means ± SD. ‡p<0.007 vs. 1; *p<0.01 vs. 2; §p<0.03 vs. 3.
60 1 = 07mmHg < DBP 2 = 14mmHg < DBP 3 = 21mmHg < DBP 50 4 = 28mmHg < DBP 5 = 35mmHg < DBP 40 ‡ ‡ ‡ § 30 * *
Venous outflow †
(ml·100ml tissue-1·min-1 20
10 12345 Venous occlusion pressures (mmHg)
Figure 2.1b. Venous outflow using different pressures. Values are in means ± SD. ‡p<0.02 vs. 1; *p<0.02 vs. 2; §p<0.05 vs. 3; †p<0.06 vs. 4.
15
40 5
)
4 30 ‡
‡
3 outflow Venous 20
Venous capacitanceVenous (ml·100ml tissue-1·min-1 (ml·100ml (ml·100ml tissue-1·min-1) (ml·100ml
2 10 5Minutes 10Minutes 5Minutes 10Minutes
Time Time
Figure 2.2. Venous capacitance and outflow following 5 & 10 minutes of venous occlusion. Values are in means ± SD. ‡p<0.05 vs. 10 minutes.
5 ‡ ‡ ‡ 4 ‡ ‡ ‡ ‡ ‡ 3 ‡
2
Venous capacitance 1 (ml·100ml tissue-1·min-1
0 12345678910 Time (Minutes)
Figure 2.3. Venous capacitance over a 10 minute venous occlusion period. Values are in means ± SD. ‡p<0.05 vs. minute 1.
16
Table 2.3. Between-days ICCC for forearm arterial and venous indices Conditions RBF FVR Venous Venous (ml/100ml/min) (U) Capacitance Outflow (ml/100ml/min) (ml/100ml/min) Rest 0.29 0.30 0.78 0.94 Post occlusion 0.81 0.75 0.70 0.91 ICCC = interclass correlation coefficient; RBF = resting blood inflow; FVR = forearm vascular resistance.
Experiment 3 & 4: Relationship between physical fitness measures and arterial and venous function indices.
The associations between measures of physical fitness and vascular function are presented in Table 2.4 and depicted in Figures 2.4 and 2.5. Estimated VO2peak was significantly correlated with peak forearm blood inflow and vascular resistance, and with venous outflow. Further analyses revealed a significant difference in vascular function between those with higher VO2peak estimates (46.4 ± 6.6 ml/kg/min) compared to the lower fit group (32.3 ± 3.4 ml/kg/min).
65 0.54; p = 0.012
55
45 (mL/kg/min) 2
VO 35
25 10 20 30 Arterial inflow (mL· 100mL tissue-1· min-
Figure 2.4a. Relationship between estimated VO2 & post arterial occlusion blood inflow.
17 60 0.56; p = 0.016
50
40 (mL/kg/min) 2
VO 30
20 25 35 45 55 65 Venous outflow (mL· 100mL tissue-1· min-
Figure 2.4b. Relationship between estimated VO2 & resting venous outflow.
Handgrip strength was significantly correlated with venous capacitance and outflow at
rest and following occlusion (see Figures 2.5 a & b). Moreover, as presented in Table 2.6,
venous function indices were different between the HI (121.1 ± 10.64 lb) vs. LO (82.14 ±
14.9 lb) handgrip strength. No significant associations were found between handgrip
strength and forearm blood inflow at rest or following occlusion (see Table 2.4).
Finally, resting and peak forearm arterial inflow and vascular resistance correlated with resting and post occlusion venous capacitance and outflow (see Table 2.4).
2.4. DISCUSSION
This study was designed to examine relationships between measures of physical fitness and arterial and venous function. In addition, methodological concerns and the reliability of arterial and venous function measures using MSGP were also addressed. Uniquely, the data indicate the magnitude of forearm venous outflow is positively associated with both cardiovascular (estimated VO2peak) and musculoskeletal (handgrip strength) fitness.
Moreover, venous function measures are significantly influenced by the magnitude of the
occlusion pressure and length of occlusion period without changes in arterial inflow.
18 Finally, measures of arterial as well as venous function appear to be reasonably reproducible over a 10-day period.
7 r=0.57; p=0.007
6
5
Venous outflow 4 (mL·100mL tissue-1·min-1)
3 40 60 80 100 120 140 Strength (lb)
Figure 2.5a. Relationship between handgrip strength & resting venous capacitance.
r=0.67; p=0.001 70
60
50
40 Venous outflow 30 (mL·100mL tissue-1·min-1)
20 40 60 80 100 120 140 Strength (lb)
Figure 2.5b. Relationship between handgrip strength & resting venous outflow.
19 Table 2.4. Pearson product moment correlation between arterial, venous and physical fitness measures
Conditions Resting vascular indices Post occlusion vascular indices Fitness measures Blood Vascular Venous Venous Blood Vascular Venous Venous Estimated Handgrip Variables inflow resistance capacitance outflow inflow resistance capacitance outflow VO2peak strength Blood inflow 1.0 (mL/100mL/min) Vascular resistance r=-0.9; 1.0 (U) p=0.001
indices Venous capacitance r=0.5; r=-0.4; 1.0 (mL/100mL/min) p=0.001 p=0.012
Resting vascular vascular Resting Venous outflow r=0.4; r=-0.5; r=0.76; 1.0 (mL/100mL/min) p=0.013 p=0.002 p=0.0001 r Blood inflow r=-0.007; r=-0.18; r=0.18; r=0.38; 1.0 (mL/100mL/min) p=0.9 p=0.26 p=0.27 p=0.018 Vascular resistance r=-0.3; r=0.06; r=0.13; r=-0.2; r=-0.83; 1.0 (U) p=0.13 p=0.7 p=0.45 p=0.19 p=0.0001
indices Venous capacitance r=0.52; r=-0.54; r=0.7; r=0.6; r=0.2; r=0.005; 1.0 (mL/100mL/min) p=0.001 p=0.0001 p=0.0001 p=0.0001 p=0.2 p=0.975 Venous outflow r=0.54; r=-0.63; r=0.77; r=0.85; r=0.4; r=-0.27; r=0.64; 1.0
Post occlusion vascula (mL/100mL/min) p=0.001 p=0.0001 p=0.0001 p=0.0001 p=0.02 p=0.12 p=0.0001
Estimated VO2peak r=0.04; r=-0.3; r=-0.08; r=0.56; r=0.54; r=-0.7; r=0.2; r=0.44; 1.0 xxxx (mL/kg/min) p=0.876 p=0.2 p=0.74 p=0.016 p=0.012 p=0.001 p=0.410 p=0.07
Fitness Handgrip strength r=0.02; r=-0.05; r=0.57; r=0.67; r=0.2; r=0.07; r=0.6; r=0.5; xxxx 1.0 measures (kg) p=0.93 p=0.8 p=0.007 p=0.001 p=0.4 p=0.76 p=0.004 p=0.005
20 Table 2.5. Differences between the cardiorespiratory groups in forearm arterial and venous indices Group Resting Condition Post Occlusion. Condition Binf Vres Binf Vres Arterial LO 1.6 ± 0.7 52.1 ± 18.7 18.5 ± 2.87 4.3 ± 0.7 Indices HI 1.6 ± 0.5 46.7 ± 13.8 22.2 ± 4.9 3.4 ± 0.8 p-value 0.46 0.91 0.05 0.013 Vcap Vout Vcap Vout Venous LO 3.8 ± 0.8 40.7 ± 7.9 2.6 ± 0.7 36.9 ± 8.2 Indices HI 4.0 ± 0.7 49.5 ± 9.7 2.9 ± 0.8 48.9 ± 9.7 P-value 0.54 0.05 0.30 0.014 Binf = blood inflow; Vres = vascular resistance; Vcap = venous capacitance; Vout = venous outflow.
Table 2.6. Differences between the handgrip strength groups in forearm arterial and venous indices Group Resting Condition Post Occlusion. Condition Binf Vres Binf Vres Arterial LO 3.48 ± 1.2 26.3 ± 12.4 19.39 ± 3.7 4.27 ± 1.0 Indices HI 3.53 ± 0.9 24.32 ± 4.9 20.60 ± 4.1 4.24 ± 0.8 p-value 0.94 0.73 0.55 0.94 Vcap Vout Vcap Vout Venous LO 4.9 ± 0.8 45.0 ± 10.7 3.61 ± 0.60 43.8 ± 10.0 Indices HI 5.8 ± 0.6 56.2 ± 5.3 4.47 ± 0.60 54.6 ± 7.2 P-value 0.030 0.038 0.014 0.04 Binf = blood inflow; Vres = vascular resistance; Vcap = venous capacitance; Vout = venous outflow.
2.4.1. Technical Aspects and Reliability of Vascular Measures
In recent years the post occlusion reactive hyperemia model has been developed as a non- invasive surrogate marker of cardiovascular risk (4, 6). Previous work from our laboratory has indicated adequate stability and reproducibility of the post-occlusion technique for the study of arterial reactivity under strictly controlled conditions using
21 both ultrasonography (34) and plethysmography (20). Given recent information regarding
the importance of the venous system on arterial function (3, 11, 31), the present study
was designed to include information regarding venous function. Clearly, if the post-
occlusion model is extended to include information regarding venous function, it is
essential the reproducibility and limitations of the technique be established.
Data from experiment 1 indicates the importance of the selection of an
appropriate venous occlusion pressure in the study of venous capacitance and outflow.
The data reveals a stepwise decrease in venous capacitance and outflow as the magnitude
of venous occlusion pressure from resting diastolic pressure was increased (see Figures
2.1 a & b). The rational for the examination of varying venous occlusion pressures is the lack of consensus regarding the selection of a standard pressure. Two different approaches in the selection of venous pressure appear to exist. The first is the use of a standard (usually 50 mmHg) venous occlusion pressure (7, 12) and the second is the use of a venous occlusion pressure based on the individual’s diastolic pressure (33). The present findings indicate that a venous occlusion pressure of 7 mmHg below diastolic blood pressure appears to yield greater venous capacitance and outflow compared to occlusion pressures further below the diastolic pressure. This is particularly important when considering longitudinal trials in which blood pressure is subject to change.
A second inconsistency in the literature is the duration of venous occlusion.
Standard recommendations suggest venous occlusion periods of 2 minutes to examine venous capacitance and outflow (7, 12). The present data (see Figure 2.2) clearly indicate that venous capacitance and outflow were greater following 10 minutes of venous occlusion at 50 mmHg compared to 5 minutes. The magnitude of the difference was
22 particularly evident when comparing occlusion periods of 1 through 6 minutes, whereas
the difference in the venous measures was less “drastic” with occlusion periods between
7 and 10 minutes (see Figure 2.3).
Thus, when implementing these measures in longitudinal trials careful
consideration should be given regarding the venous occlusion pressure and period.
Interestingly, alterations in venous occlusion pressures and times did not have a
significant effect on the arterial inflow measures.
Experiment 2 examined the reliability of measuring arterial as well as venous function using MSGP at rest and following occlusion over a 10 day period (see Table
2.3). The magnitude of the forearm blood flow values at rest and following occlusion are consistent with data from our laboratory (1) as well as others (29). The reliability measures used in the current study confirm adequate consistency between days for the post occlusion in flow measures (20, 24). In addition, the ICCC results indicate adequate reliability for venous capacitance and outflow at rest and following arterial occlusion.
Resting blood in flow measures appear to vary considerably from visit to visit, despite the fact careful standardization of many external factors, such as meal timing, testing
schedule, testing environment, subject placements and clothing. This appears to be a
consistent inconsistency in the literature and is difficult to rationalize (20, 24, 29). The
values for these measures and the range from day to day are consistent with previously reported values from our (20) as well as other laboratories (24). However, it is important to consider the influence of the “wandering” baseline on the calculation of the blood flow difference between the peak blood flow response following the period of arterial occlusion and resting blood flow. The use of the absolute reactive hyperemic response
23 would therefore appear to be more appropriate when using this model during longitudinal
trials.
2.4.2. Relationship Between Fitness and Vascular Function
The ability to distribute and deliver adequate blood flow, oxygen and nutrients to
contracting skeletal muscle is recognized as a major contributor to exercise performance
(25, 27). Data from the current study are consistent with this idea in that forearm vascular
reactivity defined as the peak blood flow response following arterial occlusion was
significantly related to estimated VO2peak (see Figure 2.4). It is believed the ability for
greater performance is in part secondary to the repeated stimulus of exercise training
which over time contributes to a series of arterial adaptations aimed at increasing
vascular conductance and oxygen and nutrient delivery to working skeletal muscle (18).
The main finding in the present study is the consistent associations between
measures of venous function and cardiovascular and musculoskeletal fitness. These findings further confirm data from our laboratory suggesting that exercise tolerance in
heart failure patients is related to venous function (35). The associations suggest the
contribution of the venous system to overall cardiovascular performance and skeletal
muscle activities is beyond the typical role as a “passive volume reservoir”.
Uniquely, the data of this study reveal an association between venous outflow and
estimated VO2peak; and greater venous outflow in the HI cardiovascular fitness group.
These results suggest that factors associated with venous function might play a role in
exercise performance. Considering the cardiovascular system is a close circuit, as well as
the venous circulation enormous potential to act as a volume reservoir and its contribution to the end-diastolic volume (8), this segment of the vasculature might have a
24 profound impact on exercise performance. We hypothesize improved venous outflow
may contribute to enhanced exercise performance secondary to greater metabolic waste
removal from exercising muscles, increased venous return, and/or enable greater arterial
inflow (3, 8, 11, 31).
Another unique finding in this study is the association between measures of
venous function and handgrip strength suggesting a link between muscular fitness and
venous circulation. Further comparison reveals greater venous function measures in the
HI handgrip strength group suggesting the importance of muscular fitness to venous function. Interestingly, individuals in the HI strength group reported long-term involvement in weight training. Given the numerous benefits of resistive exercise training in cardiovascular performance (9, 21, 37), as well as modification of cardiovascular disease risk profile and overall health (15, 32, 36), the current findings suggest that resistance training might also benefit the venous system. This is particularly relevant considering venous abnormalities in aging (17, 23), and patients with heart failure (14,
28, 35) and venous diseases (10, 30). Therefore, recognizing the contribution of skeletal muscle in venous return and overall cardiovascular performance during exercise, factors associated with muscle strength modification might contribute to increased overall cardiovascular health subsequent to improved venous function, namely venous outflow and capacitance. However, it is recognized that the current study is limited to a cross- sectional comparison and future studies should be designed to directly examine whether resistance training and subsequent strength modifications can indeed alter venous function in both health and disease.
25 The association between arterial and venous measures suggests an
intercommunication link between the arterial and venous circulations. Interestingly,
compelling evidence has emerged illustrating the importance of various aspects of the
venous system beyond its typical role as a “passive volume reservoir”. For example,
Barclay et al. demonstrated that muscle fatigue was delayed as a result of increased blood flow across the muscle bed and subsequent increased metabolic removal independent of oxygen and/or nutrient delivery (3). In a more recent study, Tschakovsky & Hughson reported greater arterial inflow patterns with increased venous emptying following arm elevation (31). The authors linked these findings to the local venoarteriolar sympathetic
axon reflex. The presence of nerve fiber collaterals from the sympathetic arteriolar plexus to adjacent venules reflex was initially described in 1991 by Rygaard et al. and may serve as the anatomical substrate for the local venoarteriolar sympathetic axon reflex (26).
Finally, evidences of cross-communication between the arterial and venous circulations
indicate that venular endothelium releases a relaxing factor (s) that contributes to the
vasodilatation of adjacent arterioles (11). The optimal position of the venous circulation
might provide a monitor of the tissue metabolic status and a potentially valuable feedback
mechanism to control the arterial circulation. Therefore, the relationship between arterial
and venous function indices, suggests that vasoreactivity and muscle perfusion might be
influenced by venous hemodynamics confirming the essential role of venous function in
overall cardiovascular control, muscle contraction and exercise performance (3, 11, 31).
2.5. CONCLUSION
These findings indicate the importance of recognizing the possible influence of venous
occlusion times and pressures on venous capacitance and outflow. Additionally, the
26 ICCC results indicate that MSGP is a reliable technique for measuring venous function on different days under controlled conditions and useful to differentiate between populations. Considering these technical aspects and establishing the reliability of MSGP are particularly relevant during cross-sectional as well as repeated measures designs, especially in cases when blood pressure may be altered. Finally, forearm venous outflow is positively associated with measures of cardiovascular (estimated VO2peak) and musculoskeletal (handgrip strength) fitness. The association between handgrip strength and venous system hemodynamics implies that muscular conditioning might be important for venous health, which warrants future studies examining the effect of muscular strength modifications on venous function.
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30 CHAPTER 3. INFLUENCE OF VENOUS FUNCTION ON EXERCISE TOLERANCE IN CHRONIC HEART FAILURE*
*Reprinted by permission of Lippincott Williams & Wilkins publisher (See appendix C)
31 3.1. INTRODUCTION
Chronic heart failure (HF) is a complex clinical syndrome characterized by an
abnormality of the heart and by a recognizable pattern of hemodynamic, renal, neural and
hormonal responses (24). The manifestations of the profound multi-organ changes often become most evident during acute bouts of physical activity as many patients experience shortness of breath and early fatigue compared to age-matched controls. Indeed, much
research has focused on the role of vascular and skeletal muscle alterations on exercise
tolerance. Blood flow to exercising skeletal muscle is reduced in chronic HF patients
(32). The proposed mechanisms for the reduction in blood flow appear to be quite
complex but include cardiac insufficiency, neurohumoral hyperactivity (eg, sympathetic
nervous system, angiotensin and endothelin mediated), and neurohumoral hypoactivity
(eg, endothelium-derived-relaxing factors), and vascular remodeling (3). In addition, decreased skeletal muscle mass, alterations in fiber type composition and metabolic milieu further modify the regional distribution of blood flow (3). As a result of these findings, additional treatment strategies (eg, angiotensin- converting enzyme inhibitors
and exercise training) have been developed and implemented in the care of the chronic
HF patient. Yet, despite these interventions many patients continue to experience activity-related symptoms, possibly suggesting the involvement of other factors.
Interestingly, the role of the venous system on exercise tolerance in patients with HF has not received much attention, despite evidence of changes in venous structure and
function in patients with HF (14, 27) and in patients with a variety of vascular diseases
(20, 29). Considering that the cardiovascular system is a closed circuit, cardiac
insufficiency also may have a profound effect on venous function. Accordingly, the
32 purpose of this study was to examine the relationships between indices of arterial and venous function and exercise tolerance in chronic HF patients and age-matched controls.
Given the complexity of the HF syndrome, we hypothesized a direct relationship between indices of vascular function and exercise tolerance.
3.2. METHODS
3.2.1. Subjects Class I and III (New York Heart Association Functional Classification) HF patients and age-matched controls were recruited to participate in the study. Individuals recovering from recent hospitalization, with unstable myocardial ischemia and/or angina, uncontrolled diabetes mellitus and/or hypertension, anemia, lung disease, or renal failure were excluded from participation. The patients continued to take the dose of medications as prescribed by their physicians. After comprehensive explanation of the proposed study, it benefits, inherent risks, and expected commitments, all participants signed an informed consent approved by the Institutional Review Board of Louisiana State
University.
3.2.1. Assessment of Vascular Function Forearm vascular indices were obtained using mercury in-silastic strain-gauge plethysmography (EC-5R system, Hokanson, Wash). Prior to the experiment, blood pressure cuffs were positioned around the participant’s upper arm and wrist, and a mercury-in-silastic strain gauge placed around the forearm approximately 10 cm distal to the olecranon process (1). Resting vascular indices were made in duplicate, after 15 minutes of supine rest, with the measures separated by 5 minutes. Immediately before obtaining measurements, hand circulation was occluded for 1 minute by inflating the cuff at the wrist to 240mmHg (1). Forearm arterial inflow was obtained using an upper arm
33 venous collecting pressure of 50mmHg (1). Forearm venous capacitance and venous
outflow were measured after an additional 2 minutes of venous occlusion (1).
Subsequently, forearm vasoreactivity was examined after 5 minutes of forearm arterial
occlusion achieved by inflating the cuff on the upper arm to 240mmHg (1). Forearm
vascular indices were then determined as described above. Throughout the experiment,
heart rate and blood pressure were obtained at rest, during and immediately after forearm
occlusion.
3.2.3. Assessment of Exercise Tolerance Exercise tolerance was measured on a motor driven treadmill as the maximum walking
distance achieved on a 6-minute walk test (9). Participants were instructed to walk as far
as possible in 6 minutes, avoiding chest pain, shortness of breath, or other symptoms.
During the test the participants controlled the speed of the treadmill and allowed to
change the speed at any time. The participants was not coached during the test, but made
aware of time remaining to completion. Throughout the test, blood pressure and heart rate
were monitored continuously by an exercise physiologist.
3.2.4. Data Analysis Resting forearm arterial inflow was recorded at a paper speed of 5 mm/sec. and values
were derived from the slope drawn at a best-fit tangent using the first 3 pulses.
Calculations were made as a function of 60 seconds divided by the horizontal distance
(mm) needed for the slope to rise vertically from baseline to the top of the recording paper and multiplied by the full chart range (4). Forearm arterial inflow responses after occlusion were recorded at a paper speed of 25 mm/sec. Analyses was performed using a slope drawn at a best-fit tangent to the curves of the first 3-pulse flows post cuff release.
The blood flows were then calculated from 60 seconds multiplied by the paper speed (25
34 mm/sec.) divided by the horizontal distance (mm) needed for the volume slope to
increase by 20 mm vertically (24). Forearm vascular resistance was calculated by
dividing mean arterial pressure by forearm arterial inflow. Forearm venous indices at rest
and following occlusion were recorded at a paper speed of 25mm/sec. Forearm venous
capacitance was calculated as the vertical distance (mm) representing the increase in
forearm-volume graph after 2 minutes of venous filling (4). Analysis of forearm venous
outflow was derived from a tangent line that represents the vertical drop in the volume-
graph from the excursion line and drawn at 0.5 seconds after the release of the venous
occlusion pressure (4). All vascular indices are expressed in mL • tissue-1 • min-1.
3.2.5. Statistical Analysis Statistical analysis was performed using SPSS for Windows (version 10.0,Chicago, Ill).
Group values are expressed as mean ± SD. Independent t tests were used to compare
vascular indices and maximum walking distance between the HF and control groups.
Pearson product moment correlation was used to examine relationships between vascular
indices and maximum walking distance. Alpha was set a priori at p<.05.
3.3. RESULTS
3.3.1. Patient Characteristics and Compliance Twenty HF patients (age 57 ± 15) and age 10 age-matched controls (age 54 ± 17 years)
volunteered to participate in the study (Table 1). Seventy-five percent of the patients were
black and 55% were women. All patients were recruited from lower socioeconomic areas.
The major etiologies of HF in this population included coronary artery disease (5 of 20),
hypertensive heart disease (15 of 20), and idiopathic cardiomyopathy (5 of 20). Due to
35 equipment failure, forearm venous capacitance and venous capacitance and venous outflow after occlusion were not obtained in one and three HF subjects, respectively.
Table 3.1. Participant characteristics CHF (n=20) Control (n=9) Mean ± sd Range Mean ± sd Range p-value Age (yr.) 59 ± 13 33-80 51 ± 16 32-74 0.18
Height (cm) 170 ± 8 155-180 169 ± 10 152-180 0.78
Weight (kg) 97 ± 27 58-165 74 ± 12 57-91 0.03
Resting Heart Rate 72 ± 17 45-98 64 ± 8 55-82 0.24 (beats/min) Resting Systolic Blood Pressure 139 ± 12 117-164 131 ± 13 114-155 0.14 (mmHg) Resting Diastolic Blood Pressure 79 ± 9 60-95 75 ± 6 69-86 0.19 (mmHg) Maximum Walking Distance 178 ± 65 75-321 563 ± 136 346-756 0.0001 (m) Etiology of CHF CAD 5/20 N/A HTN 15/20 ICM 5/20 Other 4/20 Systolic Failure 18/20
Left Ventricular Ejection 38 ± 18 10-70 N/A Fraction (%) CAD= Coronary Artery Disease; HTN=Hypertension; ICM=Idiopathic Cardiomyopathy.
3.3.2. Exercise Tolerance The maximum walking distance achieved on the 6-min walk test was significantly lower in the patients with HF compared to the control group (HF: 171 ± 75 m versus controls:
541.1 ± 145 m; p=0.0001).
36 3.3.3. Vascular Function As depicted in Table 2, forearm vascular indices at rest were not different between
Table 3.2. Independent t-test comparisons between CHF and control groups. Heart Failure Control t p (n=20) (n=9) Forearm arterial inflow (ml•100ml tissue-1 •min-1) 3.20 ± 1.20 3.46 ± 1.17 -0.63 0.53 Rest 14.8 ± 6.31 22.20 ± 6.30 -2.94 0.007 Post Occlusion Forearm vascular resistance (U) 36.00 ± 14.87 29.44 ± 10.26 1.2 0.22 Rest 8.40 ± 4.50 4.50 ± 1.34 2.69 0.012 Post Occlusion Forearm venous capacitance (ml•100ml tissue-1 •min-1) 1.90 ± 0.36 2.25 ± 0.13 -2.20 0.031 Rest 1.40 ± 0.54 1.88 ± 0.40 -3.43 0.022 Post Occlusion Forearm venous outflow (ml•100ml tissue-1 •min-1) 24.30 ± 8.00 27.25 ± 4.40 -1.10 0.3 Rest 29.50 ± 11.70 42.30 ± 10.37 -2.80 0.009 Post Occlusion groups. However, forearm arterial inflow, forearm venous capacitance, and forearm venous outflow after occlusion were significantly lower in the patients with HF.
Additionally, forearm vascular resistance was significantly higher in the patients with
HF.
3.3.4. Relationship Between Forearm Vascular Indices and Exercise Tolerance Correlation analysis revealed significant associations between all indices of vasoreactivity and exercise tolerance (forearm arterial inflow: r = 0.58, P = .001; forearm vascular resistance: r = -0.52, P = .004). Lastly, as shown in Figure 1, indices of venous function after occlusion were significantly related to maximum walking distance (forearm venous capacitance: r = 0.57, P = .001). Forearm venous outflow is shown in Figure 2 (r
= 0.67, P = .0001).
37 3.4. DISCUSSION
The unique finding of this study is the association between forearm venous capacitance
and venous outflow and the maximum walking distance achieved on a 6-minute walk
test. These data suggest factors that contribute to exercise impairment in patients with HF
may extend to the venous system. Given the important contributions of the venous
circulation to metabolic waste removal and end-diastolic volume, changes in this segment
of the vasculature could have a profound impact on the exercise response.
800 700 r =0.57; p =001 600 500 400
MWD (m) 300 200 100
.25 .75 1.25 1.75 2.25 2.75 Vcap (ml·100ml tissue-1·min-1)
Figure 3.1. Relationship between forearm venous capacitance and maximum walking distance.
As expected, the maximum walking distance achieved on the 6-minute walking test was significantly lower in the HF patients, suggesting compromised exercise tolerance.
The reduced maximum walking distance reported in the current study is consistent with previous reports (18, 23, 30) and have been attributed to pathophysiological abnormalities
that accompany the disease (28).
38 800 700 r =0.67; p =0001 600 500 400
MWD (m) 300 200 100
10 15 20 25 30 35 40 45 50 55 60 Vout(ml·100ml tissue-1·min-1)
Figure 3.2. Relationship between forearm venous outflow and maximum walking distance.
The use of the 6-minute walking test to examine exercise tolerance in patients with HF has received a lot of attention recently, due to the relative ease of its administration. The test is also thought to reflect a realistic effort as performed in daily life, and highly expectable by patients (9). Furthermore, the test appears to have adequate construct,
criterion, and predicted validity, as well as have a high degree of reproduceability.16
Construct validity has been shown by several investigators as the six-minute walk test
appears to discriminate between various populations (8, 15, 16, 23). Criterion validity
stems from studies showing adequate associations between exercise capacity (as defined
by VO2peak) and maximum walking distance on the six-minute walk test (11, 26). Yet, as
these findings are not consistent from one study to another, further work is needed to
verify the validity of the test. Finally, several studies have reported the prognostic
significance 6-min walking test in patients with HF indicating its predictive validity (16,
19).
39 A number of studies have reported adequate test, retest reliability for the maximum
walking distance achieved on the 6-min walking test (9, 11, 21). In the present study the
average distance walked increased by 5% from the first to the second test. However, the
interclass correlation coefficient of 0.97 and the CV of 8.9% indicate adequate stability of
maximum walking distance over a one week period. These data support research by
Guyatt et al, who recommend 2 walks be performed, and if the difference between the
first and second walk is greater than 10% a third walk should be undertaken and used as
the baseline measure (9).
The reduction in forearm arterial inflow and elevation in vascular resistance observed
in the current study are similar to previous reports (31, 33). Although the precise
mechanisms for the alterations in arterial vasoreactivity is not entirely understood, proposed mechanism for the reduction in peripheral blood flow include cardiac
insufficiency, neurohuromal hyper- (e.g. sympathetic nervous system, angiotensin and endothelin mediated vasoconstriction) and hypo-activity (e.g. endothelium- derived- relaxing factors), and vascular remodeling. In addition, decreased skeletal muscle mass,
alterations in fiber type composition and metabolic milieu further modify the regional
distribution of blood flow.
Previous investigations have related diminished venous function to respiratory-
cardiovascular hemodynamics, plasma norepinephrine (14) and severity of disease in
patients with HF (14, 27). Data from the current study confirm differences in venous
function following forearm occlusion between HF patients aged-matched controls. The unique finding of the current study is the demonstration that venous function are
40 associated with a measure of exercise tolerance, suggesting factors that contribute to
exercise impairment in chronic HF patients may extend to the venous system.
Examination of the literature regarding the reproducibility and validity of strain gauge
plethysmography to investigate arterial and venous function is difficult due to variations
in study aims, protocol, equipment, and interpretation of results. However in general the
literature appears to indicate adequate reproducibility for measures of arterial
vasoreactivity. In fact, unpublished data from our laboratory for within and between day
reliability indicate interclass correlation coefficients between 0.82 and 0.95 for forearm
arterial inflow and vascular resistance at rest and following forearm occlusion. The
stability of the venous function measurements is currently not clear. However, strain
gauge plethysmography is routinely used to detect venous dysfunction (12, 22, 25). The
technique appears to have adequate construct validity as it is able to differentiate between
patients with venous insufficiency and age-matched controls (7), and between patients of
different age groups (13). Moreover, high sensitivity, specificity, and predictive values were reported when strain gauged plethysmography technique was tested against
venography, the “gold dash standard”, to detect deep venous thrombosis (5). Preliminary
data from our laboratory revealed less than 7% difference for within and between day
measures at rest and following forearm occlusion.
Given the important contributions of the venous circulation to metabolic waste
removal and end-diastolic volume, changes in this segment of the vasculature could have
a profound impact on the exercise response. Evidence indicates that many HF patients are hyperadrenergic (34) and have elevated levels of endothelin-1 (17) and angiotensin II
(6) suggesting a possible neurohumoral influence on venous function (20). Considering
41 the cardiovascular system is a closed circuit, we speculate cardiac insufficiency,
decreased arterial reactivity, reduced muscle blood flow, venous alterations and elevated
cardiac pressures may contribute to a reduction in the driving pressure across metabolic
tissue. Consequently, decreased driving pressure could effect metabolic waste removal
(2), alter cardiac filling kinetics (10) and decrease exercise tolerance.
3.5. CONCLUSION
This study shows both arterial and venous indices are altered in HF patients and are associated with impaired exercise tolerance. Given the important contributions of the venous circulation to metabolic waste removal and end-diastolic volume, and the association of venous function with exercise tolerance among patients with HF demonstrated herein, future investigations should address venous health in the management of HF.
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45 CHAPTER 4. MODIFICATION OF FOREARM VASCULAR FUNCTION FOLLOWING SHORT-TERM HANDGRIP EXERCISE TRAINING*
*Reprinted by permission of Georg Thieme Verlag Stuttgart Publisher (See appendix C)
46 4.1. INTRODUCTION
Endurance exercise training promotes adaptations of vascular control mechanisms that contribute to an increase in vasodilatory capacity thereby facilitating the delivery of blood to exercising muscle. The precise mechanism for vascular adaptations following exercise training are not fully understood, however, changes in the responsiveness of vascular endothelial and/or smooth muscle may, in part, enhance the vasodilatory capacity. For example, Delp & Laughlin (3) reported enhanced endothelium-dependent dilation of the rat aorta by the fourth week of endurance training and concluded that the adaptation was in part mediated through the nitric oxide (NO) synthase pathway.
Interestingly, the enhanced forearm peak vasodilatory capacity observed following four weeks of handgrip (HG) exercise training occurred without changes in the activity of the
NO system as reported by Green et al. (4). This suggests that the exercise stimulus to induce changes in vasoresponsiveness may, in part, be dependent on the exercise training modality. Indeed the above-mentioned studies appear to indicate that exercise modality has a differential effect on vascular adaptations (2, 3). More recently, Hepple et al. reported a dissociation of peak vascular conductance and VO2max in body builders and
endurance runners compared to healthy sedentary control, suggesting training-specific
effects of the cardiovascular system to local skeletal muscle metabolic demand (6).
An alternate explanation for the difference in vasoresponsiveness between
training protocols is that these adaptations are, in part, intensity-dependent. Interestingly,
vascular adaptations following small muscle group exercise training programs using different intensities have not been adequately explored. We are aware of only one study by Yasuda et al. (15) where subjects performed 6-weeks of HG exercise training at
47 intensities of 33 or 50% of maximum voluntary contraction (MVC). Peak forearm blood
flow adaptations increased significantly, but similarly in both groups. However, the
difference between training intensities was only 17% and may have been insufficient to
determine whether the adaptations are indeed intensity-dependent. Accordingly, the
purpose of this study was to determine the effects of low (LO) and high (HI) intensity short-term HG exercise training programs on localized vascular function in healthy young men. It was hypothesized that a training exercise intensity of 75% of MVC would promote greater vasodilatory capacity compared to a training exercise intensity of 25% of
MVC.
4.2. METHODS
4.2.1. Experimental Groups and Screening Measures Twenty-eight males (Age: 23±4.3yrs) were recruited and randomly assigned to either a
LO (n=15) or HI training group (n=13). Smokers and individuals with acute medical
conditions (e.g. orthopedic injury), active infection and/or on pharmacotherapy with
known vascular effects (e.g. anti-inflammatory therapy, cardiovascular medications) were
excluded. Following comprehensive explanation of the proposed study, its benefits,
inherent risks and expected commitments with regard to time, all participants signed an
informed consent approved by the Institutional Review Board of Louisiana State
University. In addition, each subject filled out a 24-hour medical and activity history questionnaire.
4.2.2. Experimental Design The study was a randomized, prospective design consisting of two experimental sessions
separated by a 4-week intervention period. The experimental sessions involved the
measurement of forearm blood flow (FBF) at rest (RBF), following 5 minutes of forearm
48 occlusion (OCC) and OCC combined with 3 minutes of HG exercise (ROCC). The
subjects exercised 5 days per week for 4 consecutive weeks at the exercise physiology
laboratory of Louisiana State University. Each training session consisted of 20 minutes of non-dominant HG exercise at a cadence of one contraction every 4 seconds.
Participants in the LO group trained at 25% of MVC, participants in the HI group trained
at 75% of MVC.
4.2.3. Experimental Measurements Upon arrival to the laboratory, the subject’s height and weight were obtained. In addition,
MVC was determined for both arms as the average of three maximum consecutive
contraction trials using a Stoelting hand-dynamometer. Thereafter, each subject was placed in a supine position for 20 minutes prior to evaluation of forearm vascular blood flow. Forearm blood flow measurements were obtained from both arms (the non- dominant first) using an EC-5R plethysmography system (Hokanson, 1995). Prior to each experiment, blood pressure cuffs were positioned around the participant’s upper arm and wrist, and a mercury-in-silastic strain gauge (7) placed around the forearm approximately
10 cm distal to the olecranon process (13). The forearm was extended and slightly supinated and supported by a Styrofoam block. Immediately before the blood flow measurements, hand circulation was occluded for 1 min by inflating the cuff at the wrist to 240mmHg (10). All blood flow measures were made with an upper arm venous collecting pressure of 50mmHg (11) and reported in ml per min per 100 ml of tissue (13).
Reactive hyperemia following OCC was achieved by inflating the cuff on the upper arm to 240mmHg (13). Blood flow measures were then determined as described above.
Finally, vasoresponsiveness following ROCC was obtained. Handgrip exercise for the
ROCC condition was performed at an intensity of 25% MVC and a cadence of 1
49 contraction per 4 sec during the 2nd, 3rd, and 4th minutes of occlusion. ROCC
measurements were obtained following OCC and were separated by 15 minutes.
Throughout the testing procedures, heart rate and blood pressure were obtained at the
following time points: rest, during and immediately after OCC and ROCC. Throughout
the testing procedures, which lasted approximately 120 minutes the subject remained in
the supine position.
4.2.4. Data Analysis Resting blood flow was recorded at a paper speed of 5 mm/sec. and values were derived
from the slope drawn at a best-fit tangent using the first 3 pulses. Calculations were made
as a function of 60 seconds divided by the horizontal distance (mm) needed for the slope
to rise vertically from baseline to the top of the recording paper and multiplied by the full
chart range. Blood flow responses following OCC and ROCC were recorded at a paper
speed of 25 cm/sec. Analyses were performed using a slope drawn at a best-fit tangent to the curves of the first 3-pulse flows post cuff release. The blood flows were then calculated from 60 sec. multiplied by the paper speed (25 cm/sec) divided by the horizontal distance (mm) needed for the volume slope to increase by 20 mm vertically.
Mean arterial pressure was calculated from the difference between SBP and DBP divided by three and added to DBP {MAP = DBP+[(SBP-DBP)/3]}. Forearm vascular resistance was calculated by dividing mean arterial pressure by the forearm blood flow (FVR =
MAP/FBF).
50 4.2.5. Statistical Analysis
Grip strength Dependent t-tests were used to compare the grip strengths of dominant and non-dominant
arms observed prior to and following the training period. Separate t-tests were conducted
for the LO and HI intensity training groups.
Comparison of dominant to non-dominant arms Two 3x2 ANOVA’s were used to compare the FBF and FVR during each condition (rest vs. OCC vs. ROCC) in the dominant and non-dominant arms. The pre-training and post- training data were treated separately.
Effect of training on vascular responses. A 3x2x2 ANOVA was used to examine the effects of the training period on FBF and
FVR as measured under the three conditions (rest vs. OCC vs. ROCC), and across the
two training groups (LO vs. HI intensity).
Alpha was set a-priori at p<0.05 for all statistical analyses. The LSD method of post-hoc
testing was employed where indicated.
4.3. RESULTS
4.3.1. Participant Characteristics The initial number of subjects was thirty, however one participant was excluded because
of noncompliance and one participant for violation of exclusion criteria (initiated
smoking). As shown in Table 1 there were no statistically significant differences between
the LO and the HI groups in the pre-training measurements for age, height, weight,
resting heart rate, and blood pressure.
51 Table 4.1. Subject characteristics LO HI (n=15) (n=13) Age (yrs) 22 ± 2.5 24 ± 5.8 Weight (kg) 86 ± 7.5 70 ± 5.4 Height (cm) 175 ± 15.4 174 ± 10
SBPrest (mmHg) 106.5 ± 18.78 106.9 ± 9
DBPrest (mmHg) 68.3 ± 12.3 68.3 ± 7.4
MAPrest (mmHg) 81 ± 10 81.5 ± 6.3 -1 HRrest (bts•min ) 62.1 ± 6.3 64.5 ± 7 Values are mean ± SD. LO, low training intensity = 25% of maximum voluntary contraction (MVC); HI, high training intensity = 75% of MVC; SBPrest, systolic blood pressure at rest; DBPrest, diastolic blood pressure at rest; MAPrest, mean arterial blood pressure at rest; HRrest, heart rate at rest.
4.3.2. Grip Strength All participants completed at least 18 sessions of HG exercise training over the 4-week
period. The result of the t-test revealed no change in maximal grip strength in the trained
and non-trained arms of both groups. However, there was trend towards an improvement
in the trained arm strength in the HI group (35.3±8.8 to 38.3±8.8 kg; 8%; p=0.095).
