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De sid e r io , M a r y A lic e

INTERACTIONS OF NEUROHYPOPHYSEAL, ADRENERGIC AND ESTROGENIC AGENTS ON THE CANINE CARDIOVASCULAR SYSTEM

The Ohio State University PH.D, 1980

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University M kronlm s international 30C \ '?== »D- AR90S wi 106'3!3: 7 6U700 INTERACTIONS OF NEUROHYPOPHYSEAL,

ADRENERGIC AND ESTROGENIC AGENTS

ON THE CANINE CARDIOVASCULAR SYSTEM

DISSERTATION

Presented in Partial Fulfillment of the

Requirements for the Degree Doctor of

Philosophy in the Graduate School

of the Ohio State University

By

Mary Alice Desiderio, B.S., M.S.

* * * * *

The Ohio State University

1980

Reading Committee: Approved By

Kenneth M. Hanson, Ph.D.

Lawrence T. Paul, Ph.D.

Margaret T. Nishikavara, Ph.D.

Milton A. Lessler, Ph.D. Adviser Department of Physiology ACKNOWLEDGEMENTS

John E. and Sandra K. Powell of The Ohio State University Hospital

Obstetrics and Gynecology Research Laboratories kindly performed the estrone and estradiol radioimmunoassays for us. The National

Institutes of Heart, Lung and Blood Disease and the Central Ohio

Chapter of the American Heart Association provided research support.

Smith, Kline and French provided generous supplies of Dibenamine.

I would like to extend a warm appreciation to Eileen 0. Enabnit and

Daniel Burchfield for their capable technical assistance and to

Lorry Ash for her excellent typing. I would also like to thank all the rest of you who made this possible and especially my adviser,

Kenneth Hanson, without whose patience, understanding and generosity,

I would not have survived.

ii VITA

November 26, 195^...... Born - Newark, Nev Jersey

1976...... B.S., Florida State University, Tallahassee, Florida

1977-1978 ...... Teaching Assistant, Department of Physiology, The Ohio State University, Columbus, Ohio

1978-1979 ...... Research Assistant, Department of Physiology, The Ohio State University, Columbus, Ohio

1979...... M.S., Department of Physiology, The Ohio State University, Columbus, Ohio

1979-198 0 ...... Teaching Assistant, Department of Physiology, The Ohio State University, Columbus, Ohio

PUBLICATIONS

Hanson, K.M., J.A. Post and M.A. Desiderio. Alpha-Adrenergic Blocking Agents and the Cardiovascular Response to Pharmacological Doses of Vasopressin. Pharmacology (in press).

Desiderio, M.A. and K.M. Hanson. Small Intestinal and Hepatic Arterial Vascular Responses to Oxytocin in the Canine. Ohio Journal of Science. April Program Abstracts. 80:Ul, 1980.

Desiderio, M.A. and K.M. Hanson. Mechanisms Involved in the Biphasic Response of the Canine Hepatic Artery to Vasopressin Infusion. Pharmacology (submitted for publication).

FIELDS OF STUDY

Major Field: Medical Physiology

Studies in Cardiovascular Physiology. Professor Kenneth M. Hanson

Studies in Cardiovascular Pharmacology. Professor Kenneth M. Hanson

iii TABLE OF CONTENTS

Page ACKNOWLEDGEMENTS...... ii

VITA ...... iii

LIST OF TABLES ...... ix

LIST OF FIGURES...... xi

INTRODUCTION ...... 1

PEPTIDE HORMONES OF THE HYPOTHALAMIC- NEUROHYPOPHYSEAL SYSTEM ...... 1

CARDIOVASCULAR EFFECTS OF VASOPRESSIN ...... 2

Pharmacologic applications and the nature of vasopressin tachyphylaxis...... 2 Interactions between vasopressin and adrenergic agents at peripheral vascular sites...... 6 Effects of vasopressin on the splanchnic vasculature...... * ...... 9

CARDIOVASCULAR EFFECTS OF OXYTOCIN...... 11

General effects on cardiac function and mean systemic arterial pressure ...... 11 Effects of oxytocin on renal blood flow and kidney function...... IT Effect of oxytocin on the coronary circulation and the influence of preservatives used in pharmaceutical preparations ...... 18 Effect of oxytocin on uterine blood flow...... 19 Effects of oxytocin on other peripheral vascular beds ...... 20 Interactions of oxytocin with adrenergic and ovarian agents upon the cardiovascular system...... 21 Objectives of the Present Study ...... 26

iv Page METHODS

GENERAL DESCRIPTION...... * ...... 28

SUPERIOR (CRANIAL) MESENTERIC ARTERIAL VASCULATURE...... 29

HEPATIC ARTERIAL VASCULATURE AND SHUNTING OF PORTAL VENOUS FL O W ...... 31

PRESENTATION OF DATA...... 32

RESULTS

INTERACTIONS BETWEEN ADRENERGIC BLOCKING AGENTS AND THE RESPONSE OF THE INTESTINAL VASCULATURE TO PHARMACOLOGIC DOSES OF VASOPRESSIN...... * ...... 33

Effect of i.v. administered phentolamine (Regitine) upon the response of the superior mesenteric arterial vasculature to vasopressin infusion...... 33 Effect of i.v. administered Dihenamine upon the response of the isolated, autoperfused, small intestine segment to vasopressin infusion...... 35 Effect of i.a. administered Dibenamine upon the response of the isolated, autoperfused, small intestine segment to vasopressin infusion...... 1+1

EFFECTS OF PHENTOLAMINE AND DIBENAMINE UPON THE GENERAL SYSTEMIC CARDIOVASCULAR RESPONSE TO VASOPRESSIN INFUSION...... 1+5

Alpha-adrenergic blockade vith i.v. phentolamine and the mean systemic arterial pressure response to vasopressin infusion...... 1+5 Alpha-adrenergic blockade vith i.v. dibenamine and the mean systemic arterial pressure response to vasopressin infusion...... 1+8

v Page Effect of the i.a. administration of dibenamine on the response of mean systemic arterial pressure to vaso­ pressin infusion...... 53 Correlation between initial mean systemic arterial, pressure and extent of its increase following vasopressin infusion...... 53 Effect of alpha-adrenergic blocking agents on the response of heart rate to vasopressin .... 56

EFFECT OF NOREPINEPHRINE UPON THE PRESSOR RESPONSE TO VASOPRESSIN...... 56

THE ROLE OF VASOPRESSIN TACHYPHYLAXIS IN THE EXPERIMENTAL DESIGN...... 58

HEPATIC HEMODYNAMIC RESPONSE TO THE I.V. INFUSION OF VASOPRESSIN...... 6h

Effect of alpha-adrenergic blockade with dibenamine on the hepatic arterial response to i.v. infusion of vasopressin with portal venous flow intact ...... 6h Effect of alpha-adrenergic blockade with dibenamine on the hepatic arterial response to i.v. infusion of vasopressin with portal venous flow shunted into the systemic venous system...... T3 Effect of alpha-adrenergic blocking agents on the response of mean systemic arterial pressure and portal venous pressure to i.v. infusion of vasopressin...... 75

THE CARDIOVASCULAR EFFECTS OF I.V. INFUSION OF OXYTOCIN...... 78

Effect of I.v. infusion of oxytocin on mean systemic arterial pressure ...... 78 Effect of i.v. infusion of oxytocin on the vasculature of the isolated dog small intestine segment...... 86 Effect of i.v. infusion of oxytocin on heart r a t e ...... 101

THE CARDIOVASCULAR EFFECTS OF I.V. INFUSION OF OXYTOCIN IN MALE DOGS PRETREATED WITH ESTRONE...... 101 vi Page Estrone and estradiol plasma levels after chronic estrone pretreatment ...... 101 Effect of estrone pretreatment on the response of mean systemic arterial pressure to i.v. oxytocin infusion ...... 10U Effect of estrone pretreatment on the vascular response of isolated dog small intestine segments to i.v. oxytocin infusion...... 1 0 U

EFFECTS OF I.V. INFUSION OF OXYTOCIN ON HEPATIC ARTERIAL HEMODYNAMICS AND ON PORTAL VENOUS PRESSURE...... 107

DISCUSSION

INTERACTIONS BETWEEN ADRENERGIC MECHANISMS AND THE CARDIOVASCULAR RESPONSE TO VASOPRESSIN ...... 11U

Introduction and significance...... 11U Effect of adrenergic blocking agents on the pressor response to vasopressin...... 115 Effect of alpha-adrenergic blockade on the response of specific vascular beds to vasopressin...... 120 Role of vasopressin tachyphylaxis in the Interpretation of experimental findings...... 122 Interactions between catecholamines and neurohypophyseal hormones at peripheral vascular sites ...... 123 Summary of findings in the present s t u d y ...... 126

RESPONSE OF THE HEPATIC ARTERIAL VASCULATURE TO THE ADMINISTRATION OF VASOPRESSIN...... 127

Introduction and significance...... 127 Responses of the hepatic artery to various modes of vasopressin infusion...... 130 Effect of alpha-adrenergic blockade with dibenamine on the response of hepatic hemodynamics to vasopressin infusion with intact portal venous flow...... 132 Effect of alpha-adrenergic blockade with dibenamine on the response of hepatic hemodynamics to vasopressin infusion during shunting of portal flow into the systemic venous system...... 135 Summary and conclusions from present study ...... 137

vii Page THE CARDIOVASCULAR EFFECTS OF OXYTOCIN...... 138

Introduction and significance...... 138 Effects of i.v. oxytocin on mean systemic arterial pressure...... 139 Effect of oxytocin on heart r a t e ...... lU2 Effect of oxytocin on the peripheral vasculature...... lU3 Effects of i.v. oxytocin infusion upon the canine splanchnic vasculature as observed in the present s t u d y ...... 1^6 Effect of estrogens on the cardiovascular response to oxytocin...... 150

viii LIST OF TABLES

Table Page

1. Effect of phentolamine (O.U mg/kg) on the response of superior mesenteric arterial vasculature to i.v. vasopressin ...... 38

2. Effect of dibenamine (20 mg/kg) on the response of small intestine segment vasculature to i.v. vasopressin...... UU

3. Effect of i.a. dibenamine {8 mg/kg) on the small intestinal segment vasculature to a 10 min i.v. infusion of vasopressin (7 6U mU/min)...... U7

U. Effect of phentolamine (O.U mg/kg) on the response of mean systemic arterial pressure to i.v. vaso­ pressin ...... 50

5. Effect of i.v. dibenamine (20 mg/kg) on the response of mean systemic arterial pressure to i.v, vaso­ pressin infusion * . . 52

6 . Effect of vasopressin on heart rate (beats/min) before and after phentolamine (O.U mg/kg) ...... 57

7. Changes in mean systemic arterial pressure (Pg) as seen in two successive 10 min i.v. infusions of vasopressin...... 60

8. Changes in isolated small intestinal segment vascular resistance (R) as seen in two successive i.v. infusions of vasopressin...... 62

9- Effects of 10 min i.v. infusions of vasopressin on hepatic artery flow (Fj^), perfusion pressure (Pjj^) and resistance (R^J before and after dibenamine...... 68

10. Relative changes in hepatic artery flow ( , perfusion pressure (?Pha) and resistance during 10 min i.v. infusions of vasopressin before and after dibenamine...... 7 U

ix Table Page

11. Relative changes in hepatic artery flow (%FjjaK perfusion pressure (SPha.) and resistance (/SRjja) during 10 min i.v. infusion of vasopressin (?6U mU/min) before and after dibenamine and with portal venous flow shunted...... 77

12. Effect of i.v. dibenamine (20 mg/kg) on the response of portal venous pressure (Ppy) to i.v. infusion of vasopressin...... 80

13. Effect of 10 min i.v. infusion of oxytocin on mean systemic arterial pressure (Ps» mmHg) ...... 83

lU. Effect of 10 min i.v. infusion of oxytocin on gut segment arterial perfusion pressure (Pa* mmHg).... 88

15. Effect of 10 min i.v. infusion of oxytocin on gut segment blood flow (ml/min* lOOg) ...... 93

16. Effect of 10 min i.v. infusion of oxytocin on gut segment vascular resistance (R, mmHg/ml-min*100g). . . 97

17- Estrone and estradiole plasma levels (picograms/ml) before and after estrone U mg/day i.m. for 5 to 7 d a y s ...... 103

18. Effect of estrone pretreatment on the response of mean systemic arterial pressure (Ps) to 10 min i.v. oxytocin at 0 -9 5 U / m i n ...... 106

19. Effect of estrone pretreatment on the response of gut segment vascular resistance (R) to 10 min i.v. infusion of oxytocin at 0.95 U/min...... 108

20. Effect of i.v. infusion of oxytocin on hepatic artery flow (Fjja)...... 109

21. Effect of i.v. infusion of oxytocin on hepatic artery perfusion pressure (Ph a ^......

22. Effect of i.v. infusion of oxytocin on hepatic artery resistance (Rjia^...... m

23. Effect of i.v. infusion of oxytocin on portal venous pressure (Ppy)...... 118

x LIST OF FIGURES

Figures Page

1. The response of mean systemic arterial pressure (Pg) and superior mesenteric vascular resistance (R) to 5 min i.v. infusion of norepinephrine (20 pg/min) "both before and after alpha-adrenergic blockade with phentolamine (i.v., O.U mg/kg)...... 3U

2. The response of superior mesenteric resistance (R) to 10 min i.v. infusion of vasopressin at four different dose rates before and after alpha-adrenergic blockade with i.v, phentolamine (O.U m g / k g ) ...... 36

3. The mean (+^ S.E.) percent change in resistance (&AR) after 10 min i.v. infusion of vasopressin at U8, 95, 191, 76U and 1528 mU/min before and after alpha-adrenergic blockade with phentolamine (O.U m g / k g ) ...... 37

U. The mean (+_ S.E.) percent changes in mean systemic arterial pressure (^AFg) and gut segment vascular resistance ($AR) during 5 min i.v. infusion of norepinephrine (20 pg/min) both before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg)...... Uo

5. The response of gut segment vascular resistance (R) to 10 min infusion of vasopressin at three dose rates both before and after alpha-adrenergic blockade with i.v. dibenamine (2 0 mg/kg)...... U2

6 . The mean percent change (+ S.E.) in gut segment vascular resistance (AR^) to 10 min i.v. vasopressin infusion at 95, 191 and 76U mU/min before and after alpha-adrenergic blockade with i.v. dibenamine ( 20 mg/kg)...... U3

7. The responses of gut segment blood flow (F) and gut segment vascular resistance (R) to 10 min i.v. infusion of vasopressin (76U mU/min) both before and after alpha-adrenergic blockade with i.a. dibenamine (3 mg/kg)...... U6 Figure Page

8. The response of mean systemic arterial pressure (Pc) to 10 min i.v. infusions of vasopressin at 5 dose rates both before and after alpha-adrenergic blockade with i.v. phentolamine (O.U mg/kg)...... U9

9. The response of mean systemic arterial pressure to 10 min i.v. infusion of vasopressin at three dose rates both before and after alpha-adrenergic blockade with i.v. dibenamine (2 0 mg/kg)...... 51

10. Correlation (r) between mean systemic arterial pressure (Pg) existing before the onset of vasopressin infusion (initial Pg) and its percent change (£APg) after 10 min i.v, infusion of vasopressin (T6U mU/min) under four different experimental conditions ...... 55

11. The response of mean systemic arterial pressure (Pg) and gut segment vascular resistance (R) to vasopressin alone 191 (mU/min), norepinephrine alone (19 ug/min) and both simultaneously...... 59

12. The mean {+_ S.E.) percent changes in mean systemic arterial pressure (^APg) after two successive 10 min infusions of vasopressin at dose rates of 9 5, 1 9 1* 382 and 76U mU/min. The first and second infusions were separated by a recovery period of approximately 60 m i n ...... 61

13. The mean percent (+_ S.E.) changes in gut segment vascular resistance (iSAR) after two successive 10 min infusions of vasopressin at dose rates of 95, 191, 382 and J6h mU/min. The first and second infusions were separated by a recovery period of about 60 m i n ...... 63

lU. The mean (+ S.E.) changes in systemic arterial pressure (Pg), portal venous pressure (Ppy), hepatic artery flow ( hepatic artery resistance (^HA^ during 10 min i.v. infusions of vasopressin at 382 mU/min before and after alpha-adrenergic blockade with i.v. dibenamine (20 m g / k g ) ...... 66

15. The mean (+_ S.E.) changes in systemic arterial pressure (Pg), portal venous pressure (Ppy), hepatic artery flow (Fj^) and hepatic artery resistance (Rj£a ) during 10 min i.v. infusions of vasopressin at j6k mU/min before and after alpha-adrenergic blockade with i.v. dibenamine (2 0 mg/kg)...... 67 xii Figure Page

l6 . The mean (+_ S.E.) percent changes in hepatic artery perfusion pressure (^APj^) throughout 10 min i.v. vasopressin infusion at dose rates of 332 and 7 6k mU/min both before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg)...... 69

17- The mean (_+ S.E.) percent changes in hepatic artery blood flow (^AFj™) to vasopressin infusion at dose rates of 382 and 76^ mU/min with before and after alpha-adrenergic blockade with i.v, dibenamine (20 mg/kg)...... 71

1 8. The mean (+_ S.E.) percent change in hepatic artery resistance ($ARi™) during vasopressin infusion at dose rates of 382 and 76k mU/min both before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg)...... 72

19. The mean (+_ S.E.) percent changes in hepatic artery resistance (JJARg^) to 10 min i.v. infusion of ■ vasopressin (761 raU/min) under four different experimental conditions ...... 76

20. The mean (+_ S.E.) percent change in mean systemic arterial pressure (^APg) to 10 min i.v. infusion of vasopressin at dose rates of 382 mU/min and 761+ mU/min both before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg)...... 79

21. The response of mean systemic arterial pressure (P3 ) to two successive 10 min i.v. infusions of oxytocin at dose rates of 0.95, 1-9* 3.8 and 7.6 U/min. The first and second infusions were separated by a recovery period of approximately 60 m i n ...... 8l

22. The mean (_+ S.E.) percent change in mean systemic arterial pressure (/SAPg) to two successive i.v. infusions of oxytocin at dose rates of 0 -9 5, 1 .9* 3.8 and 7*6 U/min. The first and second infusions were separated by a recovery period of approximately one hour...... 8U

23. Dose-response curves showing the mean (+ S.E.) percent changes in mean systemic arterial pressure (?SAPg) after 3 min i.v. infusions of oxytocin at U dose rates (0.95* 1.9, 3.8 and 7.6 U/min). The second infusion followed the first by approximately one hour...... 05

xiii Figure Page

2U. The response of gut segment arterial perfusion pressure (Pa^ ^wo successive i.v. infusions of oxytocin at four dose rates (0 .9 5* 1 *9* 3 .8 and 7-6 U/min). The second infusion followed the first by approximately one hour...... 8?