4.3.3. Comparison of Dominant to Non-Dominant Arm As could be depicted from figure 1 and 2 and table 2, the results of the ANOVA on the
pre-training values of FBF revealed an interaction for test condition (p<0.0001) and arm
(p=0.002). This interaction was such that OCC and ROCC resulted in a graded FBF
response, with the greatest hyperemic FBF occurring following ROCC. Furthermore FBF
following OCC and ROCC were greater in the dominant arm compared to the non-
dominant arm. There was also a main effect of test condition (p<0.0001) on FVR. FVR
was lower following OCC and ROCC compared to rest, but not different from each other.
However, FVR in the dominant arm was not different from that of the non-dominant arm.
52 ‡ † 45 § * § § 40 * § * ‡ † * 35 * * * 30 *
25
20 Pre Post 15
10
Forearm Blood Flow (ml.100ml-1.min-1) 5
0 Rest OCC ROCC Rest OCC ROCC Dominant Arm Non-dominant Arm
Figure 4.1. FBF changes in the LO group following HG exercise training. Values are means ± SD. *p<0.05 vs. rest; §p<0.05 vs. OCC ‡p<0.05 vs. Pre Non-dominant; †p<0.05 vs.
The results of the ANOVA on the post-training FBF values revealed a main effect of test
condition (p<0.0001) but not for arm (p=0.718). This main effect was such that the OCC
and ROCC once again resulted in a graded response. However, following training the
response of the non-dominant arm was not different from that of the dominant arm. FVR
responses following OCC and ROCC were similar to pre-training values for both arms.
4.3.3. Effect of Training on Vascular Responses. The results of the ANOVA on the effects of training on FBF in the non-dominant arm revealed a main effect for training period (p=0.005). However, the ANOVA did not
reveal a group by training interaction (p=0.632). The main effect was such that FBF values following OCC and ROCC were higher after the training period as compared to pre-training values. However, there was no interaction with training intensity, suggesting
53 that both training groups responded similarly. The results of the ANOVA on FVR
revealed only a main effect of the test condition (i.e. rest, OCC, ROCC). These main
effects are the same as described above, and did not change significantly as a result of the
training period in either the LO or HI intensity group.
† 45 ‡ § § § § * 40 ‡ * * † * 35 * * * 30 *
25
20 Pre 15 Post 10 Forearm Blood Flow (ml.100ml-1.min-1) 5
0 Rest OCC ROCC Rest OCC ROCC Dominant Arm Non-dominant Arm
Figure 4.2. FBF changes in the HI group following HG exercise training. Values are means ± SD. *p<0.05 vs. rest; §p<0.05 vs. OCC ‡p<0.05 vs. Pre Non-dominant; †p<0.05 vs. Pre
4.4. DISCUSSION
This study examined the role of a short-term HG exercise program on FBF and FVR. Our findings indicate that the degree of vascular responsiveness is directly related to the intensity of the vasodilatory stimulus, given that the blood flow measures were significantly different for all 3 conditions (ROCC > OCC > RBF). Furthermore, vasodilatory responsiveness prior to 4 weeks of HG exercise was significantly greater in the dominant compared to the non-dominant arm. Finally, following 4 weeks of HG exercise
54 performed 5 days per week for 20 minutes vasodilatory responsiveness improved unilaterally but similarly for the LO and HI intensity groups.
Table 4.2. Dominant and non-dominant FVR changes in the LO and HI groups at rest, following OCC and following ROCC. Dominant Non-dominant Pre Post Pre Post
LO group (15) FVRrest 47.85 ± 13 42.2 ± 13.4 42.5 ± 15.4 44.6 ±17.0 FVROCC 3.3 ± 1* 3.5 ± 0.5* 3.5 ± 0.6* 3.5 ± 0.8* FVRROCC 2.5 ± 0.5* 2.8 ± 0.4* 3 ± 0.4* 2.9 ± 0.7* HI group (13) FVRrest 44.5 ± 19 45.7 ± 22.7 39.4 ± 15 34.4 ± 8.8 FVROCC 3.1 ± 0.5∗ 3.1 ± 0.5∗ 3.7 ± 0.6∗ 3.3 ±0.7∗ FVRROCC 2.7 ± 0.3∗ 2.77 ± 0.4∗ 3.3 ± 0.7∗ 2.5 ± 0.7∗ Values are means ± SD. LO, training intensity = 25% of maximum voluntary contraction (MVC); HI, training intensity = 75% of MVC; FVRrest, forearm vascular resistance at rest; FVROCC, forearm vascular resistance following five minute of occlusion; FVRROCC, forearm vascular resistance following OCC coupled with 3-min. of HG of exercise. ∗p < 0.05 vs. rest.
Previous studies have reported improved unilateral vasodilatory responsiveness following small muscle group conditioning programs (see Table 3) (4, 9, 14). The improvements in peak FBF following OCC in this study were similar for the HI group
(20.6%) to those reported by Green et al. (23%) but lower than Sinoway et al. (30%). The discrepancy between our findings and Sinoway et al. could be the result of a difference in total minutes of exercise performed or methods of assessing forearm blood flow. For example, the training regime used by Sinoway et al. included four, 30-minute sessions per week (120 minutes per week) versus our five, 20-minute sessions per week (100 minutes per week) for 4 weeks. In addition, Sinoway et al. evaluated the effect of short- term HG exercise on vasodilatory responsiveness after 10 minutes of arterial occlusion
(14). Lastly, the supine rest period prior to obtaining the resting blood flow varied between studies.
55
Table 4.3. Literature overview Study Population Type of training Results
Yasuda, 83 10 healthy, HG, 33 & 50% MVC, 6X/wk (6 34 & 26% ⇑ BF, respectively. young wks)
Sinoway, 87 6 healthy, HG, 70% MVC, 4X/wk, 30min ⇔ BFrest. young (4 wks) 30% ⇑ RHBF in trained arm
Green, 94 11 healthy, HG, 75% MVC, 4X/wk, 30min ⇔ BFrest young (4 wks) 23% ⇑ RHBF in trained arm
Bond, 94 7 healthy, Plantar Flexion, 75% MVC, ⇔ BFrest. young 4X/wk, 6sets of 15reps (4 wks) 22% ⇓ in RHBF, ⇑ VR.
Franke, 98 6 healthy, HG, 70% MVC, (4 wks) ⇔FVCrest. young 35% ⇑ FVCpeak. Bank, 98 11 healthy, HG, 70% MVC, 4X/wk, 30min, ⇔ in CHF. young & 7 CHF (4-6 wks) 24% ⇑ RHBF in healthy, young. BF = Blood flow; RHBF = Reactive hyperemic blood flow; VR = Vascular resistance; FVC = Forearm vascular conductance.
Our findings stand in contrast to other studies of different training duration or muscle groups (1, 12, 15). For example, Yasuda et al. reported an increase in FBF in the ipsi- and contralateral arm following 6 weeks of HG exercise training (15) and attributed the improvements to changes in neurogenic balance (15). In contrast, other investigators report a reduction in limb blood flow following short-term training programs. Bond et al.
(1) attributed reductions in leg blood flow to an increase in muscle cross-sectional area without proportional changes in microvascular density, whereas Saito et al. attributed the lower leg blood flow to a more effective metabolic process (12). Clearly, more studies are needed to further understand the vascular adaptive responses to exercise training.
Similar to the study by Yasuda et al. (15) the current study found no statistical differences in the degree of change in vasoresponsiveness between the LO and HI groups following the training period. Subsequent to training, FBF increased 16.5%, and 17.7% in the LO group, and 20.6%, and 29.2% in the HI group after OCC, and ROCC, respectively. The lack of a significant difference between the groups suggests that the
56 signal for vascular adaptations may have been similar for both groups. Moreover, it indicates that significant local vascular adaptations can be induced with small muscle group conditioning programs at intensities as low as 25% of MVC. Such adaptations may be particularly valuable in populations for whom low-intensity exercise training is indicated. In fact, recent studies have reported short-term HG training results in enhanced endothelium-dependent vasodilation in the forearm skeletal muscle circulation of patients with chronic heart failure (9). Likewise, patients with other disease conditions (e.g. diabetes mellitus, peripheral vascular disease, hypertension and coronary artery disease) who suffer from compromised vascular function might benefit from such small muscle group conditioning regimes. Clearly, further investigations are needed to verify this theory and whether such training programs can ultimately contribute to an individual’s improved exercise performance.
Another finding of this study is the observed difference in vasodilatory capacity between the dominant and non-dominant arm. This data is consistent with Sinoway et al. who reported a 25% and 29% difference following OCC and ROCC respectively in the dominant arm versus non-dominant arm of tennis players (13). Likewise, in the current study, vascular capacity in the dominant arm was greater than the non-dominant arm
(10% and 12% difference following OCC and ROCC respectively) perhaps as a consequence of preferential usage. The difference between Sinoway’s findings and ours is most likely the result of regular participation in the sport of tennis, as all the subjects in
Sinoway’s study were considered advanced players (13). Significantly, the observed differences in FBF between the dominant and the non-dominant arm prior to training in this study disappeared after training. This indicates that forearm vascular adaptations in
57 the dominant arm were matched by short-term forearm HG exercise training in the non-
dominant arm.
Although this study is not adequately or purposely designed to explain the exact
mechanism of the training adaptations, several previous investigations have indicated the
role of the endothelium. Shear stress associated with exercise has been proposed to
contribute to the increased production of endothelial derived relaxing factors, such as NO
(8). It is postulated that the contraction and relaxation phases of exercise results in brief
circulatory arrest followed by a hyperemic response (3, 5). This intermittent blood flow
might create shear stress on the vascular wall that is responsible for the vascular
adaptations demonstrated in this study. Clearly, other factors (e.g. increased muscle
capillarizations, or changes in local metabolites, neuroendocrine balance, other myogenic
factors and/or cardiac function) associated with exercise might also contribute to improved vascular function (3). The fact vasoreactivity is a complex interplay of central, vascular and local control mechanisms limits this study’s ability to precisely identify the mechanism(s) for change.
In conclusion, FBF and FVR at rest and following forearm occlusion (with and without HG exercise) showed a graded response with the greatest hyperemic flow recorded after five minutes of forearm occlusion with HG contraction. Furthermore, the non-dominant FBF was found to improve in response to low-intensity as well as high- intensity short-term HG exercise training. Furthermore, blood flow in the non-dominant forearm following training was increased to match the flow in the dominant forearm. The exact mechanism(s) behind these vascular adaptations are not known and warrants further investigations.
58 4.5. REFERENCES
1.Bond V, Jr., Wang P, Adams RG, Johnson AT, Vaccaro P, Tearney RJ, Millis RM, Franks BD and Bassett DR, Jr. Lower leg high-intensity resistance training and peripheral hemodynamic adaptations. Can J Appl Physiol 21: 209-217, 1996.
2.Copeland SR, Mills MC, Lerner JL, Crizer MF, Thompson CW and Sullivan JM. Hemodynamic effects of aerobic vs resistance exercise. J Hum Hypertens 10: 747-753., 1996.
3.Delp MD and Laughlin MH. Time course of enhanced endothelium-mediated dilation in aorta of trained rats. Med Sci Sports Exerc 29: 1454-1461, 1997.
4.Green DJ, Cable NT, Fox C, Rankin JM and Taylor RR. Modification of forearm resistance vessels by exercise training in young men. J Appl Physiol 77: 1829-1833, 1994.
5.Green DJ, O'Driscoll JG, Blanksby BA and Taylor RR. Effect of casting on forearm resistance vessels in young men. Med Sci Sports Exerc 29: 1325-1331, 1997.
6.Hepple RT, Babits TL, Plyley MJ and Goodman JM. Dissociation of peak vascular conductance and V(O2) max among highly trained athletes. J Appl Physiol 87: 1368- 1372, 1999.
7.Holling HE, Boland HC and Russ E. Investigation of arterial obstruction using a mercury-in-rubber strain gauge. Am Heart J 63: 194-205, 1961.
8.Hornig B, Maier V and Drexler H. Physical training improves endothelial function in patients with chronic heart failure. Circulation 93: 210-214, 1996.
9.Katz SD, Yuen J, Bijou R and LeJemtel TH. Training improves endothelium-dependent vasodilation in resistance vessels of patients with heart failure. J Appl Physiol 82: 1488- 1492, 1997.
10.Kerslake DM. The effect of the application of an arterial occlusion cuff to the wrist on the blood flow in the human forearm. J Physiol 108: 451-461, 1949.
11.Patterson GC and Whelan RF. Reactive hyperemia in the human forearm. Clin Sci Lond 14: 197-211, 1955.
12.Saito M, Matsui H and Miyamura M. Effects of physical training on the calf and thigh blood flows. Jpn J Physiol 30: 955-959, 1980.
13.Sinoway LI, Musch TI, Minotti JR and Zelis R. Enhanced maximal metabolic vasodilatation in the dominant forearms of tennis players. Journal of Applied Physiology 61: 673-678, 1986.
59
14.Sinoway LI, Shenberger J, Wilson J, McLaughlin D, Musch T and Zelis R. A 30-day forearm work protocol increases maximal forearm blood flow. J Appl Physiol 62: 1063- 1067, 1987.
15.Yasuda Y and Miyamura M. Cross transfer effects of muscular training on blood flow in the ipsilateral and contralateral forearms. Eur J Appl Physiol Occup Physiol 51: 321- 329, 1983.
60 CHAPTER 5. ARTERIAL AND VENOUS ADAPTATIONS TO SHORT-TERM HANDGRIP EXERCISE TRAINING
61 5.1. INTRODUCTION
Structural and functional changes in the arterial system following exercise training are
well documented. These adaptations are associated with reduced vascular resistance,
improved blood delivery and diffusion in the contracting muscle and contribute to
improved vascular modulation and exercise performance during subsequent exercise sessions. The mechanism(s) by which these adaptations are induced are not fully understood, but are thought to include alterations in the arterial endothelium and/or
smooth muscle (4, 8, 12, 18, 32, 65). Recent studies report training-induced adaptations
may be observed following one week of training in animal models (35, 61), and in human
single large conduit arteries (i.e. brachial) (2).
Interestingly, the effect of exercise training on venous function has received
considerably less attention despite compelling evidence demonstrating its dynamic role
beyond a typical role as a “passive volume reservoir” (22, 72). Most of the available
studies have linked training-induced improvements in venous function to greater blood
volume associated with increased VO2 (10, 29, 41), as well as increased venular cross-
sectional area (49) and density (70). On the other hand, Wecht et al. attributed enhanced
venous function in “aerobically” trained individuals to improved venous hemodynamics,
including venous emptying, outflow and compliance (76). However no studies have
examined the effect of localized exercise training (i.e. handgrip) on regional venous
function.
Therefore, the purpose of this study was to examine the influence of 4-weeks of
handgrip exercise training on arterial and venous function. A secondary aim of this study
62 was to determine the time course of change for both arterial and venous indices associated with the handgrip exercise training program.
5.2. HYPOTHESES
This study tested two specific hypotheses regarding the influence of handgrip exercise on arterial (Hypothesis I) and veno-dynamics (Hypothesis II).
5.2.1. Hypothesis I Four-weeks of handgrip exercise training will result in an increase in arterial reactivity following 5-minutes of arterial occlusion;
5.2.2. Hypothesis II Four-weeks of handgrip exercise training will result in an increase in venous capacitance, compliance and outflow at rest and following 5-minutes of arterial occlusion.
5.3. METHODS
5.3.1. Participants and Design Apparently healthy sedentary young adult males were recruited to participate in the study. Individuals with manifestations of cardiovascular, metabolic, orthopedic or neurological disease or on any medication (e.g., digitalis) which could affect the results of this study were excluded.
The study was executed in 4 weeks, and involved once a week evaluation session and 5-time-a-week exercise training sessions. Prior to and at the end of each week, participants underwent handgrip strength and vascular function evaluations in the dominant and non-dominant forearms. The primary outcomes of this study include: (1)
Peak blood inflow, defined as vascular response to 5 minutes of upper arm arterial occlusion (64); (2) Venous capacitance; defined as the changes in limb blood volume following 6 minutes of venous occlusion; (3) Venous outflow; defined as the changes in
63 limb blood volume following the release of venous occlusion and; (4) Venous
compliance; defined as the ratio of change in volume to concomitant change in
transmural distending pressure (i.e. occlusion pressure) (11, 23, 54, 76). Additionally,
handgrip strength, arm exercise tolerance, and hemodynamic measures (i.e. blood
pressure, heart rate and heart rate variability) were also assessed. Following the
completion of the initial assessment, each subject was asked to report to the exercise
physiology laboratory 5 days/week to perform 20 minutes of forearm flexion exercise at a cadence of 1 contraction every 4 seconds starting at an intensity of 60% of maximum
handgrip strength. The non-dominant arm was trained while the dominant arm served as control.
5.3.2. Study Measurements:
Vascular function assessments Vascular function indices were obtained in the dominant and non-dominant forearms
using mercury strain gauge plethysmography at rest and following occlusion (64) at the
beginning of the study, and at the end of each week throughout the study period. This
technique is noninvasive, and based on the assumption that alterations of pressures in
strategically placed cuffs allows examination of the rate of change of limb volume
thought to reflect venous function indices including blood inflow, vascular resistance,
venous capacitance, venous outflow and venous compliance (11, 23). Prior to the
experiment, blood pressure cuffs were positioned around the participant’s upper arm and
wrist, and a mercury-in-Silastic strain gauge placed around the forearm approximately 10
cm distal to the olecranon process (4). Resting forearm vascular function measures were
obtained following 15 minutes of supine rest. Immediately before the measurements, hand
circulation was occluded for 1 minute by inflating the cuff at the wrist to 240 mmHg.
64 Forearm blood inflow was obtained using an upper arm venous occluding pressure of 7 mmHg below diastolic blood pressure. Forearm venous capacitance was measured after
an additional 6 minutes of venous occlusion and venous outflow following the release of
pressure (3, 66). Subsequently, peak forearm blood was examined following arterial
occlusion achieved by inflating the cuff on the upper arm to 240 mmHg for 5 minutes.
Forearm vascular indices were then determined as described above. These were then
performed on the other arm. Prior to, during and following each procedure blood pressure and heart rate were obtained.
Autonomic nervous function assessments Indices of autonomic function were assessed using measures of heart rate variability.
These measures were obtained at the same time as the vascular assessments described
above. The ECG electrodes were placed on the chest and interfaced with a Biopac
MP100 and its companion software Acqknowledge (model MP100A, Biopac Inc., Santa
Barbara, CA) to allow for continuous data acquisition. Five minutes (at rest and during
forearm occlusion) of ECG data were analyzed for mean heart period (mean R-R
interval), standard deviation (SDNN), low frequency power (LF) and high frequency
power (HF) using the guidelines set forth by the Task Force of the European Society of
Cardiology and the North American Society of Pacing and Electrophysiology (7, 71).
These data were obtained at the beginning and at the end of the study period.
Arm ergometer test Arm exercise tolerance was evaluated using a graded arm ergometer exercise test at the
beginning and at the end of the study period (5, 48). The test was performed by initially
turning the arm ergometer at a speed of 50 rpm and a resistance of 0.5 kp for 2 minutes
65 with the participant in a seated position. The resistance was then increased by 0.25 kp
every 2 minutes until the subject reached exhaustion, requested to stop, or exhibited signs
or symptoms, which require test termination. Prior to, during, and following the exercise
test, heart rate, blood pressure measurements, and ratings of perceived exertion were also
obtained at the end of each minute throughout the test using a standard sphygmomanometer and the Borg's perceived exertion scale, respectively.
Forearm circumference and handgrip strength assessments The participant’s forearm circumferences and handgrip maximal voluntary contraction
were evaluated at baseline and at the end of each week throughout the study period.
Forearm circumference was examined using a weighted measuring tape 10 cm distal to
the midpoint between the lateral epicondyle and olecranon process. Handgrip strength
was evaluated using a handgrip dynamometer (Lafayette instruments, OH, 02) with the
participant upright, but slightly bent forward at the waist. The test involved an all-out
gripping effort for 3 seconds, without movement of the arm. The average of three
consecutive trials was used as the measure of strength.
Body composition Three-site skinfold method was used to estimate body percents of fat and fat-free masses.
The recommended body sites for this method were chest, abdominal and thigh (80).
Additionally dominant and non-dominant forearm skinfolds were obtained at the upper
1/3 of the forearm.
5.3.3. Data Reduction and Analysis Resting blood inflow was recorded at a paper speed of 5 mm/seconds and values were
derived from the slope drawn at a best-fit tangent using the first 2-3 pulses. Calculations
66 were made as a function of 60 seconds divided by the horizontal distance (mm) needed for the slope to rise vertically from baseline to the top of the recording paper and multiplied by the full chart range (11, 23). Peak forearm blood inflow was recorded at a paper speed of 25 mm/second. Analyses were performed using a slope drawn at a best-fit tangent to the curves of the first 3-pulse flows post cuff release. Blood flows were then calculated from 60 seconds multiplied by the paper speed, and divided by the horizontal distance (mm) needed for the volume slope to increase by 20 mm vertically (4). Forearm vascular resistance was then determined by dividing mean arterial pressure by the blood inflow. Forearm venous capacitance was calculated as the vertical distance (mm) representing the change in forearm-volume graph following 6 minutes of venous filling
(11). Forearm venous compliance was calculated as the ratio of venous capacitance and occlusion pressure multiplied by 100 (76). Forearm venous outflow was derived from a tangent line representing the vertical drop in the volume-graph from the excursion line and drawn at 0.5 second and 2 second following the release of the venous occlusion pressure (11). All vascular indices were expressed in ml•100ml tissue-1•min-1.
5.3.4. Statistical Analysis All statistical analyses were completed using the SPSS statistical program (version 11.00,
Chicago, Ill). To examine the influence of 4-weeks of handgrip exercise training on arterial reactivity (Hypothesis I) a 2 (Arms) * 5 (visit 1-vist 5) split-plot ANOVA was used. To examine the influence of 4-weeks of handgrip exercise training on venous capacitance, compliance and outflow at rest and following 5-minutes of arm occlusion
(Hypothesis II) a second 2 (Arms) * 5 (visit 1-vist 5) split-plot ANOVA was used.
Pearson product moment correlations were used to examine the relationships between
67 handgrip strength and vascular function indices, between arterial and venous function
indices as well as between arterial and venous adaptations. Finally a paired t-test was
used to compare between pre and post training measures of HRV indices, the skinfold
measures, and handgrip strength. Alpha was set a-priori at p<0.05.
5.4. RESULTS
5.4.1. Participants Twenty-two men participated in the study. Five participants did not complete the study
due to personal decision (3 individuals) or lack of compliance (2 individuals). Participant
descriptions are presented in Table 5.1.
Table 5.1. Subject characteristics Visit 1 Age (yrs) 22.6 ± 3.5 Height (cm) 175.4 ± 1.6 Weight (kg) 75.5 ± 4.0 Control arm length (cm) 27.5 ± 0.3 Trained arm length (cm) 27.4 ± 0.3 Sum of Skin fold (mm) 54.4 ± 7.7 Values are mean ± SE.
5.4.2. Hemodynamic, Anthropometric and Exercise Measures Table 5.2. Cardiovascular hemodynamics and HRV before and after 4-week handgrip exercise training Variable Visit 1 Visit 5 HR (bpm) 58.2 ±1.6 57.5 ± 1.8 SBP (mmHg) 112.6 ± 2.4 111.1 ± 2.1 DBP (mmHg) 60.7 ± 1.9 59.7 ± 1.8 MAP (mmHg) 78.0 ± 8.1 76.8 ±7.6 SDNN (ms) 73.4 ± 4.1 79.6 ± 4.0 LF power 36.9 ± 1.3 37.2 ± 1.4 HF power 63.1 ± 1.3 62.8 ± 1.4 Values are mean ± SE.
68 Hemodynamic, anthropometric and exercise measures at baseline (visit 1) and the end of the study (visit 5) are presented in Tables 5.2 and 5.3. No significant changes in heart rate, blood pressure, and heart rate variability indices were observed. Moreover, there were no changes in anthropometric measures in the non-trained arm. In contrast, forearm circumference increased by 1.56% (p=0.03) in the trained arm. Handgrip strength was not modified in the non-trained arm, whereas the trained arm showed a 14.5% (p=0.014) increase in maximal voluntary contraction. Finally, there were no changes in the time to fatigue for the symptom-limited graded arm ergometer exercise test.
Table 5.3. Arm ergometer exercise test, forearm skinfolds, and sum of three skinfolds before and after 4-week handgrip exercise training Visit 1 Visit 5 Arm ergometer test (mints.) 10.3 ± 0.6 9.75 ± 0.9 Control handgrip strength (kg) 40.6 ± 2.9 42.8 ± 3.2 Trained handgrip strength (kg) 36.1 ± 2.1 41.3 ± 1.7 * Control arm circumference (cm) 26.3 ± 0.5 26.5 ± 0.5 Trained arm circumference (cm) 25.6 ± 0.6 26.0 ± 0.5 * Control arm skin fold (mm) 1.1 ± 0.2 1.1 ± 0.13 Trained arm skin fold (mm) 1.2 ± 0.3 1.0 ± 0.13 Values are mean ± SE. *=p<0.05 vs. visit 1
5.4.3. Effect of Training on Vascular Function Values for the blood flow responses at rest and following occlusion for visits 1 and 5 are
presented in table 5.4 and 5.5. Furthermore, Figures 1 through 5 present data for the
vascular measures across the entire study.
69
Table 5.4. Resting arterial and venous indices Control arm Trained arm Visit 1 Visit 5 Visit 1 Visit 5 Arterial inflow 1 1 2.9 ± 0.2 2.6 ± 0.2 3.0 ± 0.2 3.0 ± 0.2 (ml·100ml tissue- ·min- ) Vascular resistance 29.4 ± 2.3 31.7 ± 2.3 28.1 ± 2.3 28.6 ± 2.3 (U) Venous capacitance 1 1 4.6 ± 0.13 4.5 ± 0.13 4.8 ± 0.13 4.5 ± 0.13 (ml·100ml tissue- ·min- ) Venous compliance 8.6 ± 0.32 9.1 ± 0.33 9.9 ± 0.32 9.2 ± 0.33 (%) Venous outflow 1 1 42.5 ± 1.9 43.3 ± 1.9 44.8 ± 1.8 45.4 ± 1.8 (ml·100ml tissue- ·min- ) Values are mean ± SE.
Table 5.5. Post occlusion arterial and venous indices Control arm Trained arm Visit 1 Visit 5 Visit 1 Visit 5 Arterial inflow 1 1 22.1 ± 0.7‡ 21.6 ± 0.7 19.0 ± 0.7 22.6 ± 0.7* (ml·100ml tissue- ·min- ) Vascular resistance 3.7 ± 0.2‡ 3.9 ± 0.2 4.3 ± 0.2 3.5 ± 2.3* (U) Venous capacitance 1 1 3.4 ± 0.13 3.4 ± 0.13 3.6 ± 0.13 3.5 ± 0.13 (ml·100ml tissue- ·min- ) Venous compliance 6.2 ± 0.25 6.2 ± 0.26 6.4 ± 0.24 6.3 ± 0.25 (%) Venous outflow 1 1 42.1 ± 1.7 44.9 ± 1.9 45.1 ± 1.7 45.5 ± 1.7 (ml·100ml tissue- ·min- ) Values are mean ± SE. *=p<0.05 vs. visit 1; ‡=p<0.05 vs. Trained arm.
Resting arterial function indices. The 2 X 5 split-plot ANOVA tests revealed no significant main effects for arms or visits, or for the arms by visits interactions for resting forearm arterial inflow and vascular resistance (see Table 5.4).
70 Control arm 25 Trained arm * ‡ * 1 * *
20 Arterial inflow (ml.100ml-1.min-
15 12345 Vis it
Figure 5.1. Changes in peak forearm blood inflow. Values are mean ± SE. *=p<0.05 vs. visit 1; ‡=p<0.05 vs. Trained arm.
Control arm 5 Trained arm
‡ 4 * * * * (U) 3 Vascular resistance
2 12345 Visit
Figure 5.2. Changes in peak forearm vascular resistance. Values are mean ± SE. *=p<0.05 vs. visit1; ‡=p<0.05 vs. Trained arm.
Peak arterial function indices. As depicted in Figures 5.1 and 5.2, the 2 X 5 split-plot ANOVA test revealed no
significant main effects for arms or visits for peak arterial indices, however there were
significant in the arms by visits interaction for trained forearm peak forearm arterial
inflow (p=0.02) and vascular resistance (p=0.009). Subsequent LSD post-hoc comparison
demonstrated increased peak arterial inflow, and decreased peak vascular resistance
71 following the 1st week in the trained forearm only. Finally, post-hoc comparison revealed greater peak forearm blood inflow (p=0.0013) and vascular resistance (p=0.05) in the
untrained verses the trained arm at visit 1, however, no differences were observed in the
subsequent visits.
Control arm 11 Trained arm † 10 † † ‡ * 9 (%) *
Venous compliance 8
7 12345 Vis it s
Figure 5.3. Changes in resting forearm venous compliance. Values are mean ± SE. *=p<0.05 vs. visit 1; ‡=p<0.05 vs. Trained arm; † =p<0.05 vs. visit 2.
Resting venous function indices. The 2 X 5 split-plot ANOVA tests revealed no significant main effects of arms or visits,
or arms by visits interactions for resting forearm venous capacitance and outflow (see
Table 4), however revealed significant (p=0.04) arms by visits interaction for venous
compliance. As depicted in Figure 5.3, subsequent LSD post-hoc comparison
demonstrated a decrease in trained arm venous compliance in visit 2 followed by a
gradual return to baseline by visit 5.
72 4
Control arm 3.8 ) Trained arm
3.6
3.4 (ml.100ml-1.min-1 Venous capacitance 3.2
3 12345 Vis it
Figure 5.4. Changes in post occlusion forearm venous capacitance. Values are mean ± SE.
Post-occlusion venous function indices The 2 X 5 split-plot ANOVA tests revealed no significant main effects for arms or visits,
or arms by visits interactions for post-occlusion forearm venous compliance and outflow.
However there was a significant main effect of visits (p=0.04) for venous outflow (see
Table 5.5 and Figure 5.4). As depicted in Figure 5.5, Subsequent LSD post-hoc
comparison demonstrated a decrease in trained forearm venous outflow in visit 2
followed by a gradual return to baseline by visit 5. The control forearm venous outflow
was greater at visit 2 than at visit 5.
Relationships between handgrip strength, and arterial and venous function indices The Pearson product moment correlations are presented in Table 5.6 and Figures 5.6 through 5.9. Significant associations were noted between handgrip strength and venous function indices (see Table 5.6 and Figures 5.6 and 5.7), and between arterial and venous function indices (see Table 5.6 and Figures 5.8 and 5.9).
73 50 Control arm Trained arm † † † † 1 45
40 Venous outflow (ml.100ml-1.min-
35 12345 Vis it
Figure 5.5. Changes in post occlusion forearm venous outflow. Values are mean ± SE. † =p<0.05 vs. visit 2.
7 r=0.58; p=0.001
6
5
4 Venous capacitance (mL·100mL tissue-1·min-1) 3 20 30 40 50 60 Strength (kg)
Figure 5.6. Relationship between handgrip strength & resting venous capacitance
74
Table 5.6. Pearson product moment correlation between arterial, venous and physical fitness measures Variables Resting vascular indices Post occlusion vascular indices Handgrip Blood Vascular Venous Venous Venous Blood Vascular Venous Venous Venous
strength inflow resistance capacitance compliance outflow inflow resistance capacitance compliance outflow
Handgrip strength 1.0 (kg) Blood inflow r=0.4* 1.0 (mL/100mL/min) Vascular resistance r=-0.26 r=-0.84* 1.0 (U) Venous capacitance r=0.58* r=0.35* r=-0.35* 1.0 (mL/100mL/min) Venous compliance r=0.27 r=-0.007 r=-0.3 r=0.67* 1.0 (%)
Resting vascular indices Resting vascular Venous outflow r=0.6* r=0.44* r=-0.5* r=0.81* r=0.52* 1.0 (mL/100mL/min) Blood inflow r=0.30 r=0.05 r=-0.2 r=0.4* r=0.45* r=0.4* 1.0 (mL/100mL/min) Vascular resistance r=-0.05 r=0.08 r=0.17 r=-0.26 r=-0.5* r=-0.27 r=-0.87* 1.0 (U) Venous capacitance r=0.57* r=0.23 r=-0.15 r=0.63* r=0.42* r=0.6* r=0.34# r=-0.15 1.0 (mL/100mL/min) Venous compliance r=0.25 r=-0.059 r=-0.07 r=0.43* r=0.55* r=0.5* r=0.5* r=-0.52* r=0.74* 1.0 (%) Venous outflow r=0.53* r=0.42* r=-0.51* r=0.76* r=0.56* r=0.95* r=0.43* r=-0.3‡ r=0.6* r=0.5* 1.0 Post occlusion vascular indices Post occlusion (mL/100mL/min)
*p<0.05; #p=0.06 † p= 0.07. ‡p=0.08
75 5.5. DISCUSSION
The main finding of this study was a unilateral improvement in regional peak forearm
arterial inflow and vascular resistance following handgrip exercise training. Furthermore,
the improvements were evident after the first week of training and appeared to plateau
during the remainder of the study. These findings confirm data from animal models (35,
61) as well as from human studies involving single conduit artery reactivity (2).
Uniquely, this study indicates a significant decrease in venous compliance after 1 week of training followed by a gradual return to baseline measures over the remainder of the study.
5.5.1. Effect of Handgrip Training on Arterial Indices Four weeks of handgrip training resulted in a 19% increase in regional peak arterial
inflow. This increase confirms previous data from our laboratory (21%) (4) as well as
others (25%) (65). Additionally, differences in peak arterial inflow and vascular
resistance between the dominant (non-trained) and non-dominant (trained) forearms at
visit 1 diminished by visit 2 suggesting vascular adaptations to habitual activities can be
matched following only 1 week of training.
The increased peak blood inflow in the trained arm without alterations in the
contra-lateral arm vasoreactivity, cardiovascular hemodynamics (i.e. blood pressure,
heart rate, and heart rate variability indices) and measures of cardiorespiratory fitness suggests that these adaptations are locally modulated. Although it is beyond the scope of this study to determine the mechanisms involved, these localized adaptations could involve either mechanical or chemical stimuli. The muscle contraction and relaxation phases associated with exercise training increase shear stress against the muscle bed arterial walls causing the release of endothelium-derived relaxing factor(s) (46). The
76 location of these changes is currently unclear. However, several studies have reported
increase in vasodilator mRNA and proteins at the site of large conduit vessels (13, 14)
while more recent studies suggest these changes occur in smaller distal vessels (36).
Alternatively, local accumulations of metabolites and vasoactive substances in distal
tissue beds could play a role in these adaptations. In fact, previous studies have shown an
increase in oxidative enzyme activity may precede the increases in vasoresponsiveness
(13).
Other studies have suggested regional neurohumoral changes following handgrip
exercise training (77). For example up-regulation of nitric oxide/prostacyclin is believed
to inhibit the release of norepinephrine from adrenergic nerves (9) and blunt norepinephrine-mediated vasoconstriction (74). Handgrip exercise training also appears to result in attenuated sympathetic vasoconstriction (56) attributed to changes in muscle metaboreceptors (67), mechanoreceptors (63) or a combination of both (53).
Additionally, Sinoway and colleagues demonstrated that this attenuation is accompanied by reduced norepinephrine arterial spillover and venous clearance (63).
Additional potential candidates that may contribute to the changes in vascular reactivity include up-regulation of prostaglandin (35, 61) and endothelium-derived hyperpolarizing factor (47), increased sensitivity of arterial smooth muscle to vasodilatory agents (34, 69), smooth muscle sarcoplasmic reticulum calcium unloading
(73) and reduced arterial wall sensitivity to endothelin-1 (28, 38). Clearly the speculations regarding the possible mechanism(s) for these adaptations need further refinement and experimental confirmation. Moreover, these speculations do not exclude the possibility of other plausible explanations for the regional vasoreactivity changes.
77 As depicted in Figure 5.1, post-occlusion blood inflow increased 13% after the first week, followed by a more gradual increase over the remainder of the training period.
These results confirm previous data from our laboratory (2) and appear to be consistent with the hypothesis advanced by McAllister and Laughlin. The authors speculate that vascular endothelial function is enhanced after just a few days of training and these adaptations could serve to buffer the increase in shear stress experienced during exercise
(47). Given that nitric oxide is an important signaling mechanism during exercise, and appears to be modifiable with exercise training, the nitric oxide system has emerged as a strong candidate responsible for the rapid changes in vascular reactivity. Both Wang et al.
(75), and Sessa et al. (59) report an enhanced nitric oxide-mediated dilation in the dog circumflex coronary artery in vivo, following 7 and 10 days of chronic exercise training.
More recently Johnson et al. (27) indicate increased acetylcholine induced relaxation and eNOS protein in porcine pulmonary arteries following 1 week of short-term training.
Kingwell and colleagues speculate that exercise-induced adaptations aimed at
meeting increased metabolic demands evolve from alterations in vasodilation to
adaptations in metabolic enzymes and vascular restructuring (31). In fact, recent data
suggest that nitric oxide contribution to exercise-training-induced arterial adaptations
might be a signal for subsequent increase in arterial wall dimensions (40). Increased shear stress (6) and flow-mediated nitric oxide production (46), associated with rhythmic exercise, inhibits intimal thickening and promotes arterial diameter enlargement (12, 40,
45). Wijnen and colleagues reported greater diameter of femoral but not brachial artery in
cyclists as compared to age-matched controls (78). In a more recent study, the diameter
of subclavian artery in the racket arm of tennis players was 19% greater than in the
78 opposite arm (26). Moreover, numerous studies have described exercise training-induced
increases in cross-sectional area by either recruiting already existing or developing new
vessels (24). These arterial structural alterations are associated with elevated skeletal
muscle blood inflow (57), and are believed to normalize increased blood shear stress and
to meet the increased metabolic demands associated with exercise training (12, 40, 52).
The plateau following the 1st week suggests a possible diminishing role of the
mechanism(s) involved in these early arterial adaptations. Since the individuals
participated in the study were sedentary, it is not clear whether the current training
regimen is sufficient to induce vascular adaptations in trained individuals. Therefore,
future studies should examine the effect of greater intensity and/or longer exercise
training program on arterial adaptations in sedentary as well as trained individuals.
The unilateral improvements in vasoreactivity indicate that exercise benefits are
specific to the trained arm confirming the importance of well-rounded exercise training
programs as advocated by ACSM (1). Therefore endurance and resistive exercise training
program targeting the heart and major muscle groups might be essential to induce overall
vascular adaptations. This is particularly relevant given the role improved arterial
function plays in blood delivery and perfusion, overall cardiovascular function, and exercise performance as well as in the prevention of cardiovascular diseases. In 1991,
Silber et al. reported increased peak forearm arterial inflow following cycle endurance
exercise training program (62). These findings were further confirmed following a
combined endurance and resistance exercise training program in healthy middle age
adults (44) as well as in patients with type 2 diabetes mellitus (42) and heart failure (43).
Exercise training-induced improvements in arterial function were also observed in the
79 coronary circulation of animals (37, 51) as well as in patients with coronary artery
disease (20), and recent myocardial infarction (25). Additionally, these arterial
adaptations were associated with improved endothelial function and exercise capacity
(42-44).
5.5.2. Effect of Handgrip Training on Venous Indices As presented in Tables 5.4 and 5.5, four weeks of handgrip training did not result in an
increase in forearm venous capacitance, compliance or outflow. In fact, venous measures
in the trained arm, at rest, and following 5-minutes of arterial occlusion showed a
consistent decline at the end of the first week followed by a gradual return to pre-training
values by visit 5 (see Figures 5.3-5.5). Figure 3 depicts a significant drop in resting
venous compliance in, the trained arm at visit 2 which remained below pre-training
values during visits 3 and 4.
The venous function responses to handgrip exercise training in this study are
intriguing. Since these responses are previously unreported, they are difficult to compare
to previous studies. However, the pattern of the data resembles the classic physiological
theory known as the Stress-Response-Adaptation. Hans Selye hypothesizes that
physiological stress causes an initial drop in physiological function followed by periods
of recovery and adaptation (58). The behavior of the data in the present study may reflect
the initial stress of training followed by a gradual recovery and beginning of an adaptive
period. Given that venous compliance is mainly affected by the sympathetic nervous
system (50, 76), regional elevation in sympathetic drive associated with the early phases of training (15, 56, 79) might result in venoconstriction. The increase in venoconstriction could lead to a reduction of venous blood volume in the forearm (15), which may account for the decline in venous function after the 1st week of training. Assuming that total
80 forearm blood volume remains the same following localized handgrip exercise training,
increased venoconstriction might decrease venous volume by driving blood towards the
arterial system. This theory may be supported by the concurrent increase in peak arterial
inflow during the 1st week of exercise training. Further analyses of the present data
revealed a weak relationship (p=0.023, r=0.58) between the magnitude of drop in venous
function and the increase in peak arterial blood inflow at visit 2; but it is premature to
suggest a direct link between those measures (see Figure 5.10).
Resting venous compliance gradually increases during visit 2-5. This response to
exercise training might be explained by two possible physiological changes. The persistence of the training stimulus may have resulted in an increased venous cross- sectional area by either recruiting already existing or developing new venules (49, 70).
Assuming the venoconstriction during the 1st week of training contributes to the increased
arterial inflow, newly accessible veins may allow for the delivery of more blood away
from the venous system into the arterial circulation. Additionally, venoconstriction may
also be attenuated subsequent to the readjustments in the sympathetic/parasympathetic
balance associated with exercise training (39). Although these mechanisms are simply
mere speculation, they reflect the recovery and adaptation phases of Selye’s theory (58).
The variations in training effect on the arterial and venous systems during the
training period might be due to the fundamental differences in the arterial and venous
structures and functions. For example, veins have thinner walls and are mainly composed
of smooth muscles, fibrous proteins, collagen, and elastin tissues (19). Additionally,
although an extensive neural network connects both sides of the vasculature, there are
differences in adrenoceptor distribution that might cause differential vascular
81 responsiveness to sympathetic stimulation (68). Endothelium-dependent dilatation also appears to operate differently in the venous side as a result of reduced relaxation, vasoagent production and/or smooth muscle responsiveness (16, 60). Given these differences and the effect of handgrip exercise training on the sympathetic nervous system (56) and endothelial tissue (17, 33, 46), it is reasonable to observe greater stiffness, less compliance and subsequent lower venous outflow under such conditions.
However, these speculations need further verifications in future studies.
Moreover, the unilateral changes in venous compliance suggest a stress response mediated by regional neural reflex and/or adrenoceptor sensitivity changes. Alterations in metaboreceptors (67), mechanoreceptors (63) stimulation and/or a combination of both
(53) have been proposed to induce attenuation in regional sympathetic activities. In the current study, cardiac autonomic modulation, supposedly a measure of autonomic nervous balance, did not change during the 4-week training program. However, heart rate variability might not be sensitive enough to detect localized neurohumoral changes.
Therefore, future studies should probably examine the effect of exercise training on regional neurohumoral modulations of the venous circulation, particularly the venoarteriolar sympathetic axon reflex.
The effect of an exercise training program with longer duration or higher intensity is unclear. Since the apparent increase in venous measures after visit 2 may lead to speculations regarding the adaptive changes using longer duration and/or higher intensity exercise training programs. Previous data from our laboratory showed a significant correlation between venous indices and handgrip strength as well as greater venous capacitance and outflow in individuals with higher handgrip strength. Individuals in the
82 higher strength group reported long-term involvements in resistance exercise training (3).
Moreover, Wecht et al reported greater venous compliance and total venous outflow in
“aerobically” trained individuals (76). As presented in Table 5.6 and Figures 5.6 and 5.7, results from the current study confirm the relationships between venous function indices and handgrip strength. Additionally, as in Figures 5.3-5.5, venous measures at visit 2 gradually increased reaching baseline levels at visit 5. However since training ended after
4 weeks, further venous adaptations to longer exercise training program are not known warranting future studies to examine the effect of different exercise training modes, intensities, and durations on venous indices.
70 r=0.6; p=0.002 60
50
40
30 Venous outflow 20 (mL·100mL tissue-1·min-1) 10 20 30 40 50 60 Strength (kg)
Figure 5.7. Relationship between handgrip strength & resting venous outflow 5.5.3. Relationship Between Arterial and Venous Functions Previous findings from our laboratory showed an association between forearm arterial and venous function indices (30). The results from the current study, as presented in
Table 5.6 and Figures 5.8 and 5.9, confirm these relationships, suggesting inter- communication between the arterial and venous circulations. Recent compelling evidence has emerged illustrating the importance of various aspects of the venous system beyond its typical role as a “passive volume reservoir”. For example, Tschakovsky & Hughson
83 reported greater arterial inflow with increased venous emptying following arm elevation
(72). The authors linked these findings to the local venoarteriolar sympathetic axon
reflex. The presence of nerve fiber collaterals from the sympathetic arteriolar plexus to
adjacent venules reflex was initially described in 1991 by Rygaard et al., and may serve
as the anatomical substrate for the local venoarteriolar sympathetic axon reflex (55).
35 r=0.5; p=0.006 30
25
20
Blood inflow 15
10 (mL·100mL tissue-1·min-1) 5 46810 Venous compliance (mL·100mL tissue-1·min-1)
Figure 5.8. Relationship between post occlusion blood inflow & venous compliance
Finally, evidence of cross-communication between the arterial and venous circulations indicates that venular endothelium releases a relaxing factor (s) that contributes to the vasodilatation of adjacent arterioles (21). Therefore, the relationship between arterial and venous function indices suggests that vasoreactivity and muscle perfusion may be influenced by venous hemodynamics, confirming the essential role of venous function in overall cardiovascular control, muscle perfusion and exercise performance (21, 72). Additionally, the correlation shown in Figure 10 between the decrease in venous compliance and the increase in arterial inflow following 1 week of handgrip training might further confirm the cross-communication between the arterial and venous circulations. The optimal location of the venous system in relation to the
84 arterial circulation may provide a mechanism for monitoring tissue metabolic status and potentially provide a valuable feedback mechanism for influencing the arterial reactivities. However more studies are certainly needed to further verify these speculations.
35 r=0.43; p=0.014 30
25
20
Blood inflow 15
10 (mL·100mL tissue-1·min-1) 5 30 40 50 60 70 Venous outflow (mL·100mL tissue-1·min-1)
Figure 5.9. Relationship between post occlusion blood inflow & venous outflow
5 r=0.58; p=0.023
0 Changes in blood inflow (mL·100mL tissue-1·min-1) -5 -10-8-6-4-20 2 Changes in venous compliance (mL·100mL tissue-1·min-1)
Figure 5.10. Relationship between changes in peak blood inflow & resting venous compliance
85
5.6. CONCLUSION
Data from the current study demonstrate a unilateral improvement in regional peak forearm arterial inflow and vascular resistance after the first week of handgrip exercise training and appeared to plateau during the remainder of the study. Additionally, the 4- week training did not result in increase in venous capacitance, compliance, and outflow.
However uniquely this study indicates a significant decrease in venous measures after 1 week of training followed by a gradual return to baseline values over the remainder of the study. The unilateral responses to handgrip exercise training further advocate the importance of whole-body exercise training programs targeting major muscle groups as recommended by recent ACSM exercise guidelines.
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92 CHAPTER 6. SUMMARY
Four projects were conducted in partial fulfillment for the requirement of this degree. The main findings of these studies were arterial and venous measures are reproducible, can differentiate between populations, and are modifiable with exercise training.
The results of the first study (chapter 2) indicate that a pressure of 7 mmHg <
DBP and a period of 10 minutes venous occlusion produced the greatest venous capacitance and outflow, without altering arterial indices. Additionally, the examination of forearm vascular function indices using mercury strain-gauge plethysmography are reproducible and associated with fitness measures (estimated VO2peak and handgrip strength). The association between handgrip strength and venous system hemodynamics implies that muscular conditioning might be important for venous health, which warrants future studies examining the effect of muscular strength modifications on venous function.
The results of the second study (chapter 3) indicate that arterial function indices are reduced in heart failure patients confirming previous studies (8, 15, 16). Additionally, venous capacitance and outflow are diminished and associated with maximum walking distance on a 6-minute walk test in heart failure suggesting the importance of venous function to exercise intolerance.
The results of the third study (chapter 4) indicate unilateral improvements in arterial inflow following 4-week handgrip exercise training similarly for the LO and HI intensity groups. The unilateral improvements suggest that vascular adaptations are locally mediated confirming previous findings (11, 14).
93 The results of the final study (chapter 5) indicate unilateral improvement in arterial inflow and vascular resistance following 1 week of handgrip exercise training.
Additionally, the exercise training resulted in a decrease in venous function indices followed by gradual increase to baseline values. The one week improvement in arterial function indices confirm previous findings from animals models (3) and human single artery (2) and suggest vascular adaptations to habitual activities can be matched following only 1 week of training. The venous function responses to handgrip exercise training in this study are previously unreported thus they are difficult to compare to other studies. However, the pattern of the data resembles the classic physiological theory known as the Stress-Response-Adaptation. Hans Selye hypothesizes that physiological stress causes an initial drop in physiological function followed by periods of recovery and adaptation (10). The unilateral responses to handgrip exercise training further advocate the importance of whole-body exercise training programs targeting major muscle groups as recommended by recent ACSM exercise guidelines (1).
The findings regarding the venous system are particularly uniqe and relevant considering venous abnormalities in aging (6, 7), and patients with heart failure (5, 9, 13) and venous diseases (4, 12). Therefore, recognizing the contribution of skeletal muscle in venous return and overall cardiovascular performance during exercise, factors associated with muscle fitness modification might contribute to increased overall cardiovascular health subsequent to improved venous function, namely venous outflow and capacitance.
However, it is recognized that the aforementioned studies are limited and more investigations are cairtainly needed to futher expand our knowledge regarding the effect of exercise training on the venous system.
94 6.1. REFERENCES
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95 13.Welsch MA, Allen JD and Geaghan JP. Stability and reproducibility of brachial artery flow-mediated dilation. Med Sci Sports Exerc 34: 960-965, 2002.
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96 APPENDIX A: LITERATURE REVIEW
97 SECTION 1. INTRODUCTION
With the latest developments in technology various vascular functions can be measured
in human, noninvasively, with minimum expense and relative ease. These functions
include arterial inflow, vascular resistance, venous filling capacity, venous emptying rate,
vessel diameter, vessel density, and volume changes. These recent advancements have
created opportunities for unprecedented prospects in vascular physiology research.
Presently, research is approaching integrative understanding of vascular function. This
kind of understanding requires gathering information obtained from different vascular
investigations and applying it to different conditions including exercise, diseases,
pharmacological therapy and alternative therapy. Additionally relate this information to
other physiologically-related disciplines including cardiology, neurology, endocrinology,
microbiology as well as psychology.
The main focus of this review is to discuss peripheral vascular function and
control at rest and during activities as well as adaptations to exercise training.
Additionally, vascular adaptations to diseases, the basic structure of the vasculature,
physical characteristics of the circulation, and the available tools to assess vascular
function are also discussed. This introduction is followed by seven sections. Section two
of the review focuses on the basic structure of different segments of the vascular system
including arteries, arterioles, capillaries, venules and veins. The wall component of each
segment is described as well as the distribution and density of each segment throughout
the body. The basic physical principles govern the circulation are discussed in section
three. This section sheds the light on various vascular physical characteristics that contribute to the control of blood flow in the arterial and venous side of the circulation
98 including vessel diameter, vessel length, transmural pressure, and blood velocity. The relationship between pressure, vascular resistance, and blood flow as well as hemodynamics essential for the venous system are also explained. Section four is divided into two parts. The first part deals with central neurohormonal factors contribute to vascular function control whereas the second part addresses the peripheral aspect of vascular control. Acute responses and control of the vascular system to exercise are discussed in section five. Similar to section four the details of the central and peripheral aspects are addressed in this section. However, since the cardiovascular system is functionally integrated, cardiac as well as blood acute adjustments to exercise training are briefly discussed. Section six deals with vascular adaptations to exercise training. This section provides a review of some of the studies examined arterial as well as venous adaptations to exercise training. Exercises promote systematic vascular adaptations are first discussed followed by regional vascular adaptations to small muscle group exercises.
In section seven, peripheral vascular adaptations to various diseases are briefly discussed.
Finally section eight overviews some of the available techniques to measure peripheral vascular function.
Since the subjects covered in this review spans over broad aspects of vascular physiology the author obtained information from various original articles, review articles, as well as textbooks. Additionally, text in this review is supported with illustrative figures and pictures selected from textbooks, review articles and original articles to explain some physiological phenomena covered in this review.
This review is certainly less than complete, however it is an attempt to reveal the available knowledge pertaining to vascular physiology. Additionally, this review is
99 expected to serve as a self-learning tool and platform for future self-advancements in integrative understanding of cardiovascular physiology.
100 SECTION 2. VASCULAR SYSTEM STRUCTURE
Blood vessels of the body form a closed delivery system that allows blood to be transported from the heart to the capillaries and back to the heart. Oxygenated blood leaving the heart passes through vessels of progressively smaller diameters referred to as arteries, arterioles and capillaries [1, 2]. On the other hand, deoxygenated blood, returning to the heart from the capillaries, passes through vessels of progressively larger diameters called veins (figure 2-1). The capillaries, which link the arterial side to the venous side, permit the exchange of nutrients and waste products between blood and living cells [1, 2]. This section will focus on the general structure of the peripheral vascular system. The functional anatomy details of all the vasculature segments, including arteries, arterioles, capillaries venules and veins, will be discussed.
2.1. ARTERIAL SYSTEM
There are two great arteries that emerge from the heart, the aorta from the left ventricle and the pulmonary artery from the right ventricle. The arteries are efferent vessels that carry blood away from the heart to the capillary beds throughout the body. All arteries in the body transport oxygenated blood from the heart to body tissues except the pulmonary vascular tree that carries deoxygenated blood from the heart to the lungs [2, 3].
Therefore, the most reliable way to classify blood vessels is by the direction in which they carry blood, either toward or away from the heart. Every artery enters an organ branches into 6-8 smaller vessels to become arterioles while arterioles branch 2-5 times to eventually become capillaries [4].
101 2.1.1. Arteries The lumen, the central canal of all blood vessels, is formed from a thick wall composed of three layers or tunicae. These tunicae are tunica intima, tunica media and tunica adventitia [5]. The tunica intima, the innermost layer, has a lining of endothelial cells, a thin subendothelial layer of fine areolar tissue, and an internal elastic layer called the internal elastic lamina (figure 2-1 & 2-2). The tunica media (the middle layer) is the thickest layer of the arterial wall in large arteries [2, 3].
Figure LR2.1. Vessel wall characteristics Internal diameter, wall thickness, and relative amounts of the principle components of the vessel walls of the various blood vessels that compose the circulatory system. Taken from [6].
It is composed mainly of connective tissues, smooth muscle cells and elastic fibers. The tunica media of the largest artery walls have more elastic tissue than smooth muscle, whereas smaller arteries contain more smooth muscle cells than elastic fibers [5]. The greater prevalence of elastic tissue helps larger arteries to stretch in order to adjust for the large change in pressure as the ventricles contract forcefully and pump blood into these type of vessels. For example, the aorta and pulmonary wall elasticity allows these vessels
102 to adjust for the pressure created by the ejected blood from the ventricles [3]. The
outermost layer of the vessel, the tunica adventitious, is mainly made of collagen and
elastic fibers with some smooth muscle fibers extending longitudinally next to the outer
border of the tunica media. This layer comprises nerves and lymphatic vessels in addition
to a capillary network formed by small vessels called “vasa vasorum” to nourish the walls of large arteries (> 20 mm) [4, 7].
Figure LR2.2. Typical elastic artery. Shown different segments of a vessel. Taken from [8].
2.1.2. Arterioles Arteries located further away from the heart branch into smaller and smaller vessels, then
into arterioles shortly before merging with the capillaries. The arterioles contain more
smooth muscle cells (figure 2-1 & 2-2). Arterioles and capillaries are connected by
structurally intermediate vessels called terminal arterioles or metarterioles [7]. The
metarterioles are wrapped around by highly innervated nerves and smooth muscles to
form precapillary sphincters however they do not have an internal elastic layer or their
103 own blood vessel supply. When these sphincters contract, blood flow is routed bypassing the capillary bed, however when the sphincters are relaxed, blood flow to the capillary bed and surrounding tissues increases [7]. The opening of the sphincters depends on the
metabolic needs of the surrounding tissues. As the metabolic activities of the surrounding
tissues increase the sphincters are opened to increase blood perfusion. Whereas when the
blood supply is no longer needed in larger quantities, the sphincters close [3, 7]. [4].
Therefore arterioles and metarterioles, due to their high content of smooth muscles,
sympathetic innervations, and location are the segments of the arterial tree most
responsible for mediating flow of blood to the capillaries [7, 9].
2.2. CAPILLARIES
As terminal arteries extend further away from the heart they branch out to form
capillaries. The capillaries run throughout all body tissues to form capillary beds in each
tissue. This serves as a point of connection between the arterial system and venous system. Capillaries are the most abundant blood vessels, which give them an enormous surface area available for exchange of gases, fluids, nutrients, and waste products between blood and nearby cells (figure 2-3) [5, 10]. They are the smallest among all vessels with a diameter of 8-10 µm, which is just large enough for the passage of one erythrocyte (figure 2-1). For example, the diameter of a medium-sized capillary is about
500 and 600 times smaller than a medium-sized artery and vein respectively. However, capillary diameters vary depending on the function of the tissue that is being supplied [5].
104
Figure LR2.3. Blood volume distribution in different compartments of the circulation. Taken from [2].
Capillary walls are mainly composed of only a tunica intima layer that contains endothelial cells on a thin basement membrane of glycoprotein (figure 2-1) [3, 11]. The endothelial cells are held together by tight junctions to prevent small molecule passage.
However, these junctions have gaps of unjoined membranes called intercellular clefts to allow for molecular movement across the capillary wall [6]. However in areas where high rate of molecular exchange is required, there are fenestrations or pores that penetrate the endothelial tissue to increase the capillary permeability for small molecules. These kinds of capillaries are called fenestrated capillaries. They are found in areas like the small intestine and the synovial membranes of joints, as large quantities of fluids and nutrients are exchanged with blood. In low molecular exchange areas (i.e. skeletal muscle, skin, and the central nervous system) are fed with continuous capillaries, in which the endothelial tissues contains no pores or fenestrations [2, 3]. Most of the small molecules are transported across capillary walls through the intercellular cleft. Moreover,
105 cytoplasmic vesicles that invaginate from the plasma membrane and migrate across the cell are used to move larger molecules across. Whereas oxygen and carbon dioxide seem
to be the only important molecules transported through diffusion [2].
2.3. VENOUS SYSTEM
As blood passes through capillary beds, it gets depleted of oxygen and nutritional
molecules and loaded with by-products (i.e. carbon dioxide and hydrogen ions).
Additionally blood pressure is reduced due to the high-resistance in arterioles and
capillary beds. Moreover, blood returns back to the right atrium through the smaller
venules then to the larger veins until it reaches the largest veins in the body, which are the
superior and inferior vena cava [11, 12]. Therefore, veins receive blood from smaller
vessels instead of giving blood off to branches as arteries do. Veins are more abundant
than their counterpart arteries and are located superficially beneath the skin as well as deeply in body tissues (figure 2-3) [1, 3, 13].
2.3.1. Venules Thoroughfare channels serve as an extension and are structurally intermediate between
capillaries and venules. These channels are connected to postcapillary venules, which are
the smallest of all veins. Postcapillary venules comprise of a thin layer of endothelium
and sometimes function like capillaries as inflammatory fluids and leukocytes leave
circulation through them. Whereas the larger venules consist of one or two layers of
smooth muscle cells, a tunica media as well as a thin tunica adventitia (figure 2-1 & 2-4)
[3, 5, 12].
106
Figure LR2.4. Typical vein. Shown longitudinal cut for a medium- sized vain. Emphasized the valves and different layers compose the venous vessel. Taken from [8].
2.3.2. Veins As blood approaches the heart through the circulatory system venules get larger to form veins. Opposite to their arterial counterpart, the tunica adventitia in veins is thicker than the tunica media. Moreover, the vein walls are thinner with larger lumens and contain less elastic tissue, collagen tissue, and smooth muscle thus veins are very distensible and comprisable (Figure 2-1) [5, 13].
Blood pressure is considerably lower in the veins than in the arteries, which reduces the propelling power of the blood as it travels back to the heart [13, 14]. Venous valves prevent blood from flowing backwards and assist in moving blood toward the right atrium. These valves are formed by the in-foldings of the tunica intimae, where each fold forms a bicuspid valve [3, 13]. Venous valves are more prevalent in the lower extremities where blood is moved against gravity. Further, fewer valves are found in the head and neck veins and there are none in the veins of the thoracic and abdominal
107 cavities. Additionally the compression of the venous walls by contracting skeletal muscle aids in propelling venous blood back toward the heart [3].
108 SECTION 3. CIRCULATORY HEMODYNAMICS
The primary function of the peripheral circulation is to deliver oxygenated blood to
tissues around the body and transport deoxygenated blood back to the heart. Meanwhile it
also transports other essential chemical substances (e.g. hormones, enzymes, and
minerals) to maintain the survival of the living cells [3]. In order to accomplish these physiological essentials blood flow is shunted according to tissue metabolic requirements
for substrate delivery and waste removal, which differs during the course of various daily
life situations. For example, skeletal muscle’s demand for nutrients and oxygen during
exercise is higher than its needs during resting condition. Likewise, the nutritional
requirements for stomach smooth muscle increase following a big meal [2, 6]. Several
aspects within the cardiovascular system interact together to achieve proper substrate
delivery and waste removal including cardiac muscle pump, vascular function, and blood
volume. These functions are further controlled by neurohumoral modulation, electrolyte balance, and tissue metabolic activities [7].
The vascular system plays a profound role in controlling the flow of blood to
tissues throughout the body. The parallel arrangement throughout the vascular system
allows for proper blood flow to organs around the body [2]. Small arteries and arterioles
are the principal site of blood flow control where terminal arterioles act as “stopcocks” to
accurately alter blood distribution to these organs according to their needs (figure 3-1).
However due to its structure and arrangement the vasculature is governed by physical principles that control blood flow [6]. This section will review the functional physical
principles of the peripheral vascular system.
109
Figure LR3.1. Schematic diagram of the parallel and series arrangement of the vessels composing the circulatory system. The drawing shows small valves, “stopcocks” likes, to control blood inflow representing vascular resistance on the arterial side (on the right). Larger venous diameter is also shown indicating the capacitance specialty of the venous system. Taken from [15].
3.1. GENERAL PHYSICAL PRINCIPLES OF PERIPHERAL CIRCULATION
The vasculature is divided into arterial, capillary and venous systems and proceeds within the body in the order of arteries, arterioles, capillaries, venules and veins. Vessels of the arterial system progressively get smaller in diameters, reaching the smallest as they approach the capillary beds. This is in contrast to the diameter of the venous system, where vessels get larger as they reach the heart [16]. The capillaries operate as an exchange structure for metabolic substances between blood and tissues whereas the
110 arteries and veins serve as distribution and collecting structures, respectively, for blood
[5, 17].
The systemic circulation contains the largest blood volume (83%) of total blood volume with 64% stored in the veins, 16% in the arteries and 4% in the capillaries.
Additionally, 8% is in the pulmonary circulation and 4% is usually within the heart chambers (figure 3-2) [2].
Figure LR3.2. Blood volume distribution in different compartments of the circulation. Taken from [2].
The capillaries have the largest total cross-sectional area (2500 cm2) allowing for sufficient blood/tissue exchange. In addition, the total cross-sectional area of all veins is
338 cm2, which is about four times that of arteries (62.5 cm2) (figure 3-3)[1]. Blood pressure also fluctuates at different segments of the peripheral circulatory system averaging approximately 100 mmHg at the aortic level and progressively declining to pressures approaching 0 mmHg in the venae cavae (figure 3-3). This drop in blood
111 pressure is a function of blood flow rate and peripheral resistance within the circulation, as will be explained later [6].
Figure LR3.3. Changes in blood pressure and velocity at different cross-sectional areas. Taken from [18].
3.2. VASCULAR DISTENSIBILITY
The vascular system has an astonishing capacity to accommodate large changes in blood volume across all vessel segments due to vascular distensibility [19]. This physical
112 feature is characterized as the ability of the vasculature to expand and described as the volume of fluid needed to increase pressure by one unit. In other words, it is the ratio increase in volume for each millimeter mercury increase in pressure and could be expressed in the following formula [3, 20]:
Vascular distensibility = Increase in volume*Original volume/increase in pressure
Vascular distensibility is vital for proper blood pressure control and to accommodates the pulsatile output of the heart. Additionally, it provides smooth and continuous flow of blood through the tissues by averaging out the pressure pulsations so that tissue blood flow will not be disrupted [20]. Arteries are far stronger and less elastic than the veins due to thicker walls and a greater smooth muscle content. Therefore, veins are about 6-10 times more distensible than their arterial counterparts, making them the ultimate blood reservoir within the circulatory system [21, 22].
Capacitance is similar to and influenced by distensibility however these two physical characteristics of the vasculature are not identical. Vessel capacitance or compliance is the ability to accommodate changes in volume for changes in pressure when fluid is added. Thus capacitance describes volume-pressure relationship with respect to blood vessel size and volume added as stated in the following formula [3]:
Vascular capacitance = Increase in volume/increase in pressure
All of the cardiovascular system compartments, including the heart, are considered capacitance vessels since they contain volume. However as the veins hold
~70% of the circulating blood and large changes in veins’ volume cause little changes in pressure they provide the major capacitance function in the circulation [15]. Distensibility
113 decreases with aging as the lining of the vasculature becomes infiltrated with less elastic
fibers. Additionally diseases (i.e. diabetes & atherosclerosis), changes in autonomic
nerves balance, and various drugs might further increase the rigidity of the vascular
system [2, 23, 24].
3.3. BLOOD VELOCITY AND BLOOD FLOW
It is important to distinguish between blood velocity and blood flow in order to describe
the hemodynamics of the peripheral circulatory system. Velocity is referred to the linear
velocity or rate of blood displacement with respect to time and has properties of distance
per unit time, for example, cm•sec-1. Meanwhile, flow is volume or quantity of blood passing through a certain point in the circulation in a given time and has properties of
volume per unit time, such as, cm3•sec-1 [3, 16].
For a given constant flow, the velocity varies inversely as the cross-sectional area. For
example when 10 cm3•sec-1 of blood passes through a vessel with a cross-sectional area
of 2 cm2, its velocity is 5 cm•sec-1. However velocity drops down to 1 cm•sec-1 when the
cross-sectional area is 10 cm2 and it elevates to 10 cm•sec-1 when the cross-sectional area
is 1 cm2 (figure 3-4) [3]. This relationship is expressed in the equation [16]:
V=Q/A, Where V=velocity, Q=flow and A=area.
Velocity throughout the circulation depends on blood flow. Flow, which
decreases as blood proceeds in the circulation, depends on blood pressure, fluid
properties of blood and dimensions of the circulatory system [25]. Therefore, due to
reduction in flow, blood velocity decreases as it proceeds in the peripheral circulation until blood reaches the capillaries. However, as blood drains out of the capillaries, blood
114 progressively gains velocity again as the venules continue to propel blood toward the venae cavae and the heart (figure 3-3) [3].
Figure LR3.4. Relationship between cross-sectional area, A, and velocity, v, in the systemic circulation. Taken from [6].
3.4. FLOW, PRESSURE AND RESISTANCE
Blood flow (cardiac output) and peripheral resistance interact to create appropriate pressure (force) throughout the vasculature to deliver blood to needed tissues. This relationship is expressed in Ohm’s law [26]:
Q=P/R
Where Q is cardiac output, P is pressure and R is resistance
This law states that cardiac output is directly proportional to pressure and inversely proportional to resistance. The heart pump eventually generates cardiac output whereas resistance depends mainly on vessel shape (length and diameter), blood viscosity, and type of tissue involved [3, 16].
3.4.1. Blood flow Total body blood flow or cardiac output (Q) at rest is about 5,000 ml•min-1 and increases to 35,000 ml•min-1 with a rise in tissue activities (i.e. exercise). Its called cardiac output because it is the quantity of blood pumped by the heart in volume unit per time [15].
115 Laminar and turbulent flow Under normal conditions, the flow of fluids in a cylindrical tube is laminar. The thin fluid
layer adjacent to cylinder wall is extremely slow or even maybe motionless (figure 3-5).
Figure LR3.5. Laminar and turbulent blood flows within a vessel. Emphasized vortices develop from turbulent flow. Taken from [18].
However the fluid layer interior to this external layer must shear against this “motionless” layer and therefore move a bit faster. Similarly layers that are more interior, follow suite and travel more rapidly. Therefore, the velocity of fluid at the center of the laminar is maximal and is twice the mean velocity of flow across the entire cross-section of the tube
(figure 3-6). In laminar flow, substances remain in one lamina as the fluid proceeds longitudinally along the tube [3, 27].
Figure LR3.6. Laminar flow depicts the motion of a series of thin, concentric cylindrical shells sliding over each other. Velocity is higher as the shell position is closer to the central axis. Taken with permission from [27].
Turbulent flow of fluid in a tube however might develop forming eddies and
vortices (figure 3-5). Under these conditions, rapid mixing occurs causing fluid elements
not to remain in one definite laminae. Therefore, turbulent flow requires considerably
116 greater pressure than laminar flow to drive fluid through the same tube. This in turn requires a pump, such as the heart, to work harder to generate a given flow if turbulence develops. Reynolds’ number is a dimensionless number that represents the ratio of inertial to viscous forces for a fluid flowing through a cylindrical tube:
NR=ρDv/η Where D is the tube diameter, v is the mean velocity, ρ is the density, and η is viscosity
When NR <2000, the flow usually is laminar; and it would be turbulent when NR >3000.
Reynold’s number between 2000-3000 is in transition phase where both types of flows
might exist. Thus Reynold’s number principle indicates that large diameter, elevated
velocity, and low viscosity are sources for turbulence flow. Additionally, abrupt
variations in tube dimensions or irregularities in the tube walls may produce turbulence
[3, 27].
3.4.2. Blood pressure Blood pressure is the force exerted against any unit area of the vessel wall by the blood.
The rhythmic contraction of the heart generates this pressure during the left ventricular
contraction and relaxation phases of the cardiac cycle [6]. Blood ejected from the left
ventricle surges into to the aorta creates a pressure gradient throughout the entire vascular
system. A portion of the ejected blood is stored by the aorta, which allows it to stretch
and then recoil during ventricle relaxation, and drives the blood in pulsatile manner. In
preparation for the next contracting phase the left ventricle expands and the left atrium
contracts to drive blood to the left ventricle [4]. A pressure of 50 mmHg means that the
force exerted is sufficient to push a column of mercury up to a level 50 mm high.
However pressure is also measured in centimeters of water. Thus, a pressure of 10
117 centimeters of water means that the force exerted is sufficient to push a column of water
for 10 centimeters. 1 mm Hg = 1.36 centimeters of water [3, 26].
Systolic blood pressure (SBP) During the contraction (systole) phase of the cardiac cycle pressure generated by the
heart at rest is about 120 mm Hg. This measurement is usually referred to as pressure in
the brachial artery at the level of the heart. Additionally, it is an estimate of the work
generated by the heart and of the strain against the arterial walls during ventricular
contraction [26]. Following contraction, the ventricle relaxes, however the elasticity of
the arterial system maintains a driving pressure for steady flow of blood to the periphery, until the next surge of blood arrives [3].
Diastolic blood pressure (DBP) Blood pressure in the brachial artery decreases to about 80 mm Hg during the relaxation
(diastole) phase of the cardiac cycle [26]. The recoiling aorta and the contraction of the
atrium maintain this pressure. Diastolic blood pressure is indicative of peripheral vascular
resistance especially at the level of arterioles and capillaries [3].
Mean arterial pressure (MAP) Due to a longer diastolic phase, the mean pressure across the circulation is not simply an
average of systolic and diastolic pressures. However it is estimated from the following
equation [3]:
MAP = DBP+[(SBP-DBP)/3]
This formula provides an optimal estimation of MAP during rest. However when the
intensity of physical stress changes (e.g. exercise or sleeping) and the length of the
diastolic period is altered (e.g. an increase or decrease in heart rate), accuracy of the
estimation is changed [3].
118 Shear stress Shear stress is the external force applied against the arterial wall as blood is propelled down the arterial tree. This stress is the product of blood viscosity and shear rate as it would increase when viscosity and/or velocity of blood increases [28]. The stress of shear produces a differential motion in the layers of the flowing blood through the circulatory system thereby blood exerts shearing stress on the surface of the arterial wall in contact with blood. Shear stress is essential for the homeostasis of the vascular system as it stimulate the release of vasoactive agents, influence vessel wall structural remodeling and plays a role in the pathogenesis of the vascular system [29].
Volume-pressure relationship The relationship between blood volume and pressure in the peripheral circulatory system is described with volume-pressure curves. Pressure in the arterial and venous systems is influenced by volume of blood within the circulation, however due to differences in wall structure each responds differently to changes in volume [19]. The walls of the arterial system have stronger smooth muscles and are thereby far less expandable than veins.
Therefore slight modifications in blood volume result in huge changes in arterial pressure, whereas slight changes in venous pressure require tremendous changes in volume [22]. For example, mean arterial blood pressure would decrease from 100 to zero mmHg when blood volume drops from 750 ml to 500 ml. On the other hand blood pressure in the venous side decreases from 15 to 5 mmHg as a result of volume reduction of 500 ml (figure 3-7). Volume-pressure relationship is largely influenced by changes in vascular smooth muscle tone (i.e. due to changes in sympathetic drive) (figure 3-7) [3,
20].
119 Blood volume can continuously change within the circulation segments, however
blood pressure integrity must be maintained to assure proper flow of blood to needed
tissues [6]. Delayed compliance is a valuable mechanism by which changes in volume are
accommodated in the circulation. An increase in blood volume will first cause a rise in blood pressure followed by a stretch in the vasculature, allowing pressure to be adjusted
back to normal values. Vise versa, reduction in blood volume will cause a rapid drop in
blood pressure, however the smooth muscle fibers readjust their tension to allow blood
pressure to increase back to proper values. This mechanism is vital when a large loss (e.g.
hemorrhage) or extra increase (e.g. infusion) in blood volume is exhibited within the
circulation [15, 27].
Figure LR3.7. Volume-pressure curves within the circulatory system. Shown the effects of sympathetic stimulation and inhibition. Taken from [3].
Vasculature wall tension Tension in the vascular wall is a function of vessel radius, vessel thickness and
transmural pressure across the vessel. Transmural pressure in turn is the difference
between inside and outside pressure. The exerted force on the inner side of the vessel
wall causes the internal pressure, whereas external pressure is the combined pressure of
120 the vessel-surrounding environment. The relationship between these factors is expressed in LaPlace’s theory, which is [6, 27]:
Pt = Pi - Pe, Where Pt=transmural pressure, Pi=internal pressure, Pe=external pressure
PtR T = ------(LaPlace’s theory) µ Where T=Tension, R=radius, and µ=thickness
LaPlace’s theory indicates; wall tension increases as a result of transmural pressure increase, vessel radius increase or thickness decreases. In a normal vessel, radius and thickness are in approximate proportion, however wall tension differs as transmural pressure changes and the two will tend to stay in reasonable equilibrium (figure 3-8).
Reaching this equilibrium is particularly important to avoid vessel walls from bulging.
For example aortic aneurysm is due to weakening of particular point in the wall of the vessel [27]. Due to increased wall tension, this weak site will soon bulge, causing a decrease in wall thickness and an increase in the radius of the aorta, which will further weaken the aortic wall. This cycle of deterioration might require surgical restoration to avoid further damage or rupture of the wall [2].
Arterial pulse pressure The distensibility of the arterial system plays a role in tissue blood flow during the diastolic phase of the cardiac cycle. After blood surges from the heart with each vertical contraction the aorta expands then recoils to assist pumping blood through the vasculature during diastolic phase of the cardiac cycle, which causes pressure pulsation.
Pulse pressure is estimated as the difference between systolic and diastolic pressure,
121 which would be ~ 40 in young normal adults. However, vascular resistance causes
pressure pulsation to diminish as blood proceeds within the vasculature, thus minimizing
its influence on tissue blood flow. Pulse pressure is influenced mainly by stroke volume
and arterial compliance. For example, a greater stroke volume would require greater
accommodation by the arterial tree thereby greater differences between systolic and
diastolic blood pressures [2, 3].
Figure LR3.8. Pressure-volume-tension relations as described by Laplace's law Left (A); normal vessel. Right (B); aneurysmic vessel due to athrosclerotic injury. Taken from [2].
3.4.3. Resistance Resistance is the combined forces in the circulation that obstruct the flow of blood to the
periphery. Direct measurement of resistance is not available, however it could be
estimated using Ohm’s low (R=P/Q) and expressed in arbitrary unit (U). Blood viscosity,
structure of vessels (diameter, length, and arrangement), and blood flow are factors that
influence vascular resistance, which are included in Poiseuille’s law of fluid conductance
in tubes.
122 Poiseuille’s law In 1846 J.L.M. Poiseuille demonstrated the relationship between steady flow and pressure in a cylindrical tube. He discovered that the relationship between volume flow (Q) and the drop in pressure (P) along a tube of a given length (L) and inner diameter (D) was
[30]:
Q=KPD4/L.
The coefficient K was found to be independent of Q, P, L, and D and changed with alterations in temperature. However, in subsequent developments, Wiedemann in 1856 and Hagenbach in 1860 found that K was related to blood viscosity (K=π/128η); where η is viscosity and expressed in 1 dyn sec.cm2 or poise. Therefore, in the light of these developments, Poiseuille’s law has become [31]:
Q= π∆Pr4/8ηL Where ∆P is the difference in pressure between two ends of vessel, and r is radius
This edition of Poiseuille’s law states that flow is directly proportional to difference in pressure between two ends of a vessel and the radius of that vessel.
Additionally, blood flow is inversely proportional to the viscosity of blood and the length of the vessel (figure 3-9) [27].
123
Figure LR3.9. The relationship between vascular resistance and blood inflow as described by Poiseuille's law in a straight vessel. Taken from [18].
The theoretical derivation of this equation however describes steady laminar flow
in rigid cylindrical tubes and not the pulsatile flow through elastic vessels as in the body.
Additionally, Poiseuille’s law is based on the assumption that each liquid particle moves
at a constant velocity (v) in parallel to the cylinder wall, and that the force opposing this
motion is proportional to viscosity and to the velocity gradient perpendicular to the direction of flow [27].
In the vascular system, length of the vessel plays an important role in determining the magnitude of resistance to blood flow. The longer the vessel the larger the pressure
gradient needed to propel the blood further. As the vascular system progresses through
the body, vessels divide and increase in number forming an in-parallel arrangement
within each vascular bed. The increasing number of vessels causes each segmental cross- sectional area for blood/tissue exchange of materials to increase also. However if this increase in vessel number and cross-sectional area didn’t occur and each microvascular beds were connected end-to-end, an in-series vascular beds or resistance would be formed. The total resistance of these in-series resistances equals the sum of the individual resistances, which adds up to an enormous resistance against flow. Therefore, such
124 arrangement makes it extremely difficult for the cardiovascular system to function given the large pressure needed to drive blood flow through, even for resting condition, let alone under stressful condition (e.g. exercise) [15, 27]. However, typically multiple vascular beds are created from one vessel forming in-parallel resistance (figure 3-10). In this situation, the total resistance equals the sum of the reciprocals of the individual resistances, which is obviously far less than in-series resistance [6].
Figure LR3.10. Vascular resistance in-series and in-parallel circuits. Taken from [18].