25. The mean (_+ S.E.) percent change in gut segment arterial perfusion pressure to two successive i.v. infusions of oxytocin at four dose rates (0.95, 1.9, 3.8 and 7.6 U/min). One hour recovery period between the first and second infusions was allowed...... 89

26. The dose-response curves showing mean (_+ S.E.) percent changes in arterial perfusion pressure (?SAPa) at 3 and 10 min during both the first and second i.v, oxytocin infusions at dose rates of 0.95, 1.9. 3.8 and 7.6 U/min. A recovery period of approximately one hour was allowed between the first and second infusions...... 90

27- The response of gut segment blopd flow to two 10 min i.v. infusions of oxytocin at 0 .9 5* 1 *9» 3.8 and 7.6 U/min. The second infusion was given approximately one hour after the first ...... 92

28. The mean (+ S.E.) percent change in gut segment blood flow to two 10 min i.v. oxytocin infusions at dose rates of 0.95* 1>9» 3.8 and 7.6 U/min. The first and second infusions were separated by a recovery period of approximately one hour...... 9^

2 9. The response of gut segment vascular resistance (R) to two 10 rain i.v. infusions of oxytocin at dose rates of 0.95* 1.9* 3.8 and 7-6 U/min. The second infusion followed the first by approximately one hour...... 96

30. Mean percent (+ S.E.) change in gut segment resistance (?*AR) to the first 10 min i.v. infusion of oxytocin at dose rates of 0.95* 1-9* 3 .8 and 7- 6 U/min...... 98

31. The mean (+_ S.E.) percent change in gut segment resistance (5SAR) to the second 10 min i.v. infusion of oxytocin at the dose rates, 0 .9 5, 1 -9 . 3 .8 and 7-6 U/min...... 99

xiv Figure Page

32. Dose-response curves showing mean (+ S.E.) percent changes in gut segment resistance (J?AR) after 3 and 10 min during both the first and second i.v. oxytocin infusions at dose rates of 0 .9 5, 1 -9 1, 3 .8 and 7 .6 U/min. The second infusion followed the first by approximately one h o u r ...... 100

33. Dose-response curves for the mean (+ S.E.) percent change in heart rate ( heart rateT after two 10 min i.v. oxytocin infusions at dose rates of 0.95, 1-9, 3.8 and 7.6 U/min. The second infusion followed approximately one hour after the first. . . . 102

3^. Plasma estrone levels (pg/ml) in dog #3 over the course of his estrone treatment for 8 d a y s ...... 105

xv INTRODUCTION

Peptide Hormones of the Hypothalamic - Neurohypophyseal System

The two major hormones of the posterior pituitary gland, oxytocin and vasopressin, are synthesized in the cell bodies of neurons in the supraoptic and paraventricular nuclei, respectively, and transported down their axons to the posterior pituitary. In all mammals studied, except the members of the pig family, the hormones are oxytocin and arginine vasopressin. Arginine in the vasopressin molecule is replaced by lysine to form lysine vasopressin in the pig and hippopotamus. The pharmaceutical vasopressin preparation used in human medicine (Pitressin) is lysine vasopressin. The posterior lobe hormones are nonapeptides with a disulfide ring at one end (refer to reference 78 for structures):

s.

Cys - Tyr - Phe - Gin - Asn - Cys - Pro - Arg - Gly - NH^

123^56789

Arginine Vasopressin

■s-

Cys - Tyr - lie - Gin - Asn - Cys - Pro - Leu - Gly - NHg

I23U56789

Oxytocin

1 2

The neurohypophysis of the chicken, some other birds, reptiles and amphibia, contains a third hormone, vasotocin, which is different from the mammalian oxytocin in that arginine is substituted for O leucine at position eight forming arginine oxytocin.

The principal physiologic effect of vasopressin is to increase the permeability of the collecting ducts to water; thereby, conserving water and concentrating the urine. Vasopressin also exerts some physiological effect on the cardiovascular system, however. The reflex release of vasopressin in amounts large enough to have a cardiovascular effect has been experimentally observed in response to carotid occlusion and decreases in blood pressure as small as

25 mmHg following hemorrhage (2 0 ,6 8).

The primary physiological effect of oxytocin is on the myo­ epithelial cells, smooth muscle cells that line the ducts of the breast. This hormone affects the contraction of these cells, squeezing the milk out of the alveoli of the lactating breast into the large ducts (sinuses), and then out the nipple (78). Another important action of oxytocin is contraction of the smooth muscle of the uterus in

late pregnancy (7 8).

Cardiovascular Effects of Vasopressin

Pharmacologic applications and the nature of vasopressin tachy­

phylaxis -

Vasopressin in pharmacologic doses is a pressor agent and

has a potent vasoconstrictor action on many vascular beds. It is

particularly potent in this respect on the coronary (2 8,^3 ) and mesenteric

(29,^2) arterial vessels. This latter effect has led to its clinical use 3 in portal hypertension and in the relief of gastrointestinal bleeding.

Vasopressin also results in a decrease in portal pressure and flow. This effect has been ascribed to vasoconstriction in the gastrointestinal trijict where vasopressin infusion has been shown to produce a sustained in­ crease in vascular resistance (U2 ) with no evidence of autoregulatory escape as is seen during norepinephrine infusion (29). Heimburger, et al. reported (U6 ) a quantitatively similar response of portal pressure and flow after each of a series of several U unit doses of Pituitrin (posterior pituitary hormone) spaced 10 minutes apart. Drapanas, et al. (28) report a sustained reduction in hepatic artery, portal vein and superior mesenteric artery flows as well as cardiac output during a period of Pitressin (vaso­ pressin) infusion in dogs. Thus, in these studies, there was no evidence of escape or tachyphylaxis (tachyphylaxis is a term used to describe the decreasing responses to a drug which follow consecutive injections of that drug made at short intervals).

A type of tachyphylaxis to some of the cardiovascular responses pro­ duced by vasopressin preparations has been reported in most studies when

it was given as a series of single bolus injections or even as a continuous

infusion of several minutes duration. The effect of vasopressin on hepatic artery flow has been reported as an increase (2 1 ) or as a biphasic response consisting of an initial decrease (vasoconstriction)

followed by an increase (1+2). It has been suggested that at least,

in part, this secondary dilation of the hepatic artery occurs in response

to a reduction in portal venous perfusion of the liver (1+2 ,U1+).

Razzak and Naguib (80) found total liver blood flow and portal pressure

reduced less and systemic arterial pressure increased less following a

series of single injections of 3 to 5 units of synthetic vasopressin.

During a 10 min infusion (l unit/min) of the drug they (80) found 1+ recovery from the initially reduced portal pressure and liver "blood flow while systemic arterial pressure continued to rise throughout the infusion. Response to a second such infusion, however, was minimal, thus showing tachyphylaxis. This would limit the use of vasopressin as an agent for long term control of portal hypertension.

The systemic pressor effect of vasopressin has also "been reported in other studies (2 8) to "be somewhat transient during infusion of the drug and thought to he the result of depressed cardiac output which might lower blood pressure despite increased total peripheral resistance.

No cogent explanation for the various types of vasopressin tachyphylaxis is at hand. Some workers have attempted to explain it solely on the basis of a central reflex mechanism.

Gardier and coworkers (36) have studied the effects of baroreceptor activity upon the response to vasopressin. Near physiological doses of vasopressin (1-10 mU/kg) in dogs with bilateral circulatory isolation of the carotid sinuses gave transient pressor responses of 15-20 mmHg.

When the influences of both carotid and aortic baroreceptors was eliminated by carotid sinus denervation or by maintaining constant pressure in them, plus performing bilateral midcervical vagotomy, these

same relatively small doses of vasopressin produced prolonged pressor responses of U0-50 mmHg, thus eliminating vasopressin tachyphylaxis.

They conclude that when the baroreceptors are intact, there is a high

level of sympathetic activity and even pharmacological doses of vaso­ pressin may produce only modest and rather transient increases in

systemic arterial pressure. Gardier and coworkers (36) further recorded

neural activity and found that following a dose of vasopressin there was an antagonistic decrease in sympathetic tone. Why this antagonism

should become more effective in attenuating the pressor response with each succeeding dose is not clear. They ( 36 ) suggest that perhaps the progressive tolerance usually associated with vasopressin tachy­ phylaxis can he justified on the basis of a serial increase in sensitivity of the carotid sinus buffer mechanism.

Nash ( 72 ) has studied the systemic and coronary effects of repeated doses of vasopressin. He found that tachyphylaxis seen during the vasopressin response was reduced by reserpine pretreatment.

It is well known that pretreatment with reserpine will virtually eliminate the peripheral function of the sympathetic nervous system perhaps by its catecholamine depletion action on sympathetic nerve terminals allowing complete enzymatic destruction of catecholamine by catechol O-methyl transferase (COM1). Elimination of sympathetic tone then might cause increased blood pressure responses to vasopressin through loss of reflex action, and hence account for less evidence of tachyphylaxis. Nash ( 72 ) found that there was less loss of myocardial contractile force and less coronary vasoconstriction in response to vasopressin administration following reserpine pretreatment. Thus it was concluded that reserpine induced depletion of norepinephrine permitted the coronary vasoconstrictor effects of vasopressin to wane faster resulting in a more forceful myocardial contraction and less reduction in coronary blood flow after repeated doses. Hence, there was less loss of cardiac output and less loss of systemic pressor response seen with repeated doses of vasopressin.

Here we see vasopressin tachyphylaxis in the coronary circulation tending to oppose tachyphylaxis in the systemic arterial pressor response. As to an alternate mechanism which might he involved in the

effect of reserpine on the vasopressin response, it was postulated

( 12 ) that reserpine acts directly to alter the affinity of the

coronary vascular tissue for vasopressin or to reduce the binding of vasopressin leaving the tissue free to combine with the next dose of vasopressin. Reasons for the primary events, however, are still not known.

Another possible mechanism responsible for this ability of reserpine to relieve vasopressin tachyphylaxis has been suggested on the basis of results in experiments utilizing in vivo isolated aortic strips (8). It has been concluded (8) that there is a competitive

inhibition of vasopressin by norepinephrine. The degradative action

of COMT on norepinephrine mediated through the catecholamine depletion

effect of reserpine on synaptic terminals, frees this common receptor

enabling it to bind with vasopressin more readily in the absence of

competing norepinephrine. Several other substances including the

catechol O-methyl transferase inhibitor, pyrogallol (7 3 ) have been

reported to alleviate tachyphylaxis to the pressor response. Thus

inhibition of norepinephrine catabolism on the one hand and depletion

of its tissue stores with reserpine on the other appear to have

similar effects.

Interactions between vasopressin and adrenergic agents at peri­

pheral vascular sites —

The effect of sympatholytic agents upon the response to vaso­

pressin infusion has received considerable research attention.

Adrenergic blocking agents have been reported to enhance (30,80), attenuate ( 7 6 ,8 9) or have no effect ( 89 ) on the pressor action of vasopressin. Most of these studies have focused upon the changes in systemic arterial pressure and have, for the most part, not explored the behavior of specific vascular beds. Other studies report that vasopressin enhances the pressor response to adrenergic stimulation

( 8,U8 ) or activation of the autonomic system. On the other hand, adrenergic stimulation has been said to sensitize the vasculature to the constrictor action of vasopressin (19 ). The mechanisms responsible for any of these observations remain somewhat obscure.

Traber, Gary and Gardier (91 ), however, presented yet another finding in dogs which had been subjected to carotid sinus isolation and vagotomy. The alpha-adrenergic blockers, phentolamine and phenoxybenzamine significantly decreased the vasopressin pressor response in the absence of baroreceptor influence. When alpha- adrenergic blockade is found to decrease the vasoconstrictor response * to vasopressin, the results become consistent with the idea that part of the vasopressin effect is due to a release of norepinephrine which might add to or even potentiate the action of vasopressin on the vasculature ( 36,91 )•

Erker and Chan ( 30 ) present data conflicting with this concept in that they found rats treated with phenoxybenzamine or phentolamine had a much higher sensitivity to the pressor effects of vasopressin and oxytocin. If adrenergic blocking agents increase the pressor response to vasopressin as in this particular study, the situation is a bit more difficult to reconcile. Again it was suggested that there is a population of receptors for vasoconstriction which will 8 accept either vasopressin or norepinephrine. Competitive inhibition of vasopressin by norepinephrine has been suggested (30 ). The adrenergic blocking agent might protect these sites from binding with norepinephrine rather than with vasopressin. Adrenergic blocking agents would also act to abolish the influence of the carotid baroreceptor mechanism and the pressor response to vasopressin might be enhanced due to loss of the buffering action of the carotid baroreceptors.

Vasopressin also seems to interact with receptors for vasodilation.

Nash, et al. (7^ ) found that in the dog vasopressin reversed isopro­ terenol induced vasodilation in the femoral arterial vascular bed and potentiated the pressor response to epinephrine. This effect might be explained on the basis of selective blockade of peripheral vasodilating receptors without blocking cardiac stimulation. Whether or not beta receptors in the heart and beta receptors in peripheral vessels have equal affinity for vasopressin remains to be seen. In this study, however, there was no marked change in the heart rate effects of catecholamines following vasopressin nor did myocardial contractile force responses appear to be affected, thus offering no substantiation for a competitive binding mechanism with beta receptors in cardiac muscle as might exist with peripheral vasodilator and vasoconstrictor receptors in vascular smooth muscle (30,7^ ). Skivolocki and Thomford ( 8U ) advise, on the basis of their results in dogs, the combined administration of vaso­ pressin and isoproterenol in the treatment of acute gastrointestinal hemorrhage. The tendency for vasopressin to depress cardiac output was overcome and the systemic pressor response was attenuated while the

desired reduction of portal pressure was achieved. To this might 9 further be added the comment of another group of workers ( 8 ) that

"in a limited number of experiments the effect of vasopressin on the vascular response to isoproterenol while not predictable, did produce an increased depressor response in several runs." It would seem from much of these findings that the absence or presence of a pronounced pressor response to vasopressin might depend on the degree of stimulation of the baroreceptors and their subsequent effect on sympathetic output, hence on blood pressure.

Effects of vasopressin on the splanchnic vasculature -

Vasopressin is routinely given in pharmacologic doses for the emergency treatment of gastrointestinal bleeding, particularly when it is the result of portal hypertension. As was mentioned before, vaso­ pressin exerts a potent vasoconstrictor action upon the precapillary vessels of the mesenteric circulation, thereby reducing portal pressure.

Thus, if its vasoconstrictor effect upon the prehepatic splanchnic arterial vessels were to be enhanced by combination with some other pharmacologic agent, for example, an alpha-adrenergic blocking agent or even norepinephrine, then its therapeutic efficacy in the reduction of portal hypertension might be improved.

Since it is well documented that vasopressin induces sustained vasoconstriction in the superior mesenteric vascular bed with little or no autoregulatory escape, considerable attention has been given to the possibility of ischemia of the liver if the hepatic artery were to be constricted to the same extent as the pre-hepatic splanchnic vasculature. In this regard, the infusion of vasopressin for the control of gastrointestinal hemorrhage might be severely damaging. Vasopressin, however* when given in a variety of dose regimens, has since been re­ ported to produce a dilation of the hepatic artery. In some studies this dilation was preceeded by a transient constriction {7 *2 1 ,3 2 ,U2 ,5 3,5 7 *8 3)

The mechanisms responsible for the transient vasoconstriction and also the vasodilation have yet to be elucidated. As was previously mentioned, an intrinsic hepatic artery vasodilation Is said to occur as a consequence of decreased portal venous flow (UU). Thus* during vasopressin infusion con­ striction of the hepatic artery in response to vasopressin might be offset by this dilator input resulting from the decrease in portal perfusion.

According to this view, the initial constrictor component would only be seen when vasopressin infusions are made directly into the hepatic artery or by another route at a fast enough rate to allow an adequate drug con­ centration to reach the site such that transient constriction occurs before dilation supervenes. Cohen, et al, (21) report data on the response of anesthetized cats to 5 min i.v, infusions of vasopressin which appear to support this concept. Doses ranged from 0.5 to 50 mU/min*kg (milli

International Units). Optimum hepatic arterial vasodilation was seen with a dose of 10 mU/min*kg, while after 5 min infusion at higher

rates vasoconstriction became more evident and the net effect was

only a slight dilation. Lechin and coworkers (57) found a somewhat biphasic response of hepatic artery flow to single i.v. bolus injections

of 100 mU/kg vasopressin in dogs. It was observed that elevated hepatic

artery flow persisted even after full recovery from the reduction in

portal flow. They conclude that some factor other than decreased portal

flow must, in part, be responsible for the secondary hepatic artery

vasodilation. Some workers have suggested an accumulation of vasodilator 11 metabolites as a consequence of reduced liver blood flow (1+2 ) or other

"autoregulatory escape" mechanisms (53,57 ).

Another possible mechanism mediating the vasodilator component of the response might be the result of increased blood flow shifted from other areas which sire more intensely vasoconstricted by a direct and sustained action of vasopressin than is the hepatic artery (the

"steal" concept).

Cardiovascular Effects of Oxytocin

General effects on cardiac function and mean systemic arterial

pressure -

In some situations, oxytocin also has been shown to have cardiovascular effects even in doses as low as 5 mU/kg. There are few data concerning the quantitative effects of oxytocin on specific vascular beds. In most cases observations on its effects have been limited to changes in systemic arterial pressure. The cardiovascular response to oxytocin seems to vary with species, sex, reproductive status and other previous history of the animal. The matter is further complicated by the presence of an enzyme, oxytocinase, the level of which Increases progressively in the blood plasma during pregnancy, from an insignificant or very low level immediately after conception to a maximum at the time of delivery. Oxytocinase is an aminopeptidase capable of splitting amino-terminal half-cysteine residues from the neurohypophyseal hormones. Assali at al. ( U ) have shown that changes in the blood levels of oxytocinase during pregnancy in the sheep do not correlate with the Increased sensitivity of the uterus to oxytocin during labor. The concentration of this enzyme is comparatively low 12 in the sheep. In pregnant women (90), the changes in oxytocinase levels also do not correlate well with the onset of labor. Oxytocinase, therefore, is an enzyme the particular function of which has not as yet been elucidated and part of the difficulty arises from the fact that the enzyme is not present in all animals ana is present in varying concentrations in animals where it has been found.

Oxytocin is available commercially as preservative-free synthetic oxytocin powder and as a pharmaceutical preparation of synthetic oxytocin with ethanol and chlorobutanol as preservatives (Syntocinon or Pitocin).

It has been shown that an interaction occurs between oxytocin and chlorobutananol, thus masking the true effects of pure oxytocin (33).

The use of Syntocinon and pure synthetic oxytocin by various researchers therefore, has resulted in conflicting data. These findings and their implications will be discussed in detail later.

Several investigators have studied the effect of oxytocin on the cardiovascular system of man, rats, cats, dogs and other species. There

is general agreement among workers that oxytocin exerts a positive chronotropic effect in dogs (71) and in man (3,8,10,1U, 1+7,51,52,5^»55,58,81) where the magnitude of the increase in heart rate is slight to moderate.

In anesthetized dogs, the increase in heart rate is associated with an

increase in myocardial contractile force and a decrease in systemic arterial pressure (71)* Nakano and Fisher (7l) founa that oxytocin

(0.05-1.6 U/kg i.v.; did not increase the heart rate in reserpinized

dogs (male or female) or in propranolol treated dogs. They (71) con­

cluded from this that the increase in heart rate induced by oxytocin

is not due to a direct positive chronotropic action on the heart but 13 to a reflex sympathetic stimulation elicited by its depressor effect.

Katz ( 52 ) on the other hand, concluded that the increase in heart rate is not due to a baroreceptor reflex mechanism since tachycardia occurs whether systemic arterial pressure decreases or not.

In conscious human subjects, Brotanek and Kazda ( 13 ) found that

3 Units of oxytocin given i.v. increased heart rate while in patients with general anesthesia (thiopentol), it was found to have no effect.

In rabbits anesthetized with morphine, Woodbury and Abreu (93 ) found that synthetic oxytocin, namely Pitocin (oxytocin plus chlorobutanol and ethanol added as preservatives) in an Intravenous injection of up to 15 U/kg, decreased heart rate markedly. Katz (52 ) showed that the intravenous injection of 20 U/kg synthetic oxytocin (with chlorobutanol only) into anesthetized cats caused a reduction in heart rate. However, no change in heart rate occurred when pure synthetic oxytocin (without chlorobutanol) was given to cats ( 52 ). The decreased heart rate observed in cats and rabbits might not have been due to a difference among species but rather to experimental artifacts; the presence of chlorobutanol or other additives (52,92).

There have been many studies on the effect of synthetic oxytocin on systemic arterial pressure using the chicken (1 5,22)*rat (5 9,6 0,6 9)* guinea pig (7 2 ), opossum (7 2 ), rabbit (7 2 ), cat (52,93)»sheep (4 ), dog

(4,12,23,31,62,72,93), monkey (1 8) and man (3,8,10,13,14,15,47,51,52,54,55,

58,67,79,01,85 )* Some have found that Pitocin or other oxytocin preparations produced no significant change in systemic arterial pressure

in man (67,79), rats (57,6 0), dogs (4 ), and sheep (4 ), while others

observed an increase in man (15,47,85), rats (5 8,5 9,6 1 ,6 9), dogs (23,31,85),

sheep (4 ), rabbits (6 9), opossum (6 9) and guinea pigs (6 9). In n* several other studies, o^tocin vas found to decrease systemic arterial pressure in man (3,8,10,lU,15,31,U7,51,52,56,67,79,01)* rabbits (05), cats ( 52,93), dogs (62,72,93), avians (15,22 ) and in rats (6 0 ).

Thus, there exists a great deal of qualitative variation in the cardiovascular effects of oxytocin as reported in the literature.

Other investigators have noted a biphasic change in systemic arterial pressure in man with single injections of oxytocin in doses ranging from 3 to 20 U i.v. (8,10,13,51*56 )* This response consisted of a transient decrease in systemic arterial pressure followed by a slight, sustained increase in pressure over control values. On the other hand, Nakano and Fisher (71) reported dose related decreases in mean arterial blood pressure in male dogs with intravenous doses of synthetic oxytocin ranging from ,05-1.6 U/kg. Kitchen, Konsett and

Pickford (5M found that single intravenous injections or a constant infusion of oxytocin in as little as 200 mU produced a slight depressor effect in man. Moreover, in some pregnant and non-pregnant women, a mild or moderate drop in arterial blood pressure and tachycardia was observed with a single intravenous injection of 10 U synthetic oxytocin

(66 ).

Woodbury and Abreu (93) have postulated that the hypotensive action of oxytocin as seen in their experiments with dogs is due to its myo­ cardial depressant action which results in a reduction in cardiac output.

Nakano and Fisher (72) report that a single intravenous injection of oxytocin (.05-1.6 U/kg) increased cardiac output in both dog and man.

Furthermore, they (72) found that oxytocin increased myocardial con­ tractile force, stroke volume and cardiac output while both systemic

arterial pressure and total peripheral resistance were decreased. 15

The direct application of oxytocin (.2-.1+ U) to the left lateral ventricle of the brain in cats has been shown to decrease systemic arterial pressure by approximately 10 ramHg whereas higher doses increased it by 10-30 mmHg. This seems to be in contradiction with other evidence which has suggested that the vascular effects of oxytocin are directly upon the peripheral blood vessels rather than being centrally mediated.

Weis (92), in his study on the cardiovascular effects of oxytocin in women during their first trimester of pregnancy, offered yet another possible explanation as to how the effects of oxytocin on the heart and circulatory system are mediated. A bolus injection of 0.1 U/kg oxytocin given intravenously to these women elicited a decrease in mean arterial pressure with an average being 30JS of control level. Total peripheral resistance declined rapidly to approximately 50% of control

1*0 seconds following the bolus injection with an increase in heart rate

of about 30/5. Stroke volume and cardiac output initially declined after

10 seconds and then increased 25% and 50% respectively, after 60 seconds.

Oxytocin administered by continuous infusion produced no significant

change in mean arterial pressure and only a slight, statistically insig­

nificant, increase in the cardiac output. Weis concluded that since

these patients were in the early stages of pregnancy, the increase in

cardiac output observed was unlikely the result of emptying of the

vessels of the contracting uterus. He postulated that oxytocin exerts

a beta stimulating effect on the circulatory system with peripheral

vasodilation, in addition to positive inotropic and chronotropic effects

on the heart. In view of all the conflicting evidence with regard to

the effects of oxytocin on systemic arterial pressure, one can assume

that the hemodynamic status of the experimental animal and other 16 unknown factors may alter the vascular responsiveness as well as the cardiac output.

Oxytocin has been found to effect the contractility of the heart.

Woodbury and Abreu ( 93) found that addition of 10 to 100 mU of Pitocin per ml of perfusate decreased the amplitude of cardiac contractions in isolated, perfused rabbit and cat hearts. Covino (23) observed that oxytocin in concentrations of 15 to 30 mU/ml of bathing solution in­ creased the amplitude of contraction by 30?! over control in isolated cat papillary muscle, while it did not significantly change the rate and force of contraction in the isolated cat atrium in concentrations less than 60 mU/ml. In contrast, Nakano and Fisher (71) found that concentrations of oxytocin less than Uo mU/ml of bathing solution, caused no significant change in contractile force in guinea pig atria or in dog papillary muscle. They (71) also observed that higher con­ centrations of oxytocin (80-320 mU/ml) decreased myocardial contractility in proportion to the concentration given. Nakano and Fisher (7 1 ) and also Katz (52) found that the intravenous injection of .05-1*6 U/kg doses of oxytocin increased myocardial contractile force and cardiac output in dogs with intact circulation. However, Nakano and Fisher (71) concluded that these therapeutic doses did not have a direct positive or negative inotropic action on the heart of anesthetized dogs. They

(71) suggested that the increased myocardial contractile force was caused by a reflex increase in sympathetic activity due to the vaso­ dilation and hypotension caused by oxytocin and not by an increase in coronary blood flow. Nakano end Fisher (71) also observed that oxytocin decreased left atrial pressure and systemic arterial pressure while cardiac output and myocardial contractile force were increased. 17

An antiarrythmic action of oxytocin has been demonstrated by

Katz (52). In the cat and dog (1 - 10 U/kg IV) and in man (10 - 20

U i.v.) oxytocin was found to restore normal sinus rhythm and to decrease the frequency of ectopic beats. However, Spurgeon et al.

(86) have recently reported some information relating to serious and even fatal cardiovascular reactions to single i.v. injections of oxytocin in the dose range of 10 Units when given to patients postpartum.

Effects of oxytocin on renal blood flow and kidney function -

Barnes (6 ) has reported increased renal blood flow, as measured by the technique, following infusion of 1, 50 or 100 mU synthetic oxytocin into the renal artery of female dogs. It was further concluded that the vessels responsible for this increase in renal blood flow are those in the cortex and papillae as well as the medullary vasa rectae. Assali et_ al. (^ ) found that administration of a single injection of oxytocin i.v. in doses up to 60 Units did not change renal blood flow in ewes between 60 and 120 days of pregnancy, at term or during early labor. Likewise, constant infusion of oxytocin did not change renal blood flow significantly. In addition to its effect on blood flow in the kidney, oxytocin appears to have some anti- diuretic activity; however, In comparison to vasopressin (ADH) it is very likely physiologically insignificant. Nevertheless, given in a dose (.025 - 2.5 U i.v.) that does not effect blood pressure, oxytocin is an anti-diuretic in rats (11,77)- Even more recently, it has been shown that oxytocin given to rats in doses of .025 to 2.5 U i.v. during water diuresis, causes first an anti-diuresis which is then followed by diuresis and total excretion which is greater than that of the controls 18

(11*77). This biphasic response occurs without any disturbance in

"blood pressure. It has been proposed (77) that the anti-diuretic effect

is probably a result of the chemical similarity between oxytocin and

anti-diuretic hormone (vasopressin) and that the diuretic effect might be of an osmotic origin, since in rats, oxytocin increases the excretion of sodium and chloride. Oxytocin (150 mU i.v.) has been found to increase

renal plasma flow in dogs and in larger doses to also increase glomerular

filtration rate (1,12,77)- It has been suggested (1,12,77 ) that this might explain the immediate rise in electrolyte excretion accompanying

the administration of oxytocin.

Effect of oxytocin on the coronary circulation and the Influence

of preservatives used in pharmaceutical preparations -

Nakano and Fisher (71) have reported that i.a. administration

of a small dose (.005 U/kg) of synthetic oxytocin dilated the coronary

arteries in the dog. A ten-fold increase in this dose, however, pro­

duced an initial and transient vasodilation followed by a more sustained

constrictor effect which they (7l) attributed to reflex sympathetic

activity. Fortner et al. (33) experimented with the effect of synthetic

oxytocin (preservative-free) and oxytocin with preservative (ethanol

and chlorobutanol) on coronary blood flow in the dog. With a series

of single i.a. injections of 0.5U of preservative-free oxytocin at

1 5-minute intervals, they observed that after the first injection, the

end-diastolic coronary blood flow and the mean coronary blood flow

were reduced and that these reductions occurred 12 to 15 seconds after

the injection. Subsequent injections caused progressively smaller

decreases in mean coronary blood flow and a shorter duration of reduction. 19

Thus, oxytocin without added preservatives appeared to cause coronary

vasoconstriction and the response showed a certain degree of tachy­

phylaxis. In other experiments, Fortner et al. ( 33) observed the effects of

ethanol (0.25 mg) and chlorobutanol (0.5 mg) first administered alone

as 3 single i.a, injections at 5-minute intervals and then in combina­

tion with pure oxytocin. Chlorobutanol in a dose of .25 mg combined with alcohol did not cause appreciable change in coronary blood flow when administered in the same way as in the previous experiments.

However, a mixture of pure oxytocin and chlorobutanol administered

i.a. as a single injection in the same dose as mentioned above,

elicited a biphasic coronary blood flow response similar to that seen with the commercial preparation of synthetic oxytocin. These investi­

gators concluded that the constriction of the coronary vasculature

produced by preservative-free synthetic oxytocin is a direct action,

since myocardial contractile force, perfusion pressure and heart

rates in the presence of the reduced flow did not change. Since animals

exposed to chlorobutanol alone experienced no change in coronary blood

flow, they suggest a possible interaction between synthetic oxytocin

and chlorobutanol, in which the product is capable of producing a

transient vasodilation followed by a more prolonged vasoconstriction.

Effect of oxytocin on uterine blood flow -

JCiingenberg ( 56) has observed the effects of oxytocin on

uterine blood flow in non-pregnant women. Oxytocin was given intra­

venously in doses of 5 U and 10 U. A fall in uterine blood flow occurred

within the first minute after injection and uterine vascular resistance

was increased. There was also a slight decrease in blood pressure which could not account for the whole reduction of flow, indicating active or passive uterine vasoconstriction. In another study, Assali et. al.

( ^ ) showed that doses of oxytocin as small as 3 mU/min/kg produced uterine contractions In ewes in labor which became longer and more regular with increasing doses. With each contraction, there was a marked fall in uterine blood flow and an Increase In uterine vascular resistance. The systemic arterial pressure, renal, carotid, femoral and Iliac blood flows and cardiac output did not change. It was also found that pregnant ewes not in labor, did not respond appreciably to oxytocin regardless of the period of gestation and regardless of the dose and the method of administration.

These findings clearly indicate that the fall in uterine blood flow which accompanied spontaneous labor or labor induced with oxytocin is related to uterine contractions and does not depend on a systemic action involving cardiac function. This is in accordance with

ICLIngenberg (56) whose experiments indicate that the circulating effects of oxytocin are limited to the uterus since the systemic arterial pressure decrease was not sufficient to explain the whole reduction in uterine blood flow.

Effects of oxytocin on other peripheral vascular beds -

Nakano (6 9) found that the intraarterial injection of synthetic oxytocin (0.1 U/kg) Increased perfusion pressure in vascular areas distal to the cannulated lower abdominal aorta in which blood flow was kept constant by a Sigmamotor pump while systemic arterial pres­ sure remained unchanged. This indicates, at least in rats and rabbits, that oxytocin constricts arterioles and increases peripheral resistance. 21

Some observations were also made on regional blood flow, Davies and tfithrington (25) report that in dogs (sex not specified) a close arterial injection of .5 U/kg of synthetic oxytocin into the spleen resulted in a reduction in arterial flow and an increase in vascular resistance over controls. Nakano and Fisher (71) examined the influence of oxytocin upon regional arterial peripheral resistance in 7 dogs in which the brachial blood flow was kept constant using a Sigmamotor pump. The intraarterial administration of .005 U/kg oxytocin decreased regional vascular resistance significantly. Since the mean arterial blood pressure in the remainder of the circulation remained constant, it was concluded that the vasodilation action of oxytocin is the result of a peripheral effect.

Deis, Kitchen and Pickford (27) observed that in humans oxytocin, given i.v. in a dose of 500 mU decreased systemic arterial pressure and increased hand blood flow as measured by venous occlusion plethysmo­ graphy. Fallowing local nerve block by various agents,oxytocin produced a fall in hand blood flow.

Interactions of oxytocin with adrenergic and ovarian agents upon

the cardiovascular system -

The cardiovascular effects of oxytocin have been shown to be greatly modified by the autonomic nervous system, by the sex or repro­ ductive status of the animal and by the administration of estrogens, stilbestrol, progesterone, epinephrine or norepinephrine. Lloyd (59,60),

Lloyd and Pickford (6l) and Pickford (78) found that oxytocin had little or no effect on blood pressure in male or diestrus female rats. Doses of 10 - 30 mU i.v., however, produced a pressor response during pregnancy, during estrous and following the administration of stilbestrol or 22 progesterone- In estrous female dogs pretreated with stilbestrol and in pregnant dogs, Nakano and Fisher (71) found that a single intravenous injection of .5 U/kg oxytocin produced slight transient (1-2 minutes after Injection) increases in heart rate, contractile force arid cardiac output; while later (3-*+ minutes after injection) there were significant decreases in these parameters. A second injection at that same dose failed to elicit the transient response; however, the decrease in cardiac function persisted. Nakano and Fisher (7l) also reported that changes in heart rate, contractile force and cardiac output in male dogs were the same as in pretreated estrous and pregnant dogs although the magnitude of the changes were smaller. Honore and Lloyd C h-9) report that in rats, pithing and autonomic nervous system blockade abolished the depressor response to oxytocin in doses of 10 - 100 mU. In hens, Lloyd and

Pickford (6l) have reported a depressor response to 10 - 20 mU oxytocin even after decerebration, autonomic blocking agents, ganglionic blockade or administration of ovarian hormones, in fact, in some cases the fall in blood pressure was actually potentiated under these circumstances.

Lloyd and Pickford (63) measured hind limb blood flow in lU dogs

(male and female) using a Pavlov stromuhr. In some cases oxytocin

(250 - 500 mU) increased blood flow while after estrogen administration, sympathetic blocking agents or acute sympathectomy, oxytocin usually caused a decrease in hind limb blood flow. They propose that oxytocin has the potential of exerting a weak constrictor action at peripheral vascular sites and that this is somehow overridden by its central action which is vasodilator in nature. This central action is inhibited by estrogens. In both dogs and rats it was found that sympathetic stimula­ tion (in the rat by eserine administration) eliminated the reversal from dilator to constrictor and pressor in response to oxytocin which usually occurs following acute surgical or chemical sympathectomy (35,39).

Eserine is unique in that it has a central sympathetic stimulating action and crosses the blood-brain barrier while other acetylcholinesterase inhibitors do not. Haigh, et al. (39) have also shown that in sympathectomi zed dogs epinephrine infusions (U pg/min) restored the dilator action of oxytocin. Lloyd and Pickford (65) present evidence that in the rat infusions of epinephrine eliminate the pressor effect of oxytocin seen in estrous, after treatment with estrogens and after chemical sympathectomy.

They (65) measured hind limb blood flows in male and female dogs and in female monkeys and found that when oxytocin (single i.v. injection 300-350 mU) is induced to cause a reduction in leg blood flow by the administration of an estrogen (.15 - *2 mg/kg body weight) the dilator effect was restored by stimulation of the lumbar sympathetic chain in the presence of atropine (J+ - 5 mg) and also by an infusion of epinephrine in the presence of atropine (1-5 pg/min for five minutes). Epinephrine alone tended merely to reduce or annul the constrictor response to oxytocin.

They (6 5) assumed that the effect of estrogens on the blood vessels is bound up with some cholinergic process combined with a reduction or inhibition of epinephrine release and that estrogens might interfere with the release or production of epinephrine. With regard to the source of epinephrine, it was suggested (39,65 ) that this substance was supplied from the sympathetic nerve endings and not from the adrenal medulla since in the acutely sympathectomized dog with intact adrenal glands the dilator effect of oxytocin was seen only when exogenous epinephrine was supplied, or when the sympathetic chain was stimulated. It should also be stated 2k that in these experiments, norepinephrine did not restore the dilator effect of oxytocin in sympathectomized or estrogen treated animals as did epinephrine. Thus, these workers (39*62,65) have presented two different theories to explain the change in the oxytocin response from dilator to constrictor after sympathectomy, denervation and the admini­ stration of estrogens. That is, estrogens could inhibit the central action of oxytocin which is primarily vasodilator in nature or the effect of estrogens on the blood vessels could be at peripheral sites and bound up with some cholinergic process combined with a reduction or inhibition of epinephrine release.

With regard to the potentiation of the vasoconstrictor effect of oxytocin by estrogens, Altura { 2 ) has demonstrated that pretreatment of male rats with a single dose of lT8-estradiol (10 ug/lOOg) 18-2^ hours prior to observation, significantly enhanced the vasoconstrictor action of oxytocin on mesenteric arterioles. In fact, there was a five-fold leftward shift in the dose response curves after treatment with single doses of estradiol (10 ug/lOOg) indicating that estradiol significantly potentiated the vasoconstrictor action of oxytocin.

According to the dose-response curves, pretreatment with estradiol also potentiates the vasoconstrictor actions of both norepinephrine and epinephrine. The concentration-effeet curves are displaced in a leftward parallel manner six-fold in the case for norepinephrine and two-fold for epinephrine. Altura suggests that some sort of change in the sensitivity of alpha-adrenergic and neurohypophyseal receptors in rat mesenteric arterioles is induced by the presence of sex hormones. Another explanation for the enhancement of catecholamines 25 by estrogens was given by Nicol and Rae (75)- The experimental evidence given by these workers seems to indicate that estrogens can inhibit extra-neuronal uptake of norepinephrine, uptake 2, in perfused rabbit ears, thus freeing norepinephrine from the degradative effects of catecholamjne-0-methyl transferase (COMT). Furthermore, Salt and

Iverson have shown that uptake 2 of norepinephrine in isolated rat cardiac muscle was Inhibited not only by adrenal and gonadal steroids

(50) but also by cholesterol (82). It would be difficult to account for the catecholamine potentiation (and parallel displacement of concentration - effect curves) as a possible mechanism in all cases however, since extra-neuronal uptake does not affect the adrenergic response of all blood vessels (63)* Certainly, the same mechanism is not involved in the potentiation of neurohypophyseal peptides by estrogens.