According to Poiseuille’s law, a slight change in vessel diameter causes a tremendous change in flow due to the fourth power and is attributed to the physical characteristics of blood flow [27]. In laminar flow, blood velocity is minimum in the layer adjacent to the vessel wall and progressively increases until it reaches maximum at the center of the lamina. In a narrow vessel, where a lot of the blood is very close to the vessel wall and minimum volume of blood is flowing centrally blood would be flowing slowly. In contrast vessels with larger diameter where more blood is flowing centrally, average blood flow would be faster (figure 3-12).
125
Figure LR3.11. A; Relationship between blood flow and vessel diameter. B; Velocities of different concentric layers of blood flow, layers in the center are the fastest. Taken from [3].
For example when a pressure gradient of 100 mmHg is applied to drive blood through three separate blood vessels with diameters of 1, 2 and 4, blood flow will be in these three perspective vessels 1 ml/min, 16 ml/min and 256 ml/min, respectively (figure
3-12) [3]. One third of peripheral resistance is in the small arterioles, which have internal diameter that ranges between 8 to 30 µm with a tremendous capability of changing their diameter by about 4 folds. A 4-fold increase in vessel diameter could increase the flow by as much as 256 folds. This phenomenon allows arterioles to have an important role in vascular resistance and to be very effective in changing blood flow to tissues as a result of central nervous system or local neurohumoral signals [3].
Viscosity of blood also contributes to total vascular resistance. Percent of blood cells and solid matters in blood vessels are called hematocrit and determines viscosity of blood [6]. Hematocrit level usually averages about 40% of blood plasma, which is about
3 times as great as viscosity of water. However an increase in hematocrit to 60 or 70% form a viscosity as great as 10 times of water [3]. Concentration of hematocrit in plasma
126 depends on gender, disease status, degree of bodily activity and altitude at which a person reside. Higher blood viscosity generates greater friction created by dragging blood cells and matters against adjacent cells and vessel wall. This friction is the source of resistance created by viscosity of blood to flow across the circulation [27].
3.5. VENOUS HEMODYNAMICS
According to the Frank-Starling theory of contraction, the heart can pump only what passively fills it during diastole. The venous system operates as a collecting reservoir for blood and controls the quantity of blood returning back to the right atrium. Consequently cardiac output is primary regulated by venous return and the resultant end-diastolic volume by a simple mechanism of filling and stretching the left ventricular muscle fibers
[32]. The Frank-Starling law involving the end-diastolic volume however is not the only factor influences cardiac output. Cardiac contractility, heart rate, and arterial pressure are also important factors [33].
Low intra-vascular pressure, large volume present, intermittent nature of the flow, gravitational effect, venous valves, and the collapsible nature of the venous wall add complicity to flow in the venous cavity. Therefore, there are supplementary factors assisting the flow of blood in within the venous tree [20].
Flow of blood runs with no resistance in large veins when they are distended however as veins enter the thorax and abdominal cavities flow is impeded. Most veins are compressed at many points by the surrounding tissues and the intra-thorax and abdominal pressures, which obstructs flow of blood to the heart chambers [27]. Pressure in the right atrium also hinders blood returning back to the heart from the systemic circulation.
Pressure in the right atrium might sometimes increase above 0 mm Hg (e.g. heart failure),
127 causing blood to backup [32]. However pressure in the venous system doesn’t increase until all collapsed segments between peripheral veins and large veins have opened, which doesn’t occur until right arterial pressure reaches 4-6 mm Hg. A further increase in right atrial pressure would reflect a progressive pressure increase in large veins [33, 34].
However, pressure elevation in the right atrium doesn’t usually occur except under acute circumstances, such as exercise and heart failure [35].
During quiet standing, gravitational effects cause the drainage of thorax blood to the lower parts. Instantly skeletal muscles surrounding the veins start contracting and pushing against the veins to propel blood toward the heart [2, 15]. Meanwhile, the veins refill during muscle relaxation. In this case skeletal muscle is acting as a pump, which is clearly demonstrated during exercise as it plays a more effective role in driving blood toward the heart during exercise. While exercising, larger cardiac output is indeed required for tissue metabolic needs. This is facilitated in part by increasing skeletal muscle contraction force to enhance venous blood flow to the heart thereby increasing end-diastolic volume and cardiac output. However increased venous return and filling capacity is offset due to increased heart rate during exercise [15].
The contraction of the venous smooth muscle also plays a role in driving blood toward the heart, however its contraction depends on the intra-vascular pressure. The venous wall movement is mainly passive at low pressure, however when venous pressure increases above 5 mmHg wall tension increases providing an active role of the venous tone. Venous tone is controlled mainly by central and local neurohumoral signals [2, 27].
While the skeletal muscle pump and venous tone is important to propel blood forward, the venous valves prevent retrograded flow. Following skeletal muscle
128 contraction and propelling blood forward, the valve cusps close and the veins refill
rapidly, which allow for unidirectional flow [13, 20]. However in some venous disorders
(e. g. inflammation of the vein or congenital defects), venous valves become
incompetent. As a result of this venous deficiency valves don’t close properly allowing
for blood to back flow, which tends to increase venous congestion and venous pressure.
Consequently venous return is reduced causing fatigue associated with painful sensation
in the congested areas of the venous system [36].
The intra-thorax pressure varies during quiet breathing from –2 to –4 mm Hg
during expiration to about –5 to –7 during inspiration. The intra-abdominal pressure
decreases when intra-thorax pressure increases, which facilitates venous return form the
intra-thorax veins thereby increase thoracic vena cavae flow of blood and venous return.
The resultant changes in flow inside the veins cause ±25% changes in cardiac output
during respiration. This change in cardiac output is even more pronounced during exercise due to the increase in the rate and force of perspiration [27].
129 SECTION 4. CONTROL OF VASCULAR FUNCTION
Changes in blood vessel diameters are the underlying factor that control blood flows in the arterial and venous systems throughout the circulation. These changes in vessel diameter are the result of contraction and relaxation of vascular smooth muscle. The strategic location and the unique arrangement of the vascular smooth muscle throughout the peripheral circulation allow for phenomenal modulation of the vascular system and subsequently blood flow modifications to and from body tissues (figure 4-1).
The smooth muscle cells wrapped around the vessels of the circulatory system are small, mononucleate, and spindle shaped. They are generally arranged in helical or circular layers around the larger blood vessels and in a single circular layer around arterioles [7, 37]. The vascular smooth muscle cells contain small numbers of thick myosin filaments and large number of thin actin filaments forming sarcomeres with non- visible striations. These filaments are aligned in the long axis of the cell and the sliding filament mechanism operates by means of phosphorylation of cross-bridges to regulate their rate of cycling [3, 7]. Parts of the endothelial cells project into the vascular smooth muscle layer forming myoendothelial junctions at various points along the vessels. These junctions are used for functional communication between endothelium and adjacent vascular smooth muscle (figure 4-1) [7].
In contrast to skeletal muscles, vascular smooth muscles contract very slowly, develops high forces, maintains force for long periods with low adenosine triphosphate
(ATP) utilization, and operates over a considerable range of lengths under physiologic conditions. On the other hand similar to cardiac muscles, cell-to-cell signal conduction is responsible for synergistic contraction smooth muscle [3]. As in skeletal and cardiac
130 muscles, contraction caused by cross-bridge formation is controlled by myoplasmic Ca+2
concentration [38], however the Ca+2 mechanism of action for regulating contraction is
slightly different in vascular smooth. Contraction is elicited by the increase in
myoplasmic Ca+2 that come via voltage-gated calcium channels, through receptor- mediated calcium channels in the sarcolemma, and by the release from the sarcoplasmic
reticulum. Several neurohormonal and metabolic pharmacologic stimuli have been
proposed to be involved in modulating intracellular Ca+2 level including catecholamines
[39, 40], angiotensin II [41], vasopressin [41, 42], histamine [40], serotonin [43],
+ + adenosine [44], CO2 [45], K [46], H [47], nitric oxide [48], and prostaglandins [49].
Figure LR4.1. Typical elastic artery. Shown different segments of a vessel. Taken from [8].
131
Figure LR3.2. Schematic diagram of signal transudation involved in vascular smooth muscle modulation. PKC; protein kinase C, PLC; phospholipase C, G; guanine nucletide binding protein, PIP2; phosphatidyl inositol bisphophate, DAG; diacylglycerol, IP3; inositol triphosphate, Ca+2; Calcium ions, CaM; calmodulin, MLCK; myosin light chain kinase. Taken from [50].
The binding of agonists to vascular smooth muscle membrane receptors activates
phospholipase C (PLC) in a reaction coupled with guanine nucletide binding proteins (G
protein). Phospholipase C hydrolyzes phosphatidyl inositol bisphophate (PIP2) in the
membrane to yield diacylglycerol (DAG) and inositol triphosphate (IP3). Diacylglycerol
+2 +2 and IP3 increase Ca by activating voltage-gated calcium channels and releasing Ca
sarcoplasmic reticulum, respectively. Calcium ions bind to calmodulin, which in turn
binds to myosin light chain kinase. This activated Ca+2-calmodulin-myosin kinase
complex phosphorylates the light chains (20,000 daltons) of myosin. The phophorylated
myosin ATPase is then activated by actin, and the resulting cross-bridge cycling initiates
contraction (figure 4-2). Relaxation on the other hand occurs when the myosin light chain
132 phosphatases dephosphorylate the light chains of myosin and the cytosolic Ca+2 is lowered by sarcoplasmic reticulum uptake and by Ca+2 extrusion by the Ca pump and Na-
Ca exchanger [51, 52].
Vascular smooth muscle is normally tonically contracted and physiological
responses of increased and decreased contraction are a function of the integrated signals
from autonomic nerves, circulating and local hormones, endothelium tissue and local
metabolites [7, 53]. The endothelium, for example, regulates vascular smooth muscle
function by releasing a variety of vasoactive factors [54]. Endothelial cell calcium
regulates the activities of nitric oxide synthase, which metabolizes L-arginine to nitric
oxide, the most important endothelium-derived relaxing factor [55, 56]. After diffusing to
smooth muscle cell nitric oxide exerts its effects by negatively influencing calcium
release from the sarcoplasmic reticulum and its entry through calcium channels, as well as by inhibiting calcium activated contractile process itself [57, 58]. Endothelin-1 is
another endothelial-borne molecule however promotes vasoconstriction by sustaining
elevated levels of Ca+2 and activating muscle cell contractile process [56, 59].
Most of the body arteries and veins are supplied to different degrees by autonomic
nervous system fibers that exert a tonic effect on the blood vessels. Activation of the
sympathetic nerves either directly or reflexly causes smooth muscle contraction or
enhances vascular resistance by increasing cytoplasmic Ca+2 levels. In contrast, the
parasympathetic nerves tend to decrease vascular resistance, but they innervate smaller
number of the body blood vessels. Moreover, changes in the local environment alter the
contractile state of vascular smooth muscle, and alterations such as increased temperature
or increased carbon dioxide levels induce relaxation of this tissue [60, 61]. This section
133 will provide an overview of the mechanisms involved in vascular function modulation under normal conditions. Since vasoregulation is a complex process and include systemic
and local factors, the regulatory effect of each factor will be discussed.
4.1. SYSTEMIC CONTROL OF THE VASCULAR SYSTEM
Systemic modulation is maintained by homestatic reflex mechanisms that receive
mechanical and chemical signals from peripheral sensors within the circulation causing
either a local reaction or activation of the central nervous system [3, 15, 62]. The signals
sent to the central nervous system are processed then forwarded to the command centers
to take appropriate action in the periphery. These command centers send signals to either
the autonomic nervous system or glands around the body. Signals from both arms of the
systemic vascular control are reflected on total peripheral resistance, blood pressure and
blood flow [3, 62].
4.1.1. Nervous Regulation of the Circulation Neural control of the cardiovascular system affects mainly global functions such as blood
redistribution (blood shunting), cardiac output, metabolic adjustments and particularly
important for maintaining constant pressure within the circulation [15]. This control is
inflicted on the vascular system entirely by the autonomic nervous system. Both branches
of the autonomic nervous system innervate the heart however the vasculature has
extensive sympathetic network whereas the parasympathetic innervation is minimum.
Thus the sympathetic nervous system is very important in the control of the heart and
vasculature whereas the parasympathetic system is particularly essential for the
regulation of the heart and cardiac output with some control to the vasculature (figures 4-
3 and 4-4) [3, 32, 62].
134
Figure LR4.3. Functional anatomy of the sympathetic nervous control of the circulation. Taken from [3].
The most remarkable characteristic of the autonomic nervous system is probably the ability to modify heart rate, vascular resistance and blood pressure within few seconds. For example the sympathetic nervous system double heart rate within 3-5 seconds whereas blood pressure is increased to double normal within 15 seconds [15, 63].
This exceptional capability is due to the sophisticated and extensive neural network that connects the central nervous system to the heart and almost all segments of the vasculature (figures 3-3 and 3-4) [61, 64]. The innervation of the small arteries and arterioles allows the sympathetic stimulation to increase vascular resistance, blood pressure and blood flow to areas where needed. Whereas the innervation of the large vessels, particularly the veins, assists in propelling blood from these vessels to the heart
135 thus generating larger cardiac output [20]. Additionally sympathetic activities are especially important to increase heart rate and contractility force [61, 64]. Although, due to minimum vascular intervention, the parasympathetic stimulation is not particularly important for vasoregulation it plays a crucial role in the reduction of heart rate and contractility thereby blood pressure [65].
.
Figure LR4.4. Sympathetic innervation of the systemic circulation. Taken from [3].
The sympathetic vasomotor nerves project their fibers from almost all the thoracic and two lumbar spinal nerves. These fibers connect to the heart and its vessels and to the peripheral vessels (figures 4-3 and 4-4). Additionally a sympathetic nerve fiber branches of the intermediolateral horn cells of the spinal cord passing through the sympathetic chains and the splanchnic nerves directly into the adrenal medulla, which is responsible for the secretion of epinephrine and norepinephrine into the blood stream [8, 66]. The signals transmitted via the sympathetic fibers are initiated in the vasomotor center located in the pons and medulla (figure 4-5). The vasomotor center is controlled by locations within the central nerves system including the hypothalamus and cerebral cortex [8].
Within the vasomotor center there are areas responsible for initiating vasoconstriction
136 whereas others initiate vasodilation. Generally however increased sympathetic nervous system activities induce strong vasoconstriction throughout the circulation except in the heart and skeletal muscle vasoconstriction is less pronounced [15, 67, 68]. In addition to vasoconstriction and vasodilation signaling the vasomotor center is also responsible for receiving and coordinating reflexes from the sensory receptors around the circulation
(e.g. barareceptors and chemoreceptors). This particular function orchestrates a rather sophisticated feedback loop between the periphery and the central nerves system [65, 69].
The vasoconstrictor area, within the vasomotor center, fires vasoconstriction impulses to vessels around the circulation to form a sympathetic vasoconstrictor tone. These signals are to maintain a vasomotor tone that is important to preserve adequate blood pressure and flow. At rest this tone allows for partial state of contraction of blood vessels however during stress the firing of vasoconstriction impulses increases causing a corresponding increase in blood pressure (figure 4-5) [15].
The sympathetic terminal nerve endings are called adrenergic nerve fibers because they secrete the neurotransmitter norepinephrine. The manufacturing of norepinephrine is initiated in the axoplasm of the terminal nerve endings of adrenergic nerve fibers but is completed inside the vesicles [66, 69]. Once released norepinephrine binds to cellular membrane receptors called alpha (α) and beta (β) adrenergic. These receptors are highly specific, located in the vascular smooth muscle cell and penetrate through the cell membrane [8]. Upon norepinephrine binding to the receptor it cause conformational changes to the penetrating receptor thereby triggering cellular activities within the vascular smooth muscle cell by either increasing membrane permeability to ions or activate/deactivating enzymatic reaction (s) [3, 66]. In general α-adrenergic
137 receptors cause vasoconstriction β2 receptors cause vasodilation whereas β1 cause
increase in heart rate and contractility. The skin and internal organs vasculature has
mostly α receptors, skeletal muscle and splanchnic vasculature contains both α and β2
receptors however α are more abundant and dominant than β2 whereas the heart muscle
contains β1 and the heart vessels contain α and β2 receptors [3, 63]. The action of β2
receptors has probably been most confusing since it induces vasodilation. When
norepinephrine infused into tissues contain α and β2 receptors it activates only α
receptors, whereas β2 receptors are activated only when α receptors are blocked.
However norepinephrine released from nerve endings does not activate β2 receptors even
if α receptors are blocked. [15]. Therefore increase sympathetic nervous system activities
increase heart rate and contractility, heart and skeletal muscle vasoconstriction and strong
skin and internal organ vasoconstriction [70]. Although some reports indicated β2
receptors’ action might contribute to initial vasodilation observed at onset of exercise, skeletal muscle vasodilation is sustained by local mechanisms [15]. Since the autonomic nervous system is responsible for the flight-or-fight response, these cardiovascular adjustments are particularly important for the initial increase in blood delivery to the skeletal muscles during times of physical stress such as exercise [15, 70]. Dilation of the eye pupils, increased activities of the sweating glands, and increased metabolism are also implicated in the flight-or-fight response and stimulated by the sympathetic nervous system [1, 71].
138
Figure LR4.5. Systemic circulation regulatory areas within the brain. Taken from [3].
The removal of norepinephrine is accomplished by either reuptake by the nerve endings, diffuse to the circulation or destroyed by the enzyme monoamine oxidase or one of its derivatives. The reuptake mechanism accounts for 50-80% of the unused norepinephrine whereas norepinephrine diffuses to the blood accounts for most of the reminder. Finally norepinephrine destroyed by monoamine oxidase accounts for only a small fraction of the reminder [3, 69]. Norepinephrine released directly into the tissue stays active for only few seconds before its removal by one of the mechanisms. However norepinephrine released into the blood stream by the adrenal medulla or escaped from the nerve terminal stays active for 10-30 seconds until it diffuses into some tissue (e.g. liver) where its destroyed by monoamine oxidase [3, 15].
The parasympathetic nervous system projects its nerve fibers to the heart through the vagus nerve (cranial nerve X). The nerve endings at the heart release the neurotransmitter acetylcholine, which binds to cholinergic receptors, located mostly at the atria [66]. This system serves a double purpose, as it is particularly important when
139 the heart is excited and functioning under great workload. Activation of cholinergic
receptors of the heart decreases heart rate and contractility within a few seconds [1]. In fact a strong parasympathetic stimulus can stop heart pumping for a few seconds and reduce the strength of the heart contraction by 20-30% [3]. The parasympathetic contribution to the control of the vasculature is very limited and confined to a few tissue beds including the brain, heart, erectile tissue of the genitalia, some glands and, to a lesser extend, the kidney [72].
The activities of both branches of the autonomic nerves system are in constant
fluctuation mainly to defend blood pressure within a narrow range however this maintenance of blood pressure is intended only for short term whereas other mechanisms
have to be activated for long-term blood pressure maintenance [27]. The slightest change
in blood pressure is normally sensed by the baroreceptors and chemoreceptors within the
circulation, subsequently they send informative singles to the vasomotor center to unleash
a reflex to restore blood pressure back to normal [6]. This reflex usually includes a
withdrawal and/or activation of either of the autonomic nervous system branches. For
example in the case of elevated blood pressure, the autonomic nervous system deceases
sympathetic activities and increase parasympathetic activities to elicit a decrease in blood
pressure to normal [3].
4.1.2. Systemic Hormonal Control of the Circulation Systemic hormones are released into the bloodstream by the major endocrine glands and
bind to receptors located on the vascular smooth muscle. These hormones include
catecholamines, rennin-angiotensin-system hormones and arginine vasopressin [1, 73].
Hormonal agents released into the blood stream act directly on vascular smooth muscle
causing either dilation or constriction [73]. However hormones might also inflect their
140 effects on vascular tone indirectly vie modulating neural control or other hormonal
mechanistic [62]. For example angiotensin II binds to the neuronal membrane and
modulate the release of norepinephrine [74]. As well, angiotensin II controls blood pressure by modulating salt and water levels in the body thereby plasma volume [75].
Therefore considering the constant communication between the periphery and central
nervous system in addition to the interaction between hormonal and neural control, the
action of hormones in the body is often multifaceted and highly complex [62].
Catecholamines Due to a lot of similarities the adrenal medulla serves as an extension to the sympathetic
nerves system. In addition the sympathetic nerves system activities at the adrenal
medullae stimulate the release of large quantities of epinephrine and norepinephrine into
the blood stream. However catecholamines effects last 5 to 10 times longer as a result of
slower removal of the hormones [3]. Similar to the sympathetic nerves activities,
circulating catecholamines increase when the body is stressed mentally or by hunger,
exercise, or hypothermia. In addition they contribute to the fight-or-flight response as
they have profound and rapid effect on the heart, vascular system, and metabolism [15].
Both catecholamines hormones are derivatives of the amino acid tyrosine, have the same production procedures, and similar structure [18]. In addition at the same sites both epinephrine and norepinephrine have the same effect however mole for mole epinephrine is several times more powerful than norepinephrine. On average 80-90% of the adrenal medulla secretion is epinephrine whereas 10-20% is norepinephrine. Both catecholamines are extremely important in blood shunting and maintenance of adequate blood pressure thereby assuring sufficient blood delivery and removal during times of rest and stress
(e.g. exercise) [3, 76].
141 Epinephrine acts on the peripheral arteries by activation of α and β2 adrenal receptors throughout the body. Alpha-receptors cause vasoconstriction whereas β2- receptors induce vasodilatation. The concentration of these receptors is more abundant and dominant in areas than others. For example α is more abundant and predominant than
β2 receptors in the skeletal muscle and splanchnic arteries whereas β2 don’t exist in the skin and internal organs arteries [3]. Alpha-receptors on the other hand are highly concentrated in the skin and internal organs. Therefore increase in circulating epinephrine, such as during exercise, results strong vasoconstriction throughout the body except the skeletal muscle vasoconstriction is moderate [3, 76]. Increase in epinephrine to concentrations of 100 pg ml-1 induces elevation in heart rate and systolic blood pressure
-1 increases at concentrations of 125 pm ml . Activation of β2 causes vasodilatation followed by a drop in diastolic blood pressure in skeletal muscle however this drop doesn’t occur until epinephrine reaches 200 pg ml-1 which is ~ 135 pm ml-1 higher than basal levels [76].
In the venous side of the circulation existence and action of β2 and α adrenal receptors are not clear yet in human. However splanchnic and cutaneous veins contain α adrenal receptors and were found to induce venoconstriction. On the other hand the action of β adrenal receptors in veins is not known yet in human [77].
Norepinephrine is considered a neurotransmitter since most of it is derived from the sympathetic nerve terminals in the body [76]. However as a result of high sympathetic nervous activities its blood concentration might reaches four or five times above normal values (~1.8 ng ml-1) [15]. At these high levels norepinephrine acts as α
142 adrenal receptor hormone and induces vasoconstriction. In areas where α and β2 adrenal
receptors present vasoconstriction occur since β2 has low affinity to norepinephrine [3].
Angiotensin II The action of angiotensin has been revealed in the 1950s as the agent responsible for the
pressor responses observed following intravenous injection of renal extracts [62].
However aside from its capability to control arterial pressure through changes in vascular
tone, angiotensin inflect its effect on blood pressure by several mechanisms involves
controlling fluid volumes and interaction with the central and peripheral nerves systems,
thus its essential for long-term pressure control [78].
The production of angiotensin II is triggered by peripheral signals initiated by
reduction in blood pressure [67]. This reduction in blood pressure provoke the conversion
of prorenin to renin within the juxtaglomerular cells in the walls of the afferent arterioles
immediately proximal to the glomeruli in the kidney [79]. Following its release into the
blood stream the enzyme renin converts angiotensinogen to angiotensin I, a mild
vasoconstrictor with minimum effect in circulatory function. Renin persists in the blood
stream for 30 to 60 minutes as it continues forming angiotensin I [79, 80]. After few
seconds of producing angiotensin I, angiotensin-converting enzyme (ACE) converts it to
angiotensin II in the small vessels of the lungs. Angiotensin II persists in the blood for
only one or two minutes, then it gets converted to the inactive form by the enzyme
angiotensinase [80].
Angiotensin II powerful vasoconstriction capability is probably the main
mechanism by which it effect vascular resistance and blood pressure [81].
Vasoconstriction occurs mainly in the arterial side and to a lesser extends in the venous
side. Contraction of the arterial smooth muscle cells generates increase in vascular
143 resistance and blood pressure, whereas the mild contraction of the venous smooth muscle cells causes increase in venous return and cardiac output to help the heart pump against the angiotensin II-induced increase in blood pressure [15, 81].
Angiotensin II is not held to its vasoconstriction capability but also affects blood pressure by changing plasma volume. Angiotensin II acts directly on the kidney causing retention of salt and water and indirectly by inducing aldosterone secretion from the adrenal glands [62]. The effect of angiotensin II on fluid volume is particularly important for long-term control of arterial blood pressure [64]. High concentration of angiotensin II constricts the renal blood vessels thereby diminishing blood flow through the kidneys as a result, less fluid filters thorough the glomeruli into the tubules. Additionally the reduced blood flow into the peritubular capillaries reduces pressure causing rapid osmotic reabsorption of fluid from the tubules [3]. Angiotensin II also directly affects the tubular by increasing reabsorption of salt and water. The combined effect of these three mechanisms result in decreasing urinary output as much as four- to six-fold.
The reduction in kidney blood flow causes the release of aldosterone. Aldosterone increases sodium reabsorption by the kidney tubules, which increases the extracellular fluid sodium thereby increase in fluid retention. The total effect of direct and indirect mechanisms of fluid control is crucial for long-term control of arterial blood pressure.
However to the opposite of the popular belief, angiotensin II-induced retention of fluid by the direct-effect mechanism is probably three or more times as potent as the indirect effect [3].
Angiotensin II presser response vie vascular and fluid control has been confirmed for decades. However research have also revealed that angiotensin II facilitate the release
144 of sympathetic nerves system neurotransmitters in the periphery and adrenal medullary
catecholamines in addition to its possible influence on the central nervous system [62].
The first observation of the central effect of angiotensin II was observed when a dog head
was isolated from its circulation and connected to another animal while the brain
remained connected to the body only by the spinal cord. Following angiotensin II
infusion in the donor animal arterial pressure was increased in both animals. This presser
effect was inhibited by blocking the sympathetic activities, which indicate the effect was mediated by sympathetic nerves system [82]. Arterial pressure increased also in rabbits following vertebral arterial infusion of angiotensin II and failed to rise following systemic infusion confirming the central role of angiotensin II [83]. However the angiotensin II- induced augmentation of blood pressure in greyhounds was greatly hampered by atropine
(parasympathetic antagonist) or vagotomy indicating parasympathetically mediated response [84]. This difference might be due to species differences given the autonomic nerves system in greyhounds is predominantly vagal. The central site for angiotensin II effect is not clearly determined. However since angiotensin II doesn’t cross the blood brain barrier the site of action must be reachable from the circulation. Several investigations have found traces of angiotensin at varies location within the circumventricular organs including postrema area of the hind brain, the subfornical organ in the forebrain and the organum vasculosum of the lamina terminalis [85, 86].
The high level of interaction of these mechanisms demonstrates the multiple and complex action of angiotensin II in the control of blood pressure. Discovering the interaction of angiotensin II with several body systems have propelled the research related to hypertension and therapy pertain to (e.g. ACE inhibitors).
145 Arginine vasopressin (antidiuretic hormone) Arginine vasopressin is a nonapeptide hormone released by the posterior lobe of the
pituitary gland. Increased osmolality (increased plasma concentration) stimulates
osmoreceptors near the hypothalamus, which send singles to posterior pituitary to release arginine vasopressin into the circulation [87]. However reduction in central blood volume
as a result of fall in blood volume (hemorrhage) [88], orthostatic and/or positive pressure
breathing, are also potent stimuli of arginine vasopressin [89, 90]. Whereas mechanisms
by which either osmolality is decreased or central blood volume is increased inhibit the
production of the nonapeptide [91].
Arginine vasopressin receptors are located on the peritubular surface of the cells
of the distal convoluted tubules and medullary collecting ducts in the kidney. The binding
of the hormone to these receptors stimulates adenylyl cyclase activity and form cAMP
[92]. Thereafter cAMP initiates protein kinase activity and a phosphorylation cascade
resulting in the insertion of the protein, aquaporin, in the luminal membrane causing
antidiuretic action, the main effect of the hormone. Vasopressin inflicts its effect on the
kidney by increasing fluid permeability of the collecting ducts and tubules allowing water
to be reabsorbed back to the body [64].
In rats arginine vasopressin is several folds more potent vasoconstrictor than
angiotensin II, on a mole-for-mole basis [64, 93]. However the vasoconstrictor effects of
arginine vasopressin in human appear to be pathological rather than physiological as
vasopressin plasma levels are unlikely to reach those levels required to achieve
vasoconstriction [94]. At lower concentration of arginine vasopressin the contractile and
pressor responses to adrenergic and neural stimuli appear to be augmented [95].
146 Vasopressin once released remains in the plasma for as little as 2-3 minutes before
removed by the kidney and liver.
4.2. THE REFLEX CONTROL
The circulation is a mechanical feedback system thus any alteration in pumping or
peripheral vascular tone will influencethe intravascular pressures in various sections of
the circulation [96]. Continuous information about the state of the circulation is
transmitted from different sensory receptors strategically located around the circulation.
Any disturbances has direct effects on the heart and blood vessels which will tend to alter
cardiac output and tissue blood flow through local regulatory mechanisms [97]. These
peripheral sensory receptors are the baroreceptors and chemoreceptors.
4.2.1. Baroreceptors Baroreceptors are found in almost every large artery in the thoracic and neck areas
however they are found in abundant in the carotid artery and aortic arc. These sensors are
stimulated when these arteries are stretched [98]. Signals from the carotid receptors are
transmitted vie the Hering’s nerve that connects to the glossopharyngeal nerve, then to
the tractus solitarius in the medullary area, whereas the aortic baroreceptors connect with
the tractus solitarius by the vagus nerves (figure 4-6) [3, 9, 65].
The baroreceptors appear to have pressure-dose-response [3, 9]. The aortic
baroreceptors are stimulated at pressures around 30 mmHg whereas the carotid ones start
sending signals to the brain when pressure decrease below 60 mmHg and their activities
progressively intensify as they reach maximum at 180 mmHg. However the normal operating pressure is around 100 mmHg and a slight change in pressure calls for the autonomic nervous system for adjustments. The remarkable characteristic of the baroreceptors is the extremely rapid respond to changes in blood pressures. Additionally
147 constantly and rapidly increasing blood pressure triggers baroreceptors’ response far more rapidly than stationary pressure [3, 99]. For example a progressively increasing blood pressure of 150 mmHg cause twice as much increase in the baroreceptors’ firing rate as a stationary blood pressure of 150 mmHg. Whereas a falling pressure cause one quarter firing rate of that for the stationary pressure [3, 100]. Once the signals are received in the vasomotor center the sympathetic vasoconstrictor tone is inhibited and the vagus tone is activated (figure 4-5). This would decrease cardiac output and vascular resistance followed by blood pressure decrease. Additionally the baroreceptors firing rate increases during systolic and decreases during diastolic phases of the cardiac cycle [3,
15]. The magnitude of sympathetic tone inhibition depends on the firing rate from the arterial baroreceptors where higher firing rate cause more inhibition. Therefore a decrease in the firing rate, such as during a fall in blood pressure, cause an increase in sympathetic tone thereby elevation in blood pressure. This is apparent when blood pressure in the head and upper portion of the body falls upon standing up after laying down the baroreceptors elicit a reflex that triggers a strong and immediate sympathetic discharge result in rapid restoration of normal blood pressure [15, 68]. The baroreceptors serve as a short-term mechanism to defend blood pressure within a narrow range. However for long-term regulation of blood pressure other control systems, such as antidiuretic and associated hormonal systems, have to get involved [9, 98].
4.2.2. Chemoreceptors The chemoreceptors are highly vascularized, extremely sensitive to changes in blood gas proportion, and easily accessible by the blood [68]. The bifurcation of the common carotid artery and the aorta are the host of these really small (1-2 millimeters) organs.
Each common carotid artery harbor one sensor called carotid bodies whereas several
148 similar receptors, called the aortic bodies, cling to the wall of the aorta (figure 4-6) [9,
67]. The accessible location of these nerve endings allows for constant sensing of changes in blood gas continent, such as lack of oxygen, excess carbon dioxide and excess hydrogen ions. Additionally the small nutrient artery runs through these bodies supply them with abundant blood flow and keep them with close contact with arterial blood [3,
101]. Below a critical value fall in blood pressure changes blood flow in the nutrient artery, which results in a diminished oxygen and accumulation of carbon dioxide contents. Consequently these sensors are stimulated and start firing signals to the vasomotor center in the central nervous system [15, 101]. Within the vasomotor center these signals are processed and a stimulus is transmitted to the sympathetic nervous system to counteract this drop in arterial blood pressure. However these sensors play an important role in preventing a fall in blood pressure below 80 mm Hg whereas they are not acutely activated during normal blood pressures [3, 9, 101].
Figure LR3.6. Peripheral baroreceptors. Taken from [3].
149 4.3. LOCAL REGULATION OF VASCULAR TONE
The cardiovascular system delivers blood throughout the body according to tissue demands. This process involves constant adjustment of the heart and the vessel functions to assure sufficient blood pressure and volume. However an integral part of this adjustment occurs primarily in the capillaries, arterioles, venules, small arteries and small veins. This group of vessels is controlled by chemical substances mainly produced in the tissues surrounding theses vessels. Additionally these chemical substances are produced in accordance with the demands of the metabolic tissues. This mechanism is the local regulation of vascular function and involves mainly the contribution of the endothelium and metabolic tissues.
4.3.1. Endothelium-Dependent Regulation of Vascular Tone The endothelium is a monolayer of squamous cells located strategically at the interface
between the blood and vascular muscle cells implying an intimal and direct interaction
between these three compartments [56, 102, 103]. The endothelium is a very interactive
tissue capable of responding to numerous mechanical, chemical and cellar stimuli. The
surface of the vascular endothelium expresses a verity of receptors for potent vasotonic
agents that have rapid and profound effects on the vascular wall function.
Recent advances in cardiovascular science have demonstrated the integral role of
endothelium cell in the cardiovascular function beyond just a passive barrier of the
vasculature. Since the illustration by Furchgott and Zawadzki of the obligatory role of the
endothelium in the relaxation to acetylcholine in the isolated aorta of the rabbit [104], it
has become obvious that the endothelium modulates the degree of constriction of the
underlying vascular smooth muscle. It does so by liberating vasoactive substances
including endothelium-dependant relaxing factors [104, 105], endothelium-dependant
150 hyperpolarizing factors [106-108], and endothelium-dependent constriction factors [59,
109].
It is also clear now that the endothelium possesses a variety of modulatory
functions beyond the control of vascular tone including endothelium-dependent
modulation of platelet aggregation [110] and endothelium-dependent modulation of
vascular growth [111]. The endothelium tissue is also involved in vascular function
alterations associated with the pathogenesis of many cardiovascular diseases [112].
Endothelium-dependent relaxation Among a number of vasotonic factors, the endothelium lining of the vascular system
releases relaxing mediators including nitric oxide, prostacyclin, and hyperpolarization
factors. In addition to smooth muscle level of responsiveness, these vasodilatory agents
play an integral part in peripheral vasodilation at rest and under various stress conditions.
In a lot of cases these agents interact together to achieve proper peripheral blood flow.
However paradoxically Ca+2 ion, the very same agent that promote vasoconstriction, is
also involved in vascular smooth relaxation. All endothelium-dependent relaxation factors are released as a result of increasing Ca+2 concentrations in the endothelial cell.
This increase is triggered by either agonists (e.g. acetylcholine, histamine, bradykinin,
arginine vasopressin and ADP) or sheer stress (figure 4-7).
Nitric oxide Nitric oxide is a substance released continuously by the endothelial lining of the vascular
system and prominently directed to induce vasodilation [48]. Additionally it is currently
considered as one of the most endothelial function markers as its release believed to
induce a compensatory relaxing response to a stimulus of a constricting nature [112] and
prevent platelet aggregation [113]. Due to its gaseous nature, nitric oxide is rapidly
151 produced, metabolized and highly diffusible without even direct contact with vascular
smooth muscle, which makes it hard to detect it [104]. However using invasive and
difficult techniques, animal models have allowed researchers to reveal a lot of
information to help us understand the important role of nitric oxide [55, 114].
Furthermore, pharmaceutical infusion of substances to modify molecular markers of nitric oxide function have been used as simple and noninvasive method to assess nitric oxide production and synthesis in human [115, 116].
Interestingly the role of nitric oxide was discovered as a result of searching the
effect of acetylcholine. For a long time the role of acetylcholine in vascular tone was a
subject of controversy. Some investigators noted acetylcholine effect ranged from slight
to substantial vasodilatation, [117-119] whereas the majority of the literature reported
only additional contraction [120]. However the importance of an intact endothelium for
the acetylcholine-induced vasodilation in rabbit aorta rings was not demonstrated until
the accidental finding by Furchgott and Zawadzki in 1980 [104]. A failure to follow the
standard procedures for the effect of drugs on vascular smooth muscle tone resulted in
preserving the intimal surface (endothelium tissue) of the vascular smooth muscle and
subsequent muscle relaxation to acetylcholine. These unexpected results were followed
by another experiment in which extra care was taken during vascular smooth muscle
preparations to avoid rubbing the endothelium tissue. In this experiment a transverse strip
of rabbit aorta with endothelium removed was mounted between spring-clips with a
longitudinal strip of the same length and width but with endothelial cells present, intimal
surface against intimal surface. In this so-called “sandwich” preparation, the
endothelium-free transverse strip, which alone did not relax in response to acetylcholine,
152 exhibited good relaxation. This relaxation was attributed, at the time, to the release of
substance(s) from the endothelial cells of the longitudinal strip, and called endothelium-
dependent relaxation factor [104]. Independently Ignarro and coworkers in their
continued study of nitrovasodilator action discovered that nitric oxide could relax
vascular smooth muscle through stimulation of soluble guanylate cyclase [121].
Afterward the findings of these two groups were matched to conclude that the
endothelium-dependent relaxation factor was indeed nitric oxide [122, 123].
Like the rest of endothelium-dependent relaxation factors, production of nitric
oxide is initiated as a result of increasing Ca+2 ions concentration in the endothelium cell.
Epinephrine, arachodonic acid, acetylcholine, ADP, ATP, arginine vasopressin,
bradykinin (AVP), endothelin, histamine, serotonin, thrombin, in addition to sheer stress
are factors responsible for this increase in Ca+2 ions.
The binding of one of these agents or the physical pressure of sheer stress trigger
intracellular signals (e.g. cyclin adenosin monophosphate, phospholipase C, inositol
triphosphate and diacylglycerol) causing an increase Ca+2 ions within the cell. This
increase in Ca+2 activates nitric oxide synthase, which reduces L-arginine to nitric oxide
and L-citrulline [125-127].
Following the release of nitric oxide from the endothelium cell, it easily diffuses
to the adjacent vascular smooth muscle cell and produces vasodilation. This vascular
relaxation occur as a result of nitric oxide binding to the heme portion of guanylyl
cyclase in muscle cells and increase production of cyclic guanosine monophosphate
(cGMP) from guanosine triphosphate. The accumulation of cGMP activates cGMP-
dependent protein kinase, which phosphorylates myosin light chain kinase and regulates
153 activity of action-myosin ATPase (figure 4-8). Phosphorylation of myosin light chain
kinase lowers its affinity for the calcium-calmodulin complex, thus decreasing
phosphorylation of myosin light chain and promoting the formation of comparatively less
active, dephosphorylated form of myosin. Activation of cGMP-dependent protein kinases
can also result in the phosphorylation of calcium transporters, complexing intracellular
calcium and allowing relaxation of vascular smooth muscle [128]. However other
investigators have suggested that cGMP-mediated relaxation might involve other
mechanisms. These mechanisms include inhibition of a sarcolemmal calcium channel to
inhibit Ca+2 influx [57, 129], or the activation of sarcolemmal calcium extrusion pump
[130].
Figure LR4.7. Agents cause the release of endothelium-derived relaxing factors. NO; nitric oxide, PGI2; prostacyclin, AVP; arginine vasopressin, NA; norepinephrine. Taken from [124].