The renal response to oxytocin has also been modified by various agents and procedures. However, Barnes ( 6) found that autonomic blocking agents did not alter the usual renal vasodilator response to oxytocin. She claims this finding indicates that oxytocin induced vasodilation in the kidney involves neither autonomic reflexes nor autonomic receptor sites within the renal vasculature. Croxatto and

Zamorano ( ) , however, have shown that a change in the state of activity of other endocrine glands can modify certain renal function responses to oxytocin. In adrenalectomized and in hypophysectomized rats, they (2h) found that 50 mU subcutaneous oxytocin suppresses the

saliuretic and antidiuretic effects of oxytocin where the normal response reappears in hypophysectomized rats if they are treated with

deoxycorticosterone, or given saline to drink. In another study (26) 26

dogs receiving subcutaneous stilbestrol or progesterone (.033 mg/kg/body vt ) showed the normal antidiuretic response to oxytocin (single injection

50 - 100 mU) but in the presence of this drug, the effect of oxytocin given intravenously almost completely abolished electrolyte excretion.

When oxytocin was given into the carotid artery, a delayed and centrally originating rise in sodium and chloride excretion occurred. This

suggests that in the presence of a high concentration of estrogen, the peripheral effect of oxytocin antagonizes or masks the responses to the central effect.

Objectives of the Present Study

The present study was designed to investigate various aspects of the direct effects of vasopressin and oxytocin upon the splanchnic vasculature in anesthetized dogs. Observations were made utilizing three different regional peripheral vascular preparations: the intact

superior (cranial) mesenteric arterial circulation, completely isolated autoperfused segments of small intestine (ileum) and the hepatic arterial circulation. Changes in arterial perfusion pressure, regional blood flow and vascular resistance were noted during 10 min i.v. infusions of vasopressin or oxytocin at various dose rates. Mean systemic arterial pressure, heart rate, portal venous pressure and inferior vena cava pressure were also recorded.

The phenomenon of tachyphylaxis was examined by watching changes

in the response throughout a single infusion or by comparing the response

as seen in repeated successive doses at the same infusion rate. The

possible interaction between vasopressin and adrenergic mechanisms upon

the cardiovascular system was investigated by looking at the response 27 to vasopressin after alpha-adrenergic blockade with Dibenamine or phentolamine (Regitine) or when given in combination with norepinephrine.

The mechanisms involved in the peculiar biphasic response of the hepatic artery to vasopressin infusion were explored by looking at the effects of alpha-adrenergic blockade and shunting of portal venous flow upon this response.

The effects of oxytocin in pharmacologic doses upon the vasculature of the small intestine and liver also were studied in male dogs. The possibility of the cardiovascular response to oxytocin being modified by circulating levels of female sex hormones was examined by comparing the response to oxytocin as seen in control male dogs with that in male dogs which had been chronically pretreated with estrone. METHODS

General Description

Male mongrel dogs weighing 18-25 kg were anesthetized with i.v. sodium pentobarbital (Nembutal, 60 mg/ml, Abbott) at an initial dose of

30 mg/kg. Subsequent amounts were given throughout the experiments as necessary. An i.v. infusion of 5 percent dextrose in 0.9 percent saline was started. Each animal usually received about 500 ml of this solution during an experiment of U to 5 hours duration. Immediately prior to doing the first cannulation,in any case,500 U/kg heparin (A.H. Robins) was given as an anticoagulant. Phentolamine mesylate (Regitine, Ciba),

Dibenamine hydrochloride (Smith, Kline and French), synthetic lysine®- vasopressin (Calbiochem), oxytocin (Calbiochem), and norepinephrine bitartrate (Levophed, Winthrop) were made up in buffered saline immediately before infusion. Drugs were infused into a short cannula by way of a femoral vein (i.v.), or into an arterial perfusion circuit

(i.a.) as indicated in each case. A Harvard syringe pump was used.

Dogs which were chronically treated with estrogens received intramuscular

injections of U mg of estrone (Theelin Aqueous suspension, Parke-Davis) each day for several days prior to the acute experiment. Plasma levels of estrone and estradiol were followed using standard radioimmunoassay procedures. These assays were done in the laboratory of the Department of Obstetrics and Gynecology, Ohio State University Hospital.

28 29 Blood flows were measured using a Biotronex electromagnetic blood flowmeter. A catheter was inserted into the abdominal aorta by way of a femoral artery for measurement of mean systemic arterial pressure.

Statham strain gauge pressure transducers were used for all pressure measurements. Heart rate was recorded immediately before the onset and again at the conclusion of each drug infusion. Recording was done on a

Gilson Macropolygraph.

Superior (Cranial) Mesenteric Arterial Vasculature

The abdomen was opened by a ventral midline incision. The small intestine was wrapped in gauze moistened with warmed (3T°C) saline and retracted to the left thus exposing the superior (cranial) mesenteric artery at its Junction with the aorta. Its investing sheath and accompanying nerves was carefully slit in a longitudinal direction exposing about 1.5 cm of the vessel. In this fashion the periarterial innervation was, for the most part, spared damage. The artery was ligated, cannulated and perfused with blood from a femoral artery by way of a Silastic rubber tubing extracorporeal circuit. The circuit contained a cannulating-type electromagnetic blood flow transducer and a T-tube which was connected to a transducer for measurement of superior mesenteric artery perfusion pressure. Portal venous pressure was measured from a catheter inserted by way of one of its pancreatic or splenic tributaries. Superior mesenteric artery blood flow was expressed as ml/min. Resistance in this vascular bed was calculated as superior mesenteric artery perfusion pressure-port ad venous pressure/superior mesenteric artery blood flow. Hence, resistance units were mmHg/ml*min. 30

Surgically Isolated, Denervated, Autoperfused Segments of Small Intestine

The abdomen was opened by a ventral midline incision exposing the distal small intestine and the lymph nodes about the major mesenteric blood vessels. A segment of ileum served by a single artery and vein was surgically isolated by resection. Its artery and vein were cleared of sheath and lymph nodes. All perivascular nerves were doubly tied and sectioned. Pieces of plastic tubing were tied into the cut ends of the segment using umbilical tape as a ligature. This controlled bleeding and allowed drainage of secretions from the lumen of the gut segment.

The cut ends of the remaining intestin were spliced together in a similar fashion using a piece of plastic tubing and umbilical tape.

The artery serving the isolated gut segment was ligated, cannulated and perfused with blood from a femoral artery by way of an extracorporeal circuit which contained a cannulating-type electromagnetic flow transducer and a lateral connection for measurement of gut segment arterial perfusion pressure. Arterial inflow was then briefly occluded while the vein serving the segment was ligated and cannulated. Venous effluent was carried by a second extracorporeal circuit, drained into a plastic reservoir and returned to the animal by way of a cannula in a Jugular vein. Venous outflow pressure was measured from a side port in this circuit. The gut segment was placed on a plastic platform, covered with gauze moistened with saline and kept at a temperature of approximately

37°C by means of a heat lamp.

At the end of each experiment the gut segments were drained and weighed. Their weights were in the range of 150-300 grams. Segment blood flows were then expressed as ml/min*100g of tissue. Gut segment vascular resistance was calculated as arterial perfusion pressure—venous 31 outflow pressure/blood flow. Thus resistance units were ramHg/ml*niln* 100 grams. Refer to reference U2 for diagrams of both in situ and isolated autoperfused small intestinal preparations.

Hepatic Arterial Vasculature and Shunting of Portal Venous Flow

Those dogs in which hepatic arterial blood flow was to be measured were opened by an incision immediately below the costal margin on the right side exposing the hilum of the liver and the major abdominal blood vessels. The common hepatic artery was cleared of sheath and the periarterial nerve trunk tied and sectioned. In order that all common hepatic artery blood flow would go to the liver only, the gastroduodenal artery was ligated between the most distal proper hepatic artery and the origin of the right gastric artery. Next, the common hepatic artery was ligated, cannulated and perfused with blood from a femoral artery by way of a length of Silastic tubing. A cannulating type electromagnetic flow transducer and a lateral connection for the measurement of hepatic artery perfusion pressure were included in this circuit. A catheter was inserted into a femoral vein and its tip positioned in the inferior vena cava at the level of the hepatic vein orifices. The livers were drained and weighed at the end of each experiment. Blood flow was expressed as ml/min*100g of liver. Hepatic artery resistance (R^a ) was

calculated as hepatic artery perfusion pressure (Ph a ^ ~ inferior vena

cava pressure ( P ^ ^ )/hepatic artery flow (Fh a >* Its units were mmHg/ml*min*100g of liver. Portal pressure at the hilum of the liver was detected from a catheter inserted by way of a pancreatic or splenic

tributary. 32

In some of the liver experiments, portal venous inflow was shunted away from the liver and into the systemic venous system by way of an extracorporeal circuit. A splenectomy was performed and the ligated cleaned stump of the splenic vein left intact. A Jugular vein and the proximal portion of the splenic vein were then cannulated and connected by Silastic tubing. The portal vein at the hilum of the liver was occluded with a snare and portal flow was shunted into the tubing circuit emptying into the Jugular vein. Refer to reference Uh for diagrams of both intact liver portal venous flow and shunted liver portal flow preparations.

Presentation of Data

Raw data are expressed as mean value l+_) S.E. Similarly, data expressed as mean percent change from control value are presented as mean percent change S.E. The standard Fisher t-test applied to the mean of differences between paired values was used to compare the changes from control observed in each experimental animal. A value of P < 0.05 was considered to be significant. RESULTS

Interactions Between Adrenergic Blocking Agents and the Response of the Intestinal Vasculature to Pharmacologic Doses of Vasopressin

Effect of i.v. administered phentolamine (Regitine) upon the

response of the superior mesenteric arterial vasculature to

vasopressin infusion -

The effect of the alpha-adrenergic hlocking agent phentolamine

(0.i+ mg/kg) upon the constrictor response of the superior mesenteric vasculature to the i.v. infusion of vasopressin was studied in a total of 35 dogs. Vasopressin was infused for 10 min at one of five different rates ranging from L8 to 1528 mU/min. Changes in superior mesenteric artery flow C^SMA) ’ pressure ^PSMA^ an

from an experiment showing the nature of this alpha-adrenergic blocking

effect are shown in Figure l . However, this dose of phentolamine usually produced a decrease in mean systemic arterial pressure (oa. 2h%),

33 31

i.v, NOREPINEPHRIHE 20 ug/ain 250

200, C Control • After Phentolamine i.v. 0.1 tag/kg d?° 1501-

lOOi

1.0

0.8

0.6

0.1

5 min

TIME (min)

Figure 1. The response of mean systemic arterial pressure (?g) and superior mesenteric vascular resistance (R) to 5 min i.v. infusion of norepinephrine (20 pg/min) both before (open circles) and after (.closed circles) alpha-adrenergic blockade vith phentolamine (i.v., 0.1 mg/kg). 35 a decrease in superior mesenteric arterial pressure (ca. 33#) and a decrease in superior mesenteric blood flow to a lesser extent (ca.

21#) such that generally there was an apparent increase (ca. 23#) in calculated superior mesenteric vascular resistance. The vasopressin

infusion was then repeated. In about TO# of the experiments the mesenteric vascular resistance response appeared less than that seen before phentolamine. Figure 2 shows the response of resistance during 10 min i.v. infusions of vasopressin at 95, 191* 76U and 1528 mU/min both before and after the administration of phentolamine.

Table 1 gives the mean (+_ S.E.) values for superior mesenteric blood

flow arterial perfusion pressure (Pgj^) and resistance (R) under control conditions and after 10 min i.v. infusion of vasopressin at all five dose rates used, both before and after phentolamine.

Figure 3 shows the mean {+_ S.E.) percent change In resistance (#AR)

from control after 10 min i.v. infusion of vasopressin at U8, 95* 1 9 1*

T6U and 1528 mU/min before and after alpha-adrenergic blockade with phentolamine pretreatment. Its effect on resistance at the three lower doses was not significantly different from that seen before phentolamine.

At the two higher doses, however, its constrictor effect on the mesen­ teric vasculature was significantly less. This is largely manifested

as a smaller decrease in blood flow during vasopressin infusion after alpha-adrenergic blockade (see Table l ).

Effect i.v. administered dibenamine upon the response of the

isolated, autoperfused small intestine segment to vasopressin

infusion - Figure R (mnHg/nl-min) 0. 0. 0. 1. 1. 1. 1. 1. 2 2 2. 2

,

.8- Ai- ■ . Theresponse of superior2. mesenteric resistance (R) to 0L 6 , withi.v. phentolamine (0.UDoserates are mg/kg). doserates before and after alpha-adrenergic10 mini.v.blockade Infusion of vasopressinat four different indicatedat theright eachof curve. ©— ® Control ® ©— After Phentolamine TIME (min)TIME VASOPRESSININFUSION 2 min I 191 T6U

3 6 +300 CONTROL AFTER PHENTOLAMINE

+200

<1 S'

+ 100

10 100 1000 5000 VASOPRESSIN (mU/mtn)

Figure 3. The mean (+ S.E.) percent change in resistance (foR) after 10 min i.v. infusion of vasopressin at lj8t 95, 191, 761i and 1520 mll/mln before and after alpha-adrenergic blockade with phentolamine (O.Ji mg/kg). u> Table Effect of phentolamine (0.H m o n the response of superior mesenteric arterial vasculature to i.v. vasopressin*

Vasopressin PSMA(jmnHc) Rftmdllj/ml-rain)

Infusion Rate a 10 min 0 10 mlu 0 10 min

Control 1*0 lbl + 13 U ? + 10 90 + 6 100 t 8 0.6 6 + 0.11 0 .9 6 + 0.11

After Phentolamine rail/min 98 ♦ lb 9b + 13 5 6 + 6 83+6 0.63 + 0.17 1.0b + 0.31 H = 6

Control 95 190 i lb 126 + lb 8b ♦ b 9b + b 0.33 + 0.03 0 .7 6 + 0 .1 0

After Phentolamine mU/min 126 + 20 9? + 11 59+5 86 + 5 0.50 + 0,10 O.98 + 0,16 It * 6

Control 191 187 ♦ lb lib ♦ 6 72 + 8 105 + 7 0.36 + 0.05 0.91 + 0.09

After Phentolamine mll/mln 110 + 10 81 + b« 59 t 9 93+9 0.b9 + 0.08 1.10 + 0.1b N > 8

Control 76b 171 + 13 75 + 10 83* 6 118 + 7 0.b7 + 0.07 1.71 + 0,33

After Phentolamine mlf/mln 119 + 9 72 ♦ 6* 6 9 + 9 106+8 0.55 ♦ 0.11 l.b9 + 0.25* H - 8

Control i5?8 183 + 27 79 + 15 78 + 13 119 + 13 o.b? + 0.15 1.86 + 0.50

After Phentolamine MU/min l?b + 30 8b + 18* 58 + 12 101 +15 0.55 + 0.18 1.5b + 0.b6* n = T

’Mean + S.E. values for superior mesenteric artery flow ( ) , pressure (^fiMA) an<* rcDi3tflnce (R) before and after 10 min vasopressin Infusion.

•Significantly (P = 0.05 or less) different from response seen before phentolamine. 39 The effect of another alpha-adrenergic "blocking agent, dibenamine, on the vasoconstrictor response to i.v, infusion of vasopressin was investigated. The test system in this case was somewhat different, consisting of completely isolated, autoperfused segments of

small intestine (ileum) from a total of 22 dogs. Vasopressin was

infused for 10 min at one of three different rates, 95* 191 or ?6U mU/min.

After allowing a 30-60 min recovery period, 20 mg/kg dibenamine was

infused i.v. over a period of 15 min. Another 30 min was allowed for

alpha-adrenergic blockade to take affect. This dibenamine regimen was

found to produce a very effective alpha-adrenergic blockade. The mean

systemic arterial pressor response and the gut segment vasoconstrictor

response usually seen when norepinephrine was infused i.v. at a rate of

20 ug/min for 5 min were both actually reversed following dibenamine

pretreatment. The mean (+_ S.E.) percent changes in systemic arterial

pressure (^APg) and gut segment resistance (&AR) seen during norepine­ phrine infusion in 10 experiments are shown in Figure ^ . Note that vasoconstriction was converted to vasodilation after dibenamine. However,

dibenamine under these circumstances usually produced marked hemodynamic

effects upon the experimental preparation. There was a marked decrease

in mean systemic arterial pressure (ca. h'j%), in gut segment arterial

perfusion pressure (ca. Uo*) and in gut segment blood flow to an extent

(ca. 35%) such that generally calculated gut segment vascular resistance

appeared to increase (ca. 21%).

Following dibenamine the vasopressin infusion was then repeated.

The responses of gut segment vascular resistance to 10 min i.v. infusion

of vasopressin at rates of 95» 191 and l6h mU/min both before and after Figure as < .*AP,03 -10 +10 +20 -20 -10 +10 +20 +30 +U0 b.

The ( mean + S.E.)percent change* in meanaysteaic arterial pressure Cj&Pc) andgut sequent vascular resistance (J5AR) both (solidbefore lines) and after (broken lines) alpha- adrenergicblockade i.v.with dibenamine (20 mg/kg). during5 I.v. min infusion of norepinephrine (20 Vg/ain) 0 k 1 ■ I.v.NOREPINEPHRINE TIME(ola) i 20ug/mln AFTER DIBENAMINE A

20 mg/kg *

bo

Ul dibanamine are shown in Figure 5 . When the vasopressin infusion was repeated the gut segment resistance response was less than that seen before dibenamine in only about 50^ of the experiments. Table 2 gives the mean (£ S.E.) values for gut segment blood flow (F), arterial perfusion pressure (?A ) and resistance (R) under control conditions and after 10 min i.v. infusion of vasopressin at the three dose rates.

Figure 6 shows the mean (+ S.E.) percent change in resistance f?5AR) from control after 10 min i.v. infusion of vasopressin at 95* 191 and

76U mU/min both before and after alpha-adrenergic blockade with dibenamine pretreatment. The mean change in resistance during vasopressin infusion after dibenamine was not significantly different from that seen during the control infusion at any dose. This was due to much less of a decrease in flow during the second vasopressin infusion* accompanied by a significantly greater increase in perfusion pressure (see Table 2 ).