154
Figure LR4.8. Effect of endothelium-derived relaxing factors. NO; nitric oxide, PGI2; prostacyclin, R; receptor, cGMP; cyclic guanosine monophosphate, cAMP; cyclic adenosine monophosphate. Taken from [124].
Nitric oxide is also involved in deactivation of platelets aggregation. This process
is mediated by increasing intercellular levels of cGMP [58]. However the effect of nitric
oxide is strongly synergistic with that of prostacyclin. Thus the combined secretion of
nitric oxide and prostacyclin by the endothelin cells at the interface with the blood must
constitute a powerful antiaggregatory barrier (figure 4-8) [131].
Nitric oxide-independent relaxation maybe reduced by hemoglobin or methylene
blue, which antagonize the rise in cGMP either by inhibiting guanylate cyclase or by
scavenging nitric oxide and preventing its stimulation of the enzyme [54, 56].
Infusion of 0.8 mg ml-1 of nitrates was enough to produce venodilation but 3-4 mg
ml-1 was required to produce dilation in the arterial side in pre-constricted vessels [132],
whereas nitroglycerin produced venodilation even in unconstricted veins [133]. The effect of nitric oxide however varies in different segments of the venous system. In a study to compare the effect of nitric oxide in different veins found that acetylcholine
produced significant dilation in the external jugular, superior vena cava, brachiocephalic
veins and two portions of the inferior caval vein. Whereas the azygos, pulmonary,
155 splenic, renal, femoral and lateral saphenous veins exhibited 10-20% relaxation. On the
other hand the portal, mesenteric and the third segment of the inferior caval veins showed
constriction [134]. Similar observations were also found in human [135], as 0.4 of
nitroglycerine had no effect on superficial hand veins whereas produced dilation in the
calf and forearm veins.
Prostacyclin Physiologically, prostacyclin is locally produced and serves a double purpose as it causes
vascular smooth muscle relaxation and prevents platelets aggregation. In addition it is
responsible for metabolizing cholesterol esters in smooth muscle cell, prevent the
accumulation of cholesterol esters and suppress the release of vascular smooth muscle growth factors [105, 136]. Arachidonic acid is a membrane-bound fatty acid processed by cyclooxygenase into endoperoxides then to prostacyclin by prostacyclin synthase. This process is initiated by the break down of cell phospholipids by phospholipase A2 within
the endothelium cell then degraded to 6-keto-PGF1α, a biologically inactive form of
prostaglandin [137]. Amongst several substances proposed, bradykinin and choline esters
seem to be the strongest candidates to stimulate prostacyclin formation and release in
endothelium cells of human and animals [138, 139]. Angiotensin I and II also participate
in the production process, however they don’t seem to be as potent generators as
bradykinin and choline esters [139]. Nicotinic acid derivatives and the sheer stress of
blood on vessel walls have also been suggested to mediate the release of prostacyclin in
human and animal endothelium cells [140-142]. Moreover histamine, arginine
vasopressin, platelet activating factor, interleukin-1, leukotrienes and thrombin are
believed to be involved in the production of prostacyclin [31]. However regardless of the
external stimulus, prostacyclin biosynthesis is triggered by activating membrane G-
156 protein and increasing cytoplasmic Ca2+. This increase in [Ca2+] activates phospholipase
A2, the initial rate-limiting enzyme in prostacyclin production pathway (figure 4-8) [105].
Glucocorticosteroids and lipcortin inhibit the production of prostacyclin via
inhibiting phospholipase A2 [143, 144]. Whereas low-density lipoproteins [145] and
vitamin K [146] inhibit prostacyclin biosynthesis by interfering with prostacyclin
synthase function because they produce oxygen free radicals. On the other hand aspirin
and aspirin-like drugs affect prostacyclin production by blocking the activities of
cyclooxygenase thereby inhibiting the conversion of arachidonic acid to endoperoxides
[147].
Following binding to the receptors prostacyclin induces relaxation and prevents
platelet aggregation by increasing cAMP activities in vascular smooth muscle and in
platelets [105]. However some investigators suggest the contribution of prostacyclin in vascular function is not as significant as nitric oxide and might be considered negligible
[148].
The effect of prostacyclin on venous smooth muscle is a disperse, mainly due to minimum information available. Sinzinger and Fitscha questioned venodilation to prostacyclin as they found venoconstriction in human saphenous veins [149]. In general however prostacyclin was found to produce venous smooth muscle relaxation [150, 151].
For example in a study by Hiremath et al. prostacyclin produced dose-dependent relaxation [150] confirming previous observations by Robinson and colleagues [152].
Endothelium-dependent hyperpolarization factor In 1978 Kuriyama and Suzuki reported hyperpolarization of the cell membrane of smooth
muscle when exposed to acetylcholine [153]. Bolton et al. verified this finding in 1984
while hyperpolarization was converted to depolarization as a result endothelial tissue
157 removal suggesting the involvement of the endothelium tissue [154]. Bioassay experiments thereafter revealed that the endothelium indeed releases highly diffusible substance(s) causing hyperpolarization followed by smooth muscle relaxation [107].
Despite infusing endothelium-dependent relaxant inhibitors (e.g. methylene blue and hemoglobin) [155, 156] or exposing the endothelium tissue to substances don’t cause the release of nitric oxide or proctacyclin (e.g. substance P and oxotremorine) [106, 157] hyperpolarization-induced relaxation was maintained indicating additional endothelium- dependent relaxation mechanism other than nitric oxide and prostacyclin.
Several substances have been identified to cause the release of endothelium- dependent hyperpolarization factor. These substances include acetylcholine, the primary smooth vessel dilator, in addition to histamine [155] bradykinin [158, 159], substance P
[160] [107], calcitonin gene-related peptide [161], ADP [162], endothelin [163-165] and vasoactive intestinal polypeptide [160]. As nitric oxide, endothelium-dependent hyperpolarization factor is released as a result of an increase in free [Ca+2] within the endothelium cell. The initial response to a stimulus can be attributed to the release of intercellular endothelial calcium stores, whereas the entry of extracellular calcium into the cell prolongs the response to acetylcholine or bradykinin [166, 167]. Additionally endothelium-dependent hyperpolarization apparently involves calcium calmodulin, as inhibitors of this enzyme decrease hyperpolarization in porcine coronary artery to bradykinin (figure 4-8)[168, 169].
Endothelium-dependent hyperpolarization of smooth muscle is induced by an increase in K+ conductance of the cell membrane, which leads to relaxation by increasing the membrane potential of the muscle cell [154]. This has been evident as membrane
158 conductance of K+ in smooth muscle is increased during hyperpolarization [153, 154,
170], the magnitude of hyperpolarization is inversely related to [K] [170], radioactive
rubidium (marker of K+ activities) efflux is increased during hyperpolarization [171,
172], hyperpolarization has been inhibited by K+-channels blockers [173], acetylcholine
produces a transient increase in the K+ efflux, associated with the transient
hyperpolarization of the membrane [171] and K+-channels openers have caused
hyperpolarization-induced relaxation [170]. Potassium channels have been proposed to
account for most of the potassium trafficking across the smooth muscle membrane
because resting membrane potential relay mostly on the membrane potassium gradient.
This has been more evident in large conduits than small ones [174]. Moreover, several
investigations have demonstrated that endothelium-dependant hyperpolarization is
insensitive to ATP-sensitive potassium channel inhibitors, blocked by small conductance
calcium-activated potassium channel and voltage-dependent potassium channel inhibitors
but not affected by large conductance calcium-activated potassium channels inhibitors
[175]. However other mechanisms might be involved in inducing hyperpolarization. For
example Busse et al, demonstrated that the increase in intracellular free Ca+2 might be the
preliminary event leading to the secondary membrane hyperpolarization [176].
Several characteristics have been identified for the hyperpolarization mediator including high solubility and transferability, and ability to produce hyperpolarization- induced relaxation [107, 177]. Additionally the hyperpolarization of the smooth muscle usually is more transient than the accompanying relaxation induced by endothelium- dependent vasodilators [155, 170, 171, 178]. The exact nature of the endothelium- dependent hyperpolarization factor however has not been identified as several substances
159 have been proposed to cause hyperpolarization in smooth muscle cell. Prostacyclin, nitric
oxide, and products of cytochrome P450 have been reported to cause smooth muscle
hyperpolarization in different species at various locations in the vascular system. Fore
example prostacyclin and nitric oxide have been reported to increase potassium
trafficking across the smooth muscle membrane by activating ATP-sensitive [179] and
calcium-activated channels [180]. Moreover, experiments performed mainly in bovine
and porcine coronary arteries show that smooth muscle hyperpolarization is inhibited by
inhibitors of cytochrome P450 monooxygenases and associated with the release from
endothelial cells of epoxyeicosatrienoic acid (EET) (substance produces
hyperpolarization of vascular smooth muscle) [181, 182]. Alternatively,
hyperpolarization of endothelial cell causes accumulation of potassium ions in the
intercellular space between endothelial and smooth cells. This increase potassium
concentration can provoke the hyperpolarization of vascular smooth muscle cell by
activating the inwardly rectifying potassium conductance [183] and Na/K pump [184].
Considering all these dynamics, potassium could also be regarded as an endothelium-
dependent hyperpolarization factor. However these facts, about the nature of
endothelium-independent hyperpolarization factor, are not true in all blood vessels of
different species. For example in blood vessels of the pig as well as in various arteries
from human, chemically unrelated inhibitors of cytochrome P450 do not produce an
inhibition of hyperpolarization-responses [185-187]. Additionally these proposed factors might all be responsible for hyperpolarization-responses in the smooth muscle cell of vessels as they all contribute at different stages and to various degrees to the
160 hyperpolarization process. Therefore, more research is needed to further identify the
nature of this factor(s) in different metabolic tissues of different species.
Resent investigations have shown the importance of endothelium-dependent
hyperpolarization factor increases as the vessel size decreases in rat mesenteric as well as
in human gastroepiploic arteries [186, 188]. Additionally endothelium-independent
hyperpolarization factor is impaired in aging and hypercholesterolemic human and
animals [186, 189, 190] and in hypertensive and diabetic animals [189, 191].
Endothelium-dependent contraction It was as early as 1982 when the vasoconstrictory role of the endothelium was
demonstrated as arachidonic acid augmented smooth muscle contraction evoked by
norepinephrine in the canine systemic and pulmonary veins [192]. This augmentation disappeared when the endothelium tissue was mechanically removed suggesting that the endothelial cell can be a source of vasoconstrictor signals [192]. Endothelium-dependent
smooth muscle contraction has been described in a verity of blood vessels under various
conditions and plays a role in maintaining proper vascular tone without implying endothelium dysfunction [56, 193].
Endothelin The existence of a contractile factor released by the endothelium cell was demonstrated as early as 1985 with the presence of extracellular calcium and independent of the adrenergic, cholinergic, serotoninergic, or hitaminergic systems [195, 196]. This factor was then characterized and named endothelin, as it was isolated from the endothelial cells of pig aorta in 1988 by Yanagisawa et al. Endothelial was described as a peptide made up of 21 amino acids with a N-terminal domain provided a very strong affinity to the receptor [59]. Subsequently three isoforms of the peptide were identified and named
161 endothelin-1, endothelin-2 and endothelin-3. All subtypes are present in all mammalian
species, however endothelin-1 is the only one made in the endothelial cell [197].
Endothelin-1 is ten times more potent vasoconstrictor then angiotensin II and induces a
very long lasting hypertensive action [198]. However endothelin-2 is the most potent
among all of them and has longer duration of action on blood pressure whereas
endothelin-3 has 1/20th of the vasoconstrictor activity of endothelin-1 [59].
Figure LR4.9. Agents cause the release of endothelium-derived contracting factors. EDCF1; endothelium-derived contracting factor, PO2; partial pressure of oxygen, AI&II; angiotensin I & II, AA; arachidonic acid, SP; substance P, 5-HT; 5-hydroxytryptamine, PGH2; prostaglandin H2, TXA2; thromboxane A2, AVP; arginine vasopressin, TGFα; transforming growth factor beta, ACE; angiotensin converting enzyme. Taken from [194].
The secretion of endothelin-1 is regulated at the level of peptide synthesis due to the lack
of dense secretary granules where it can be stored then released as needed. The
expression and release of endothelin-1 is induced by several factors including adrenaline,
thrombin, vasopressin, angiotensin II, insulin, cytokines, growth factors and physical
stimuli (figure 4-9). The production of endothelin-1 involves the increase in intracellular calcium and protein kinase C activity, which is induced by many of these agents [199].
162 On the other hand nitric oxide [200], prostacyclin [201], natriuretic peptides [202],
heparin [203] and protein kinase C inhibitors reduce endothelin-1 expression and release
[199].
Two types of endothelin receptors were identified and termed ETA and ETB
receptors. The affinity of the three isopeptide to ETA is endothelin-1 > endothelin-2 >>
endothelin-3, where the affinity of endothelin-1 in man is 1000-fold more than
endothelin-3 [204]. However the ETB receptors show no selectivity for any of the
endothelin isoforms [205]. The vasoconstrictor response is produced by stimulation of
ETA receptors present in vascular smooth muscle and relaxing response by stimulation of
ETB receptors in the endothelium. However studies reported the existence of ETB in
vascular smooth muscle and produces vasoconstriction [206, 207].
Administration of endothelin-1 induces an initial reduction in arterial blood
pressure followed by prolonged increase. This reduction is transient and most noticed
during elevations in resting blood pressures however have been attributed to the release
of vasodilator substances including prostacyclin and nitric oxide from the vascular
endothelium [126, 208, 209]. Similar to other vasoconstrictors, endothelin induces
smooth muscle contraction by increasing intracellular C+2. The initial phase of this
+2 contraction is achieved by activating IP3 system thereby mobilizing intracellular C
stores. The prolonged phase of the contraction is maintained by stimulating the entrance
of extracellular C+2. However the mechanism by which the entrance of extracellular C+2
is achieved is not clear yet. Several investigations have suggested that the formation of
+2 inositol tetrakis phosphate (IP4) might trigger the entrance of C [210-212].
163 In addition to its vasodilator and vasoconstrictor effects endothelin-1 promotes
proliferation of vascular endothelium and smooth muscle cells and fibroblasts, which
results in structural changes to the vascular wall. The mitogenic effect of endothelin-1 is
attributed to stimulation of tyrosine residues phosphorylation and activation of two other
mitogen-activated protein kinases in the vascular smooth muscle [213, 214].
Numerous reports have been published regarding the possible role of endothelin
in a variety of phathophysiological conditions involving several body systems. These
systems include the renal system, pulmonary system, heart and vasculature. This
involvement in a wide range of diseases and conditions is due mainly to the endothelin capacity to induce constriction and cell proliferation. In the vascular system, endothelin
increases vascular tone, vascular resistance thereby blood pressure. It is also believed to
be involved in the atherosclerotic process and vascular structure alterations. Endothelin is
also involved in increasing the production of other vasoconstrictor including rennin,
angiotensin II, epinephrine, and aldosterone as well as increasing sympathetic activity.
Besides its vasoconstrictor and pressor effect, endothelin has positive inotropic properties
that increase the contractility of vascular smooth muscle [215, 216].
Veins appear to be super sensitive to endothelin as it was 5- to 10-fold more
potent constrictor in isolated large veins than isolated large arteries from dog coronary,
mesenteric, femoral, renal, and internal mammary vessels [217, 218]. Similar
observations were also noted in human as endothelin produced greater contraction in
human saphenuos vein then the gastroepiploic artery [219]. Endothelin effect in human
is described as slow onset venoconstriction. This constriction was maintained even
following the administration of Ca+2-blockers [220, 221]. On the other hand
164 venoconstriction was reversed when nitric oxide was administrated but to a lesser extent
than as observed in the arteries [219]. Additionally infusion of potassium channels opener
drugs and calcium channels antagonists prior to endothelin administration prevented
endothelin-induced constriction [222]. In a comparison of the constrictor effect in human superficial hand veins, endothelin-1 was 26 times and enodhtlin-3 was 3 times more potent than sympathomimetic drug (phenylephrine) [223, 224].
Activation of voltage-dependent channels [59, 225], closure of ATP-sensitive potassium channels [222], and most importantly mobilization of Ca+2 from internal
sources [226] are the mechanisms by which endothelin increase Ca+2 levels and
consequent venoconstriction.
4.4. METABOLIC VASOREGULATION
The metabolic theory of vascular control is characterized as changes in the concentration
of dilator substances(s) alter the excitation-contraction coupling of vascular smooth
muscle cells. Increased vascular resistance and decreased blood flow to metabolic tissues
causes increased vasoactive metabolic substance concentrations in the interstitial space
[97, 227]. This theory indicate that imbalanced blood demand/supply induce the
production and accumulation of vasoregulation mediators. These mediators interact with
the vascular smooth muscle of the resistance vessels inducing alterations in vascular tone
thereby changes in local tissue blood flow [228].
Hypoxia has been implicated as a vasomediator since oxygen might be a limiting
factor in aerobic ATP production. This hypothesis indicate that reduction in cellular
[ATP] during muscle exertion induces vasodilation [229]. However studies suggested
that respiratory chain function and ATP production wouldn’t be affected until
165 mitochondrial PO2 reaches ~0.05 mmHg [230, 231]. This drop in PO2 can’t be found in
normal situation even under hypoxic conditions experienced during exercise [232, 233].
Hypoxia-induced smooth muscle contractions in isolated peripheral, coronary and
cerebral arteries of the dog however have been documented. This response was attributed
to an unknown substance produced from the endothelium to the smooth muscle [234-
236]. Endothelium-dependent vasoconstriction to hypoxia appears to be dependent on activation of voltage-dependent calcium channels in smooth muscle because the contraction was blocked by calcium antagonists [237].
Upon the discovery of endothelin, as a vasoconstrictor, it has become an attractive candidate especially it produces a contractile effect sensitive to inhibition with calcium antagonists and does not require the activity of cyclooxygenase in animal models.
However further analysis of the characteristics of the substance(s) released by hypoxia indicated that endothelin is not responsible for the response [238]. In addition contractile respond to endothelin is slow and sustained whereas hypoxia-induced contraction was fast and transient [239]. On the contrary, hypoxia-induced contraction can be blocked when activity of nitric oxide is reduced by either methylene blue or inhibition of nitric oxide synthesis. Thus endothelium-dependent contractions to hypoxia might be due to the reduced activities of nitric oxide caused by hypoxia rather than the release of a diffusible contracting factor(s) [240, 241]. In human internal mammary artery hypoxia-induced contractions can be attenuated by indomethacin or inhibition of nitric oxide synthesis and are abolished by a combination of both. Therefore in human endothelium, two contracting components might be involved during hypoxia; a product of cyclooxygenase and the reduced activity of nitric oxide [242]. Interestingly in re-perfused canine coronary
166 arteries, the hypoxia-dependent contraction is augmented indicating the response may
contribute to myocardial ischemia in coronary artery disease [193].
On the other hand endothelium-dependent hypoxic contractions are not observed in peripheral veins [192]. This lack of response however might be due less sensitivity of the venous smooth muscle to the contracting effect as coronary arterial smooth muscle
“sandwiched” with venous endothelium exhibits hypoxic contraction [235].
Since carbon dioxide production is coupled with oxygen consumption and ATP production [243-245] it might be an attractive candidate as a vasoregulator. Assuming venous PCO2 is representative of tissue PCO2 investigators have found a direct
relationship between venous PCO2 and vasoactivity in both direction constriction and
dilation [45, 246, 247]. However since these investigations where not designed
+ particularly to distinguish between the effect of CO2 and H , changes in pH might also
play a role in vasodilation.
Potassium is constantly floating around during metabolism and muscle contraction as it plays an integral role in cell membrane depolarization and repolarization.
The progressive increase in interstitial [K+] noticed following increased metabolism is
associated with a vasodilator effect [248]. However this vasodilator effect continues as
the rise in [K+] disappears indicating transitory, complimentary or negligible role of potassium in the vasodilation attributed to metabolic out-products [249, 250]. A moderate increase in [K+] has minimum effect on smooth muscles of venous wall in animals [251,
252]. However, higher concentrations (above 12-20 meq/liter) seem to potentiate the
venoconstrictor effects of catecholamines and directly caused vasoconstriction of isolated veins [253-255]. These venoconstrictor effects of K+ ions have been attributed to
167 membrane depolarization and consequent influx of extracellular Ca+2 [256].
Concentrations above 30 meq/liter induce indirect venoconstrictor effect of K+ by stimulating norepinephrine from the nerve endings [257, 258]. Additionally concentrations of 10 or 15 meq/liter seem to inhibit the neuronal uptake of catecholamines. This inhibition was related to the Na+/K+ ATPase of the cell membrane
[257]. This venoconstrictor effect of potassium is appears not significant as compared to the effect to sympathetic or norepinephrine activities.
The relaxing role of adenosine appears to be significant during the early phase of vasodilation [259, 260]. This adenosine-induced vascular relaxation is activated by specific receptors located on the vascular smooth muscle and inhibited by methylxanthines [261, 262]. However the inhibition of adenosine activities didn’t prevent vasodilation to other factors including myogenic [228, 263] and acidosis [264].
Adenosine appears to promote dilation and to inhibit the effect of venoconstrictors, such as norepinephrine, in the venous side of the circulation in dogs [265, 266]. Similar observations were also found in human veins as adenosine promotes venodilation in a concentration-dependent manner [267]. In vivo, human hand veins precontracted with phenylephrine (a sympathomimetic) are relaxed by infusion of adenosine, showing no effect of aging on the venous respond to adenosine [268]. Venous relaxation in respond to adenosine or adenosine products was observed in several other tissue beds including erectile [269], portal [270] and jugular [271] veins of different species.
It appears that several metabolic out-product are involved concurrently in the control of the vascular function. However, it’s not known which of these substances is the sole mediator of metabolic regulation. Evidence from experiments that perturb a steady
168 state suggests that vascular function don’t depend on a single metabolite. Rather the
regulation of resistance vessels may reflect the combined influences of several
+ + vasodilators and the most likely candidates seem to be CO2/H , K , and adenosine [228].
Identifying the attribution of each mediator individually requires a series of
straightforward experiments designed to study the effect of each metabolic out-product in
isolation from the rest of out-products. In addition this relationship has to be established
under all physiological as well as pathological conditions [272]. However such a
designed is not easy to accomplish and requires complex preparations.
4.5. THE MYOGENIC VASOREGULATION
Blood vessels respond to elevation in transmural pressure with constriction and to
pressure reduction with dilation. These changes in the vessel diameter are induced independent from neural, hormonal, or metabolic influence and attributed to the
autoregulations of the myogenic response. This behavior is most observed in the
arterioles however it has been demonstrated in arteries, venules, veins, and lymphatics
[97, 273, 274].
Jones et al. reported the myogenic response as early as 1852 [275], and followed
by similar observations by Ostroumoff et al. in 1868 [276], and Gaskell in 1881 [277].
However in 1902 William Bayliss was the one who clearly attributed a considerable
contribution of vascular control to changes in transmural pressure. Bayliss concluded that
changes in transmural pressure induce changes in the diameter of the vessels too rapid to
be mediated by metabolic accumulation and is similar to isolated arteries constriction
observed following sudden distension [278]. These findings were a matter of debate for a
long time until 1949 Folkow demonstrated that denervated vessel preparations developed
169 pressure-dependant vascular tone [279] and autoregulation of blood flow was attributed
in part to pressure-dependent mechanism [280]. These findings were followed by
intensive investigations resulted in developing sophisticated techniques to study whole
organ circulation [34, 281] and quantify the myogenic responses in the microcirculation
[282, 283]. These investigations confirmed previous reports and concluded that the
vascular responses to changes in transmural pressure are rapid [284] and induce
considerable changes in vascular resistance [34, 281]. Development of isolated vessel
techniques [285] has also allowed for more accurate quantification of myogenic mechanism in small arteries [286, 287] and arterioles [288, 289] where the effects of pressure could be clearly distinguished form flow, metabolic, neural and endothelial influences.
The magnitude of the myogenic respond is inversely related to vessel size among arterioles of a given vascular network [14, 290]. However the magnitude of the myogenic respond of similar size vessels varies in different vascular beds. For example the rat small mesenteric vessels have weak myogenic respond whereas it is strong in similar sized
cerebral [291] and skeletal muscle [292] vessels. These differences in the response were also observed between vessels with similar diameter in the same vascular bed [289].
Therefore the existence of the myogenic response is well documented however its strength varies between vessels even in vessels of similar diameter.
Several studies, performed in isolated and in vivo vessel preparations, have attributed part of the basal vascular tone to the myogenic respond [14, 293]. In these studies increase in vascular transmural pressure resulted in vasoconstriction and increased vascular resistance independent of other controlling factors [294]. The myogenic
170 response is also postulated to play an important role in maintaining constant blood flow
and capillary hydrostatic pressure during variations in systemic arterial pressure [97].
Whole organ data suggested that changes in arterial inflow and venous outflow pressures
produced changes in arterial resistance that would serve to minimize changes in capillary
hydrostatic pressure. Mellander et al. have reported constant cat hand-limp volume
during wide range changes in systemic pressure [294]. Given the tissue volume is a
reflection of capillary hydrostatic pressure, it was assumed that constant capillary
hydrostatic pressure was achieved by the autoregulation of the myogenic respond [295].
However these assumptions have disregarded the role of Frank-Starling law and other
local regulatory mechanisms. The myogenic response might be involved in maintaining
blood flow and capillary hydrostatic pressure, however other regulatory mechanisms
might play a more important role [14].
Vascular smooth muscle contraction and relaxation associated with the myogenic
response is thought to be the results of vascular smooth muscle membrane depolarization
and increased calcium ions permeability [296, 297]. However recently researchers have
found that myogenic contraction is initiated by depolarization of the vascular smooth muscle, which modulate calcium ions entry to the cell through voltage-gated calcium channels [298]. Data from cat cerebral artery showed graded (by –20m) vascular smooth muscle depolarization occurred as pressure was changed from 0 to 160 mmHg [299].
Similar observations were recorded in pig coronary [300], bladder myocytes [301] and mesenteric artery myocytes following longitudinal stretch [302]. This depolarization was
associated with activation of voltage-gated calcium channels and subsequent myogenic
behavior [14].
171 SECTION 5. ACUTE VASCULAR RESPONSES TO EXERCISE
Greater blood flow to skeletal muscles during exercise is a combination of increased cardiac output, increased inactive tissue vascular resistance and decreased skeletal muscle vascular resistance. Certainly this increase is in accordance with active muscle metabolic demands [303] and the result of the integration of central and peripheral mechanisms
[15]. Cardiac output increases as a result of greater venous return and stroke volume, additionally increased sympathetic activities and parasympathetic withdrawal strengthen contractility and accelerates heart rate, thereby larger cardiac output [304].
Vascular adjustments to exercise however are controlled centrally and peripherally. The large sympathetic discharge and increased hormonal activities cause total body vasoconstriction thereby increased vascular resistance. This “global” vasoconstriction is in favor of active muscle blood flow as local vasodilatory mechanisms are activated causing vascular smooth muscle relaxation during exercise [305]. This dynamic behavior of the vasculature, systemic vasoconstriction and local vasodilation, causes blood redistribution. Since blood is more needed in the contracting muscles the majority of cardiac output is redistributed or shunted toward the active muscles while blood flow to most of the inactive tissues is reduced [306].
Vascular adjustments are often regulated by different mechanisms during different phases of exercise as none of these mechanisms could alone explain the integrated control of blood flow to the active muscles [307]. While vascular control during exercise is complex this chapter will try to explain the mechanisms responsible for increasing blood flow to the active muscles. The systemic control of vascular function
172 will be briefly discussed whereas the peripheral vasoregulatory mechanisms will receive greater attention.
5.1. SYSTEMIC VASOREGULATION DURING EXERCISE
Muscle contractions stimulate the vasomotor center to increase sympathetic activities and to cause parasympathetic attenuation [308]. The mechanism (s) by which the vasomotor center is stimulated during physical activities is still a mater of debate.
Since heart rate and ventilation at onset of exercise increase almost immediately whereas electrically induced exercise (central command inoperative) developed a slower tachycardic response some researchers have suggested a centrally mediated sympathetic control of the cardiovascular function. Freyschuss et al. reported that static contraction increased heart rate and blood pressure with or without neuromuscular blockade, however this increase was one-half greater without blockade [309]. Therefore the supporters of this theory propose that motor outflow from the cerebral cortex interacts with the central neuron pools that regulate the cardiovascular responses to exercise. On the other hand, others reported electrically induced contraction caused an increase in heart rate and blood pressure indicating an activation of the sympathetic nervous system without central command involvement [310-312]. Additionally this increase in sympathetic nervous activities was intensity related [313, 314]. The supporters of this theory indicate that peripheral mechanical and chemical receptors are responsible for increased sympathetic activities associated with exercise. They further explain that reflexes from the contracting muscles seam to be the primary mechanism that activates the vasomotor center to initiate a sympathetic drive [315]. It seems muscle mechanoreceptors are the main source of peripheral reflexes during initial phase of exercise (0-15 sec.) whereas vascular
173 baroreceptors, carotid bodies, thermoreceptors and muscle chemoreceptors are activated
next (15 sec.-2 or 3 min.) by exercise. Peripheral reflexes during steady-state (3 min.
onwards) seems to be derived from the activation of all peripheral mechanisms [316].
However both sides of the argument have ignored the integration of central and
peripheral mechanisms to provoke sympathetic activities. In a recent study, Wichester et
al., [317] demonstrated the importance of both mechanisms to evoke sympathetic
discharge during exercise as demonstrated in patients with Brown-Sequard syndrome.
The spinal cord in these patients is hemisectioned leaving one side of the body with
sensory loss and normal motor function and the other side with normal sensation and
reduced motor function. Voluntary and electrically stimulated static knee extension of
either legs (with either deficit) elicited increases in heart rate and blood pressure, except
during electrical stimulation of the leg with sensory deficit heart rate and blood pressure
didn’t increase. In summary, neural mechanisms of cardiovascular control is redundant
[318] additionally modifications in autonomic nervous system activity could always find a strategy for stimulating the cardiovascular system stressing the plasticity of the neural
network [317].
Regardless of the mechanism (s) responsible for activating the vasomotor center, exercise appears to initiate sympathetic release that is rapidly propagated to the heart and most vascular networks. Probably the most remarkable characteristic of the autonomic nervous system is to elicit cardiovascular adjustments within few seconds. For example the sympathetic nervous system double heart rate within 3-5 seconds whereas blood pressure is increased to double normal within 15 seconds [15, 63]. This exceptional capability is due to the sophisticated and extensive neural network that connects the
174 central nervous system to the heart and almost all segments of the vasculature [61, 64].
Increased sympathetic activities during exercise to small arteries and arterioles increase
vascular resistance, blood pressure and blood flow to areas of need. Whereas the
sympathetic innervation to the large vessels, particularly the veins, assists in propelling
blood from these vessels to the heart thus generating larger venous return, stroke volume
and cardiac output [20]. For example the sympathetic stimulation increases arteriolar
resistance by ~eight-folds and can expel approximately 40% of the blood in the
capacitance vessels [319].
Although sympathetic outflow help increasing blood flow to the contracting muscles by triggering systematic vasoconstriction, substantial increase in sympathetic drive as a result of engaging in near maximal exercises involving large number of muscle groups might induce skeletal muscle vasoconstriction [320, 321]. In a study by Clausen et al., exercising upper and lower body at near maximal intensity resulted in decrease in lower body blood flow. This could be an active reflex redistribution of cardiac output and would indicate that all sympathetic efferent activity is not locally blocked or attenuated
[322]. This increase vascular resistance might be essential to maintain sufficient blood pressure with the circulation [314, 323]. However vasoactive agents resulting from the contracting muscles usually override skeletal muscle vasoconstriction by reducing sensitivity to [324] and maybe even inhibit the release of norepinephrine [102, 325, 326] thereby maintaining sufficient blood flow. Actually norepinephrine might stimulate the release of some of these agents thereby induce vascular smooth muscle relaxation and increase active muscle blood flow [327].
175 Cardiovascular responses to dynamic upper body exercise are different from lower body exercise. Due to the lack of lower body musculature involvement during upper body exercise a large vasculature remains undilated providing resistance to peripheral circulation [328]. This larger peripheral resistance induces substantially higher blood pressure at a given exercise intensity. Additionally at a given submaximal steady state oxygen consumption, heart rate during upper body exercise is higher. Due to less musculature involvement in upper body exercise venous return and stroke volume are lower. Therefore, since cardiac output is similar for upper and lower body exercises higher heart rate compensate for the reduced stroke volume [329, 330].
The complete parasympathetic withdrawal is achieved at heart rate of ~100 b/min.
This withdrawal causes an increase in cardiac output however it has minimum effect on the vasculature since they have very few vagal receptors [72]. Therefore as far as neural control, the sympathetic nervous system plays a greater role in controlling the vascular during exercise [65].
Increased sympathetic drive, during exercise, activates the medullary portion of the adrenal glands to release relatively large quantities of epinephrine and a smaller amount of norepinephrine. Essentially both adrenal hormones activate adrenergic receptors of the cardiovascular system causing increased cardiac output and generalized vasoconstriction. Hormonal control however seems to play a relatively minor role in regional blood flow during exercise compared with the more direct, rapid and potent sympathetic neural control. Furthermore it is more essential during stead-state exercise as it support the neural activities to maintain appropriate cardiovascular respond.
Additionally other physiological requirements, including thermoregulation, fluid
176 homeostasis and metabolic activities, compete for the adrenal medulla hormones [316,
331].
5.2. PERIPHERAL VASOREGULATION DURING EXERCISE
Increased blood flow and oxygen delivery to the contracting muscles are essential in order for exercise to continue. The first few seconds of exercise are associated with substantial increase in muscle blood flow, perfusion and oxygen extraction while these elements remain elevated as long as muscles are contracting [305]. In fact peak muscle blood flow during exercise is estimated to increase ~20 folds and to peak at 300-400 ml/min/100 g [306]. Certainly this increase is in accordance to muscle metabolic demands and related to muscle oxygen consumption [332], muscle contracting frequency
[333, 334], and running speed [335, 336]. Additionally within the active muscle group the recruited muscle fibers appear to receive most of muscle blood flow [334, 336].
Sympathetic vasoconstriction is induced during exercise throughout the body except in the contracting skeletal muscles. In a study by Johnson and Rowell blood flow to forearm decreased whereas it increased to the leg muscles during prolonged lower body exercise [337]. In fact blood flow was observed to increase to red portions and decrease to white portions of the same muscle during low-intensity exercise [338, 339].
The increased sympathetic vasoconstriction and decreased blood flow to inactive tissues while blood flow to the contracting remains elevated is achieved by functional sympatholysis. This process involves deactivating sympathetically induced vasoconstriction during exercise [325].
5.2.1. Local Mechanisms Contribute to Vasoregulation Various mechanisms have been proposed to contribute to locally induced increase in
muscle blood flow, as none of these mechanisms can be solely held accountable for this
177 substantial increase. Muscle pump, metabolic factors, myogenic response, and
endothelial-derived relaxing factors have been implicated as vasoregulatory mechanisms
responsible for the increased blood flow during exercise. These mechanisms are a
reflection of the contracting muscle activities and primarily to offset the vascular
responses to systematically induced vasoconstriction [254, 340-342].
Metabolic Mechanisms The oldest mechanism have been considered to contribute to hyperemic response
associated with exercise is metabolic vasodilation. As previously discussed, contracting
muscle metabolic byproducts are one of the mechanisms involved in peripheral reflexes
that initiate sympathetic vasoconstriction. Ironically however these metabolites also
appear to be involved in local muscle vasodilation during exercise. The byproducts of
metabolism accumulate in the interstitial fluid surrounding resistance vessels, thereby
establishing a local mechanism that is capable of matching oxygen delivery to the level of
contractile activity. The mechanism(s) by which the sympathetic vasoconstriction in the
active muscle is deactivated varies however the metabolic byproducts generally compete
with the sympathetic vascular regulation causing vascular smooth muscle to be less
sensitive to catecholamines [325]. For example Dodd and Johnson reported that the
metabolic byproducts reduced vasoconstriction mediated by post-junction α1-
adrenorecepoters but not by circulating catecholamines [343]. There is also a differential
sensitivity of α1- and α2-adrenoceptor constriction to metabolic inhibition during skeletal
muscle contraction as metabolic release of dilator substances attenuated α2-adrenoceptor
constriction 10-folds more than α1-adrenoceptor constriction [327].
Anyway the apparent relationship between muscle metabolism and muscle blood flow during exercise indicates a factor related to muscle metabolism might contribute to
178 vascular tone regulation. This relationship becomes evident when vasodilation was observed in resting recipient muscle upon the infusion of venous blood from a contracting muscle [344, 345]. It appears metabolites produced by the contracting muscles diffuse to the resistance vessels causing vasodilation and increased capillary recruitments. Adenosine, magnesium, potassium, calcium, oxygen and overall osmolarity are among the vasoactive factors associated with skeletal muscle metabolism and have been implicated in elevation of muscle blood flow during exercise [325]. Isolating the vasoactivity of each factor is a challenge however some studies have successfully observed the action of some of these factors.
Adenosine appears to accumulate in the vicinity of the contracting muscles [346].
The source of adenosine could be muscle tissue [347], endothelium tissue [348, 349], or a property of nervous activities [350]. However the muscle seem to be the major source of adenosine during contraction as electrically stimulated muscle produced adenosine without the present of nerve and endothelial cells [347].
Regardless of the source during contraction adenosine seems to induce vasodilation associated with increased muscle blood flow [15]. Arterial infusion of adenosine has been shown to produce vasodilation in most vascular beds including in human forearm and leg muscles [351] [352]. The magnitude of this vasodilation is comparable to the one observed during peak effort and greater than induced by acetylcholine [353].
Adenosine blockade seems to show the importance of adenosine vasodilatory role as exercise hyperemia is reduced in animals. During electrically induced muscle contraction exercise hyperemic response was reduced in cat [354], dog [355] and rabbit
179 [356] when adenosine receptors were blocked. Additionally peak and average arteriolar diameter and volume flow in hamster was reduced [357]. Data from human exercise investigations are however insufficient to form a conclusion. Systemic infusion of nonspecific adenosine receptor blockers in human reduced total reactive hyperemia after
5-10 min. of forearm occlusion [352]. However minimum information available regarding adenosine receptor blockade on exercise-induced hyperemia in human skeletal muscle. Additionally since the only drug used in human trials to block adenosine receptors is nonselective therefore other vasodilator mechanisms might be inhibited or activated and mask the blockade effect [358].
Muscle fiber type and intensity of exercise might play a role in the vasodilatory effect of adenosine. In a study by Schwartz and McKenzie the administration of adenosine deaminase (promotes the degradation of vasoactive adenosine) reduced muscle blood flow by ~40% during high intensity contractions of highly oxidative slow-twitch cat soleus muscle followed by reduction in muscle oxygen consumption and force production. Whereas the same variables in the same muscle were not effected by adenosine deaminase during low-intensity contraction. Moreover adenosine deaminase had no effect on these variables during neither low- or high-intensity contraction of fast- twitch cat gracilis muscle [359]. These data indicate that the vasodilatory effect of adenosine appears to be most evident during high-intensity contraction of highly oxidative slow-twitch muscles. This might be related to adenosine formation from AMP, which is proportional to oxidative capacity of the tissue, which greatest in slow oxidative muscle fibers during high intensity contraction. [359].
180 Since potassium is one of the byproducts of skeletal muscle it has been implicated as one of the contributors to the vasodilation associated with exercise [341, 345]. Several studies have illustrated the relation of accumulated potassium to reduced vascular resistance [333, 360] and increased muscle blood flow during exercise [345]. The vasodilatory effect of potassium however seems to be transient. The increase in venous potassium [361] as well as potassium infusion [345] were found to be effective for a short period of time indicating involvement in the initial phase but not in the maintenance of functional vasodilation. Additionally resting blood flow or the relationship between oxygen consumption and tissue blood during contraction were not changed as a result of potassium infusion [362]. Therefore potassium has been rolled out as a primary vasodilator during steady-state exercise [362, 363]. However the increase in systemic potassium observed during large mass muscle maximal contraction might be sufficient to induce vasodilation in other vascular beds [363].
While changes in hydrogen and subsequent reduction in pH during exercise are well established [345] no relationship was found between venous blood concentration of hydrogen ions and vascular resistance [360]. Additionally vasodilation observed during exercise was not associated with increased arteriolar hydorogen ions nor superfusion with highly acidic mixture resulted in significant vasodilation [364]. Therefore alterations in local pH don’t seem to play a role in increasing blood flow to contracting muscle.
Since ions and metabolic byproducts increase in the venous blood the subsequent increased in osmolarity [341, 345, 365] might be presumed as a vasodilator agent during muscle contraction. Although raise in osmolarity can result in relative increase in skeletal muscle vasodilation [366], elevated venous blood osmolarity during contraction is not
181 related to the magnitude of the decrease in vascular resistance [333, 360] and observed
dilation is transient [345, 365]. Additionally artificially increasing osmolarity in the
venous blood of resting muscle does not result in vasodilation similar to the one observed
in exercising muscles [365] nor alters the relationship between blood flow and tissue
oxygen consumption [362]. Moreover blood flow remained elevated while osmolality
returned to control values during long-lasting muscle contraction [367], whereas blood
flow returned to control values as osmolality in venous blood remained high during close
arterial infusion of hyperosmotic solution [368]. Therefore local osmolarity might play
minor role in the vasodilation observed during exercise other factors associated with
exercise seem to have greater influence.
As important as each metabolite in vasodilation associated with muscle
contraction simultaneous alterations of multiple chemical factors have a synergistic effect
on vascular function. In a study by Skinner and Costin, simultaneous manipulations of
oxygen content, potassium concentration, and osmolarity of arterial blood could produce
vasodilation comparable to that seen during contraction while manipulation of single
chemical factor resulted in weak vasodilation [341]. Similarly correcting venous blood
PO2 and pH completely abolished the vasodilator effect of venous blood from the
contracting muscle [361]. Additionally Mohrman and Regal found the combined effect
insufficient PO2 and PCO2 is profound and may account for most the metabolic
vasodilation observed during contraction [332]. These and other studies confirm the importance of each muscle metabolic byproduct during contraction however they also
emphasize the importance of the combined effect of these byproducts. They also suggest
that a variety of chemical combinations can result in profound increases in skeletal
182 muscle blood flow, which might differ according to muscle fiber, contraction speed,
stimulation parameters, species or other factors associated with muscle properties during
exercise.
As much as the metabolic theory is accepted, it doesn’t not explain all the hyperemia observed during exercise. Additionally the vasodilation generated by metabolic factors (e.g. potassium, PO2, adenosine, osmolarity) seem to be sufficient to
initiate exercise hyperemia however don’t persist as exercise hyperemia continues. This
indicate that they are either are not responsible for exercise vasodilation or are
superseded as exercise continue [250, 363]. Additionally vasodilation occurs at location
not within the metabolic vasodilatory factor reach. Therefore other mechanism(s) should
be involved in vasodilation observed during exercise especially after the initial phase of
exercise hyperemia and in locations away from interstitial fluid [369].
Mechanical Mechanisms Skeletal muscle during contraction undergoes a cycle of shortening and lengthening causing the muscle contracts and relaxes, respectively. The contracted muscle produces mechanical tension that reduces and, in some occasions, pauses blood flow to the active tissue even during maximal vasodilation [344, 370]. Any reduction in blood flow during contraction is rapidly compensated for by an overshoot during the relaxation phase.
Additionally muscle force development during rhythmic contraction fell sharply with small reduction in perfusion pressure and blood flow [371]. In the venous system, the muscle pump propels blood toward the heart as well as maintains venous pressure.
During quiet standing, gravitational effects cause the drainage of thorax blood to the lower parts. Instantly skeletal muscles surrounding the veins start contracting and pushing against the veins to propel blood toward the heart [2, 15]. Meanwhile with the help of the
183 venous valves the veins refill during muscle relaxation. In this case skeletal muscle is
acting as a pump, which is clearly demonstrated during exercise as it plays a more
effective role in driving blood toward the heart during exercise.
Transmural pressure is the pressure gradient across the vessel wall and calculated
as the difference between internal pressure and external pressure. Elevation in transmural
pressure causes vasodilation whereas reduction causes vasoconstriction. These changes in
the vessel diameter are induced independent from neural, hormonal, or metabolic
influence and attributed to the autoregulations of the myogenic response. This behavior is
most observed in the arterioles however it has been demonstrated in arteries, venules,
veins, and lymphatics [97, 273, 274]. Due to increase in byproducts within the muscle
tissue pressure increases during exercise causing a decrease in transmural pressure
thereby vasodilation. The reduction in transmural pressure and subsequent increase in
vasodilation and blood flow can even occur at intensities as low as 15-30% of MVC [372,
373]. In fact Mohrman and Sparks reported that vascular myogenic respond accounts for
~50% dilation observed following 1 sec tetanic contractions in the dog gastrocnemius
muscle [374]. Myogenic vasoconstriction, caused by rapid increased in intravascular
pressure, seems to be very potent and can’t be overridden by local vasodilators at rest and
during contraction. In a study by Meininger et al., the myogenic vasoconstriction is
maintained in the presence of extremely low tissue PO2 in resting skeletal muscle [375].
Moreover Lash and Shoukas reported reduction in functional vasodilation by 60-70% as a
result myogenic vasoconstriction during rest and exercise [376].
Increased vascular resistance and cardiac output associated with exercise result in systematic increase in blood pressure, which is also another mechanical factor contributes
184 to vascular control. Elevated mean arterial blood pressure increases perfusion pressure
across the vascular bed thereby increasing muscle blood flow [377, 378].
Endothelial Mechanisms While metabolites with vasodilator properties have been considered to amply contribute to vasodilation it doesn’t fully explain the magnitude, persistence and the sites of hyperemia observed during exercise. For example the vasodilatory effect of metabolites
seems to diminish after the initial stage of exercise (~ 1st 3 minutes) [250, 325, 363] as
well as hyperemia observed during exercise is more than the maximum possible
hyperemic respond can be produced by metabolic byproducts [305, 369]. Additionally
although it is generally assumed that small arterioles are the major resistance vessels
recent investigations revealed that 40-55% of the total vascular resistance reside in large
arterioles and small arteries. Metabolic byproducts can’t account for the vasodilation
detected in the larger upstream vessels, which are not bathed with interstitial fluid or
subjected to skeletal muscle metabolic factors [369, 379]. These observations indicate the
involvements of and have commenced the search for other mechanisms to explain all
aspects of the tremendous increase in skeletal muscle blood flow during exercise.
The involvement of the endothelium tissue in the regulation of blood flow has
been suggested first by Furchgott and Zawadzki [104] and has provided further
explanations to aspects associated with exercise hyperemia. Pharmacological agents such
norepinephrine, acetylcholine and bradykinin as well as physical factors such as shear
stress have been proposed to cause the release of nitric oxide. Both of these stimuli are
augmented during exercise [380, 381] and increase intracellular level of Ca2+, which
triggers nitric oxide synthase activities and subsequent nitric oxide production.
185 Indirect evidence, from pharmacological studies of large conduits, strongly
suggests that α2-adrenergic receptors are present on vascular endothelial cells [382, 383].
Since catecholamines increase during exercise, it would be justifiably predicted that they
might induce the release of nitric oxide. Additionally nitric oxide released has been
observed to diminish contraction induced by α1- and/or α2-adrenergic receptors located on
the vascular smooth muscle cell [102, 383]. These data imply stimulation of α2-
adrenergic receptors might be involved in the release of endothelial nitric oxide.
Flow-mediated dilation or shear stress-induced vasodilation is another function of
the endothelial cell. During exercise, increased blood flow velocity within the vascular
lumin causes an increase in the blood drag force against the vascular walls creating a
shear stress. The increased stress of blood shearing against the vessels is a signal that
causes vasodilation to reduce stress against the vascular wall. However, intact functional endothelium is obligatory for optimal regulation of shear stress [384, 385]. Stress of blood sheering against the endothelium layer produces vasodilatory factors, including
nitric oxide, from the endothelium tissue resulting in vasodilation of the proximal smooth
muscle cells [305]. Once released nitric oxide diffuse to the underlying vascular smooth
muscle cells, which activates guanylate cyclase to convert GTP to cGMP, thereby
vasodilation occurs [126].
Skeletal muscle blood flow at rest is estimated to be 2-4 ml/100g/minute whereas
it might exceeds 50-80 ml/100g/minute and values as high as 500 ml/100g/minute have been reported during strenuous exercise [306, 386]. This tremendous increase in muscle
blood flow is highly associated with the metabolic demands of skeletal muscle. Although
terminal arterioles and small arterioles regulate blood flow to the capillaries and
186 contribute largely to vascular resistance large arterioles and small arteries control the
amount of blood received by the terminal arterioles and account for 40-55% of vascular resistance [369]. Since they are within the influence of the muscle metabolic signals, the diameter of the smaller vessels change according to the muscle metabolic demands.
However upstream feed arteries are not within reach of the metabolic influence yet their
vasoreactivity are still in accordance to the contracting muscle metabolic demands [305].
Moreover smaller vessels are mainly controlled by changes in local metabolites thus they
regulate intra-organ blood flow whereas neural and endothelium regulation is
concentrated on the larger arteries which are better placed to regulate the organ perfusion
and systemic blood pressure [305]. Smaller vessel dilation is capable of increasing blood
flow by a factor of only 2-3 times more than resting levels, which is simply not adequate
to attain increase in flow of blood observed during exercise. Additionally without a
decrease in feed artery resistance, increased perfusion pressure through the microvessels
would be prohibited, thereby limiting the driving force for flow. Therefore concomitant
and coordinated dilation of the feed arteries located upstream seems detrimental to meet
the metabolic demands of the exercising muscle [305, 369].
Consequently it is clear, in order to match blood flow with muscle metabolic
requirements, the vasodilatory response to exercise has to be controlled by coupling
microvessel dilation with that of upstream arteries. This could be achieved by ascending
signals from the downstream vasculature to that of the upstream to induce feed artery
dilation [305]. These signals could be a functional vasoactive respond that propagates
vasodilation by direct cell-to-cell conduction along the vascular wall vie gap junctions of
either the endothelium tissue [387] or the smooth muscle cell [388]. Recent work
187 suggests that the signal might be a depolarization propagation triggered by acetylcholine along the endothelial cell layer ascending dilation along the arterial network [387, 389], however more research is needed to verify these findings. Therefore functional hyperemia seems to be originated in the smallest pre-capillary arterioles, which are generally presumed to be the most sensitive to vasodilator metabolites and spreads towards the lager arteries.
With flow-mediated dilation mechanism, sheer stress causes the release of endothelium-relaxing factor(s), which relaxes adjacent smooth muscle cells. Through this mechanism, the initial dilation of downstream vessels will increase flow through upstream vessels, inducing dilation of the upstream vessels [379, 390]. In fact cell-to-cell communication during exercise has also been reported to be influenced by venous factors. Venular endothelial cells can also release factors that relax adjacent arterioles and thereby contribute to functional hyperemia, without the present of arterial endothelium
[391].
Nitric oxide is clearly essential for the vasodilatory respond to exercise, recent evident however suggest the contribution of endothelin-1 in the increased blood flow to the contracting muscle. Maeda et al., demonstrated increased circulating endothelin-1 following 30-minute exercises at 90 and 130% of ventilatory threshold of young healthy volunteers. The concentration of endothelin-1 was the greatest after 30 minutes of exercise cessation and was greater following the higher intensity [392]. In a follow-up study, Maede et al. showed that circulating endothelin-1 increased in femoral artery of the noncontracting muscle and decreased in the contracting muscle during strenuous 30- minitues-leg exercise [393]. The authors of these studies concluded that endothelin-1
188 increase during exercise is intensity dependent and contribute to the distribution of
increased cardiac output during exercising.
5.3. SUMMARY
Muscle contraction triggers systemic and local mechanisms involved in vascular control.
Systemic factors, including neural and hormonal, appear to maintain the homeostasis of
all aspects of the cardiovascular system including the heart, vessels and blood
distribution. The raised sympathetic drive increases cardiac output, total vascular
resistance and direct blood toward the working muscle. The shift of the increased cardiac
output toward the active muscles is also facilitated by the vasodilatory factors produced
at the site of contraction by overriding the vasoconstrictory effect induced by the
sympathetic discharge. These factors include metabolic, endothelium and mechanical factors. Metabolic vasodilators tend to influence the arteries within the interstitial space, which includes capillaries, terminal and small arterioles. Whereas endothelium-induced vasodilation seems to be involved in flow-mediated dilation, which occurs mainly in the large arterioles and small arteries and triggered mainly by the dilation in the distal vessels and sheer stress. However these factors jointly increase the diameter of the vessels
thereby blood delivery to meet the metabolic demands of the muscle. The relative
importance of each of these factors to the vasodilatory response is not well understood,
however each factor seems to play a role and to contribute to increased blood flow to the
contracting muscle either throughout the exercise period or limited to only initiation of
the blood increase. Moreover some factors appear to be more important than others
depending on the muscle fibers involved in contraction. It seems, as might make sense,
muscle blood flow increase in oxidative fibers more than nonoxidative. Finally systemic
189 and local factors compete to takeover the control of the vessels however it appears as long as the muscle is contracting, factors produced locally usually claim “victory”.
5.4. VENOUS RESPONSES TO EXERCISE
According to the Frank-Starling theory of contraction, the heart can pump only what passively fills it during diastole. The venous system operates as a collecting reservoir for blood and controls the quantity of blood returning back to the right atrium. The increase in the amount of blood mobilized in the veins and propelled back to the heart during exercise is well established. This increase in venous return is essential to accommodate the required increase in cardiac output [32]. While the controlling mechanisms of blood flow in the arterial vasculature during muscle contraction are complex and interrelated more controlling mechanisms are even involved in the venous system. Low intra-vascular pressure, large volume present, intermittent nature of the flow, gravitational effect, venous valves, cardiac pressure, and the collapsible nature of the venous wall add complicity to flow in the venous cavity as all these factors interact with each other during exercise to achieve blood return to the heart [20].
Flow of blood runs with no resistance in large veins when they are distended however as veins enter the thorax and abdominal cavities flow is impeded. Most veins are compressed at many points by the surrounding tissues as a result of the intra-thorax and abdominal pressures, which obstructs flow of blood to the heart chambers [27]. Due to the increase in intercostals muscle contractions during exercise the physical pressor in the intra-thorax cavity increases causing further obstruction of blood in the veins.
Pressure in the right atrium might also hinder blood returning back to the heart from the systemic circulation. The increase in speed and force of cardiac contraction
190 during exercise causes elevation in cardiac pressure including pressure in the right atrium resulting in venous return blood to backup [32, 35]. However pressure against flow of blood in the venous system doesn’t increase until all collapsed segments between peripheral veins and large veins have opened, which doesn’t occur until right atrial pressure reaches 4-6 mmHg. A further increase in right atrial pressure would reflect a progressive pressure increase in large veins [33, 34].
Upon presuming standing position, gravitational effects cause the drainage of thorax blood to the lower parts. Instantly skeletal muscles surrounding the veins start contracting and pushing against the veins to propel blood toward the heart meanwhile, the veins refill during muscle relaxation [2, 15]. In this case skeletal muscle is acting as a pump, which is clearly demonstrated during exercise as it plays a more effective role in driving blood toward the heart. The efficacy of this pump is intensity dependent and certainly enhances venous blood flow [15].
The contraction of the venous smooth muscle also plays a role in driving blood toward the heart, however its contraction depends on the intra-vascular pressure. The venous wall movement is mainly passive at low pressure, however when venous pressure increases above 5 mmHg wall tension increases providing an active role of the venous tone. Venous tone is controlled mainly by central and local neurohumoral signals [2, 27].
While the skeletal muscle pump and venous tone is important to propel blood forward, the venous valves prevent flow regurgitation. Following skeletal muscle contraction and propelling blood forward, the valve cusps close and the veins refill rapidly allowing unidirectional flow only [13, 20].
191 The intra-thorax pressure varies during quiet breathing from –2 to –4 mm Hg during expiration to about –5 to –7 during inspiration. The intra-abdominal pressure decreases when intra-thorax pressure increases facilitate venous return form the intra- thorax veins thereby increase thoracic vena cavae flow of blood and venous return. The resultant changes in flow inside the veins cause ±25% changes in cardiac output during respiration. This change in cardiac output is even more pronounced during exercise due to the increase in the rate and force of perspiration [27].
In a series of studies by Bevegard and colleagues venous function was examined during and following exercise. Forearm and calf venous capacitance decreased and pressure increased during leg exercise and combined leg and handgrip exercises.
Additionally forearm and hand venous capacitance decreased in accordance to leg exercise intensity throughout the experiment. The authors attributed these changes to increased venous tone induced by sympathetic venoconstriction and with minimum contribution of the muscle pump especially since venous tone remained elevated even following cessation of exercise [394, 395]. In subsequent studies, Bevegard reported increase in forearm blood inflow without concomitant increase in venous capacitance following cessation of exercise indicating that local vasodilators contributed to arterial dilation but didn’t change venous tone confirming previous observations [396]. Similarly
Seaman and colleagues investigated forearm venous function during handgrip exercise at
20, 30 and 40 % of most voluntary contraction. Venous filling decreased and venous pressure increased indicating greater venous wall tension during exercise. The authors concluded that this increase was intensity dependent and is a reflection of venoconstriction essential to increase venous emptying and right atrial and ventricle
192 fillings [397]. In 1968 Robinson and Wilson examined the effect of exercise on venous function of nonworking muscle with and without propranolol (β-blocker) administration in young healthy males. Forearm venous compliance decreased during exercise than it had been at rest. Moreover despite reduction in heart rate, an indictor of cardiac performance, propranolol administration didn’t cause changes in forearm venous compliance however forearm blood flow and vascular resistance decreased and increased respectively. The authors concluded that venous compliance in the nonworking muscle is decreased during exercise however its not affected by reductions in cardiac output whereas the arterial system is affected by changes in cardiac performance [398]. Detry et al. investigated the effect of exercise on the venular blood volume of nonworking muscles with and without nitroglycerine, a potent vasodilator, in normal young men.
Forearm venous volume decreased by 25% during moderate-intensity leg exercise however this volume increased following nitroglycerine administration indicating volume changes might be due to venoreactivity. The authors concluded that the venous system in the nonworking muscle constricts during exercise however nitroglycerine administration causes venodilation. Additionally the effect of exercise on venous distensibility is essential to increase venous return however the effect of nitroglycerine caused venous relaxation and pooling thereby reducing preload which might partially explain the reason nitroglycerine is helpful for individuals with angina patients [399]. In order to determine variations in venous function hemodynamics during the course of the day, changes in valvular function, calf muscle pump and ambulatory venous pressures were examined during performing daily activities in the early morning and late after noon. These functions were examined during standing up from supine position and after tiptoe
193 exercises in males and females. Indexes of valvular venous function in males and females were lower in the afternoon than in the early morning whereas calf muscle pump and ambulatory venous pressure indexes remained the same. These data indicate that venous function is not affected by either daily circadian rhythm or gender, with the exception of the effect of daily circadian rhythm on venous valvular function [400].
In the contrary other investigations have reported venous dilation. The increase in sympathetic activity associated with exercise causes systemic arterial and venous constriction. However infusion of vasodilator substances as well as applying heat resulted in significant dilation in the arterial vessels but did not change venous tone [401-403].
These results indicate the involvement of other factors. Marshall and Tandon investigated venous function during exercise using direct measurements. The internal diameter of individual arterioles and venules of rat spinotrapezius were measured immediately after a period of electrical stimulation. The arteries as well as veins similarly dilated within the region of muscle contraction. Venodilation was faster and larger in magnitude in the smallest vessels as well as in individual venous and was contraction frequency dependant indicating metabolic vasodilation. The authors concluded that venodilation results from factors associated with muscle contraction and may play a role in outward capillary filtration and preventing edema formation following exercise [404]. Other investigators reported endothelium involvement in venodilation. Administration of nitric oxide inhibitor in rats resulted in a dose-dependent increase and decrease in venous tone and capacitance, respectively. The authors concluded that endothelium tissue contributes to venodilation observed in animals [405]. Inhibition of nitric oxide was also found to elevate venous tone and reduce return and cardiac filling, confirming the involvement of
194 endothelium-derived nitric oxide in the modulation of venous function [406]. Koller and
collogues examined the effect of shear stress on the walls of isolated venous diameter and reported increased nitric oxide and prostaglandin release and venodilation. The authors concluded that shear stress, as observed in the arteries, induces endothelium-derived nitric oxide and prostaglandin release, which contribute to the regulation of venous function. Additionally, since shear stress increases during exercise, the authors expected that endothelium is also involved in venous regulation during muscle contraction [407].
There is a general certainty that venous emptying increases during exercise to suffice the demanded increase in cardiac output however the mechanism by which venous emptying increases is still a debate. Contrary to that of the resistance vessels some investigators indicate sympathetically-induced increases in venous tone is not abolished by local vasodilators. Additionally this increase in venous tone propels blood toward the heart [304, 408]. Whereas other studies suggest the involvement of local vasodilators, including metabolic and endothelial, result in increased venous relaxation and venous diameter [404, 406, 407]. The mechanisms, suggested by both sides of the debate panel, seem to contribute to blood return to the heart during exercise. However there is a clear lack of well-designed studies and conflicting results, which limits our
understanding of the role of venous function during exercise. Additionally the control of
the venous system under resting conditions is complex and involves several mechanisms,
let alone during exercise. Therefore there is a tremendous need for more investigations
that examine each mechanism separately to have an integrative understanding of venous
function and overall cardiovascular physiology.
195 SECTION 6. CHRONIC CIRCULATORY ADAPTATIONS TO EXERCISE
TRAINING
Repeatedly challenging the body systems with exercise will result in adaptations of all body systems to assure maximum performance. While increased exercise tolerance is an obvious feature of the adaptations to exercise training a wide range of cellular, mechanical, chemical, structural, and regulatory changes associated with this increase.
These changes occur in a verity of systems and body parts including, cardiovascular, skeletal muscles, nervous and hormonal systems and depend largely on the type of exercise chosen for training. Changes in these systems are eventually integrated to achieve increase in exercise tolerance [409, 410].
Generally, exercise training involves rhythmic contractions of large muscle groups for a long period of time improves oxygen consumption. This improvement, first and foremost, corresponds to and is to meet the changes in the metabolic characteristics of the skeletal muscle involved and, for the most part, achieved by improvements in the cardiovascular system capacity to deliver oxygen and nutrients and remove carbon dioxide and byproducts. Essentially the delivery and removal aspects of the cardiovascular system become more efficient and result in reduced the workload placed on all components of the cardiovascular system at any given intensity, including resting.
This decrease in workload allows the cardiovascular system to function efficiently for longer period of time, which results in increased exercise tolerance [322, 411].
A variety of physiological changes occur in the cardiovascular system following long-term exercise training. These changes involve each aspect of the system including the heart, vessels and blood and all directly influence each other. For example decreased
196 heart rate following exercise training is attributed to increased stroke volume, which
attributed to changes in cardiac structure and increased blood volume. Moreover exercise
training results in increased vessel capabilities to deliver and remove blood to and from
skeletal muscles. This increase is due to the contribution of all aspects of the
cardiovascular system, however studies examining the effect of localized small muscle
group reported increase in blood flow without the involvement of the heart and blood
[322, 412-414]. Since the cardiovascular system is a continuum and its components directly interact with each other changes in the heart and blood will be briefly and
generally described. However this chapter will mainly focus on the vasoregulatory
mechanism adaptations associated with exercise training.
6.1. CENTRAL CIRCULATORY ADAPTATIONS TO EXERCISE TRAINING
6.1.1. Changes in the Heart and Blood Constituents Repeated bouts of exercise place the cardiovascular system under stress and induce
physiological adaptations. Maximum oxygen consumption has been the most useful to
quantify the results of exercise training. Values of oxygen consumption range from 30 ml-1●kg-1●min-1 in young sedentary, to 85 ml-1●kg-1●min-1 in elite athletes, and values of
45-50 ml-1●kg-1●min-1 have been observed in young active individuals [322]. Meanwhile
2 to 3 months of exercise training resulted in increases ranging between 15 and 30%,
depending on the initial level of fitness [413]. Oxygen consumption increases is
attributed to increase in maximal cardiac output and muscle tissue capabilities to extract
oxygen (A-Vd). Exercise training for 2-3 months resulted in 16% increase in oxygen
consumption as a consequence of 8% increase in cardiac output and 8% increase in A-
Vd. The increase in cardiac output has been attributed completely to increases in stroke
volume whereas maximum heart rate doesn’t change, if anything, it might slightly
197 decrease following exercise [322, 412, 413]. Whereas the increase in A-Vd might be due to several reasons including increased muscular vascular density, vasoreactivity and muscle oxidative capacity [415].
Reduced heart rate in athletes and in individuals following exercise training is
well documented. The decrease in resting and sub-maximal heart rate might results from structural, neural and hormonal changes [412, 416] including alterations in sympathetic/parasympathetic balance toward greater dominance of the parasympathetic component [417, 418]. However the reduction in heart rate has been reported to decrease during 7-weeks training program without concomitant reduction in plasma catecholamines indicating the involvement of other factors [419]. The reduction in resting heart rate might also results from changes in stroke volume, which might result from changes in cardiac structure and blood volume. Cross-sectional date indicate that cardiac dimensions and end-diastolic volume are larger in endurance athletes than athletes involved in activities of a short durations, such as resistance training (figure 6-1) [420].
These observations were also reported following training at 85-90% of maximum oxygen consumption for 7-weeks in sedentary individuals [421]. In fact increase in heart dimensions were observed following only 1 week of exercise training [422]. In addition to cardiac structure, Gillen et al. reported 10% increase in blood volume within 24 hours following only one bout of ergometry-cycling at 85% of maximum oxygen consumption
[423]. Additionally one-half of the difference in stroke volume normally observed between untrained and highly endurance-trained men during upright exercise is due to a suboptimal blood volume in the untrained men [424]. Coyle EF et al., concluded that the reduction in cardiovascular function following detraining is due, in large, to the decline in
198 blood volume [425]. Therefore increased cardiac dimensions and blood volume result in increased stroke volumes and subsequent reduction in heart rate.
Figure LR6.1. Cardiac mass index in elite athletes. Taken from [18].
Training-induced increase in blood volume and myocardial hypertrophy contributes also to reduced heart rate during submaximal exercise. However heart rate reductions might not exclusively due to central adaptations. In a study by Saltin et al., heart rate responses to one-leg submaximal exercise in the trained leg were lower than in the untrained leg [426]. Similarly, heart rate responses to leg exercise were higher than to arm exercise following arm exercise training program and vise versa following leg exercise [416, 427]. These studies suggest at least part of the fall in heart rate during submaximal exercise is induced by peripheral factors and specifically related to the trained body part. The relationship between heart rate and the trained muscle group might be mainly due to the decrease in sympathetic stimulation [417, 418]. Exercising with untrained leg or arm might result in greater feed back reflex induced by greater accumulation of metabolic byproducts. Specific nerve fibers within the skeletal muscle of most species can be activated by many different metabolic stimuli. Substances including
199 lactate, osmolality, and potassium that trigger these fibers appear in different concentrations in the extracellular space of trained and untrained muscle while exercise.
Actually the magnitude of heart rate increase during exercise in the trained and untrained muscles has been related to the concentrations of metabolites produced by the muscle
[318]. Thus cardiac adaptations to exercise training appear to result from central and peripheral changes. These changes include a shift in autonomic nerves system balance, structural changes in the cardiac muscle, increased in blood volume and reduced metabolic stimuli from the trained muscle.
6.2. PERIPHERAL VASCULAR ADAPTATIONS TO EXERCISE TRAINING
Resting muscle blood flow appears not to be affected by exercise training however endurance, high-intensity sprint and isometric training have been shown repeatedly to increase blood flow under varies conditions, including anticipatory state. These training effects might be the results of alterations in vascular smooth muscle and vascular endothelium structure and function, however the influence of cardiac and skeletal muscle adaptations cannot be totally excluded [428-432].
One of the early study examined the effect of exercise training on regional vascular function reported a decrease in blood flow in humans. In this study male subjects jogged 4-5km at 80% of maximum running capacity 5-6X/wk for five weeks.
Blood flows, measured immediately after maximal and submaximal exercise, were lowered following the training program. The authors attributed lowered leg blood flow to a more “effective” metabolic process [433]. Bond et al. also reported reduction in hyperemic blood flow following a training program consists 6 sets of 15 one-leg maximal isokinetic concentric contractions of planter flexion for 4 weeks while the nontrained leg
200 served as control. The authors attributed reductions in trained leg blood flow to an increase in muscle cross-sectional area without proportional changes in microvascular density [434]. More recently another study reported reduction in leg submaximal blood
flow following 9-12 wk high intensity lower body endurance training and this decreased
was coupled with decrease in submaximal oxygen consumption and unchanged cardiac
output. The authors attributed the decrease to unchanged cardiac output and blunted
splanchnic vasoconstriction [435].
Several other investigations however revealed conflicting results. For example
Klausen and colleagues reported that trained leg blood flow in six young healthy male
subjects increased during maximal and submaximal exercises following eight week one-
leg exercise training at ~85% of heart rate maximum [436]. The effect of endurance
training on the regional blood flow response to maximal exercise was also investigated in
foxhound by Musch et al. Training consisted of 8-12 wk of treadmill running at 80% of
maximal heart rate 1 h/day for 5 days/wk. Gastrocnemius muscle blood flow increased
and systemic vascular resistance decreased during maximal exercise following training
[437]. In another study by Musch et al., regional blood flow adaptations were determined
for rats after 6 days/wk for 6 wk of high-intensity sprint training or limited cage activity.
Training consisted of five 1-min bouts of treadmill running at work loads (15% grade, 97
m/min) in excess of the animals' maximal oxygen uptake interspersed with 90s of rest.
Blood flow to the different organs of the abdominal region was greatly reduced during
maximal exercise although. Furthermore blood flow to the entire hindlimb and to several
individual muscles (soleus, plantaris, gastrocnemius, flexor hallicus longus, vastus
lateralis, rectus femoris, biceps femoris, and adductor magnus and brevis muscles) was
201 greater in the exercise group during maximal exercise than in the sedentary control rats.
The authors concluded that increased cardiac output following high-intensity sprint training is primarily directed toward the working skeletal muscle and not toward the organs found in the abdominal region in rat [438]. Armstrong et al., reported increase also in preexercise anticipatory blood flow after running exercise training in rats. In fact this increase was intensity dependent, as the rats were assigned to a low-speed (daily treadmill walking up a 12 degree incline at 15 m/min), and high-speed (daily treadmill galloping up a 12 degree incline at 50 m/min) training program regimens for 2-4 wk. The authors also demonstrated that regional changes in blood flow to long-term endurance exercise training that elicits systemic adaptations were more pronounced in red muscles whereas short term elicited adaptations in only the white muscles indicating that blood flow adaptations are dependent on muscle type and the length the exercise training program [431, 439]. Blood flow was also reported to increase at onset of exercise following 10 days of cycle ergometer exercise training at 65% of maximal oxygen consumption [440]. The results of the above-cited studies are indeed very tempting to conclude that exercise training induce vascular adaptations. However the concomitant modifications in blood volume, stroke volume, heart rate, cardiac output, blood pressure, muscle metabolic capacity, and oxygen consumption demonstrate central cardiovascular involvement. Since the exercise training mode selected to examine vascular adaptations elicited systemic responses, the observed improvements in regional blood flow might be the result of increased blood volume, stroke volume, cardiac output, capillarization, A-Vd and oxygen consumption or might be basically due to improved cardiac output shunting to the active muscles rather than improved regional vascular function per say. Therefore
202 the question whether exercise training leads to regional vascular changes without central influence, remained unanswered.
TABLE LR6.1. Vascular function adaptations following small muscle group conditioning programs Study Population Type of training Results Yasuda, 83 10 HY HG, 33 & 50% MVC, 6X/wk (6 34 & 26% ⇑ BF, wks) respectively.
Sinoway, 87 6 HY HG, 70% MVC, 4X/wk, 30min (4 ⇔ BFrest. wks) 30% ⇑ RHBF in trained arm
Green, 94 11 HY HG, 75% MVC, 4X/wk, 30min (4 ⇔ BFrest wks) 23% ⇑ RHBF in trained arm
Franke, 98 6 HY HG, 70% MVC, (4 wks) ⇔FVCrest. 35% ⇑ FVCpeak. Bank, 98 11 HY & 7 HG, 70% MVC, 4X/wk, 30min, ⇔ in CHF. CHF (4-6 wks) 24% ⇑ RHBF in HY. Alomari, 01 28 HY HG, 25 & 75% MVC, 5X/wk (4 18, 29% ⇑ RHBF in trained wks) arm, respectively. BF = Blood flow; RHBF = Reactive hyperemic blood flow; VR = Vascular resistance; FVC = Forearm vascular conductance; healthy & young = HY.
In one of the early studies that examined the effect of localized exercise training on peripheral vasculature without central influence, Yasuda et al reported improvements in hyperemic forearm blood flow. The training program consisted of performing 6X/wk isometric handgrip exercise for 6 weeks at 33 and 50% of MVC to fatigue. Both training regimens resulted in similar increase in blood flow however adaptations after 6 months were greater than 3 months indicating training-period dependency [441]. Sinoway and colleagues also showed localized vascular adaptations to small muscle group training without systemic influence. In a cross-sectional study forearm blood flow, measured following 5-minutes of ischemia and 5-minutes of ischemia coupled with exercise, was higher in the dominant arm of tennis players whereas it was similar in both arms of a control group of comparable oxygen consumption. Additionally the difference in
203 hyperemic blood flow between 5-minutes ischemia and 5-minutes ischemia coupled with exercise in the dominant forearm of the tennis players was greater than the rest of the measured arms (nondominate forearm of the tennis players and both forearms of the control group) indicating greater vasodilatory reserve in the dominant forearm of the tennis players. The experiment populations and protocol allowed viewing the peripheral vasculature as an isolated local entity independent of the systemic cardiovascular involvements such as cardiac output and mean arterial blood pressure. Although the study was not proposed to reveal the mechanisms responsible for these adaptations, the authors attributed increase in dominant forearm blood flow of tennis players to peripheral vascular phenomena (increased capillarization), but also didn’t totally exclude systemic influence such as changes in sympathetic tone [428]. In a longitudinal study, Sinoway et al. further demonstrated modifications in regional vascular function following a small muscle group exercise-training program without eliciting systemic cardiovascular adaptations. The training regimen included four-week isometric handgrip exercise at 70% of maximum workload (workload was determined as the maximum amount of work the subjects could perform while maintaining 30 contractions/min for 3 minutes). The subjects exercised 4 times a week for 30 minutes and intensity was progressively increased as needed. The authors reported increase in forearm blood flow and reduction in forearm vascular resistance, measured immediately after ischemia and ischemia coupled with exercise, in the trained arm while oxygen consumption and vascular function in the untrained arm remained unchanged. The authors concluded that regional vascular modifications without systemic adaptations are attainable and attributed to enhanced intrinsic skeletal muscle vasodilatory capacity [442]. In an attempt to examine
204 the effect of training intensity on vascular function, subjects exercised at 25 and 75% of
MVC of isometric handgrip in our laboratory. The training program consisted of exercising 20 minutes 5X/wk for 4 weeks and both intensities resulted in similar increase in forearm blood flow, although the magnitude of increase was greater in the high intensity group [443].
Some investigations however have found that vascular adaptations to localized exercise training might be transferable to untrained body parts. Forearm blood flow in the ipsi- as well as the contra-lateral arms following 6 weeks were increased in the study of
Yasuda and colleagues. The authors attributed the improvements to changes in neurogenic balance however the metabolic contribution was not totally excluded [441].
Silber et al. also reported a 50% increase in forearm blood flow following leg cycle ergometer exercise however these adaptations might be the results of systemic adaptations rather than localized vascular adaptations as changes in oxygen consumption and heart rate were reported [444]. Therefore these studies have certainly demonstrated that peripheral muscle blood flow adaptation to exercise training program is a local entity independent of systemic cardiovascular modifications. However all the above studies were not designed to explain the mechanism(s) involved in peripheral vasculature adaptations although speculations attributed the observed blood flow adaptations to unclear local phenomena.
Hashimoto was the first to examine the effect of exercise training on endothelium- induced dilation [445]. The investigator reported no effect of exercise training on endothelium-dependent modulation (vasoconstriction or vasodilation) following 1- hour/day treadmill exercise training for 12 weeks in young (16wks) and old (98wks) rats.
205 However no documentations were provided to quantify the effect of exercise training
although it did induce lipoprotein profile changes (↑HDL- and ↓LDL-cholesterol). On the
other hand several other investigators reported different results. In well-designed
sequence of studies, Delp and colleagues repeatedly demonstrated training-induced
improvements in endothelium-dependent vasodilatory capacity. In one particular study
male rats ran on a treadmill at 30 m/min with 15O incline for 1 h/day 5 days/wk for 10-12
wk. Maximal vasodilator responses induced by acetylcholine were greater in vessel rings
from trained rats whereas dilatory responses by sodium nitroprusside were not different
following training, indicating that the augmented acetylcholine-induced dilatory response
resulted from endothelium adaptation. Additionally blockade of nitric oxide synthase activity, by L-NMMA, diminished acetylcholine-induced vasodilation by 100 and 79% in trained and nontrained rats, respectively [446]. Similar exercise training protocol was used to examine the effect of exercise training in hyperthyroid rats. Vasodilatory responses to acetylcholine and sodium nitroprusside were enhanced and not altered, respectively, in rat abdominal aortas following the exercise program [447]. Vascular smooth muscle dilation is mediated by increasing cellular cyclic monophosphate, which is stimulated by activating guanylate cyclase. Acetylcholine indirectly increases the levels of cyclic monophosphate by stimulating endothelium-derived nitric oxide release, which activates smooth muscle guanylate cyclase whereas sodium nitroprusside increase the levels of cyclic monophosphate by directly activating smooth muscle guanylate cyclase. Thus the unaltered vessel response to sodium nitroprusside in the trained animals indicates that adaptations might be independent of cyclic monophosphate and related to the endothelium tissue. Additionally inhibiting nitric oxide activities, by L-NMMA,
206 completely obliterated the acetylcholine-induced vascular relaxation, which further
demonstrated the involvement of endothelium tissue and nitric oxide pathway.
Similar observations were reported by Chen and Li following only eight weeks of
treadmill exercise training. Acetylcholine- and sodium nitroprusside-induced relaxations were enhanced and unaltered, respectively, in thoracic aortae and pulmonary arteries, but not in carotid arteries from rabbits [448]. Vascular endothelium adaptations were reported following even shorter exercise program. Sun et al. found that vasodilatory responses to acetylcholine and L-NMMA were augmented in gracilis muscle of rats following treadmill exercise training as short as 2-4wks [449]. Therefore in an attempt to determine the time course of improvements in vasodilatory capacity in rats, Delp and et al. examined training-induced adaptation of vascular endothelium at different times of a 10- wk exercise-training program in rats. The exercise group was further subdivided into postexercise, 1-day, 1-wk, 2-wk, 4-wk, and 10-wk groups, whereas another group of animals didn’t exercise and served as control. The exercise program was similar to the previous studies [446, 447] and resulted in enhanced maximal vasodilation to acetylcholine and expression of endothelium nitric oxide synthase protein in aortas from
4-wk and 10-wk rats. These data indicate that the enhanced endothelium-dependent vasodilation of the rat aorta is present by 4 wk of endurance exercise training.
Additionally adaptations appear to be mediated primarily through the nitric oxide synthase pathway and are associated with an increased expression of endothelium nitric oxide synthase protein [450]. These studies clearly indicate that exercise training enhances vascular sensitivity and vasomotor function through adaptations of the endothelium as well as smooth muscle in animals. Additionally training-induced increase
207 in blood flow observed following exercise training could be partially allocated to greater vasodilatory capacity of nitric oxide. However thus far, the effect of exercise training on vascular endothelium function is not as clear in human as in animals therefore these findings still need to be confirmed in humans.