Effect of i.a. administered dibenamine upon the response of the

isolated, autoperfused, small intestine segment to vasopressin

infusion -

In another series of eight isolated gut segment experiments, a different route of dibenamine administration was utilized in an attempt to achieve local alpha-adrenergic blockade of the vessels in the segment with less of a systemic effect on the dog. Vasopressin was infused i.v. at a rate of T6k mU/min for 10 min. After a 30-60 min recovery period dibenamine was infused into the arterial perfusion circuit of the gut segment preparation over a period of about 20 min duration such that the dose given was on the order of 8 mg/kg. Another 30 min was allowed for adrenergic blockade to occur. The effects of dibenamine iue5 Theresponseof gut segment vascular resistance (R) Figure5. to

Rfmni Hg/ml-min*100g) 10 11.0 k.Q 5.0 6.0 i.oi 3-0 2 9.0 3.0 . . 0 0 * . w . right of each curve. each of right andafterbefore alpha-adrenergic blockade withi.v. ieaie (20dibenamine mg/kg). 10infusion min of vasopressinat threedose rates both VASOPRESSIN INFUSION VASOPRESSIN <# After Dibenamine» Control o o— Dose rates are indicated at the the at indicated are rates Dose IE (min) TIME

191 191

U2 CONTROL +200 AFTER DIBENAMINE

+ 100

+ 50

0 50 100 500 1000 VASOPRESSIN DOSE < m U /m in)

Figure 6. The mean percent change £ S.E.) in gut segment vascular resistance (ARJO to 10 min i.v. vasopressin infusion at 95, 191 and f&i mU/min before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg). ^ LO Table 2 . Effect of dibenamine (20 mg/kg) on the response of small intestine segment vasculature to i.v. vasopressin*

Vasopressin F(ml/mi n*100g) PA(mmHg) R(mmUg/ml•min•lOOg)

Infusion Rate 0 10 min 0 10 min 0 10 min

Control 95 1*3 + 7 29 + It 113 + 2 123 + 2 2.78 + 0.50 U. 36 + 0.68

After Dibenamine mU/min 20 + 3 18 + 3* 79 + 10 112 + 10* 3.70 + 0.1*1* 5.96 + 0.51 N = 8

Control 191 3 2 + 3 1 9 + 3 123 + 6 138 + 1* 3.58 + 0.3l* 7.36 + 0.96

After Dibenamine mU/min 12 + 2 11 + 1* 62 + 11 102 + 11* **.59 + 0.80 8.30 + 1.19 N = 7

Control 761* 27 + 2 11 + 2 103 + 7 112 + 6 3.1*0 + 0.33 10.03 + 1.56

After Dibenamine mU/min 15 + 1 12 + 1* 6l + 6 112 + 5* 3.51 + 0.57 9.51 + 1.17 N = 7

*Mean + S.E. values for intestinal segment blood flov (F), arterial perfusion pressure (PA ) and resistance (R) before and after 10 min vasopressin infusion.

•Significantly (p = 0.05 or less) different from response seen after 10 min. under control conditions. ^5 upon hemodynamics of a segment were somewhat different than seen when it was given i.v. at the larger dose. Arterial perfusion pressure was decreased, but by only about 19%, while segment blood flow was slightly increased (ca. 835). Thus calculated gut segment vascular resistance appeared to decrease by about 15%- When the vasopressin infusion was repeated, however; its effects on segment hemodynamics appeared to be modified by alpha-adrenergic blockade in a manner somewhat similar to that which occurred when dibenamine was given at the larger dose. The mean (+S.E.) changes in gut segment blood flow (F) and vascular resistance

(R) seen during 10 min i.v. infusion of vasopressin at a rate of 76U mU/min both before and after i.a. dibenamine are shown in Figure 7- Under control conditions the mean (+S.E.1 percent changes in segment blood flow (F), arterial perfusion pressure and resistance (R) after 10 min vasopressin infusion were, -50 +_ 3%, +20 +. and +160 _+ 25%, respectively. Following the i.a. dibenamine pretreatment the mean (+S.E.1 percent change in flow after 10 min vasopressin infusion was -U6 +_ 5%, which was slightly less than seen under control conditions. The change in arterial perfusion pressure, +30 was significantly greater, while the change in resistance, +157 +, l8£, was essentially the same as that seen during the control vasopressin infusion. These data are

summarized in Table 3.

Effects of Phentolamine and Dibenamine upon the General Systemic Cardio­

vascular Response to Vasopressin Infusion

Alpha-adrenergic blockade with i.v. phentolamine and the mean

systemic arterial pressure response to vasopressin infusion - 46

4- i.v* VASOPRESSIN 764 mU/min ^

30 o o r a. —«e •20 B t a 10 r O Control • After Dibenamine (i.a. 8 mg/kg) 12.0 f 11.0 m 10.0 I I 60 9*0 §

■ 8.0 I e 7-0 1 a t 6.0 CO 5.0 ti i 4.0 r 5 mis i 3*0 i 2.0

TIME (min)

Figure 7. The responses of gut segment blood flow (F) and gut segment vascular resistance (R) to 10 min i.v. infusion of vasopressin (764 mCJ/aln) both, before and after alpha-adrenergic blockade vlth. i.a. dibenamine (3 ag/kg). kl

Table 3. Effect of i.a. dibenamine (8 mg/kg) on the small intestinal segment vasculature to a 10 min i.v. infusion of vasopressin (76k mU/min)1

Control After i.a. dibenamine

0 10 min 0 10 min

F(ml/min) 23.3 + 1.8 11.6 + 1.1 25-2 + 3.3 13.5 + 1.8

%&T -50 + 3 -U 6 + 5

PA (mmHg) 100 + k 130 + 3 87 + 6 113 + 7

*a p a +20 +. k +30 _+ U*

R(mmHg/ml*min) U.38 + 0.2k 11.1+0 + 1.23 3.66 + 0.67 9.U0 + 1.5U

+160 + 25 ----- +157 + 18

*Mean (+_ S.E.) values for blood flow (F), arterial perfusion pressure (P^) and resistance (R) and their respective percent changes after 10 vaso­ pressin infusion.

•Percent change significantly (p = 0.05 or less) different after dibenamine as compared to that seen under control conditions. U8

Figure 8 shows the response of mean systemic arterial pressure (Pg) during 10 min i.v. infusions of vasopressin at rates of US, 95j 191, 76U and 1528 mU/min both before and after i.v. phentolamine (O.U mg/kg). These data are further summarized in

Table k where the mean changes in systemic arterial pressure after

10 min vasopressin infusions before and after phentolamine pretreatment are statistically compared. Note that under control conditions even when vasopressin was given in very large doses for a 20 kg dog, the increase in systemic arterial pressure was only about 20 mmHg; a mean increase of less than 20%. Administration of phentolamine itself resulted in a decrease (ca. 25%) in systemic arterial pressure, however; after phentolamine the effect of vasopressin on arterial pressure was signifi­ cantly greater. When the highest dose (1520 mU/min for 10 min) of vaso­ pressin was repeated after phentolamine systemic arterial pressure was increased by 36 mmHg; a mean increase of over 50% (see Table U ).

Alpha adrenergic blockade with i.v. dibenamine and the mean

systemic arterial response to vasopressin infusion -

The effect of i.v. dibenamine (20 mg/kg) on the response of mean systemic arterial pressure to 10 min i.v. infusions of vasopressin at rates of 95, 191 and 76U mU/min was studied in a series of 22 experi­ ments. Figure 9 shows the response of mean systemic arterial pressure

(Pg) during 10 min i.v. infusions of vasopressin at the three dose rates both before and after i.v. dibenamine. These data are further summarized in Table 5 where the mean changes in systemic arterial pressure (Pg) after 10 min vasopressin infusions before and after i.v. dibenamine

(20 mg/kg) pretreatment are statistically compared. The i.v. administration Figure

Ps (mHg) 150 110 120 . 130 1U0 100 8 0 ■ 90 TO Theresponse of .meansystemic arterial pressure (?g) to10 - . mini.v. infusions of vasopressinat phentolamine (0.1 mg/kg). beforeand after alpha-adrenergicblockade with right of each curve. each of right •— • • After Phentolamine •— O Control O— i VASOPRESSIN INFUSION VASOPRESSIN

TIME(min) Dose rates are indicated at the at indicated are rates Dose 2 min 5 dose ratesboth -• 191 -• 95-O Ud —Q 191-© 76U -© -01528 i.v. 76U 1528

U9

50

Table 4. Effect of phentolamine (0.4 mg/kg) on the response of mean systemic arterial pressure to i.v. vasopressin1

Vasopressin PgCmmHg) * Infusion Rate 0 10 min %&

Control 48 122 + 3 126 + 5 + 3 + 2

After Phentolamine mU/min 7 7 + 7 9 9 + 7 +28 + 8» N = 6

Control 95 108 + U 122 + 4 +13 + 1

After Phentolamine mU/min 8 3 + 6 104 + 6 +25 + 5* H = 6

Control 191 120 1 6 135 + 5 +12 _+ 4 After Phentolamine mU/min 87 + 10 113 + 9 +30 + 8* N = 8

Control 764 123 + 4 138 + 8 +14 + 5

After Phentolamine mU/min 9 1 + 9 118 + 10 +30 +_ 6* N = 8

Control 1528 124 + 9 1^5 + 10 +17 + 3

After Phentolamine mU/min 88 + 12 124 + 13 +4l +_ 9* N = T

1Mean + S.E. values for systemic arterial pressure (Pg) "before and after 10 min vasopressin infusion. Mean +_ S.E. percent changes CfSA) are also shown.

•Significantly (p = 0.05 or less) different from response seen "before phentolamine. iue9 Theresponse Figure of mean 9.systemic arterial pressure to10 min

Pg (m nHg) 150 130 lUo 100 110 120 o L 6o curve. and after alpha-adrenergicblockade with i.v. dibenamine i.v. infusion of vasopressinat threedose rates both before 2 gk) Doserates are indicated at theright each of (20 mg/kg). m m m AORSI NUIN ’V VASOPRESSININFUSION IE (min) TIME —• Afteri.v. Dibenamine(20 • mg/kg) •— —- Control -O ©— •o KD 191

95 191 T6U T6U

51

52

Table 5* Effect of i.v. dibenamine (20 mg/kg) on the response of mean systemic arterial pressure to i.v. vasopressin infusion1

Vasopressin Pg (tnmHg)

Infusion Bate 0 10 min %&

Control 95 129 + U 135 + 3 + 5 + 1

After i.v. Dibenamine mU/min 06 + 10 118 + 10 +37 + 8* N = 8

Control 191 136 + 5 1U5 +_ h + 7 + 3 * After i.v. Dibenamine mU/min 65 + 11 103 + 11 +58 16* N - 7

Control 76U 113 + 8 12 U + 8 +10 + 2

After i.v. Dibenamine mU/min 6 3 + 6 11U + 5 +81 + 18* N = 7

^Mean (^S.E.) values for mean systemic arterial pressure (Pg) before and after 10 min i.v. vasopressin infusion. Mean (+ S.E.) percent changes (J£A) are also shown.

•Significantly (p = 0.05 or less) different from response as seen before i.v. dibenamine (20 mg/kg). 53 of 20 mg/kg dibenamine resulted in a greater decrease (ca. hj%) in mean systemic arterial pressure than did phentolamine. As in the case of phentolamine, alpha-adrenergic blockade with dibenamine signifi­ cantly enhanced the response of mean systemic arterial pressure to vasopressin. A 10 min infusion of vasopressin at a rate of 761+ mU/min given before i.v. dibenamine increased systemic arterial pressure by about only 10 mmHg; a mean increase of 10#. When this dose of vasopressin was repeated after i.v. dibenamine, systemic arterial pressure was increased by 51 mmHg; an increase of over 80#. These results are shown in Figure 9 in the form of doses response curves.

Effect of the i.a. administration of dibenamine on the response

of mean systemic arterial pressure to vasopressin infusion -

The i.a. administration of dibenamine (8 mg/kg) resulted in a decrease in systemic arterial pressure of only about 20#. Following this regimen of dibenamine pretreatment the effect of i.v. vasopressin infusion on mean systemic arterial pressure was again significantly enhanced; but not to the extent seen following i.v, dibenamine at a dose of 20 mg/kg. Under control conditions 10 min vasopressin infusion at 761+ mU/min increased mean systemic arterial pressure from 121 + 5 to

135 ± 1+ mmHg, or 12 +_ 3#, while after i.a, dibenamine it increased pressure from 97 +, 5 to 121 +_ 5 mmHg, or 25 +. 3#. These are the mean

(+ S.E.) values from 8 experiments.

Correlation between initial mean systemic arterial pressure and

extent of its increase following vasopressin infusion - 5U

Figure 10 gives data expressing the correlation (r) between mean systemic arterial pressure existing before the onset of vasopressin infusion (initial Pg) and its percent change after 10 min vasopressin infusion at a rate of J6h mU/min. The data are divided into four groups: (l) control (N * 21), (2) i.a. dibenamine, 8 mg/kg (N = 8),

(3) i.v. phentolamine, 0.U mg/kg (N * 6) and (U) i.v. dibenamine,

20 mg/kg (IT = T). Initial mean systemic arterial pressures in the con­ trol group ranged from 91 to 150 mmHg (average, 118 mmHg), the mean change after vasopressin was +9% and the correlation coefficient a nonsignificant - 0.2U. Regression equation for this group was y = -O.lUx + 26.1. Initial mean systemic arterial pressures in the i.a. dibenamine (8 mg/kg) group ranged from 78 to 117 mmHg (average,

97 mmHg), mean change after vasopressin was +26# and r = -0.66.

Regression equation was y = -0.36x + 60.3. Figures for the i.v. phentolamine group were: 68-125 mmHg (average, 90 mmHg), +30#, —0.60 and y =-0.l2x + 68.6, respectively, little different from those for group 2. Note that much better correlation was seen in groups 2 and 3 between initial mean systemic arterial pressure and its percent change after a 10 min infusion of vasopressin. The data from the i.v. dibenamine

(20 mg/kg) group gave yet a different picture. Initial pressure range was the lowest, U1+-90 mmHg (average, 63 mmHg); pressure change after vasopressin the greatest, +90#; correlation between initial mean systemic arterial pressure and its change after 10 min infusion of vasopressin the highest, r = -0 .8 2; and negative slope of the regression line the steepest, y = -2.6lx + 253. INITIAL Pt % AP* f CROUP DRUG DOSE {RANGE) (RANGE) 1. • CONTROL 118( 81*160) +9 (-6+130) - 0 .2 4 2 .0 U MENAMNE 97178-117) +26 - 0 .6 6 8 m g /l| (+13+40) - 0 .6 0 + 180 3 • I.v. PHENTOLAMME 90(68+23) +30 0.4 mg/kg (+4 + 30)

+ 1 6 0 4 .6 I.v DIBENAMINE 63(44-90) +90 -0 .8 2 20 m g / I cq ff30+173)

+ 140

+ 120

+ 100

+ 60

+ 4 0

+ 20

O

- 20 40 60 80 100 120 140 160 Pt mmHf

Figure 10 . Correlation (r) between mean systemic arterial pressure (Pg) existing before the onset of vasopressin infusion (initial Ps ) and its percent change (>APg ) after 10 min. i.v. infusion cf vasopressin ( mU/min) under four different experimental conditions. 56

Effect of alpha-adrenergic blocking agents on the response of

heart rate to vasopressin -

Generally, 10 min i.v. infusions of vasopressin at rates of

95 mU/ain and above produced a significant bradycardia. Modest reduc­ tions in heart rate of 10 to 30/E were seen; however, no clear dose response pattern was seen. Alpha-adrenergic blockade with either phentolamine or dibenamine had no significant effect on this vasopressin

induced bradycardia when expressed as mean percent {%&) change. Table 6 shows the mean (_+ S.E. ) changes in heart rate (beats/min) seen after

10 min i.v. infusion of vasopressin at rates of U8 to 1520 mU/min both before and after i.v. phentolamine (0.1+ mg/kg). Note that except in the case of the lowest dose rate vasopressin produced a significant decrease in heart rate both before and after phentolamine. Phentolamine

itself resulted in a significant increase in heart rate; however, the mean percent change in heart rate caused by vasopressin was not signifi- i cantly altered by phentolamine pretreatment. Similar results with

respect to changes in heart rate were seen in the series of experiments

in which vasopressin was infused before and after i.v. dibenamine

(20 mg/kg).

Effect of Norepinephrine upon the Pressor Response to Vasopressin

The combined effects of norepinephrine and vasopressin were studied

in a series of 9 isolated autoperfused dog small intestine preparations.

Norepinephrine was infused i.v. for 10 min at a rate of 19 yg/min

followed by a 30 min recovery period. Then vasopressin was infused

i.v. for 10 min at a rate of 191 mU/min with 60 min allowed for recovery.

Finally, norepinephrine and vasopressin were given as a simultaneous Table 6 . Effect of vasopressin on heart rate (beats/min) before and after phentolamine (0.1* mg/kg)1

Vasopressin Control After phentolamine

0 10 min %t 0 10 min *A

1*8 mU/min 1I18 + 8 ll*8 + 7 0 173 + 12 170 + 11 -2 + 2 N = 6

95 mU/min 1U8 + 1 132 + 13* -10 + 3 191 + 13 1^5 + 12* -2k + 7 N = T

191 mU/min m + 8 12*t + ll** -27 + 10 217 + 19 165 + 20* -21* + 10 N = 7

382 mU/min 169 + 1* 121 + 7* -28 + 5 190 + 7 150 + 8* -22 + 5 N = 7

761* mU/min 152 + 11 123 + 6* -19 + 3 182 + 18 ll*5 + 8* -20 + 6 N = 7

1528 mU/rain 161 + 6 117 + 8* -27 + 1* 171 + 6 lhl + 8* -17 + 2 N = 6

— ...

!Mean {^S.E.) values for heart rate (beats/min) after 10 min. i.v. vasopressin infusion. Mean (+^S.E.) percent changes (?A) are also given.

•Heart rate significantly (p = 0.05 or less) decreased after 10 min vasopressin infusion. 58 i.v. infusion- Figure 11 shows the effects of these infusions on gut segment vascular resistance and mean systemic arterial pressure.

Norepinephrine and vasopressin, when given separately, produced mean increases in resistance of 0.95 (or 21 +_ 5$) and 2.62 mmHg/ml*min*lOOg

(or +_ 9%), respectively- When they were given simultaneously the resistance increase was 1.93 nrmHg/ml• min• lOOg or 73 +, 1356. Even though the combined effect was greater, it did not prove to be statistically significant from the sum of their individual effects (p > 0-3). Increases in mean systemic arterial pressure were 20 (or 17 ,+ 3%) and 7 mmHg (or

6 + 1%) after 10 min infusion of norepinephrine or vasopressin, respec­ tively. Combined Infusion produced a 33 mmHg increase in pressure or

26 +_ U/S; again somewhat greater than, but not significantly different from, the sum of their independent effects.

The Role of Vasopressin Tachyphylaxis in the Experimental Design

A series of 23 isolated, autoperfused small intestinal segment experiments was done in which a 10 min vasopressin infusion was given, about 60 min allowed for recovery, and then the vasopressin infusion at the same rate was repeated. Four different infusion rates were used;

95* 191* 382 and 761 mU/min. Changes in mean systemic arterial pressure during the second infusion were not significantly different from those seen during the first; thus showing no evidence of tachyphylaxis. These data are summarized in Table 7 . Figure 12 shows dose response curves for the mean (+^ S.E.) percent changes in systemic arterial pressure ($APg) seen after 10 min in two successive i.v. infusions of vasopressin separated by a 60 min recovery period. Data relating to changes in resistance in this series of experiments are similarly presented in Table 8 and

Figure 13 . The mean percent changes in gut segment vascular resistance INFUSION PERIOD * * 170

150 E E 130

110

i .v NOREPINEPHRINE 19>ug/mln 14.0 • • 1 v VASOPRESSIN 191 m U /m in D— 0 BOTH SIMUTANEOUSLY _ T

100

6.0

6.0

4.0 5 Mir

TIME

Figure 11. The response of mean systemic arterial pressure (Pg) and gut segment vascular resistance (R) to vasopressin alone 191 (mU/min), norepinephrine alone (19 pg/min) and both s intuit aneous ly . Table 7. Changes in mean systemic arterial pressure (Pg) as seen in two successive 10 min i.v. infusions of vasopressin1

Vasopressin First infusion Second infusion

Dose Pg(mmHg) %t Pg(mmHg) %L

0 10 min 0 10 min

95 mU/min 129 + 1** 135 + 9 + 5 + ** 136 + 13 1**3 + l*i + 5 + 1 N = *♦

191 mU/min 118 + 12 132 + 12 +12 + 2 132 + 13 1*»*» + 13 + 9 + 2 N = 6

382 mU/min 128 + 7 1U3 + 6 +12 + 3 13** + 6 1U9 + 7 +11 + 1 N = 7

761* mU/min 11^ + 6 133 + 3 +17 + 5 119 + 1 137 + *♦ +15 + ** N = 6

1Mean (+_ 5.E.) values for mean systemic arterial pressure (Pg) before and after 10 min i.v. infusion of vasopressin. Mean (+^S.E.) percent changes (£d) are also shown. +20

6— •■First infusion •'— •Second infusion +15

w +10 % i t

o\ 50 100 1000 VASOPRESSIN DOSE (mU/min^ Figure 12. The aeon (+ S.E.) percent changes in ae&n systemic arterial pressure (JCaPq) after two successive 10 ain infusions of vasopressin at dose rates of 95, 191• 3&2 and 76** aU/ain. The flrBt and second infusions were separated by a recovery period of approximately 60 ain. Table 8 • Changes in isolated small intestinal segment vascular resistance (R) as seen in two successive i.v. infusions of vasopressin1

Vasopressin First infusion Second infusion

Dose R JfAR R JfAR

0 10 min 0 10 min

95 mU/min 2.76 + 0.6 I1.23 + 0 .8 + 51! + 13 3.35 + 0.5 5.2I1 + 1 .0 +57+16 N =

191 mU/min 3.Oil + 0.5 6.33 + 1.1 +108 + 16 1|.30 + 0.5 111.25 + 3*3 +119 + 19 N = 6

382 mU/min 3.65 + 0.6 8.95 + 1.2 +lh5 + 35 U. 1+2 + 0 ,6 10.76 + 1.1* +lll3 + 32 N = 7

76H mU/min 1.90 + 0.3 6.2l| + 0.6 +228 + 23 2 .7 8 + 0 .1; 7.53 + 1.1 +172 + 23* N = 6

*Mean (+_ S.E.) values for gut segment vascular resistance (ramHg/ml*min*100g) before and after 10 min i.v. infusion of vasopressin. Mean (_+ S.E.) percent changes (JtAR) are also shown.

*Mean JtAR seen in second infusion significantly (p = 0.05 or less) less than that seen in first infusion. +300

+250 Q— o First Infusion # - » Second infusion

+200

+150

§ w

+100

+50 •

0 _ ------— i------i______— . j------j 0 50 100 500 1000 VASOPRESSIN DOSE (aU/mln) o\ Figure 13. The aean percent (+ S.E.) changes in gut segment vascular resistance (JfaR) after tvo ^ successive 10 nln Infusions of vasopressin at dose rates of 95, 191* 36? and 76** ■iU/ain. The first and second infusions were separated by a recovery period of about 60 win. 6k at the three lower doses were nearly the same during the second infusion as those seen during the first. However, after 10 min infusion at 76k mU/min mean percent changes in resistance for the first and second

infusions were 228 +_ 23 and 172 +_ 23* respectively. Apparently somewhat less during the second infusion, but actually not to any highly signifi­ cant extent (p = 0.05)- Thus with vascular resistance also there was no evidence of tachyphylaxis, except possibly at the 760 mU/min dose, under the experimental protocol used in the studies described above.

Hepatic Hemodynamic Response to the i.v. Infusion of Vasopressin

Effect of alpha-adrenergic blockade with dibenamine on the hepatic

arterial response to I.v. Infusion of vasopressin with portal venous

flow Intact -

The responses of hepatic artery blood flow perfusion pressure (Pg^) and vascular resistance (Rjj^) s-s well as mean systemic arterial pressure (Pg) and portal venous pressure (Ppy) "to 10 min I.v.

infusions of vasopressin at two different dose rates, 382 and 76U mU/min, was followed in a series of 18 experiments. A recovery period of 30-60 min duration was allowed after this first vasopressin infusion. The

alpha-adrenergic blocking agent, dibenamine, was then administered i.v.

at a rate of 20 mg/min until a dose of about 20 mg/kg was achieved.

Another 30 rain waiting period was allowed. This dose of dibenamine

has been shown to block the constrictor response of the hepatic artery

to i.v. norepinephrine or electrical stimulation of the hepatic nerve

trunk (1*1 ).

The administration of dibenamine also resulted in a rather marked

(ca. k$%) decrease in mean systemic arterial pressure, hepatic artery 65 pressure (ca, 50#) and hepatic artery flow (ca. lO#) with little or no change in calculated hepatic artery resistance. Next, the vasopressin infusion was repeated at the same dose rate as given before dibenamine.

Figures l1* and 15 show the mean (.+_ S.E. ) changes in systemic arterial pressure (Pg}, portal venous pressure (P ), hepatic artery flow (F ) FV HA and hepatic artery resistance throughout 10 min i.v. infusions of vasopressin at 382 and J6h mU/min, respectively, both before and after the dibenamine pretreatment. Note that the vasopressin effect upon the hepatic artery was much different from its effect upon the prehepatic splanchnic vasculature. No marked degree of vasoconstriction sustained throughout the infusion period was seen (see Figure 15 )• Hepatic arterial resistance showed an initial transient increase which peaked after about 5 min infusion at the slower rate and after about 3 min infusion at the faster rate. It then returned to values at or slightly below its initial level as the vasopressin infusion was continued.

Following alpha-adrenergic blockade with dibenamine the initial transient increase in resistance was abolished and in its stead a modest decrease was seen. These data are statistically summarized in Table 9 .

A clearer picture of the differences in the response to vasopressin before and after dibenamine is seen if the mean percent change in the various parameters are followed throughout the infusion period. Figure 16

shows the mean (+ S.E.) percent changes in hepatic artery perfusion pressure throughout 10 min i.v. infusion of vasopressin at 382 and J6h mU/min both before and after dibenamine. Note the greater percent Increase in perfusion pressure after dibenamine pretreatment.

Under control conditions it was increased about 15# after 5 “In and then 66

VASOPRESSIN INFUSION 3S2mU/min 160

£ 120 01 a. SO ■ CONTROL 40 AFTER DIBENAMINE r.o

a.i * 5.0

3.0 o x<9 j. ■

S 9-0

< 1 « ■

3.0

2.0 L

TIMC

Figure lA The mean (+ S.S.] changes in systemic arterial pressure (?§), portal venous pressure (Pp»), hepatic artery flow (Fg*) and hepatic artery resistance durlaS 10 31111 1*v* infusions of vasopressin at 362 mU/oin before and after alpha-adrenergic blockade with i.v. dibenamine C 20 mg/kg). 67

■ VASOPRESSIN INFUSION . * 764 mU/min * 170

ISO

o 130 X i — r 1 no

9 0

7 0 CONTROL AFTER DIBENAMINE 6.0

6.0

90

§ 7 0

I SO E 3 0 i S 3 .0

2.0

f , o 2 min

TIME

Figure -2-5. The mean (+ S.E.) changes in systemic arterial pressure (Pg), portal venous pressure (Ppy), hepatic artery flow (F^) and hepatic artery resistance (834) during 10 min i.v. infusions of vasopressin at 76h mU/min before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg). Table 9 ’ Effects of 10 ain I.v. Infusions of vaunprensin on hepntic artery flow ), perfusion pressure (i’m) snil resistance (Ht|g) before and after il I lennalnr’

Vasopressin dose P (al/ain-lOOg) R.(»Kg/.l • at n* lOOg)

5 >ln 10 ain 5 ain

30? ad/aln 1 = 5

1- 50 + 0.7100 + 13 1-50 + 0.7100 k,?9 * 0.0*

After dlbenaalne

T6lt all/ai n N - 13

6 5 .2 * 10.6 6k.6 * 10.0 10k + 117

After dlbenaalne 60 +

’Kean (^S.JJ.) values at the onset and at three tlaes during vasopressin infusion.

•Statistically significant (p ■ 0.05 or leas) froa value at zero tlae.

ON CD | VASOPRESSIN INFUSION J CONTROL AFTER DIBENAMINE 382 mU/min 0— 0 I I + 120 764mU/min 0— 0

+ 100

+ 8 0 x< °- + 6 0 <3

+ 4 0

+20

0 2 min

TIME

Figure l6. The mean (+ S.E.) percent changes in hepatic artery perfusion pressure (JfaPju) throughout 10 min i.v. vasopressin infusion at dose rates of 382 and 16U roU/min both before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg). TO

subsequently decreased somewhat such that it was only U lj£ and

13 ±_ 2% above its initial level after 10 min infusion at 382 and 76U mU/min, respectively. Following the administration of dibenamine the

increase in hepatic artery pressure was significantly (p < 0.03) enhanced.

During infusion of vasopressin at 382 mU/min it steadily rose to a mean

82 +_ 22% increase after 10 min. However, during infusion at 76k mU/min

the increase in hepatic artery pressure tended to level off after about

5 min and had a mean increase of only 62 +_ lk% after 10 min. This

reflects the more pronounced tendency for resistance to decrease with

the higher vasopressin dose (see Figure 16 ).

Figure 17 similarly shows the mean (+_ S.E.) percent changes in

hepatic artery blood flow (*AF^). Note the greater percent increase

hepatic artery blood flow after dibenamine pretreatment. Flow underwent

an initial transient decrease of about 8% and then subsequently increased

throughout the remainder of the infusion. At the lower dose rate there

was relatively little increase; on the average flow had returned to only

slightly (2 +_ 2%) above its initial level by the end of the infusion

period. After 10 min vasopressin infusion at a rate of l6h mU/min, flow

had increased by 3^ ± 6%. The increase in hepatic artery flow during

vasopressin infusion at both doses after dibenamine was very significantly

(p < 0.01) greater than that seen under control conditions. The initial

transient decrease in flow was not seen. It sharply increased throughout

the entire infusion period until after 10 min it was elevated 10U +_ 30%

and 129 + 15% by the lower and higher dose rates, respectively.

Figure l8 gives the mean (+^ S.E.) percent changes in hepatic artery

resistance (SiR^) during vasopressin infusion at the two dose rates both 71

I VASOPRESSIN INFUSION £ CONTROL AFTER DIBENAMINE 3 8 2 mU/min □— 0 + 160 r 764mU/min

+ 140 -

+ 120 -

+ IO O -

< X + 8 0 - u_ 4 + 6 0 '

• + 4 0 '

+ 2 0 *

0 - r!, O 2 min

TIME

Figure IT. The mean (i S.2.) percent changes in hepatic artery hlood flow "to vasopressin infusion at dose rates of 382 and 761* mU/min both "before and after alpha-adrenergic blockade with i.v dibenamine (20 mg/kg). 72

[ VASOPRESSIN INFUSION \ CONTROL AFTER DIBENAMME 382 mU/min □— □

7 6 4 mU/min II

+ 40

+ 30

+20

+ 10

E QC 0 < s* -10

-20

-30 2 min -40

TIME

Figure J-8 . The mean (+_ S.2.) percent change in hepatic artery resistance (jSARjj^) during vasopressin infusion at dose rates of 382 and 76k mU/min hoth before and after alpha-adrenergic blockade with i.v. dibenamine (20 mg/kg). 73 before and after dibenamine. A biphasic resistance response was clearly seen under control conditions. During infusion at 382 mU/min resistance increased by 20 +_ U% after about 5 min andthen subsequently declined until it was only 4 + 5^5 above its initial level after 10 min infusion.

With 764 mU/min it peaked with a 31 +, 8% increase after only about 3 min infusion and then declined until it was 14 + 355 below its initial level after 10 min infusion. Thus the time of onset and the magnitude of the initial transient increase in resistance, as well as its subsequent decrease, all appear to be dose related. The response of hepatic artery I resistance to vasopressin infusion at both dose rates was significantly

(p < 0.02) different after dibenamine. The initial transient constrictor phase was longer in evidence and resistance tended to decline throughout the infusion period. Again this appeared to occur in a somewhat dose related fashion, being decreased by 11 +_ 3% and 29 +. 3% after 10 min infusion at 382 and 764 mU/min, respectively. These data are statistically summarized in Table 10 .

Effect of alpha-adrenergic blockade with dibenamine on the hepatic

arterial response to i.v. infusion of vasopressin with portal venous

flow shunted into the systemic venous system -

In another series of 23 experiments portal venous inflow to the liver was completely diverted by way of an extracorporeal shunt circuit running from the splenic vein to a Jugular vein. The animals were

allowed to equilibrate at this state for about 60 min and then a 10 min

I.v. infusion of vasopressin was given at a rate of 764 mU/min. The

response of the hepatic artery to this dose of vasopressin was essentially

the same as that seen in the group of dogs with portal venous inflow to Table 10 . Relative changes in hepatic artery flow {% perfusion pressure {% Pj^) and resistance (% R ^ ) during 10 min i.v. infusions of vasopressin before and after dibenamine1

Vasopressin *AFW fcpHA

Dose 3 min 5 min 10 min 3 min 5 min 10 min 3 min 5 min 10 min

382 raU/min N = 5

Control - 1* + 2 - 8 + 1* + 2 + 5 + 6 + 1 +12 + 2 + 1* + 2 +13 + !* +22 + 5 1* + 6

After dibenamine +1*9 + lit* +72 + 23* +106 + 3l** +2l* + 1** +1*6 + 11* +82 + 27* -llj + 5* -12 + 6* -1 0 + 1*

761* mU/min N = 13

Control -5+6 + 2 + 6 + 3 3 + 1 0 +13 + 1* +17 + 3 +13 + h +31 + 9 +19 + 8 -ll4 + 3

After dibenamine +65 + 11* +92 + 16* +125 + 19* +37 + T* +52 + 12* +62 + 18* -11 + 5* -20 + 5* -2 8 + 1**

^ean (+ S.E.) percent changes at three times during vasopressin infusion.

•Statistically significant {p = 0.05 or less) from value before dibenamine. 75 the liver intact- The mean (+ S.E.) percent changes in hepatic artery resistance seen during vasopressin infusion in these two groups are compared in Figure 19 . The characteristic biphasic response persisted in the absence of portal flow. After about 3 min infusion a peak increase in resistance of + 3# was reached; not significantly (p > 0 .3 ) different from the mean 31 ± 9% increase seen with portal .flow intact.

The subsequent vasodilator phase was, however, significantly (p < 0.01) less. Resistance after 10 min was still 7 ± 2% above its initial level as compared with lU +_ 3% below initial level in the group with portal flow intact.

In six of the dogs with portal venous inflow to the liver diverted an alpha-adrenergic blocking dose of dibenamine was administered as described above. The 76U mU/min vasopressin infusion was then repeated.

As in those dogs with portal flow intact, the response to vasopressin was no longer biphasic after dibenamine. The initial transient rise in hepatic artery resistance was abolished as shown in Figure 19 - It declined throughout the entire infusion period, however, to a significantly

(p = 0.05) lesser extent than seen with portal flow intact. After 10 min infusion the mean decrease in resistance was only 13 +. 6% as compared to

28 +_ h%. These data are summarized in Table 11.

Effect of alpha-adrenergic blocking agents on the response of mean

systemic arterial pressure and portal venous pressure to i.v.

infusion of vasopressin -

As in the experiments with isolated small intestinal segments as described above, the response of mean systemic arterial pressure to i.v infusion of vasopressin appeared to be significantly enhanced by i.v. , VASOPRESSIN INFUSION . * 764 mU/mfai I +40 CONTROL - with Portal Flow CONTROL- with Portal Shunt +30 ■ AFTER DIBENAMINE - with Portal Flow AFTER DIBENAMINE - with Portal Shunt

+20 ■

i-*+10 * tt < j o ■ *

■ I -10 I I

-20 *

-30 ■

-4 0

TIME

Figure 19. The mean (+ S.E.) percent changes In hepatic artery resistance (£ARjja} to 10 min i.v. infusion of vasopressin (76m mU/min) under four different experimental conditions. —i o\ Table n . Relative changes in hepatic artery flow (% Fjj*), perfusion pressure (JE P ) and resistance (% R„ ) during 10 min i.v. infusion of vasopressin (ToU mU/min) before and after aibenamine and with porta venous flow shunted1

*1FHA *apHA *arh a

3 min 5 min 10 min 3 min 5 min 10 min 3 min 5 min 10 min

Control - 5 + h + 2 + li + 17 + 3f +12 + U +18 + U +23 + h +2h + 3 +15 + li + 7 + 2+ N = IT

After dibenamine +71 + 10* +111 + 5* +116 + 5* +51* + 6* +79 + 8+ +91* + 11+ -1 0 + 5* -11 + 5+ -13 + 6+ N = 6

lMean (+ S.E.) percent changes at three times during vasopressin infusion.

•Statistically significant (p = 0.05 or less) from change seen in portal shunt preparation before dibenamine. tStatistically significant (p - 0,05 or less) from change seen in preparation with intact portal flow (see Table 12).

-q -q 78 dibenamine (20 mg/kg) pretreatment in the series of hepatic artery experi­ ments. Figure 20 shows the mean (+_S.E.) percent changes in systemic arterial pressure (&APg) throughout 10 min i.v. infusions of vasopressin at 382 mU/min (N - 5) and 76U mU/min (N = 13) both before and after dibenamine.

Portal venous pressure was significantly reduced (ca. 25%) by both doses of vasopressin after 10 min infusion. The relative change (-20 +

5%) seen after 10 min infusion at 382 mU/min following dibenamine pre­ treatment was not significantly different from that seen under control conditions. However, the 16 +_ 2% decrease in portal venous pressure seen after 10 min i.v. vasopressin infusion at 76U mU/min following dibenamine pretreatment was significantly (p < 0,02) less than that seen under control conditions. These data are summarized in Table 12 .

The Cardiovascular Effects of i.v. Infusion of Oxytocin

Effect of i.v. infusion of oxytocin on mean systemic arterial

pressure -

The effects of 10 minute i.v. infusions of oxytocin at four different dose rates (0.95, 1.9, 3.8 and 7-6 Units/min) on mean systemic arterial pressure (Pg) was studied in a series of 38 dog experiments.