In a series of studies, Green et al. reported that enhanced peak blood flow following small muscle group exercise training is not endothelium-dependent. As previously reported [442, 443] Green and colleagues found increase in peak forearm blood flow following handgrip training program however with no changes in the vasodilatory capacity of nitric oxide [430]. Similarly they reported enhanced peak blood flow in the dominant forearm of tennis players however no evidence of difference were found in vascular endothelium function in the nondominant forearm of the tennis players and both forearms of the control group [451]. In fact the same group reported that forearm blood flow responses to L-NMMA and norepinephrine remained unchanged following 6 weeks of detraining, induced by forearm casting, indicating no detraining effect on nitric oxide pathway, despite evidence of physical deconditioning [452]. The authors attributed changes in blood flow to changes in vascular structure and vascular responsiveness without alterations in capillarization. Additionally they concluded that the lack of nitric oxide adaptations to exercise training (or detraining) as shown in animals, might be due to training protocol or species differences however didn’t exclude the contribution of other vasodilator pathways.
In contrast Kingwell and his group repeatedly observed improvements in nitric- oxide-mediated vasodilation following physical conditioning among humans. In a cross- sectional study they reported that endurance-trained athletes had greater vasodilatory
208 response to acetylcholine, greater vasoconstrictor response to norepinephrine, angiotensin
II, and L-NMMA, and similar response to sodium nitroprusside as compared to sedentary control group. Nevertheless these differences were linked to the level of serum cholesterol indicating that training-induced changes in endothelium function might be the result of reduction in cholesterol level [453]. The same research group substantiated these results in a longitudinal study as the subjects engaged in a 4-week cycle exercise-training program. Training increased maximal work-load and maximal oxygen consumption, and blood pressure was reduced. Additionally L-NMMA caused a greater vasoconstriction after training whereas there was no difference in vascular response to acetylcholine or sodium nitroprusside between pre and post training. The authors concluded that the improvements in vascular endothelium tissue are due to elevated shear stress resulting from increased blood flow and blood viscosity after cycle training [454]. More recently the same research group reported endothelium changes following home-based exercise training program for 4 weeks in hypercholesterolemic volunteers. The training program consisted of cycling 3X/wk for 3 minutes at 65% of maximum oxygen consumption and resulted in greater oxygen consumption, blood flow, vasoconstriction response to L-
NMMA along with improvement in lipid/lipoprotein profiles whereas vascular responses to acetylcholine and sodium nitroprusside didn’t change before and after training [455].
Endurance training was also found to improve nitric-oxide-mediated blood flow by several other investigations in young [456], older [457] heart failure [458], and hypertension [459, 460] participants. However the involvement of other vasodilatory pathways shouldn’t be denied since nitric oxide inhibition was found to have modest
209 effect on peak vasodilation during reactive hyperemia whereas prostaglandins appeared to play greater role [461].
Despite the overwhelming evidence of nitric-oxide-induced vasodilation improvements following exercise training the current discrepancy found between different studies need to be resolved. This discrepancy might be due to differences in training stimulus and/or experimental measurements and protocol. For example Bank et al. was the only one to report improvement in nitric-oxide-dependent vasodilation and peak hyperemic forearm blood flow in normal subjects but not in patients with heart failure following handgrip exercise training [462]. Otherwise all the studies reported changes in nitric oxide production used large muscle group exercises (e.g. running or cycling) whereas Green et al. examined vascular function following small muscle group exercise training (e.g. forearm handgrip or tennis playing). Additionally these improvements were reported in muscle beds that were not involved directly in exercise training (e.g. measuring forearm vascular function following leg exercise training), which indicate that improvements might be due to systemic adaptations. Therefore the search for other mechanism(s) that might be involved in vascular adaptations observed during exercise training is still warranted.
Increased blood flow observed following exercise training might also be due to reduction in vasoconstrictor control. Wiegman et al. reported reduction in norepinephrine-mediated vasoconstriction due to the reduction in the sensitivity to norepinephrine following 6wk-swim-training program in rats [463]. However the study was not design to determine whether the alterations in norepinephrine function are related to the endothelium or smooth muscle. Delp and colleagues found that endurance training
210 reduced vascular sensitivity to norepinephrine, in rat aortic rings, only when the
endothelium tissue was intact suggesting endothelium involvement [446]. Since α2-
adrengeric receptors are found on the endothelium tissue and mediate the release of nitric oxide or other endothelium-derived relaxing factors [382], binding of norepinephrine to the receptors on vascular endothelial cells from trained animals may induce greater released of endothelium-derive factor(s), which could blunt norepinephrine-mediated vasoconstriction. Additionally evidence indicates that endothelium tissue might inhibit the release of norepinephrine from adrenergic nerves [326] therefore exercise training
might alter the endothelium-induced inhibition of norepinephrine release.
Despite the minimum knowledge of endothelin-1 function during exercise
evidence shows that its production in the working muscles decreases while it increases
and causes vasoconstriction in the nonworking muscles. Additionally this increase is
exercise intensity- and time-dependent, contributes to norepinephrine-induced
vasoconstrictions and plays a role in blood redistribution during exercise [392, 393, 464].
However, very few studies have examined the effect of exercise training on the
endothelin-1 function and these studies are certainly far from conclusive. For example
Jones et al. found endothelin-1 adaptations to exercise training in miniature swine is gender dependent. The exercise program included treadmill running for 12 wks and
resulted in reduction of coronary artery sensitivity to endothline-1 of male swine but not
of the female [465]. From the same laboratory, Laughlin and coworkers also reported
gender-dependent adaptations however exercise training was found to reduce porcine,
femoral and brachial artery sensitivities to endothelin-1 in female swine but not in the
male [466]. In an effort to examine the effect of exercise training on endothelin-1
211 function in human, Maeda et al. examined plasma concentration of endothelin-1
following 8 wks of 1 hour cycling, 3-4 days/wk, at 70% of oxygen consumption.
Enduthline-1 plasma concentration were reduced and negatively correlated with
increased plasma concentration of nitric oxide. The observed alterations in endothelin-1 and nitric oxide concentrations were maintained for 4 weeks and didn’t return to basil
level until the 8th week after cassation of exercise training [467]. These studies suggest that exercise training might has some effect on endothelin-1 function however more research is definitely needed to expand our knowledge in the role of endothline-1- mediated modulation of the vascular function.
Therefore exercise training seems to modify the peripheral effect of vasoconstrictor factors however the mechanism by which this function is modified is not known. Additionally more studies are needed to examine the effect of exercise training modality, intensity, duration, frequency as well as training period length on the function of vasoconstrictor factors in different segments of the vasculature of various species, particularly human.
6.3. VENOUS SYSTEM ADAPTATIONS TO EXERCISE TRAINING
Improved venous return is probably the most recognized feature of the venous system that improves following exercise training. However since the effect of training on the heart, arterial system as well as blood contents have been extensively investigated, increased venous return has primarily been attributed to alterations in these aspects of the cardiovascular system, with minimum considerations given to the venous system. For example cardiac contractility and blood volume thereby stroke volume have been repeatedly found to increase following exercise training. Additionally vascular resistance,
212 vasoactivity, muscle vascular density, muscle perfusion, muscle blood flow and muscle blood volume have also been reported to be favorably altered. Since the cardiovascular system is a closed circuit, the combined exercise training-alterations within the system would certainly induce venous return changes. However recognized the venous system is a reservoir for ~65% of the total body blood and repeatedly challenged during exercise, it must be affected by exercise training. However very few studies examined the effect of exercise training on venous hemodynamics, therefore conclusions regarding the vasculature performance have for the most part ignored the contribution of the venous system adaptations to physical conditioning [32, 322, 413].
The available studies investigated sporadic aspects of the venous system. For example Suzuki and colleagues examined the effect of exercise training on venous density. Soleus muscle arteriolar and venular capillary densities in young (3 wks) and middle-age (54 wks) Wistar rats were evaluated before and after 6-wk exercise training program. The exercise protocols for the young training group were: 10-22.5 m/ min 60 min/day for 6 days a week, on 7O incline during the final 2 weeks; for the middle-aged training group, the protocols were: 10-20 m/min. 50 min/day for 6 days a week. Although it was greater in the younger group, arteriolar capillary density increased similarly in both training groups whereas the venular portion didn’t [468]. The same group of researchers examined time-course changes in soleus muscle arteriolar and venular capillary densities of young female Wisters. Running exercise training started at the age of 5 weeks and lasted for 5 weeks at 25 m/min on a 25O incline, 10-60 min/day, 5 days/week.
Morphological findings were obtained from the soleus and deep portions of the medial gastrocnemius muscles. In soleus, training resulted in increased capillary-to-fiber ratio,
213 total capillary density, and density of venular capillaries at Week 1. However values for total capillary and arteriolar and venular capillaries were significantly decreased after 4 weeks of training. In gastrocnemius muscles, training significantly increased the capillary-to-fiber ratio at Week 1 and from Week 3 onwards whereas total capillary density values showed no difference between control and training groups at all points of time. These findings suggest that exercise training in Wister rats results in greater arteriolar and venular capillarization, mainly in highly oxidative muscles, as early as 1 week. However the conflicting result between the two studies might indicate that training-induced adaptations might be time-, age- or gender-dependent. Additionally the reduction in capillary growth observed after the 4th week might suggest that other mechanisms, including vascular modulator adaptations, might be involved during a delayed phase of the exercise program. However since neither study have examined arterial or venous hemodynamics or any vascular function indicators a complete conclusion cannot be made. Finally the changes in venular capillary densities might explain the growth pattern of the capillary network associated with endurance training
[469].
Kenney and Armstrong investigated venous pooling following 40 mints, 3d/wk for 8-wk running exercise training program. Venous pooling was measured in the foot as volume drainage following “Trendeleburg” maneuver (VT). The “Trendeleburg” maneuver consisted of 45o leg elevation for 15 seconds while the subjects is laying down followed by setting up to a position of 90o flexion with the legs lower than the body. The investigators assisted the subject throughout the maneuver to avoid muscle contraction.
Moreover, to take the muscle pump contribution into account, volume drainage was
214 evaluated following 15soconds of dorsi-flexion/planter exercise while the foot is lower
than the body (VE) then subtracted from volume drainage following the Trendeleburg
maneuver (VT - VE). Exercise training resulted in increased VT, VT - VE, oxygen
consumption without changes in VE. Therefore the authors concluded that increased venous pooling is the result of hypervolemia associated while ignoring structural or functional changes in the venous system [470].
Hypervolemia, associated with exercise training, was also linked to changes in central venous pressures. Exercise program consisted of 10 wk of cycle exercise for 30 min/day, 4 days/wk at 75-80% of maximal O2 uptake and resulted in 20% increase in maximal O2 uptake, decreased resting heart rate, 9% increase in blood volume and increased central venous pressure. The increase in central blood pressure was associated with the increase in blood volume without changes in total effective venous capacitance, thus the authors attributed changes to blood volume but not to alterations in venous structure or function [471]. Louisy and coworkers also attributed changes in venous hemodynamics to hypervolemia. Leg-filling volumes during 30 head-up tilt (FV), half- emptying time (T1/2) and venous output at the 6th second of emptying during return to
horizontal position (VO6) were examined in endurance trained, strength trained and
sedentary subjects. Results showed higher values for FV and VO6 in endurance trained
subjects than strength trained and sedentary subjects. Whereas no significant differences
were observed between groups in T1/2. Therefore the authors concluded that endurance training seems to increase lower limb venous distensibility. Additionally since venous
emptying and venous return from the lower limbs were not affected increased venous
distensibility is probably a physical response to hypervolemia consecutive to chronic
215 aerobic training rather than alterations of the visco-elastic properties of deep vein walls
[472].
However other investigations reported venular structural and functional changes following exercise training. In a cross-sectional study, Wecht et al examined the difference in venous hemodynamic indices between individuals with spinal cords injury, sedentary and active individuals. Venous tone, venous capacitance, venous compliance, total venous outflow but not venous emptying rate were higher in the active group. The authors concluded that endurance exercise training enhance vascular function [473].
Miyachi et al. examined venous diameter following 6 wk one-legged cycle training at
80% peak oxygen consumption, 40 min/day for 4 days/wk followed by 6 wk of detraining. Arterial and venous cross-sectional areas increased by 16% and 46%, respectively and correlated with increase in one-leg peak oxygen consumption. These changes returned to baseline during detraining. The authors attributed part of the changes in oxygen consumption to arterial adaptations and improved blood delivery [474].
Involuntarily detraining also might affect venous function such as in morbidity diseases. Ikenouchi and colleagues found that venous distensibility is reduced and related to the severity of the disease in patients with heart failure [433, 475]. Our laboratory found that reduced venous distensibility and outflow in heart failure were related to exercise tolerance indicating inactivity influence confirming the effect of heart failure on venous function [476]. While venous changes might be a direct effect of heart failure, inactivity associated with the disease might indirectly alter venous function.
The effect of exercise training on venous function seems to be ignored despite evidences of the importance of the venous system in cardiovascular function. Therefore
216 more studies are certainly needed to examine the effect of exercise training modality, intensity, duration, frequency as well as training period length on different segments of the venous system of various species, particularly human.
217 SECTION 7. VASCULAR PATHOPHYSIOLOGY
The vessel diameter is modulated by cellular calcium ion level. Calcium ions increase in
vascular smooth cell as a result of a cascade of chemical reactions initiated by an agonist
(e.g. norepinephrine or endothelin-1) binding to a receptor located on the cell membrane.
The binding of agonists to vascular smooth muscle membrane receptors activates
phospholipase C (PLC) in a reaction coupled with guanine nucletide binding proteins (G
protein). Membrane Phospholipase C hydrolyzes phosphatidyl inositol bisphophate
(PIP2) to yield diacylglycerol (DAG) and inositol triphosphate (IP3). Diacylglycerol and
+2 +2 IP3 increase Ca by activating voltage-gated calcium channels and releasing Ca from
the sarcoplasmic reticulum, respectively. Thereafter, Ca+2 ions bind to calmodulin, which in turn binds to myosin light chain kinase. This activated Ca+2-calmodulin-myosin kinase
complex phosphorylates the light chains of myosin. The phophorylated myosin ATPase is
then activated by actin and the resulting cross-bridge cycling initiates contraction.
Relaxation on the other hand occurs when the myosin light chain phosphatases
dephosphorylate the light chains of myosin and the cytosolic Ca+2 is lowered by
sarcoplasmic reticulum uptake and by Ca+2 extrusion by the Ca pump and Na-Ca
exchanger [51, 52].
Agents elicit elevation in smooth muscle calcium ions and vasoconstriction include
catecholamines [39, 40], angiotensin II [41], arginine vasopressin [41, 42], and
endothelin-1, whereas metabolic byproduct [228, 477], nitric oxide, prostacyclin, and
endothelium hyperpolarization factor [54] inhibit the effect of these vasoconstrictors.
Therefore the endothelium-derived vasodilatory factors counteract the vasoconstrictor
effect induced mainly by the sympathetic nervous system and circulating hormones. In
218 vascular disease state these mechanisms are altered as such Ca+2 levels are constantly
elevated resulting in elevated vascular tone. Vascular disease is primarily characterized as
elevated circulating vasoconstricting agents and endothelium failure to inhibit the
contraction of smooth muscle cell. Additionally the vasculature undergoes structural
changes that result blood flow kinetics. The purpose of the section is to discuss the effect
of a variety of disease state on vascular function.
7.1. SYSTEMIC NEUROHORMONAL DYSFUNCTION IN DISEASES
Abnormalities in the neuroendocrine systems have been implicated in a verity of
cardiovascular and metabolic diseases. These abnormalities include mainly impaired
sympatho-vagal balance, elevated circulating catecholamines, and hyperactivated renin-
angiotensin-aldosterone hormonal system. Overstimulated neurohormonal systems
usually result in blood pressure rise followed by cardiac and vascular structural changes.
The increase in blood pressure is due to increased cardiac output, hypervolemia, and
increased vascular tone. This segment will focus on changes in the neuroendocrine
systems observed in heart failure and hypertension.
7.1.1. Heart failure The predominant quandary in heart failure is significant reduction in cardiac output resulting in underfilling of the arterial system. This underfilling causes activation of neurohormonal reflexes that stimulate multiple system responses. These neurohormonal reflexes are initiated by mechanoreceptors located at the high-pressure sides of the circulation including the left ventricle, carotid sinus, aortic arch, and renal afferent arterioles. Decreased activation of these receptors due to a decrease in arterial blood pressure, stroke volume, renal perfusion, or peripheral vascular resistance leads to an
increase in sympathetic nervous system, activation of the renin-angiotensin-aldosterone
219 system and release of arginine vasopressin as well as stimulation of thirst. The activation of these systems results in increased sodium and water retention, greater stroke volume, elevated cardiac pressure, fluid intake, and vasoconstriction. These responses are compensatory and mainly to maintain sufficient circulatory blood pressure and tissue perfusion, however unfortunately they may lead to further deterioration in heart function.
The activation of the sympathetic nervous system in heart failure causes increased cardiac contractility, tachycardia, arterial vasoconstriction thereby increased cardiac afterload. Sympathetic nervous system activation also indirectly triggers systemic vasoconstriction by stimulating the release of catecholamines from the adrenal medulla.
Furthermore vasoconstriction induced by increased sympathetic drive and circulating catecholamines causes reduction in renal blood flow, which stimulates the proximal convoluted tubule to activate the renin-angiotensin-aldosterone hormonal system, which also contributes to further vasoconstriction.
Plasma renin activity is increased and provides prognostic index in heart failure, additionally its activity is largely related to the severity of the disease [478]. The activated renin-angiotensin-aldosterone hormonal systems serve two compensatory purposes in the circulation. Aldosterone increases blood volume by increasing sodium and water retention while angiotensin II is a potent vasoconstrictor. This increase in body fluids is to increase the amount of blood pumped out of the left ventricle thereby increasing systemic blood pressure and tissue perfusion. Whereas angiotensin II further provokes the proximal tubular to increase aldosterone secretion in addition to its potent vasoconstrictive effect [479]. Angiotensin II also seems be involved in increased thirst sensation observed in heart failure as it may to trigger the central thirst center. However
220 angiotensin II seems to have a deleterious effect on the cardiac muscle as it causes cardiac mitogenesis resulting in cardiac remodeling that may lead to a decrease in the size of the capillary network thus predisposing the patient to ishcemic injury. Angiotensin- converting enzyme inhibitors seem attenuates the affects of angiotensin II as they may reduce the vasoconstrictor effect, reverse the myocytes proliferation, and inhibit thirst sensation [480-482].
Similar to renin-angiotensin-aldosterone hormonal system vasopressin actions include vasoconstriction and decrease in water excretion. The activities of vasopressin are elevated in heart failure contributing to the increased vasoconstriction and water retention. The stimulus for vasopressin release in heart failure is not completely known yet, though angiotensin II, osmolality and fluid volume have been implicated [483, 484].
Vasopressin appears to be the least influential vasoregulatory agent activated in heart failure though it is a very potent vasoconstrictor. It seems to be the last one activated among all of the vasoregulatory systems as its concentrations are related to the severity of the disease [485]. According to Creager et al., characterization of the contribution of the three vasoconstrictors in heart failure the sympathetic nervous system produce most of the vasoconstriction followed by renin-angiotensin system and last the arginine vasopressin [486]. Theoretically vasopressin antagonists could be useful drugs to inhibit its activities. In animal investigations as well as in healthy volunteers receptor blockers have shown promising results [487, 488]. However well controlled trials are still needed to verify the efficacy of vasopressin antagonists in heart failure.
The atrial natriuretic peptide is primarily produced by the atrial and triggered by stretching of myocardium cells. This peptide is a vasodilatory hormone but it is main
221 function is to increase sodium and water secretion from the kidney. Thus it counteracts
the effect of the other activated neurohormonal systems (i.e. sympathetic, renin-
angiotensin-aldosterone and arginine vasopressin systems). Atrial natriuretic peptide
inhibits renin-angiotensin, suppresses aldosterone and vasopressin release and prevents norepinephrine release from neurotransmitter sites [489, 490]. This hormone seems to be activated at the initial stages of heart failure as higher levels are evident in asymptomatic left ventricular dysfunction patients and before renin-angiotensin-aldosterone system is activated. Thus it appears to be essential in helping to maintain a compensated state and in inhibiting the progression of the disease at early stage [491]. Elevated atrial natriuretic factor is also found in symptomatic heart failure. In fact its level increases as heart failure worsen and is the highest in sever patients [485, 489]. However he beneficial effects of the hormone appear to be is attenuated in advanced patients as it is overwhelmed by the
increased activities of the other neurohormonal systems [492].
In patients with overt heart failure the compensatory mechanisms that initially
improve cardiac output are endless and become problematic and counterproductive as
they contribute to the progression of heart failure. The net result of neurohormonal
activation is intense vasoconstriction and expanded fluid volume as the balance is tipped in favor of vasoconstrictor and antidiuretic agents. Additionally neurohormonal activities
trigger cardiac and vascular remolding that exacerbate loading conditions on the failing
left ventricle and contribute to further decrease in myocardial contractility and the
development of overt and symptomatic failure. Antagonist and inhibitory pharmaceutical
agents seem to reduce the effect of these neurohormonal activities. Especially α- and β-
blockers and ACE-inhibitors have shown a variety of benefits including decrease
222 morbidity and mortality, improve quality of life and reduce the symptoms associated with
heart failure [493].
7.1.2. Hypertension Since a variety of systems are involved, the precise mechanism(s) responsible for
elevated blood pressure is hard to define (fig. 7-1). However the genesis of hypertension
includes one or all of the following alterations; abnormal volume expansion, increased
vascular resistance or greater cardiac output. Each one of these alterations is initiated by
abnormal neurohormonal activities and cardiovascular structural changes.
Abnormality in the reflex control of the sympathetic nervous system has been
reported. Mark et al., studied cardiopulmonary baroreceptors control of forearm vascular resistance in nine hypertensive patients (> 150/90 mmHg) and in seven normotensive.
Sympathetic nervous system activities and vascular resistance were measured while
reflex activation was provoked by applying negative pressure. The authors found
attenuation in cardiopulmonary baroreceptor reflexes characterized by increased
activation of the sympathetic nervous system and vascular resistance [494]. These
findings were confirmed by another study as abnormal arterial baroreceptor activities in
hypertensive patients were similar to those normal subjects following administrating
atropine and propranolol [495]. Impaired peripheral reflexes were also found in salt-
sensitive hypertensive patients and attributed to inadequate cardiopulmonary receptor
plasticity to increased salt intake [496]. These studies indicate abnormal sympatho-vagal
balance that contributes to vascular resistance and elevated blood pressure, additionally
these abnormalities are attributed to impaired peripheral reflexes [497]. Increased
sympathetic nervous system activation also indirectly triggers systemic vasoconstriction
by stimulating the release of catecholamines from the adrenal medulla. Furthermore
223 vasoconstriction induced by increased sympathetic drive and circulating catecholamines causes reduction in renal blood flow, which stimulates the proximal convoluted tubule to activate the renin-angiotensin-aldosterone hormonal system, which also contributes to further vasoconstriction and hypervolemia, thereby further elevation in blood pressure.
Abnormal sympatho-vagal balance also contributes to elevated blood pressure by increasing cardiac output. In borderline hypertension, the patients experience elevated cardiac output attributed to elevations in heart rate and stroke volume. Since blood volume is reduced in borderline patients the increase in stroke volume is attributed to alterations in autonomic nervous system balance characterized by increased sympathetic and attenuated parasympathetic activities. However with either aging, the disease or both, elevations in cardiac output tends to diminish reaching normal levels while vascular resistance remains elevated. The normalization of cardiac output appears to be the result of reductions in stroke volume secondary to cardiac structural changes. Continual elevations in blood pressure cause cardiac hypertrophy and decrease in cardiac compliance. Likewise prolonged blood pressure elevation leads to hypertrophy of the medial part of the vasculature leading to augmented vasoconstrictive response. The changes in vessel wall thickness cause the vessel wall to encroach even more on the vessel lumen [498].
Angiotensin II is a multifaceted hormone that contributes to hypertension in several ways. Angiotensin II is a very potent vasoconstrictor, can directly increase renal retention of salt and water and indirectly increase fluid retention by stimulating aldosterone secretion. Renin-angiotensin-aldosterone system is reportedly hyperactivated
224 in hypertensive patients. This system appears to be activated mainly as a result of
increased renal vascular resistance attributed to impaired sympatho-vagal balance [499].
7.2. ENDOTHELIUM DYSFUNCTION IN DISEASES
Vascular endothelium is a key element in maintaining balanced vasomotor regulation and
vascular homeostasis. In vascular diseases the endothelium function is compromised
leading to diminished vascular function. The main futures of endothelium dysfunction are
reduction in the efficacy of endothelium vasodilatory anticoagulant agents and increased
in the vasoconstriction capacity of the endothelium [500].
7.2.1. Atherosclerosis Endothelium dependent relaxation is impaired in atherosclerotic pigs and primates [501,
502], as well as in human coronary arteries [503]. In fact this impairment was related to
the stage of atherosclerotic development [503]. Moreover Ginsburg et al. found that the
contractile process of coronary arteries were related to the severity of the atherosclerotic
lesion not to the degree of the myocardial dysfunction [504]. In another study, vasodilation response to nitroglycerin and acetylcholine were reduced in proportion to local plaque load while this plaque load was associated with basal vascular tone. These results indicate that the vascular wall sensitivity to vasoactive agents is reduced in arteries with atherosclerotic lesion. However evident of endothelium dysfunction have also been observed in atherosclerosis as production of endothelium derived relaxing factor(s) are reduced in atherosclerotic coronary arteries obtained from patients with coronary artery disease [505].
Since atherosclerotic lesion is usually associated with other disorders including hypertension, hypercholesterolemia and diabetes mellitus it is difficult to separate the effect of atherosclerotic lesion on vascular wall function from the effect of other diseases.
225 It appears however the initial stage of vascular dysfunction in atherosclerosis is the result
of reduction in nitric oxide production, as this dysfunction appears to be essential for the
development of the atherosclerotic process. However some authors argue that as the
artery thickness and stiffness increase, it becomes harder for the endothelium-derived
relaxing factor(s) to reach the vascular smooth muscle cell. The prolong exposure to varies cardiovascular disease risk factors and the mechanical effect of sheer stress promote endothelium dysfunction. Subsequently larger and larger sections of the vessel wall become disinfected leading to less effective endothelium-derived relaxing factor(s) as well as increase in endothelium-vasoconstricting factor release. This process together with increased release of growth factors lead to further endothelium damage, wall proliferation and lesion formation initiating a vicious cycle of vessel wall deterioration
[112, 500].
7.2.2. Hypercholesterolemia/Hyperlipidemia Increased serum cholesterol levels appear to be associated with blunted vasoactivity.
After feeding rabbits 2% cholesterol diet for 13-15 wks, the inhibitory effect of
endothelium to neurogenic contraction was reduced whereas endothelium-dependent
relaxation to acetylcholine was similar to normally fed rats [506]. This inhibitory effect
was further worsen in denuded vessels [507] indicating endothelium-relaxing factor(s) is
still synthesized, released and has an effect on vascular smooth muscle in
hypercholesterolemia. However the oxidative stress of low-density protein seems to
deplete the reservoir of relaxing factors. Increased levels of low-density proteins activate
macrophages and favors formation and deposition of oxidative low-density protein in the
vascular wall. Nitric oxide seems to be either naturalized by oxidized low-density
lipoproteins or scavenged by macrophages [508, 509].
226 Endothelium-dependant relaxation has repeatedly been shown to be blunted following dietary-induced hypercholesterolemia. However this endothelium impairment is reversible by dietary manipulation. Morphological improvement of the atherosclerotic lesion, and restorations of endothelium-dependent relaxation were evident while the intima remained thickened after regression of atherosclerosis following dietary program aimed at improving lipid-lipoprotein profiles [510].
7.2.3. Diabetes mellitus Alterations in vascular wall are well documented in diabetes mellitus, leading to predisposition to other cardiovascular risk factors including hypertension and atherosclerosis. Compelling evidence strongly suggest endothelial dysfunction is manifested in this metabolic disorder. This dysfunction is characterized as blunting of endothelium-derived vasodilator effects and increased generation of vasoconstrictors such as angiotensin II, endothelin-1, and arachidonic acid products [511, 512]. Following
6 weeks of alloxan-induced diabetes mellitus in rabbits, acetylcholine relaxation was normal whereas endothelium inhibition of neurogenicaly-contracted vessels was nearly absent. Additionally neurogenic contraction in normal denuded vessels was similar to those diabetic with endothelium present indicating completely diminished vasodilatory effect of endothelium in diabetic rabbits [513].
Growth factors and cytokines agents appear to be elevated in diabetes resulting in acute increases in vascular tone, increased blood pressure and vascular remodeling that contributes to the microvascular, macrovascular, and renal complications in diabetes.
Superoxide products, overly produced in diabetics, seem to serve as signals that mediate many of the cellular biochemical reactions that result in these deleterious effects.
Diminished endothelium-dependent relaxation in diabetic rats observed in diabetic
227 vessels was significantly suppressed by pretreatment with superoxide dismutase [514].
These results indicate augmentations of superoxide products in diabetics and possible
treatments with antioxidant therapy [512].
7.2.4. Hypertension Endothelium-dependant relaxation was attenuated in genetically induced hypertensive
rats [515] as this attenuation progressively increased with age [516, 517]. The
vasodilatory response to acetylcholine was also altered in hypertensive patients [518],
independent of sodium intake [519]. Whether this attenuation is due to deficiency in
synthesis and release of endothelium-dependent relaxing factors or attenuated smooth
muscle cell responsiveness to these mediators is not clear yet. However both of these
mechanisms have been observed in human and animals with hypertension [520].
Figure LR7.1. Mechanism involved in vascular dysfunction in hypertension. Taken from [124].
Endothelin-1 may also be a factor contributing to vascular resistance in hypertensive, though its role is not clear yet. Plasma endothelin levels seem normal in most patients with essential hypertension however blood vessel wall from hypertensive animals are more sensitive and contract more profoundly in response to endothelin-1 [521].
228 The endothelium plays an obligatory role in controlling vascular function as it produces varies vasodilatory agents. It is essential to counterbalancing vasoconstriction induced by vasotonic agents released by either the autonomic nervous system or the endothelium cell. Vascular wall dysfunction observed in patients with cardiovascular risk factors is due to a decline in the inhibitory effect of the endothelium tissue. This decline might be due to either reduction in endothelium release of vasodilatory agents or failure of the vascular smooth muscle to respond appropriately to these agents, or a combination of both [522, 523] (fig 7-1).
7.3. SUMMARY
A Wide rage of changes occurs to the vascular system of patients with cardiovascular diseases including alterations in systemic and local neurohormonal stimulation, vascular responsiveness and vascular structure. The net result of these alterations usually is impaired blood flow kinetics in these patients and associated with increased fatigue and morbidity and reduced quality of life. Medications seem improve quality of life and reduce mortality and morbidity caused by the disease, however patients still experience symptoms associated with diseases.
229 SECTION 8. VASCULAR FUNCTION ASSESSMENTS
Cardiac output distribution to body parts varies during different conditions and strongly
influence whole-body hemodynamics and homeostasis. Since skeletal muscle shares a
large portion of this distribution, especially during exercise, its essential to measure
skeletal muscle blood flow under various conditions for experimental and clinical
understanding. Therefore invasive and noninvasive techniques have been developed to
examine various vascular functions including arterial inflow, vascular resistance, venous
filling capacity, venous emptying rate, vessel diameter, vessel density, and volume
changes. While there are advantages and disadvantages for each technique, testing
environment, financial feasibility, staffing and purpose of testing are all deciding factors
to which of the techniques would be used. The purpose of this section is to provide an
overview of the common techniques available for examining vascular function.
8.1. PLETHYSMOGRAPHY
8.1.1. Principles The physiological base of venous occlusion plethysmography is measuring the changes in limb volume following manipulations of the arterial and/or venous systems pressures.
Arterial inflow, vascular resistance, blood pressure, venous reflux, venous capacitance, and venous outflow rate are some the vascular functions that can be measured using this technique. For example arterial inflow is measured by calculating the rate of change in limb blood volume following venous occlusion. Whereas the change in limb blood volume following maintaining venous occlusion for a period of time (2-10 minutes) allows for estimating maximum venous filling capacity and venous emptying rate can be estimated by calculating the change in limb blood volume following venous release [524,
525].
230 8.1.2. Technicalities Various modalities of this technique have been adapted to measure vascular function
including air-filled, water-filled and strain-gauge plethysmography. However while all
these modalities have provided equally sufficient estimations of various vascular
functions, recently due to its precision, simple operative procedures and small size,
strain-gauge plethysmography has gained a lot of popularity. The strain-gauge is a sensor placed around the widest part of the limb which converts the limb volume change to change in voltage. Mercury-in-silastic strain-gauge sensor has been most used among all sensors [524].
Plethysmography can be used to measure vascular function in the forearm, calf, finger and penis however forearm and calf are generally the most used model.
Measurements are usually obtained while the subject in supine position, although standing, sitting and exercise are conditions that have been used, with a rest period (20-30 minutes) prior to evaluation of vascular functions. Vascular function can be measured in the dominant and/or nondominant limb depending on the purpose of the study. When the forearm vascular functions are measured using mercury-in-silastic sensor, cuffs are strategically positioned around the participant’s upper arm and wrist, and the strain gauge sensor is placed around the forearm approximately 10 cm distal to the olecranon process.
The forearm would be extended and slightly supinated and supported by a styrofoam block. The wrist cuff is usually used to exclude the hand circulation for the forearm measurements whereas the upper arm cuff is used to occlude the venous or/and arterial vessels. Currently suprasystole (180-240 mmHg) pressures are used to exclude the hand and occlude the arterial system, and infradiastole (30-80 mmHg) pressure is used to
occlude the venous system [443, 476, 525-528].
231 Manipulations in the pressures of the upper and wrist cuffs allows examination of
the rate of change of limb volume thought to reflect vascular function measurements,
including arterial inflow, venous capacitance and venous outflow. These changes in limb
volume are depicted as volume graphs on the plethysmograph recorder. Forearm blood
inflow rate is derived from the change in the slope drawn on the forearm-volume graph
following the first 3-5 seconds of venous occlusion. Forearm venous capacitance is
calculated as the vertical distance (mm) representing the change in forearm-volume graph
following the completion of venous filling (2-10 minutes). Whereas forearm venous outflow rate is derived from the change in the slope drawn on the forearm-volume graph between at 0.5 and 2 second following the release of venous occlusion. Forearm vascular resistance is calculated using the following equation:
Vascular resistance = Mean arterial pressure/Blood inflow.
Vascular indices are usually expressed in ml•100ml tissue-1•min-1. Forearm
vasoreactivity can be examined following 5-10 minutes of forearm arterial occlusion achieved by inflating the cuff on the upper arm to 240mmHg [443, 476, 526-528].
8.1.3. Advantages and Disadvantages Low cost and noninvasive simple operative procedures are the advantages of venous occlusion plethysmography. This technique has usually been used to measure vascular function at rest, after the cessation of exercise, and during pharmaceutical intervention.
However heighten sensitivity to subject’s movements has traditionally been problematic and required the subject to be immobilized. While this technique has been refined and improved since the original model was introduced, vascular function assessment during exercise using plethysmography is still practically impossible although some researchers
232 have reported arterial inflow during exercise [395, 396]. Additionally applying sufficient
pressure to occlude the venous system without altering the arterial kinetics is the original
fundamental assumption has to be fulfilled. However occlusion pressures (30-80 mmHg)
used in various studies is arbitrary and lack physiological basis. Additionally venous
occlusion time required for maximum venous filling, in order to measure the venous
function, is not clear resulting in inaccurate interpretations of the current studies.
Therefore studies are needed to address methodological issues of the technique to provide more accurate vascular function measurements [524, 525].
8.2. ULTRASOUND
8.2.1. Principle The images created by ultrasonic flowmetery are based on stipulations made upon high
frequency sound (ultrasound) waves reflection off blood flowing in blood vessels. These
waves are usually transmitted in a beam form by a crystal transducer to an artery or vein
to record blood particle movements. Especially with advancements in Doppler ultrasound
technology the main estimates made of the reflecting waves are the velocity and direction
of flow in the vessel. Ultrasound Doppler technology allows for continues beat-to-beat
estimation of blood flow profile [529-532].
8.2.2. Technicalities Doppler echocardiography is among the most advanced modalities available currently
however M-mode, two-dimensional and contrast echocardiography are other modalities
can also provide valuable information. Doppler ultrasound technology was introduced
during the 1950’s however a lot of improvements have been made in order to provide
sufficient information, especially regarding exercise. Properly equipped Doppler probe
233 can provide continuous estimations of blood velocity and direction and compile a flow
profile while the subject is performing physical movements [532].
The angle between the direction of blood flow and the ultrasonic beam determines
the kind of information obtained from Doppler sonagrphy. Insonation angles below 60o
while using pulsed-wave mode seem to provide accurate parabolic velocity profiles at the
V=(Fr-Fo)*(c/2Fo)*(cosθ) Where V=velocity, θ= the angle of intonation, Fr=frequency of reflected sound beam, Fo= frequency of transmitted sound beam, c=speed of sound (1540).
BF=6*104*Vmean*A Where A=cross-sectional of the vessel, Vmean=weighted mean blood velocity (m/s), constant 6*104=convert m/s to L/min.
specific depth required. However perpendicular insonation angle is essential for diameter
measurements. The direction of flow is estimated such that the sound frequency
reflection is depicted positively on the computer monitor when blood is moving toward
the transducer and vise versa when blood is moving away. Whereas blood flow is a
function of volume and time (L/min.) and derived from estimating blood velocity and
vessel cross-sectional. The following standard Doppler equations are used to calculate
blood velocity and direction: [529-532]. The diameter of the vessel increases with each
pulsation and retreat during diastolic, therefore diameter measurements are related to the
systolic (1/3) and diastolic (2/3) time periods of the cardiac cycle.
8.2.3. Advantages and Disadvantages While the measurements performed noninvasively with fairly easy procedures,
continuous blood flow profile estimation with high resolution are useful attributes of ultrasound Doppler. Rapid changes in blood flow as a result of exercise, metabolic stress, orthostatic modifications and pharmacological infusion, are currently easily detected,
234 especially with the recent technological Doppler advancements [533-535]. These
measurements are considered highly stable and have sufficient within-day and between-
days reproducibility [536, 537]. However since maintaining appropriate and consistent
insonation angle is essential for accurate measurements, the measured limb and the vessel
being measured have to be in fixed position [532].
Relatively steady laminar flow is essential for accurate blood flow measurements,
when ultrasound is used. However blood streams through different sizes and shapes of
vessels and is in constant friction against the vessel walls, thus flow is described as
parabolic profile. Flow in large vessel is flatter than in small vessel. Additionally when
blood enters a vessel the flow profile is fairly flat however as it propels for long distance
the profile becomes more parabolic. When blood departs a narrow vessel into a larger
vessel, blood flow tends to changes from laminar to a turbulent, which might result in
inaccurate blood flow measurements. Due to friction against the walls blood would flow faster in the outside of a curvy vessel, resulting in change of profile with the vessel.
Friction between blood and vessel wall also causes blood velocity in the center, of even a straight vessel lumen, to be slower than near the edges of the wall, thereby flow profile also differs. These physiological concerns are largely avoided by using anatomical landmarks within the vasculature when measurements are obtained. Therefore major conduits in the body including brachial, femoral, aortic, carotid and pulmonary arteries have been used as models to examine vascular function [529-531].
Due to higher resolution, accurate estimations and easier assessments, measurements are usually obtained in larger conduits. Blood flow regulation and responses to various physiological, physical and pharmacological perturbations are
235 different in large, medium and small vessels. Therefore current ultrasonic studies should
be interpreted cautiously and maybe limited to only large vessels.
8.3. ISOTOPE CLEARANCE
8.3.1. Principle This method is based on locally tracing the clearance of an infused molecule into the
circulation while assuming clearance is flow-dependent. Obviously this molecule is
usually easily diffusible and radio-labeled [529-531].