After a recovery period of approximately 1 hour an infusion at the same dose rate was repeated in each case. As can be seen in Figure 21 the average systemic pressures under control conditions in the four dose groups ranged from 105-130 mmHg. During the first 10 min infusion the lowest dose rate, 0.95 U/min, had little or no effect on systemic pres­ sure while during infusion at the three higher dose rates systemic pressure showed a biphasic response. There was a dose-related transient 79

j VASOPRESSIN INFUSION \ CONTROL AFTER DIBENAMINE 3 8 2 mU/min □— D

+ 100 764mU/min II

+90

+ 60

+70

+60

+50

+20

+ IO

0

TIME

Figure 20. The M a n (+ S.3.) percent change in mean systemic arterial pressure (.lAPg) to 10 min i.v. infusion of vasopressin at dose rates of 382 mU/min and ?6U mU/min "both before and after alpha-adrenergic blockade with i.v. dibenonine (20 mg/kg). Table 12 . Effect of i.v. dibenamine (20 mg/kg) on the response of portal venous pressure (Ppy) to i.v. infusion of vasopressin1

Vasopressin Control After dibenamine

Dose P ^ (mmHg) Ppy(mmHg)

0 10 min %t 0 10 min *A

382 mU/min 6.0 + 0.8 1.5 + 0.5* -25 + 5 5.5 + 1.1 li.l* + 1.0* -20 + 5 N = 5

76U mU/min 7.3 + 0.6 5.3 + 0 .5* -27 + 3 6.7 + 0.5 5.6 + 0.5* -16 + 3** N = 13

^ean (+_ S.E.) values for Ppy before and after 10 min i.v. infusion of vasopressin. Mean (+ S.E.) percent changes (%K) after 10 min are also given.

*Ppy significantly (p = 0 .0 5 or less) decreased after 10 min i.v. vasopressin infusion.

**Mean JfAP after 10 min i.v. vasopressin significantly (p < 0.02) less than that seen before dibenamine pretreatment. OXYTOCIN INFUSION FIRST INFUSION SECOND INFUSION 140 i

130 ■ 0 .8 3

120 3.8 7.6

3 .8 7.6

90

8 0 2 mtn

TIME

Figure 21. The response of mean systemic arterial pressure (Pg) to tvo successive 10 min i.v. infusions of oxytocin at dose rates of 0-95, 1-9* 3.0 and 7.6 U/min. The first and second infusions were separated by a recovery period of approximately 60 min. Dose rates are indicated at the right of each curve. 82 decrease in pressure which reached its minimum after about 3 min infusion and then subsequently returned to a value at or near its control level during the remainder of the infusion period. The response seen during the second 10 min infusion was somewhat different. The transient decrease in pressure was not seen, but rather there was a tendency for pressure to increase throughout the entire infusion period. The average (+_ S.E.) values for mean systemic arterial pressure under control conditions, after 3 min infusion and after 10 min infusion, during the first and second infusion periods, are summarized in Table 13-

These same data are again shown in Figure 22 j however, in this case the percent changes in systemic arterial pressure throughout the first and second infusion (j£APg) periods at each dose rate have been plotted. The dose-related biphasic response during the first infusion, as compared to only a slight, but consistent rise during the second infusion is more clearly evident. Dose-response curves showing the average (+_ S.E.) percent changes in mean systemic arterial pressure after 3 min infusion at the four dose rates seen during the first and second infusion periods are compared in Figure 23 * Note that at this point in time the response of mean systemic arterial pressure seen during the first infusion differs from that seen during the second infusion at the three higher dose rates to a highly significant degree

(p - ca. 0.03, 0.02 and 0.01, respectively). However, after 10 min infusion there was no significant difference in the percent change in systemic pressure between the first and second infusions at any dose rate (p > 0.10). For example, after 10 min infusion of oxytocin at a rate of 7.6 U/min systemic arterial pressure had returned from its

initial 22 + 7 percent decrease to a level of 1* + 3 percent above Table 13, Effect of 10 min i.v, infusion of ocytocin on mean systemic arterial pressure (Pg» mmHg)*

First infusion Second infusion

Dose (U/min) 0-95 1.9 3.8 7.6 0.95 1.9 3.8 7-6

*

N 8 8 8 8 8 8 8 8

Time 0 127 + 3 128 + It lilt + 3 10lt + 10 132 + 3 128 + It H 3 + 6 105 + 12

3 min 128 + 3 116 + 6* 98 + 6* 81 + 12* 131! + 3 129 + 5 lilt + 6 109 + 12

10 min 129 + 3 125 + 5 110 + 5 108 + 11 135 + 3 130 + 5 118 + 6 Hit + 12

^ean (+^S.£.) values after 3 and 10 min i.v. oxytocin infusion

*Value after 3 min infusion significantly {p = 0.05 or less) different from that existing at zero time.

OD U) 8U

i OXYTOCIN INFUSION i FINST INFUSION + is - SECOND INFUSION

+ 10' 7,.»

3.1 + s ■ 7.* J.9 0. »9 0- 0 . 9 9 1.9 s.a * ■ s ' < - 10 -

- IS ■

-20 ■

- 25 ■

-3 0 ■ 2 m in - I

TIME

Figure 22. The mean (+ S.2.) percent change in mean systemic arterial pressure (^AFg) to tvo successive i.v. infusions of oxytocin at dose rates of 0-95, 1.9» 3.8 and 7.6 U/oin. The first and second infusions vere separated hy a recovery period of approximately one hour. Dose rates are indicated at the right of each curve. + 10 i

AFTER 3 MM INFUSION O— O FIRST MFUSION • — • SECOND INFUSION

-5

-15

-20

-2 5

“I- T 1.0 OXYTOCIN (U/n4n)

Figure 23. Dose-response curves shoving the mean (+_ S.E.) percent changes in mean systemic arterial pressure (jtAPg) after 3 min i.v. infusions of oxytocin at 1| dose rates (0.95, 1-9. 3.8 and 7*6 U/min).- The second infusion followed the first by approximately one hour. 86

control while during the second infusion it rose throughout to reach

a level 8+^2 percent ahove control.

Effect of i.v. infusion of oxytocin on the vasculature of the

isolated dog small intestine segment -

The effects of 10 min i.v. infusions of oxytocin at four dif­

ferent dose rates on arterial perfusion pressure (F^)* ^1°°^ flow and

vascular resistance (R) was studied in 32 surgically isolated segments

of dog small intestine. As shown in Figure Zh and Table lU the response of

gut segment arterial perfusion pressure to oxytocin infusion was much

the same as that of mean systemic arterial pressure. A transient dose-

related decrease in pressure was seen during the first 3 min of the first

infusion at the 1.9* 3.8 and 7.6 U/min dose rates with a return of pres­

sure to a value at or near its control level throughout the remainder of

the infusion period. Again the lowest dose rate, 0.95 U/min, had little

or no effect on pressure. During the second infusion the initial tran-

* sient decrease in arterial perfusion pressure was not seen at any dose

rate but rather there was a somewhat dose-related steady rise in pres­

sure throughout the entire infusion period. This becomes even more

evident in Figure 25 where the same data are given but are plotted as

percent change in gut segment arterial perfusion pressure (^AP^). The

dose-response curves showing the mean (+_ S.E.) percent changes in gut

segment arterial perfusion pressures at 3 min and at 10 min during both

the first and second oxytocin infusions are given in Figure 26. Note

that the only significant differences between the response seen during

the first infusion as compared with the second infusion occurred during

the initial portion of the infusion at the three higher dose rates

(p *= ca. 0.02). After 10 min infusion there was no significant OXYTOCIN INFUSION o— o FIRST INFUSION SECOND INFUSION 120

0.95 100 3.8 7.6 o X E 3.8 E 90 7.6

80 -o-

TO­

GO - 2 min

TIME

Figure 2*». The response of gut segment arterial perfusion pressure (P^) to two successive i.v. infusions of oxytocin at four dose rates (0.95, 1.9, 3.8 and 7.6 U/min). The second infusion followed the first by approximately one hour. Dose rates are indicated at the right of each curve. Table ill, Effect of 10 min i.v. infusion of oxytocin on gut segment arterial perfusion pressure (PA , mmHg) 1

First infusion Second infusion

Dose (U/min) 0.95 1.9 3.8 7.6 0.95 1.9 3.8 7.6

If 8 8 8 8 8 8 8 8

Time 0 101 + k 105 + 89 + It 81 + 11 108 + 1* 108 + l» 93 + 6 85 + 13

3 min 100 + 5 90 + 5* 71+ + 7* 61 + 9* 109 + 1+ 109 + k 91* + 6 91 + 13

10 min 101+ + li 105 + 3 92 + 5 91 + 11 111 + 5 11U + 5 101 + 7 97 + l1* |

^ean (+ S.E.) values after 3 and 10 min i.v. oxytocin infusion

*Value after 3 min infusion significantly (p = 0.05 or less) different from that existing at zero time.

CD CD I I OXYTOCIN INFUSION FIRST INFUSION + 20 i SECOND INFUSION « 7.6 /> 7.6 + 1 0 - / I 3.6 ^ 5 19 095 -=$3.8 0 H o 0 93 1.9

i! “ 10

- 2 0 -

- 3 0 ■ 2 min I 1

TIME

Figure 25- The mean (+ S.E.) percent change in gut segment arterial perfusion pressure to two successive i.v. infusions of oxytocin at four dose rates (0.95, 1.9, 3.8 and 7*6 U/min). One hour ( recovery period between the first and second infusions was allowed. Dose rates are v indicated at the right of each curve. +20 AFTER 3 MIN INFUSION FIRST INFUSION 410 SECOND INFUSION AFTER K) MIN INFUSION □— □ FIRST INFUSION i s : SECOND INFUSION CL* « -10 15

-20

-30

— 1~ ' r— — 1— —1 0.1 0.5 IO 50 10.0 OXYTOCIN (U/mln)

Figure 26 . The doae-response curves showing mean S.E.) percent changes in arterial perfusion pressure ({(AP^) at 3 and 10 min during both the first and second i.v. oxytocin Infusions at dose rates of 0.95* 1.9, 3.8 and 7-6 U/min. A recovery period of approximately one hour.was allowed VO between the first and second infusioirs. o 91 difference in the percent change in arterial perfusion pressure

"between the first and second infusions at any dose rate (p = ca.

0.U0).

The data showing the response of gut segment blood flow during oxytocin infusion at the four dose rates are similarly shown in

Figure 27 and Table 1 5 . Note the considerable variability in blood flow values. Flows at the onset of the first infusion varied from

20-32 ml/min*100g and at the onset of the second infusion from lU-25 ml/min*100g. During the first infusion at the three lower dose rates,

0.951 1*9 euid 3.8 U/min, there was a somewhat dose-related transient rise in flow which peaked after about 3 min of infusion. Flow then declined throughout the remainder of the infusion period reaching values at or below their control levels after 10 min infusion. No initial transient rise in flow was seen during the first infusion at the highest dose rate, 7.6 U/min, but rather it decreased throughout the entire infusion period. The latter was true during the second infusion at all dose rates except for a slight tendency for an initial transient rise in flow after 3 min infusion at the lowest dose rate,

0-95 U/min. The same data on the response of gut segment blood flow to oxytocin infusion are given in Figure 28 , but in this case are expressed as percent change from control. The only points of difference between percent change in flow responses seen during the first infusion as compared with those seen during the second infusion which proved to be statistically significant occurred at the initial transient rises in flow seen after 3 min during the first infusion at dose rates of

1.9 and 3.8 U/min. For example, the mean percent change in flow seen 92

i OXYTOCIN INFUSION 1

O——0 FIRST INFUSION 32 1 SECOND INFUSION

3 0 1

0 .9 9

24 H 8 22 1

3 .8

1.9 l« 1

14 1 7.6

2 min 7.6

TIME

Figure 2T- The response cf gut seffaent blood flov to two 10 min. i.v. infusions of oi^ocin at 0.95, 1.9, 3.3 and 7.6 U/min. The second infusion was given approximately one hour after the first. Dose rates are indicated at the right of each curve. Table 15. Effect of 10 min i.v. infusion of oxytocin on gut segment blood flow (ml/min’lOOg)1

First infusion Second infusion

Dose (U/min) 0.95 1.9 3.8 7.6 0.95 1.9 3.8 7.6

N 8 8 8 8 8 8 8 8

Time 0 27 + k 26 + 2 30 + U 20 + 2 2k + k 21 + 3 23 + 2 111 + 2

3 min 29 + k 30 + li* 31 + 5 18 + 3 25 + l» 20 + 2 22 + 2 13 + 2

10 min 26 + li 21 + 3* 22 + 2* ll* + 1* 21+3 17 + 2 19 + 1 11 + 1

*Mean (+_ S.E.) values after 3 and 10 min oxytocin infusion.

•Value after 3 or 10 min infusion significantly (p = 0.05 or less) different from that existing at zero time.

vo u> OXYTOCIN INFUSION o— « FIRST INFUSION +30 n SECOND INFUSION

+ 2 0 -

- 1 0 - 0.93

- 2 0 - 3.8 7.6 -30- 7.6 2 min

TIME

Figure 28. The mean (+ S.E.) percent change in gut segment blood flow to two 10 min i.v. oxytocin infusions at dose rates of 0.95, 1.9* 3.8 and 7.6 U/min. The first and second infusions were separated by a recovery period of approximately one hour. Dose rates are indicated at the right of each curve. 95 after 3 min during the first infusion at a rate of 1.9 U/min was

+20 _+ 8 percent as compared with -2 +_ 3 percent after 3 min during the second infusion at this same dose rate (p < 0.01).

The response of gut segment vascular resistance (R) to the i.v. infusion of oxytocin is summarized in Figure 29 and Table 16 .

Resistance values at the onset of the first infusion ranged from about 2.5 to U.O mmHg/ml*min*100g and at the onset of the second infusion from about 3-5 to 5 .5 nnnKg/ml*min*100g. During the first infusion there was a dose-related initial transient decline in resis­ tance seen during the first 3 min. This was followed by a subsequent dose-related rise in resistance throughout the remainder of the infusion period. This initial transient decrease in resistance was not seen during the second infusion except possibly to a slight extent with the lowest dose rate, 0.95 U/min. Rather there was a dose-related rise in resistance which occurred throughout the entire infusion period.

This is more graphically evident in Figures 30 and 31 where the same data are plotted as percent change in resistance (!5aR) during the first and second infusions at all four dose rates. Dose-response curves showing mean (+_ S.E.) percent change in resistance after 3 min and after 10 min during both the first and second infusions are given in Figure 32 . The percent change in resistance seen after 3 min during the first infusion was significantly different from that seen after 3 min during the second infusion at all four dose rates. For example, after 3 min during the first infusion the relative change in resistance was -23 + 10 percent, while after 3 min during the second infusion it was +26 +_ 6 percent

(p < 0.01). Note that after 10 min infusion at any dose rate the relative 96

8.9 -I OXYTOCIN INFUSION

8. 0 - 7.6 FIRST INFUSION SECOND INFUSION 7 .9 -

1.9 7 .0 -

6.0 * £ E 9.9 ■ 7.6 vE 9 3 .8 X E 9 .0 ■ E K

4 . 0 - 0 .9 9 3.8 3 . 9 -

3 .0 ■ 2 min

2 .9 ■

2.0 TIME

Figure 29. The response of gut segment vascular resistance (H) to two 10 min i.v. infusions of oxytocin at dose rates of C-95* 1-9, 3.8 and T.6 U/min. The second infusion followed the first by approximately one hour. Dose rates are indicated at the right of each curve. Table 16. Effect of 10 min I.v. infusion of oxytocin on gut segment vascular resistance (R, imnHg/ml-min’lOOg)1

First infusion Second infusion

Dose (U/min) 0.95 1.9 3.8 7.6 0.95 1.9 3.8 7.6

N 8 8 8 8 8 8 8 8

Time 0 3.6+0.5 3.9+9•5 2.7+0.2 3.5±0.8 l».3+0.5 5.3+0.8 3.6+0.3 5.1+1.0

3 min 3.3+0.5 2.9+0.6* 2.1+0.2* 2.7+0.8* It. 2+0.5 5.5+0.8 3.8+0. 6.2+1.0*

10 min *t.l+Q.lt* 5.0+0.8* 3.8+0.3* 5.6+0.9* 5.0+0.5* 7.1+1.0* 5.1+0.7* 8.0+2.0*

^ean (+_ S.E.) values after 3 and 10 min i.v. oxytocin infusion

•Value after 3 or 10 min infusion significantly (p = 0.05 or less) different from that existing at zero time. 99

OXYTOCIN INFUSION FIRST INFUSION

+ 60 7 .6

+ 30-

3 .6 + 4 0 -

< £

- 10

-20

2 min - 3 0

TIME

Figure 30 . Mean percent (+_ S.S.) change in gut segment resistance (JSAR) to the first 10 min i.v. infusion of oxytocin at dose rates of 0.95* 1.9* 3.3 and J.6 U/min. Dose rates are indicated at the right of each curve. i i OXYTOCIN INFUSION SECOND INFUSION

7.6

+50

3 0

0.95

-10

TIME

Figure 31. The mean (+ S.E.) percent change in gut segment resistance (%&R) to the second 10 min i.v. infusion of oxytocin at the dose rates, 0.95* 1.9* 3.8 and 7-6 U/min. Dose are indicated at the right of each curve. + 80i

AFTER 3 MIN INFUSION o— o FIRST INFUSION • — * SECOND INFUSION AFTER 10 MIN INFUSION +20 D—□ FIRST INFUSION oc | —■ SECOND INFUSION <3

-20

-40-

0.1 0J5 IjO 5.0 10.0 OXYTOCIN (U/mln)

Figure 32. Dose-response curves shoving mean (j^S.E.) percent changes in gut segment resistance (JJAR) 100 after 3 and 10 min during both the first and second i.v. oxytocin Infusions at dose rates of 0.95, 1.91, 3.6 and 1.6 U/min. The second infusion followed the first by approximately one hour. 101 change in resistance seen during the first infusion was essentially the same as that seen during the second infusion. For example, after

10 min during the first infusion the relative change in resistance was

+67 11 percent and after 10 min during the second infusion it was

6l +_ 10 percent (p > 0.20).

Effect of i.v. infusion of oxytocin on heart rate -

The mean initial heart rate in the group of 32 anesthetized

dogs was 130 +_ 8 beats/minute. Dose response curves for relative

changes in heart rate after 10 min infusion of oxytocin are shown in

Figure 33 . The usual response was a slight bradycardia; however, the mean responses did not prove to be statistically significant at any

dose rate after either the first or the second infusion (p = 0.3 - 0.T).

The Cardiovascular Effects of i.v. Infusion of Oxytocin in Male Dogs

Pretreated with Estrone

Estrone and estradiol plasma levels after chronic estrone pretreatment ■

A group of 15 male dogs were injected intramuscularly with

estrone aqueous suspension at a dose of U mg/day for a period of from

5 to 7 days including an injection on the day each was subjected to the

acute experiment. Blood samples were drawn before the estrone injection

on the first day and after the estrone injection on the last day; the

day of the acute experiment. Plasma levels of estrone and estradiol

were determined by standard radioimmunoassay techniques before the first

injection and after the injection on the day of the acute experiment.