8.3.2. Technicalities The tracer is injected intramuscularly to the area of interest and its clearance for that area
is externally observed. The rate of clearance is depicted as curves and assumed to
correspond to blood flow in vessels. Thus tracer is cleared faster during exercise than
during rest since blood flow is also faster. Usually catheter(s) is inserted into the muscle
to inject and measure drainage rate of the tracer mixture. Since they are not toxic and
easily diffuse, Thermodilution and 133xenon are the two most common tracers used in humans [529-531].
8.3.3. Advantages and Disadvantages The potential of measuring blood flow during exercise is an extremely important and
intriguing attribute of isotope clearance rate method. However its invasive nature and the
trauma associated with the measurement requires medical training and might actually
hinder exercise performance. Additionally in some cases isotope does not mix properly
with the blood, thus results might be erroneously interpreted. Adequately mixed isotope
and positioned catheter helps ensuring proper isotope/blood mixing and minimize errors
[529-531].
236 8.4. MAGNETIC RESONANCE IMAGING ANGIOGRAPHY
8.4.1. Principle It is a relatively recently adapted technique to measure blood flow and based on phase-
contrast magnetic resonance velocity imaging of hydrogen nuclei in blood moving through a magnetic-filed accumulate a phase shift proportional to their velocity. Moving
protons are phase-shifted in proportion to their velocity the circulation whereas stationary
protons do not change their phase [529, 530].
8.4.2. Technicalities Similar to Doppler ultrasound, blood velocity and vessel cross-sectional area are
measured to calculate blood flow. The following equation is used to calculate blood flow;
Blood flow (ml/min)Vmean * A * 60 2 Where Vmean (cm/s) is the mean velocity and A (cm ) is the cross-sectional area.
Deep vessels blood flow as well as local number of vessels can be evaluated using this
technique with extremely high quality images. The images can be obtained during the
entire cardiac cycle which provide information on the nature of the pulsatile flow. The
technique is usually used during rest and hyperemic conditions since its very sensitive to
movement. Magnetic resonance imaging has been validated and used to measure vascular
function in disease population and to detect vascular pathology [529, 530, 538].
8.4.3. Advantages and disadvantages High quality images and the noninvasive procedures are the most recognized advantages
of this technique. Blood flow can be measured in deep small individual vessels in human
as well as animals. Additionally this technique does not require infusing tracing elements
or determining the insonation angle, unlike isotope tracing or ultrasound respectively.
However a high price is tagged to each vascular evaluation additionally since it is a
237 magnetic procedure patients with any metal medical aids, such as pacemaker or orthopedic metal rods are excluded [529, 530].
Many direct as well as indirect techniques are available to measure blood flow including electromagnetic flowmetery, microdialysis ethanol outflow, near spectroscopy, positron emission tomography and laser Doppler. However since it is not expensive, simple to operate and measure blood flow during a variety of conditions, venous occlusion plethysmography is probably still considered the “gold standard” for blood flow measurements [530], however Doppler ultrasound has recently gained tremendous popularity. With recent advancements, relatively simple measurement procedures, and ability to measure blood flow even while the body is moving, Doppler ultrasound has become a favorite measurement tool of vascular function [529-531].
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279 APPENDIX B: PROPOSAL
280 CHAPTER 1. INTRODUCTION
1.1. SPECIFIC AIMS
Selye’s theory of stress-response-adaptation (3) suggests an exercise stress, when applied to the vascular system, results in an increased delivery of blood flow to active skeletal
muscle, and if the stress is repeated over time contributes to a series of vascular
adaptations. These vascular changes are aimed at increasing vascular conductance and
oxygen and nutrient delivery to working skeletal muscle during subsequent exercise
bouts (7, 10, 13, 16, 31, 33). In contrast to the extensive literature describing the
structural and functional changes in the arterial system following training, training-
induced alterations in the venous system are less clear. Given the importance of the
venous circulation in regards to metabolic waste removal, its enormous potential to act as
a volume reservoir and its contribution to the end-diastolic volume, changes in this
segment of the vasculature could have a profound impact on the exercise response. To
this extent, Louisy et al. (23) reported greater venous distensibility in endurance-trained
subjects compared to healthy controls and Miyachi et al., (26) reported an increase in
peripheral (femoral) venous size following one-legged training. Both investigators
suggested the veno-adaptations could be associated with increases in blood volume
following endurance training, and that the increased blood volume may be distributed on
the venous side of the vascular space, since the veins represent a more compliant
reservoir. Recognizing the potential influence of systemic factors such as blood volume
in several previous studies, the purpose of this proposal is to examine the effects of 4-
weeks of handgrip exercise training on arterial reactivity and venous function,
particularly venous capacitance and venous outflow.
281 1.2. HYPOTHESES
This study will test two specific hypotheses regarding the influence of physical training
on arterial (Hypothesis I) and veno-dynamics (Hypothesis II). Finally, depending on the
outcome of the first two hypotheses, the third hypothesis will test the timecourse of the expected changes in vascular dynamics.
Hypothesis I Four-weeks of handgrip exercise training will result in an increase in arterial reactivity
following 5-minutes of arm occlusion;
Hypothesis II Four-weeks of handgrip exercise training will result in an increase in venous capacitance
and venous outflow at rest and following 5-minutes of arm occlusion;
1.3. BACKGROUND AND SIGNIFICANCE
1.3.1. Previous work The effects of exercise training on hemodynamic and hematologic factors have been
extensively investigated. One of the early studies examined the effect of exercise training
on regional vascular function reported a decrease in blood flow. The authors attributed
lowered leg blood flow to a more “effective” metabolic process (30). However Sinoway and coworkers reported increased vascular reactivity in the dominant arm of tennis
players (34) and following handgrip exercise training (35). These findings were further
confirmed in a number of studies and attributed to alterations in vascular smooth muscle
and/or vascular endothelium structure and function, however the influence of cardiac and
skeletal muscle adaptations were not totally excluded (3, 17).
282 One of the most predictable hemodynamic changes following “aerobic” training is
an increase in blood volume. Consequently, venous return is higher and stroke volume is
elevated at each absolute level of work. Recognizing that the venous system is a reservoir
for ~65% of total blood volume and is repeatedly challenged during exercise, it is
reasonable to assume that this system would be affected by exercise training. However
few studies examined the effect of exercise training on venous function, therefore
conclusions regarding circulatory adaptations have for the most ignored the possible
contribution of the venous system (11, 18, 29). The following paragraphs will provide a
brief review of the available literature regarding venous adaptations following exercise
training.
Wecht et al. examined the difference in venous hemodynamics between
individuals with spinal cords injury, sedentary and active individuals. Venous tone,
venous capacitance, venous compliance, and total venous outflow were higher in the
active group, leading the authors to conclude that endurance exercise training enhanced
venous function (41). Louisy and coworkers examined venous hemodynamics in
endurance trained, strength trained and sedentary subjects and reported higher values for leg-filling volumes and venous emptying in the endurance trained subjects. The authors
concluded that endurance training increased lower limb venous hemodynamics
subsequent to hypervolemia rather than alterations of the visco-elastic properties of the
vein walls (23).
Miyachi et al. examined venous diameter following 6 weeks of one-legged cycle training at 80% peak oxygen consumption, 40 min/day for 4 days/week followed by 6 weeks of detraining. Arterial and venous cross-sectional areas increased by 16% and
283 46%, respectively and correlated with an increase in one-leg peak oxygen consumption.
These changes returned to baseline during detraining. The authors attributed part of the
changes in oxygen consumption to arterial adaptations and improved blood delivery.
Furthermore, they suggested the veno-adaptations could be associated with increases in
blood volume following endurance-training, and that the increased blood volume may be
distributed on the venous side of the vascular space, since the veins represent a more
compliant reservoir (26).
Suzuki and colleagues examined the effects of exercise training on venous
capillary density of young (3 weeks) and middle-age (54 weeks) (37). The 6-week
exercise-training program resulted in increased arteriolar capillary density in both groups
without changes in venular density. In a subsequent time-course study by the same group,
the effect of exercise training on venular density was examined in young female rats (5
weeks). Vascular density indices (capillary-to-fiber ratio, total capillary density, and
density of venular capillaries) of soleus and medial gastrocnemius muscles increased
following only 1 week of training. However values for total capillary, arteriolar, and
venular capillaries were significantly decreased after 4 weeks of training in the soleus
muscle and after 3 weeks in the medial gastrocnemius. These results suggest that 1-week of exercise training in Wistar rats results in greater arteriolar and venular capillarization, mainly in highly oxidative muscles and indicate that training-induced adaptations might be time-, age- or gender-dependent. However since neither arterial or venous hemodynamics nor any vascular function indicators have not examined, a complete
conclusion were not made (37, 38).
284 Kenney and Armstrong investigated venous pooling following an 8-week
exercise-training program (40 min/day, 3day/week). Venous pooling was measured in the
foot as volume drainage following the “Trendelenburg” maneuver (VT). The
“Trendelenburg” maneuver consists of 45º leg elevation for 15 seconds with the subjects in a supine position, followed by sitting up to a position of 90º flexion with the legs lower than the body. The investigators assisted the subject throughout the maneuver to avoid muscle contraction. Moreover, to account for the muscle pump contribution, volume drainage was evaluated following 15 seconds of dorsi-flexion/plantar exercise while the foot is lower than the body (VE) then subtracted from volume drainage following the
Trendelenburg maneuver (VT-VE). Exercise training resulted in increased VT, VT-VE, oxygen consumption without changes in VE. The authors concluded that the increased venous pooling was the result of hypervolemia. However, the possible structural and/or functional changes in the venous system following exercise training were not addressed
(21). Hypervolemia associated with exercise training, has also been linked to changes in central venous pressures as reported by Convertino et al. Convertino used an exercise program consisting of 10 week of cycle exercise for 30 min/day, 4 days/week at 75-80% to examine changes in central venous pressure. Training resulted in a 20% increase in maximal O2 uptake, decreased resting heart rate, 9% increase in blood volume and increased central venous pressure. The increase in central blood pressure was associated with the increase in blood volume without changes in total effective venous capacitance,
thus the authors attributed the observed changes in central venous pressure to an increase
in blood volume but not to alterations in venous structure or function (12).
285 Detraining has also been shown to induce changes in the venous system (20, 22,
32, 42). Louisy and Guezennec examined the effect of 30-days head-down bedrest on
lower-body venous hemodynamics and reported a progressive increase in venous
compliance. The study was not designed to reveal the mechanisms involved, however
reduction in the blood volume and subsequent dilation in the venous system and
decreased pressure in the proximity of the lower veins were stipulated. Involuntarily
detraining induced by morbidity diseases might also affect venous function (22).
Ikenouchi and colleagues found that venous distensibility is reduced and related to the
severity of the disease in patients with heart failure (20, 32). In our laboratory venous capacitance and outflow were lowered and related to exercise intolerance in heart failure, confirming the relationship between inactivity and venous dysfunction (42). While venous changes might be a direct effect of heart failure, inactivity associated with the disease might indirectly alter venous function.
In summary, the aforementioned studies suggest higher levels of fitness and/or exercise training may alter venous hemodynamic. The changes in venous hemodynamics appears to be directly related to changes in blood volume and associated with changes in exercise capacity. In contrast detraining reveres these adaptations, which may be further affected by systemic diseases such as heart failure. However, none of the above- mentioned studies have clearly addressed the issue whether exercise improves measures of venous function and the manner in which this relates to muscle performance.
Barclay et al. demonstrated the importance of metabolite removal to muscle fatigue, indicating the essential role of venous hemodynamics to performance. In this study, muscle fatigue rate in dogs was measured in 5 experimental groups subjected to
286 different perfusion conditions by manipulating O2 contents and/or muscle blood flow with a perfusion pump. The first group served as a control group and breathed room air while the muscles were self-perfused (self-perfused). Group 2, breathed 14% O2 while
increasing blood flow across the muscle bed with the perfusion pump (low O2/ high flow)
and examined the effect of hyperfusion on muscle fatigue. In group 3, the dogs breathed air saturated with 100% O2 while blood flow was at reduced perfusion rate (high O2/low
flow) and examined the effect of hypofusion on muscle fatigue. In group 4, blood flow
pump-perfused while perfusion pressure was kept constant at the animal’s blood pressure
until a steady pressure was attained. This procedure kept the plasma flow constant and
identified the effect of blood and plasma flows at constant level established by the
th animal’s blood pressure (constant flow). Finally, the 5 group breathed 100% O2 and the
blood was diluted with a volume-for-volume exchange of dextran solution while
maintaining constant plasma flow by increasing blood flow with the perfusion pump to
identify the effect of hyperfusion with a constant plasma flow (diluted/high flow). The
self-perfused group fatigued the fastest while the pump-perfused groups had slower
fatigue rate regardless of O2 delivery or plasma flow rates. The authors argued that
increased blood flow across the muscle bed resulted in changed cellular environment
and/or metabolic pattern. Additionally they concluded that the attenuated fatigue
associated with hyperfusion was independent of O2 and/or substrate delivery and is
attributed to improved metabolite removal out of the muscle bed (8). This suggests that
venous health is particular important in exercise performance subsequent to ensuring
adequate metabolite removal.
287 More recently, Tschakovsky and Hughson also examined the importance of venous emptying in the vasodilatory capacity and muscle function. Continuous forearm venous volume and blood flows were obtained following arm elevation while either venous drainage is prevented or allowed (Fig 1.1).
Figure PR1.1. Schematic illustration of the timing of arm position relative to heart level during the prolonged and the acute arm elevation protocols. This timing was the same for both venous emptying and venous congestion trials. Solid bars indicate transition period between arm positions. Taken from (40).
As demonstrated in Fig 1.2, venous emptying protocol resulted in marked decreased blood volume associated with transit increase in blood flow during acute arm elevation while the opposite was observed during venous occlusion protocol. The hyperemic respond was attributed to transit vasodilatory effect consequent to reduced forearm venous volume. Additionally, this increase in forearm blood flow was linked to venoarteriolar reflex vasoconstriction with reduction in venous volume and was
288 suggested to simulate hyperemic response subsequent to venous emptying during muscle contractions (40).
Figure PR1.2. Hemodynamic response to acute forearm elevation. Data are means ± with values at 1-s intervals. A: forearm blood flow; B: changes in forearm volume from below heart baseline; C: forearm venous pressure (antecubital vein). Forearm began below heart (time = 10 s to 0 s). Arm above heart from 2 s to 6 s. Venous emptying allowed during arm elevation ( ). Venous emptying prevented during arm elevation ( ). Solid bars indicate the 2-s transition periods between arm positions; n = 9 except for venous pressure, venous emptying allowed n = 4, venous emptying prevented n = 3. Taken from (40).
Thus, these above-mentioned studies suggest a critical role of venous outflow in
exercise performance. Consequently, studies are needed to examine the effects of acute
and chronic exercise training, as well as the modality, intensity, duration, and frequency
289 of training, on different segments of the venous system. Additionally investigations
examining the effects of a localized exercise program on venous alterations are certainly
warranted.
1.3.2. Progress report / Preliminary studies In preparation of this proposal a series of studies have been conducted to develop the
appropriate methodologies and understand the rationale for this proposal. These studies
focused on determining the reproducibility of several outcome measures pertinent to this
study, examined the ability of vascular measures to differentiate between various
populations, and evaluated the influence of interventions on changing measures of
vascular function. These projects will be summarized in chronological order in the
following section.
The first study examined the effect of 4 weeks of low (25% of MVC) and high
(75% of MVC) intensity handgrip (HG) exercise training on arterial function. Forearm blood flow (Binf) and vascular resistance (VR) were evaluated in twenty-eight young men at rest, following forearm occlusion (OCC) and following forearm occlusion combined with HG exercise (HGOCC) using mercury strain gauge plethysmography
(MSGP) (3). The 4-week program consisted of non-dominant HG exercise performed 5 d/week for 20-min at either low or high intensity. Both training intensities resulted in improved vasodilatory responsiveness in the trained arm only with no change in muscular strength (see figures 1.3 & 1.4 and table 1.1).
290 ‡ † § 45 § § * § 40 ‡ * † * Pre * 35 * * * Post 30 * (ml.100ml-1.min-1) 25 20 15 10 5 0 Rest OCC HGOCC Rest OCC HGOCC Forearm Blood Flow Dominant Arm Non-dominant Arm
Figure PR1.3. Forearm Binf changes in the low group following HG exercise training. Values are means ± SD. *p<0.05 vs. rest; §p<0.05 vs. OCC ‡p<0.05 vs. Pre Non- dominant; †p<0.0 (3).
† § ‡ * 45 § § § ‡ † 40 * * * * * 35 * * (ml.100ml-1.min-1)
30 25 Pre 20 Post 15 10 5 0 Rest OCC HGOCC Rest OCC HGOCC
Forearm Blood Flow Dominant Arm Non-dominant Arm
Figure PR1.4. Forearm Binf changes in the high group following HG exercise training. Values are means ± SD. *p<0.05 vs. rest; §p<0.05 vs. OCC ‡p<0.05 vs. Pre Non- dominant (3).
291
Table PR1.1. Dominant and non-dominant forearm VR changes in the low and high groups at rest, following OCC and following HGOCC (3). Dominant Non-dominant
Pre Post Pre Post
Low group (15)
VRrest 47.85 ± 13 42.2 ± 13.4 42.5 ± 15.4 44.6 ±17.0
VROCC 3.3 ± 1* 3.5 ± 0.5* 3.5 ± 0.6* 3.5 ± 0.8*
VRHGOCC 2.5 ± 0.5* 2.8 ± 0.4* 3 ± 0.4* 2.9 ± 0.7*
High group (13)
VRrest 44.5 ± 19 45.7 ± 22.7 39.4 ± 15 34.4 ± 8.8
VROCC 3.1 ± 0.5∗ 3.1 ± 0.5∗ 3.7 ± 0.6∗ 3.3 ±0.7∗
VRHGOCC 2.7 ± 0.3∗ 2.77 ± 0.4∗ 3.3 ± 0.7∗ 2.5 ± 0.7∗
Values are means ± SD. LO, training intensity = 25% of maximum voluntary contraction (MVC); HI, training intensity = 75% of MVC; VRrest, forearm vascular resistance at rest; VROCC, vascular resistance following five minute of occlusion; VRHGOCC, vascular resistance following OCC coupled with 3-min. of HG of exercise. ∗p < 0.05 vs. rest.
Table PR1.2. Independent t-test comparisons between CHF and Control groups (42). CHF Control p-value (n=20) (n=9) Binf (ml•100ml tissue-1 •min-1) Rest 3.10 ± 1.20 3.43 ± 1.24 0.50 Post Occlusion 15.25 ± 6.31 22.30 ± 6.70 0.018 VR (U) Rest 36.90 ± 14.87 30.40 ± 10.46 0.24 Post Occlusion 7.80 ± 3.07 4.64 ± 1.38 0.007 Vcap (ml•100ml tissue-1 •min-1) Rest 1.88 ± 0.44 2.17 ± 0.22 0.068 Post Occlusion 1.43 ± 0.50 2.04 ± 0.40 0.003 Vout (ml•100ml tissue-1 •min-1) Rest 20.50 ± 6.40 22.90 ± 10.78 0.46 Post Occlusion 24.50 ± 9.42 33.11 ± 9.97 0.04
292 In a subsequent study forearm Binf, VR, venous capacitance (Vcap) and outflow (Vout)
and maximum walking distance (MWD) were evaluated in heart failure patients and age-
matched older adults (42). Arterial and venous function indices and MWD were different
between the groups (table 1.2). Additionally, correlation analyses revealed significant
associations between vascular indices and exercise tolerance measurements (figures 1.2
& 1.3), confirming impaired forearm vasoreactivity and indicating that venous function
might contribute to exercise intolerance in heart failure.
800 700 r =0.57; p =001 600 500 400
MWD (m) 300 200 100
.25 .75 1.25 1.75 2.25 2.75 Vcap (ml·100ml tissue-1·min-1)
Figure PR1.5. Relationship between forearm venous capacitance and maximum walking distance (42).
Interestingly many of the available protocols to examine venous function using
MSGP have failed to address many physiological and methodological concerns that might directly or indirectly affect the precision of the measurements (4, 5). A particular important factor to consider is the selection of the pressure and time of venous occlusion.
For example Hokanson et al. and Cramer et al. have recommended 50mmHg for venous occlusion pressure, regardless of the pressure of the arterial system (14, 15, 19). While this pressure is arguably sufficient to prevent venous emptying, in some cases it may also
293 change the kinetics of the arterial system. In a more recent study, Wecht et al. used a
pressure “just below” diastolic blood pressure as a criterion for venous occlusion
pressure. However the authors did not quantify the precise pressure used to occlude the
venous system, thereby interpretations of their findings were ambiguous (41).
800 700 r =0.67; p =0001 600 500 400
MWD (m) 300 200 100
10 15 20 25 30 35 40 45 50 55 60 Vout(ml·100ml tissue-1·min-1)
Figure PR1.6. Relationship between forearm venous outflow and maximum walking distance (42).
Furthermore, the aforementioned studies (14, 15, 19) have also adapted two minutes of venous occlusion to evaluate the venous system, however it is not clear whether this time is sufficient to achieve maximum venous filling. Thus the next phase involved examining the effect of different venous occlusion times and pressures on Vcap and Vout. Venous indices were examined this in ten young individuals using two distinct protocols. In protocol one venous occlusion pressures of 7, 14, 21, 28 and 35 mmHg below resting diastolic blood pressure were used with a fixed occlusion time of four minutes.
294 In protocol two venous occlusion times of 5 and 10 minutes were used with a
fixed venous pressure of 50mmHg. Figures 1.5 and 1.6 depict a stepwise decrease in
Vcap (p ≤ 0.0001) and Vout (p ≤ 0.0001) with greatest values following venous occlusion
pressure 7mmHg below diastolic blood pressure. Forearm Vcap ranged between 3.7±0.7
and 2.2±0.5 ml•100ml tissue-1•min-1 and Vout ranged between 32.7±4.5 and 17.3±4.0
ml•100ml tissue-1•min-1.
6 1 = 07mmHg < DBP 2 = 14mmHg < DBP 5 3 = 21mmHg < DBP 4 = 28mmHg < DBP 5 = 35mmHg < DBP 4 ‡ ‡ ‡ * § 3 *
2 Vcap (ml·100ml tissue-1·min-1)
1 12345 Pressures (mmHg)
Figure PR1.7. Venous capacitance using different pressures. Values are in means ± SD. ‡p<0.007 vs. 1; *p<0.01 vs. 2; §p<0.03 vs. 3 (36). Furthermore, 10 minutes of venous occlusion resulted in greater Vcap (p < 0.001) and Vout (p < 0.026) values as compared to 5 minutes (Figure 1.7). Finally, the effects of altering occlusion times and pressures on resting arterial inflow and vascular resistance were minimal and not statistically significant. These findings indicate the importance of recognizing the possible influence of venous occlusion times and pressures on venous capacitance and outflow. Additionally these technical aspects are particularly relevant when considering repeated measures designs during which blood pressure may be altered
(36).
295
60 1 = 07mmHg < DBP 2 = 14mmHg < DBP 3 = 21mmHg < DBP 50 4 = 28mmHg < DBP 5 = 35mmHg < DBP 40
‡ ‡ ‡ § * 30 * †
Vout (ml·100ml tissue-1·min-1) 20
10 12345 Pressures (mmHg)
Figure PR1.8. Venous outflow using different pressures. Values are in means ± SD. ‡p<0.02 vs. 1; *p<0.02 vs. 2; §p<0.05 vs. 3; †p<0.06 vs. 4 (36).
5 40
4 30 ‡
‡
3 20 Vout (ml·100ml tissue-1·min-1) Vcap (ml·100ml tissue-1·min-1)
2 10 5Minutes 10Minutes 5Minutes 10Minutes Time Time
Figure PR1.9. Venous capacitance and outflow following 5 & 10 minute venous occlusion Values are in means ± SD. ‡p<0.026 vs. 10Minutes (36).
296 With attempts made to investigate the role of the venous system for understanding cardiovascular physiology, it is imperative to have a reliable investigative model for venous function. Our laboratory (24) as well as others (28) has reported adequate reproducibility and reliability of the plethysmographic technique to measure arterial inflow. In contrast, few studies have examined the reproducibility and reliability of plethysmography to measure venous function. Thus another aspect of the research process involved examination of the reproducibility of venous and arterial indices.
Forearm Binf, VR, Vcap and Vout were evaluated at rest and following OCC and
HGOCC on three different occasions within 10 days and expressed in ml•100ml tissue-
1•min-1. As revealed in table 1.3, between-days interclass correlation coefficients for arterial and venous indices at rest and following OCC and HGOCC ranged between 0.7 and 0.97 (2). The values for the resting Binf and VR are well within expected range, additionally no significant difference were found between the means values of the three visits. However a slight day-to-day difference might result in low statistical reproducibility (34, 35). These results indicate adequate reliability of MSGP technique for measuring arterial as well as venous function on different days under controlled conditions.
Table PR1.3. Between-days interclass correlation coefficients for forearm arterial and venous indices (2). Condition Binf VR Vcap Vout Rest 0.29 0.3 0.78 0.94 OCC 0.81 0.7 0.7 0.91 HGOCC 0.95 0.95 0.8 0.97 Binf=Blood inflow, VR=Vascular resistance, Vcap=Venous capacitance, Vout=Venous outflow
297 Finally the effects of functional ability on arterial and venous measures have been examined. This is extremely relevant as this research is ultimately trying to help clinicians understand why individuals may be intolerant to exercise. Linking a pathologic condition to a functional limitation will eventually result in the development of more appropriate strategies to preserve function and avoid disability. Consequently, several studies have attempted to link performance measures with arterial and venous function.
Most recently, HG strength on venous function, forearm Vcap and Vout were evaluated in 13 apparently healthy males at rest and following OCC using MSGP. Maximum HG strength was determined as the average of 3 maximum consecutive contraction trials using a hand-dynamometer and expressed in kilogram. Subsequently, the group was divided into above (HI) and below (LO) average HG strength according to the Canadian
Society of Exercise Physiology classification. As depected in figure 1.8 Pearson correlation coefficient showed an association between HG strength and forearm venous indices at rest {Vcap: r=0.72; p=0.006; Vout: r=0.72; p=0.006} and following OCC
{Vcap: r=0.56; p=0.058; Vout: r=0.73; p=0.007} (2).
7 70
60 6
50 5 40
4 30
Vcap (ml.100ml-1.min-1) r =0.72; p =0.006 r =0.72; p =0.006 3 Vout (ml.100ml-1.min-1) 20 30 80 130 180 30 80 130 180 Strength (kg) Strength (kg)
Figure PR1.10. Relationship between HG strength & resting forearm venous indices (2).
298 Finally, Student t-test comparison between the HI and LO HG strength groups
revealed difference in Vcap and Vout at rest and post OCC (table 1.4). These results
suggest that MSGP is useful technique to differentiate between populations. Furthermore,
HG strength appears to contribute to venous system hemodynamics implying that muscular conditioning might be important for venous health.
Table PR1.4. Differences in venous indices between low and high strength groups (2) Group HG Strength (kg) Vcap at rest Vcap post OCC Vout at rest Vout post OCC LO 80.55±16.2 4.5±0.8 3.4±0.7 38.7±9.2 38.0±7.5 HI 136.8±20.7 5.7±0.5 4.2±0.7 54.3±7.6 54.0±8.2 Alpha 0.0001 0.002 0.065 0.01 0.007
In conclusion, the completed research has determined that assessment of arterial
and venous function using plethysmography is reproducible and reliable, allows
differentiation between populations, and arterial function is modifiable with training. The
final aspect of this research is to examine the influence of localized exercise training on
venous measures.
1.5. DISCUSSION OF EXPECTED RESULTS
Due to the relatively small muscle size involved in the exercise-training program,
physiological adaptations are expected to be limited. Arm ergometer test, measures of
autonomic nervous system-HRV, body composition are expected to remain unchanged
throughout the exercise-training program (33, 34). Additionally as previously reported
arm circumference as well as MVC are expected to increase by 2 (1) and 6-8% (1, 3),
respectively.
299 Vascular indexes As previously reported, HG exercise training is expected to result in an increase in arterial reactivity following 5-minutes of arm occlusion (3, 17, 34, 35). The effect of localized exercise training program on venous function have not been evaluated.
However few studies have examined the effect of exercise-training programs that involve large muscle groups and trigger systemic adaptations, including hypervolemia. Therefore increase in venous capacitance and venous outflow were attributed to systemic changes while ignoring possible changes in the venous function (21, 23, 41). Given the importance of the venous system in overall cardiovascular physiology (8, 40) and since the venous system will be challenged with exercise training, improvement are expected in venous capacitance and venous outflow.
300 CHAPTER 2. METHODS
2.1. PARTICIPANTS AND DESIGN
Fifteen apparently healthy sedentary young (ages 18-30) adult males will be recruited to participate in the study. Individuals with manifestations of cardiovascular, metabolic, orthopedic or neurological disease or are on any medication (e.g., digitalis), which could affect the results of this study will be excluded.
The proposed study is randomized, prospective, controlled and will be executed in
4 weeks. The study involves once a week evaluation session and 5-time-a-week exercise training sessions. Prior to and at the end of each week, participants will undergo HG strength and vascular function evaluations in the dominant and non-dominant arms. The primary outcomes of this study include: (1) Vasoreactivity, defined as the change in forearm blood flow from rest to the peak blood flow response following 5 minutes of forearm occlusion (34, 35); (2) Venous capacitance; defined as the changes in limb blood volume following 6 minutes of venous occlusion; and (3) Venous outflow; defined as the changes in limb blood volume following the release of venous occlusion (15, 19).
Additionally, handgrip strength, arm exercise tolerance, and hemodynamic measures (i.e. blood pressure, heart rate and heart rate variability) will also be assessed. Following the completion of the initial assessment, each subject will be asked to report to the exercise physiology laboratory 5 days/week to perform 20 minutes of forearm flexion exercise at a cadence of 1 contraction every 4 seconds starting at intensity of 60%. The non-dominant arm will be trained while the dominant arm will serve as control.
301 2.2. STUDY MEASUREMENTS:
2.2.1. Vascular function assessments Vascular function indices will be obtained in the dominant and non-dominant forearms using MSGP at rest and following OCC (34) at the beginning of the study, and at the end of each week throughout the study period. This technique is noninvasive based on the assumption that alterations of pressures in strategically placed cuffs allows examination of the rate of change of limb volume thought to reflect venous function indices including
Binf, VR, Vcap and Vout (15, 19). Prior to the experiment, blood pressure cuffs will be positioned around the participant’s upper arm and wrist, and a mercury-in-silastic strain gauge placed around the forearm approximately 10 cm distal to the olecranon process (3).
Resting forearm vascular function measures will be obtained following 15 minutes of supine rest. Immediately before the measurements, hand circulation will be occluded for 1 minute by inflating the cuff at the wrist to 240mmHg. Forearm Binf will be obtained using an upper arm venous occluding pressure of 7mmHg below diastolic blood pressure.
Forearm Vcap will be measured after an additional 6 minutes of venous occlusion and
Vout following the release of pressure (2, 36). Subsequently, forearm vasoreactivity is examined following 5 minutes of forearm arterial occlusion achieved by inflating the cuff on the upper arm to 240mmHg. Forearm vascular indices will then be determined as described above. The exact same procedures will then be performed on the other arm.
Prior to, during and following each procedure blood pressure and heart rate will be obtained.
2.2.2. Autonomic nervous function assessments: Indices of autonomic function will be assessed using measures of heart rate variability.
These measures will be obtained at the same time as the vascular assessments described
302 above. The ECG electrodes will be placed on the chest and interfaced with a Biopac
MP100 and its companion software Acqknowledge (model MP100A, Biopac Inc., Santa
Barbara, CA) to allow for continuous data acquisition. A period of five minutes (at rest
and during forearm occlusion) of ECG data will be analyzed for mean heart period (mean
R-R interval), standard deviation (SDNN), low frequency power (LF) and high frequency
power (HF) using the guidelines set forth by the Task Force of the European Society of
Cardiology and the North American Society of Pacing and Electrophysiology (9, 39).
These variables will be obtained at the beginning of the study, and at the end of each
week of the study period.
2.2.3. Arm ergometer test Arm exercise tolerance will be evaluated using a graded arm ergometer exercise test at the beginning and at the end of the study period (6, 25). The test will be performed by
initially turning the arm ergometer at a speed of 50 rpm and a resistance of 0.5kp for two
minutes while the participant in a seated position. The resistance will then be increased
by 0.25kp every two minutes until the subject reaches exhaustion, requests to stop, or
exhibits signs or symptoms, which require test termination. Prior to, during, and
following the exercise test heart rate, blood pressure measurements and ratings of
perceived exertion will also be obtained at the end of each minute throughout the test using a standard sphygmomanometer and the Borg's perceived exertion scale,
respectively.
2.2.4. Forearm circumference and HG strength assessments Participants forearm circumferences and HG maximal voluntary contraction will be evaluated at baseline and at the end of each week throughout the study period. Forearm circumference will be examined using a weighted measuring tape 10 cm distal to the
303 midpoint between the lateral epicondyle and olecranon process. Handgrip strength will be evaluated using a handgrip dynamometer (Lafayette instruments) with the participant upright, but slightly bent forward. The test involves an all-out gripping effort for 3 seconds, without movement of the arm. The average of three consecutive trials will be used as the measure of strength.
2.2.5. Physical activity questionnaire The participants will be asked to complete the Aerobic Center Longitudinal Study
Physical Activity Questionnaire. This Questionnaire is designed to conduct physical activity pattern, including leisure and household, during the past 3 months. Information regarding walking, stair climbing, jogging, swimming, weightlifting, aerobic dance, and household activities are acquired (27).
2.2.6. Body composition Three-site skinfold method will be used to calculate body percents of fat and fat-free masses. The recommended body sites for this method are chest, abdominal and thigh.
Additionally dominant and non-dominant forearm skinfolds will be obtained at the upper
1/3 of the forearm length (43).
2.2.7. Computer assessment The Biopac system and Acknowledge computer software will be utilized to capture and digitize vascular function indices received from the plethysmograph. The data will be collected at 500 Hz. then resampled at 4 Hz.
2.3. DATA REDUCTION AND ANALYSIS
Resting Binf will be recorded at a paper speed of 5 mm/sec. and values will be derived from the slope drawn at a best-fit tangent using the first 2-3 pulses. Calculations will be made as a function of 60 seconds divided by the horizontal distance (mm) needed for the
304 slope to rise vertically from baseline to the top of the recording paper and multiplied by the full chart range (15, 19). Forearm Binf following OCC will be recorded at a paper speed of 25 mm/sec. Analyses will be performed using a slope drawn at a best-fit tangent to the curves of the first 3-pulse flows post cuff release. Blood flows will then be calculated from 60 seconds multiplied by the paper speed divided by the horizontal distance (mm) needed for the volume slope to increase by 20 mm vertically (2, 3).
Forearm VR will then be calculated by dividing mean arterial pressure by the Binf.
Forearm Vcap will calculated as the vertical distance (mm) representing the change in forearm-volume graph following 6 minutes of venous filling (15). Forearm Vout will be derived from a tangent line representing the vertical drop in the volume-graph from the excursion line and drawn at 0.5 second and 2 second following the release of the venous occlusion pressure (15). All vascular indices will be expressed in ml•100ml tissue-1•min-
1.
2.4 STATISTICAL ANALYSIS
All statistical analysis will be completed using the SPSS statistical program (version
11.00). To examine the influence of 4-weeks of handgrip exercise training on arterial
reactivity (Hypothesis I) a 2 (Arms) * 2 (Pre and Post training) ANOVA will be used. To examine the influence of 4-weeks of handgrip exercise training on venous capacitance and venous outflow at rest and following 5-minutes of arm occlusion (Hypothesis II) a
second 2 (Arms) * 2 (Pre and Post training) ANOVA will be used. Depending on the
outcome of the 2 * 2 ANOVA Hypothesis III will be examined with a 1 (trained arm) * 5
(visits) repeated measure ANOVA to examine the time-course of the arterial and venous
changes. Finally a paired t-test will be used to compare between pre and post training
305 measures of HRV indices, body composition, and physical activity questioner. Alpha will
be set a-priori al p<0.05
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309 APPENDIX C: LETTERS OF PERMISSIONS
310 Sent by: [email protected] To: [email protected] cc: Subject: RE: Urgent_Permission request_JCR, dissertation Permission is granted to reproduce the requested material for use in your academic thesis/dissertation. Permission is granted provided a prominent credit line is placed stating the original source and copyright owner, Lippincott Williams & Wilkins.
Sincerely,
Arnetta G. Queen Permissions Editor
-----Original Message----- From: O'Brien, David Sent: Wednesday, September 03, 2003 1:34 PM To: Queen, Arnetta Subject: FW: Urgent_Permission request_JCR, dissertation Importance: High
-----Original Message----- From: Mahmoud A Alomari [mailto:[email protected]] Sent: Tuesday, September 02, 2003 5:00 PM To: [email protected]; [email protected] Subject: Urgent_Permission request_JCR
Dear Mr. O?Brien I am completing a doctoral dissertation at Louisiana State University entitled " ACUTE AND CHRONIC VASCULAR RESPONSES TO EXERCISE TRAINING ". Since I was a co-author, I would like your permission to reprint in my dissertation following article:
Welsch MA, Alomari M, Parish TR, Wood RH and Kalb D. Influence of venous function on exercise tolerance in chronic heart failure. J Cardiopulm Rehabil 22: 321-326., 2002.
The requested permission extends to any future revisions and editions of my dissertation, including non-exclusive world rights in all languages, and to the prospective publication of my dissertation by UMI Company. These rights will in no way restrict republication of the material in any other form by you or by others authorized by you. Your signing of this letter will also confirm that you own the copyright to the above-described material.
If these arrangements meet with your approval, please sign this letter and email me the permission for the use of this article.
311
Since the permission has to be included in my dissertation, I would prefer an electronic copy of the permission however a fax would also serve the purpose. I have been trying to communicate with your company regarding this permission, either by email or using your online permission form, since August/28. However I havnt received any communication and your online permission form has not been working. Therefore, a quick response would be highly appreciated ?.. I have to meet the graduate school deadlines (September/3/2003) and they need to review the dissertation before I can submit it. Thank you very much.
Sincerely,
Mahmoud Alomari,
PERMISSION GRANTED FOR THE USE REQUESTED ABOVE:
Signature:
Name:
Date: ______
312 Sent by: "[email protected]" Subj: Reprint permission Date: 8/29/2003 2:05:45 AM Pacific Daylight Time From: [email protected] To: [email protected] Sent from the Internet (Details) Dear Mr. Alomari, PERMISSION GRANTED provided that full credit is given to the original publication. ACUTE AND CHRONIC VASCULAR RESPONSES TO EXERCISE TRAINING, Int J Sports Med 22: 361-365, 2001. GEORG THIEME VERLAG. Kind regards Georg Thieme Publishers Margaretha Schoo Team 1 Ignat Mayzeles, Manager International Reprint Corporation (IRC) 287 East H Street Benicia, CA 94510, US Phone: (707) 746-8740 Fax: (707) 746-1643 313 314 VITA Mahmoud Alomari was born in 1968 in Habaka, a little village of Irbid in Jordan. He received his elementary through high school education in United Arab Emirates until 1986. Subsequently Mahmoud attended the Yarmouk University, at his native Irbid, and received a Bachelor of Science in physical education in 1990. In an effort to expand his horizon, he moved to the United States to attend the Minnesota (previously named Mankato) State University, Mankato, Minnesota, and finished a Master of Science degree in exercise physiology/cardiac rehabilitation in 1995. While pursuing his masters’, he received the International Student Endowment Scholarship for two consecutive years and served as teacher assistant for one year. Mahmoud started pursuing a doctorate in exercise physiology in 1997 at the Louisiana State University, Baton Rouge, Louisiana, and finished in 2003. During his tenure he served as research/teacher assistant at the Department of Kinesiology for four years and research associate at the Baton Rouge General Hospital for two years. Additionally, in 2003 he was recipient of the Corbett Graduate Summer Research Award from the Department of Kinesiology and the Southeast American Collage of Sports Medicine Student Research Award. Mahmoud will assume his responsibilities as a faculty member in the Department of Allied Health Science at the Jordan University of Science and Technology, Irbid, Jordan, starting October 2003. 315