These data are summarized in Table 17 . In several of the dogs blood

samples were drawn after the estrone injection on each day and plasma

estrone levels determined. After 2 to It days of treatment the estrone +4 i

+2

SECOND INFUSION g 0 I- K - 2 w X <3 j? "4

-6

-a

-10 -i-p- —T~~ 0.1 0.3 1.0 3.0 10.0 OXYTOCIN (U/min)

Figure 33 . Dose-response curvea for the mean ( + S.E.) percent change in heart rate (J(A heart rate) after two 10 min i.v. oxytocin infusions at dose rates of 0.95, 1.9, 3.8 and 7.6 U/min. The second infusion followed approximately one hour after the first* SOI Table 17 • Estrone and estradiole plasma levels (picograms/ml) before and after estrone 4 mg/day i.m. for 5 to T days1

Estrone Estradiol

W - 15

Control 100 + 13 37+3

Day of experiment UU39 + 1260* 2^26 + 115^*

^■Mean (+_S.E.) plasma levels

•Significant (p = 0.05 or less) increase in plasma level. Id!* levels tended to plateau; however, the ultimate level achieved showed marked variation among the 15 dogs. Figure 3^ shows the plasma estrone levels in dog #3 over the course of his estrone treatment. Note that the plateau level was reached on the third day of treatment but the

level achieved was much lower than that usually found on the day of the acute experiment.

Effect of estrone pretreatment on the response of mean systemic

arterial pressure to i.v. oxytocin infusion -

The 15 dogs pretreated with estrone were on the day of their

acute experiment subjected to i.v. infusion of oxytocin. Two successive

10 min infusions at a rate of 0.95 U/min were given as usual separated by a 60 min recovery period. Changes in mean systemic arterial pressure

(Pg} were followed. They were compared with those seen in the group of

3 control male dogs which received the same oxytocin dose (refer to

Figures 22,23,2U and Table 13). These data are summarized in Table 18 .

The 0.95 U/min oxytocin dose was previously shown to have no significant

effect on systemic arterial pressure in the control group of male dogs

(see Table 18 ). In the group of male dogs pretreated with estrone

this same dose of oxytocin produced a mean response which was slightly

depressor, but again not statistically significant.

Effect of estrone pretreatment on the vascular response of isolated

dog small intestine segments to i.v, oxytocin infusion -

Isolated, autoperfused small intestine preparations were also

done in the 15 estrone pretreated male dogs on the day of their acute

experiment. The change in gut segment vascular resistance seen during Figure 3k Plasna estrone.levels (pg/»l) in dog #3 over the course of Idsestrone treatsentfor 8 days.

ESTRONE (pg/ml) 1000 1200 600 200 i 1 2 1 1 DAY OF SAMPLE 1 O 3 21.2 kg DOG #3 I -HD- i

1 105 Table 18. Effect of estrone pretreatment on the response of mean systemic arterial pressure (Pg) to 10 min i.v. oxytocin at 0.95 U/min1

First infusion Second infusion

0 3 min 10 min 5£a 0 3 min %L 10 min %t

Control 127 + 3 128 + 3 +1 + 1 129 + 3 +2 + 1 132 + 3 13h + 3 +2 + 1 135 + 3 +2 + 1 N *= 8

Estrone 112 + 5 102 + 6 -9 + 2 10lf + 6 -7 + 2 112 + 6 111 + 6 -1 + 1 llii + 6 +3 + 1 pretreatment N = 15

*Mean (+_ S.E.) systemic arterial pressure {ramHg) after 3 and 10 min oxytocin infusion. Mean (+_ S.E.) percent changes {JCA) are also given. 106 107 10 min i.v. infusion of oxytocin at a rate of 0.95 U/min in these dogs was compared with that seen in the control group of 8 male dogs which had received the same oxytocin dose (refer to Figures 30,31,32 and Table 19).

These data are summarized in Table 19* Resistance was significantly

increased after 10 min in both the first and second infusions in the

control group. In the estrone treated group this was true only after the second infusion. However, in spite of this there was no statistically

significant difference in the mean percent changes in resistance when the

control and estrone treated groups are compared.

Effects of i.v. Infusion of Oxytocin on Hepatic Arterial Hemodynamics

and on Portal Venous Pressure

The response of the hepatic artery in terms of blood flow (Fg^),

arterial perfusion pressure (Fg^) suid calculated vascular resistance

(Rg^) was followed during two successive 10 min i.v. infusions of

oxytocin at 1.91 and 3.82 U/min in a series of 22 experiments. The

data are summarized in Tables 20 , 21 and 22 . In general the response

was vasodilator in nature, peaking after about 3 min infusion. For

example, after 3 min infusion at 3.82 U/min resistance decreased from

5.2 + 0.9 to 3.6 _+ 0.6 mmHg/ml*min*100g (-30 + 8jt). After 10 min

infusion resistance was U.2 ^ 0.7 mmHg/ml*min’100g or 20 +_ b % less

than its initial value (see Table 22). After a recovery period of about

60 min the oxytocin infusion was repeated. During the second infusion

the response was considerably less, in fact, in general it was statistically

non-significant. For example after 3 min of second infusion at 3.82

U/min resistance decreased from 6.0 +_ 0.8 to 5*7 +1*1 mmHg/ml*min*lOOg

or only 6 +_ 3% (see Table 22 ). Table 19- Effect of estrone pretreatment on the response of gut segment vascular resistance (R) to 10 min i.v. infusion of oxytocin at 0.95 U/min1

First infusion Second infusion

0 3 min %h 10 min 0 3 min %L 10 min U

Control 3.6+0.5 3.3+0.5 -8+2 h.l+O.k* +12+6 U.3+0.5 1+.2+0.5 ■ 3+2 5.0+0.5* +16+3 N = 8

Estrone 3.9+0.5 3.5+0.5 -9+1* 3.8+0.U - 3+9 h.3+0.5 h.3+0.5 0 l».8+0.5# +13+3 pretreatment N = 15

^Mean (j^S.E.) resistance (mmHg/ml*min*100g) after 3 and 10 min oxytocin infusion. Mean (+_ S-E-) percent changes (Jti) are also given.

•Value after 10 min infusion significantly {p = 0.05 or less) greater than that existing at zero time. 108 Table 20. Effect of i.v. Infusion of oxytocin on hepatic artery flow (F^ ) 1

First infusion Second infusion

Dose (U/min) 0 3 min it 10 min %t 0 3 min %t 10 min %t

1.91 26.9+3.8 28.8+1*. 3 + 7+6 27.7+1*.! + 3+3 19.1»+2.1 20.2+2.1 +lt+2 22.6+2.8 +16+1+ N = 9

3.82 20. lt+2.7 25-2+3.2* +23+3 2k.3+2.9* +19+3 19.0+1.9 20.14+1.9 +7+2** 22.0+2.1 +16+2 N = 13

1Mean (_+ S.E.) values after 3 and 10 min infusion

*Value at 3 and 10 min significantly {p = 0.05 or less) greater than value at zero time

^•Response during second infusion significantly less than that seen during first infusion

»-• o vo Table 21. Effect of i.v. infusion of oxytocin on hepatic artery perfusion pressure (Ph^)1

First infusion Second infusion

Dose (U/min) 0 3 min 10 min 0 3 min 10 min $1

1.91 102 + 5 90 + 7 -12 + k 93 + 8 -9 + ^ 99 + 10 100 + 10 +1 + 2•• 101 + 10 +2 + 2** N = 9

3.82 81t + 8 76 + 7 -9 + 2 83 + 8 -1 + 2 97 + 10 99 + 10 +2 + 2 « 101 + 10 +1 + 2** N = 13

*Mean (+ S.E.) values after 3 and 10 min infusion

••Response during second infusion significantly different from first infusion 110 Table 22. Effect of i.v, infusion of oxytocin on hepatic artery resistance (R^ ) 1

...... t ■ ■

First infusion Second infusion

Dose (U/min) 0 3 min 10 min 0 3 min 10 min JCA

1.91 It. 5+0.7 3,6+0.5“ -20+3 •». 1+0.7 - 916 5.5+0.6 5.1++0.6 -2+5** 5.1+0.5 -7+6““ N - 9

3.82 5-2+0.9 3.6+0.6* -30+8 I*.2+0.7* -20+1+ 6.0+0.8 5.7+1.1 -6+3** 5.6+1.1 -7+5** R = 13

*Mean ( + S.E.) values after 3 and 10 min infusion

“Value at 3 and 10 min significantly (p - 0.05 or less) different from value at zero time

““Response during second infusion significantly less than during first infusion Portal venous pressure (Ppy) was also recorded in this series of experiments. It was significantly decreased, generally to a greater extent during the second infusion than during the first. These data are summarized in Table 23. Table 23. Effect of i.v. infusion of oxytocin on portal venous pressure (Ppy)1

First infusion Second infusion

Dose (U/min) 0 3 min 10 min 0 3 min 10 min %t

1.91 6.6+0 .8 6.I1+O.7 -3+2 5.9+0.7* -11+3 5.9+0.8 5.6+0 .7 ~ 511 5.3+0.7* -10+3 N a 9

3.82 5.8+0,5 5.1t+0.1* -7+3 It. 9+0.5* -15+2 5. *1+0.3 It. 5+0.3* -17+2** tt.5+0.3" -17+2** N = 13

1Mean { + S.E.) values after 3 and 10 min infusion

*Value after 3 or 10 min infusion significantly{p = 0.05 or less) different than that seen at zero time

•^Response during second infusion significantly greater than that seen during first infusion 113 DISCUSSION

Interactions Between Adrenergic Mechanisms and the Cardiovascular

Response to Vasopressin

Introduction and significance -

The possible interaction between the neurohypophyseal peptide hormones and autonomic mechanisms upon the cardiovascular system has

received considerable research attention. One approach has been to

observe the effect of sympatholytic agents upon the response to vasopressin

infusion. Adrenergic blocking agents have been variously reported to

enhance ( 30, U5 ,87,88), attenuate C91) or have no effect (7 6 ,8 9) on the pressor action of vasopressin. Most of these studies have focused upon

the changes in systemic arterial pressure under these circumstances rather

than upon the behavior of specific vascular beds. In other studies vaso­

pressin has been reported to enhance the pressor response to adrenergic

stimulation ( 8 ), or activation of the autonomic system, on the other

hand, has been said to sensitize the vasculature to the constrictor action

of vasopressin (1 9). The mechanisms responsible for any of these observa­

tions still remain somewhat obscure.

The major interest in the cardiovascular actions of vasopressin

relates to its use in pharmacologic doses for the emergency treatment of

gastrointestinal bleeding, particularly when it is the result of portal

hypertension. Vasopressin exerts a potent constrictor action upon the

11U 115 precapillary vessels of the mesenteric circulation and in this fashion causes a reduction in portal pressure. Thus if its vasoconstrictor effect upon the prehepatic splanchnic arterial vessels were to "be enhanced by combination with some other pharmacologic agent, then its therapeutic efficacy in the reduction of portal hypertension might also be improved.

Hence, the present study not only focuses upon the basic mechanisms which might be involved in adrenergic-neurohypophyseal hormone Interactions upon the cardiovascular system but also in the spirit of applied pharmacology explores the possibility of modification of the constrictor effect of vasopressin in pharmacologic doses upon the mesenteric circulation by pretreatment with alpha-adrenergic blocking agents or combination with norepinephrine infusion.

Effect of adrenergic blocking agents on the pressor response to

vasopressin -

Administration of the alpha-adrenergic blocking agents phento- lamine and dibenamine appeared to result in both a relative (percent change) and absolute (mmHg pressure rise) increase In the response of mean systemic arterial pressure to i.v. infusion of vasopressin in anesthetized dogs (see Figures 8,9 and Tables ^j?). This finding is in general agree­ ment with most other studies of a similar nature. Alpha-adrenergic blocking agents and sympatholytic drugs such as reserpine have been found to enhance the mean systemic arterial pressure response to vaso­ pressin. The administration of beta-adrenergic blocking agents have shown no such effect. Erker and Chan (30) have reported that in anesthetized rats the arterial pressure increase in response to vasopressin was nearly doubled by pretreatment with phentolamine (Regitine) or phenoxybenzamine 116

Cdibenzyline) while beta adrenergic blockade with propranolol had no effect. The alpha-adrenergic blocking agents hydergine, dibenamine and tolazoline as well as the sympatholytic compounds reserpine and guanethidine, were found by Supek, et al. (87,83) to have an enhancing effect on the blood pressure response to vasopressin in anesthetized dogs, while the beta-adrenergic antagonist, pronethalol, did not. They were unable to demonstrate any significant effect of alpha blockade

(dibenamine and tolazoline) on the blood pressure response to vasopressin in rabbits or rats (8?)* However, Hazard and coworkers (U5 ) found several alpha blocking agents including phenoxybenzamine, dihydroergotamine and tolazoline effective in this manner in both dogs and rabbits.

In spite of all that has been done, no single cogent explanation for the above described observations has emerged. Erker and Chan (30) have proposed that the effect is related to some action of alpha- adrenergic blocking agents which facilitates the extraneuronal presence of norepinephrine and the accumulation of this catecholamine compliments or even potentiates the pressor action of vasopressin. They further showed that the contractile response of isolated rat aortic strips to vasopressin was doubled by pretreatment with phenoxybenzamine and norepinephrine in combination.

If alpha-adrenergic blocking agents are found to increase the systemic arterial pressure response to vasopressin as has been the usual case and was also found to be the case in the present study, this situation might be reconciled by yet another hypothesis. This would postulate the presence of a population of receptors for vasoconstriction which would accept either vasopressin or norepinephrine, the former of 117 course, having the greatest constrictor potential. Competitive inhibi­ tion of vasopressin by norepinephrine has been suggested (30).

Norepinephrine would occupy constrictor sites which would ordinarily be available for vasopressin. The adrenergic blocking agent would then effectively enhance the constrictor response to vasopressin by protecting these sites from binding with norepinephrine rather than with vasopressin.

Supek, et_ al. (8 7) have reported that in anesthetized dogs bilateral vagotomy has no effect on the blood pressure response to vasopressin either before or after alpha-adrenergic blockade. Doses on the order of

100 mU/kg produced a rise of about 15 mmHg before, as compared to about

130 mmHg after tolazoline in both intact and vagotomized animals. Gardier and coworkers (36,37,91) have studied the effects of baroreceptor activity upon the response to vasopressin. In dogs in which there was bilateral circulatory isolation of the carotid sinuses, near physiological doses of vasopressin (1-10 mU/kg) gave transient pressor responses of 15-20 mmHg. When the influence of both carotid and aortic baroreceptors was eliminated by carotid sinus denervation or by maintaining pressure in than constant, plus performing bilateral mid-cervical vagotomy, these same relatively small doses of vasopressin produced prolonged pressor responses of U0-50 mmHg. They conclude that when the baroreceptors are intact and there is a high level of sympathetic activity, even pharmacological doses of vasopressin may produce only modest and rather transient increases in systemic arterial pressure, largely as a result of the buffering action of the baroreceptors. This could be an important feature in the enhance­ ment of the systemic arterial response to vasopressin by alpha blockade as seen in this and other studies. Alpha-adrenergic blockade should 118 indeed decrease sympathetic activity and decrease the capacity for further depressor action in response to increased baroreceptor firing. This would in turn allow a more complete expression of the vasopressin pressor response as seen in the present study where vasopressin doses on the order of 50-100 mU/kg produced pressure increases of about 5 mmHg before, and of about Uo mmHg after, the administration of dibenamine (see Figure

9 and Table 5)■

Traber, Gary and Gardier (91) have, however, presented yet another finding in dogs which had been subjected to carotid sinus isolation and vagotomy. Phentolamine (Regitine) and phenoxybenzamine (dibenzyline) significantly decreased the vasopressin pressor response in the absence of baroreceptor influence. They suggest that vasopressin produces its pressor effect, in part, either by potentiating normally released catecholamines or by causing their release from sympathetic neurons.

However,no increase in post ganglionic sympathetic nerve activity could be detected by electrical recording during vasopressin infusion in their experiments. This finding may or may not be taken as evidence precluding enhanced norepinephrine release by the action of vasopressin on post­ ganglionic vasopressor fibers. Also, in this vein, Hertting and Suko

(U8) demonstrated that vasopressin did not facilitate release or inhibit rebinding of 3H-norepinephrine in an isolated perfused cat spleen preparation. These findings have led the above mentioned workers, as well as others, to the conclusion that part of the normal vasopressin pressor response is due to its causing an increased permeability of vascular smooth muscle receptor sites to catecholamines (3 6,3 7 ,9 1)*

Thus the action of any norepinephrine present might be enhanced by 119 vasopressin rather than the latter causing increased norepinephrine release. A decrease in the pressor response to vasopressin, if this were to be the finding following alpha-adrenergic blockade, would tend to support either of these concepts.

Another factor which frequently enters into discussions concerning potentiation of the action of pressor agents relates to the influence of the level of existing control blood pressure. It has been suggested that hypotension, that is, low blood pressure per se, will enhance the response to a pressor drug. This problem is difficult to investigate, since any experimental procedure which produces hypotension, such as hemorrhage, for example, also results in a myriad of other changes in the test system. Alpha-adrenergic blocking agents produce significant hypotension as was definitely the case in the present study. Other workers have argued that it is not the decreased blood pressure itself, but rather some other specific action of the blocking agents which is responsible for the enhanced response to vasopressin. This conclusion was based on the observation in certain studies that the pressor responses to angiotensin or barium chloride were not affected by alpha blockade nor was there any significant correlation between the blood pressure effect of vasopressin and the initial control arterial pressure,initial arterial pressure after the alpha antagonists,or degree of arterial pressure fall caused by these agents (30,88). The response to vasopressin infusion at a rate of 76U mU/min for 10 min as seen in the present study was analyzed in this fashion (see Figure 10 ). Control systemic arterial pressure ranged from 91 to 150 mmHg and its response to vasopressin from -5 to

+30#. Low correlation (r = -0.2U) existed between these values which 120 would indicate that, at least under control conditions, initial level of blood pressure over the range which existed was not a significant factor in determining degree of response to vasopressin. Administration of i.a. dibenamine (8 mg/kg) or i.v. phentolamine (0.4 mg/kg) decreased initial pressure by about 21% such that it ranged from 68 to 125 mmHg.

Under these conditions correlation between initial pressure and percent change after vasopressin was considerably better, r > -0.6. The i.v. administration of dibenamine in a dose of 20 mg/kg decreased initial pres­ sure by about 475 such that it ranged from 1+4 to 90 mmHg and the response to vasopressin from +30 to +175)5. In this the correlation was even better {r = -0.82). Another facet of these relationships also emerged in that the data describe three different regression equations, the negative slopes of which increase as the initial pressures involved cover a lower range. Thus adrenergic blocking agents lower initial pressure, increase both the relative and absolute responses of mean systemic arterial pressure to vasopressin and create a significant negative correlation between these variables which also appears to im­ prove with groups of data having lower initial arterial pressures. It would be very difficult to fully evaluate the significance of alpha blocking agent induced hypotension in their production of an enhanced response of mean systemic arterial pressure to vasopressin using the test systems employed in the present study.

Effect of alpha-adrenergic blockade on the response of specific

vascular beds to vasopressin -

Although there have been a number of studies concerning the effect of adrenergic blocking agents on systemic arterial pressure 121 response to vasopressin there are only very limited data available regarding similar effects in specific vascular beds. Davies and

Withrington (2 5 ) have reported some findings using isolated, constant pressure, blood perfused dog spleens. Adrenergic blocking doses of phenoxybenzamine increased the constrictor response of splenic vascular and capsular smooth muscle to vasopressin while phentolamine had no effect. They conclude that the effect is not due to alpha- adrenergic blockade, but rather to some other action which is peculiar to phenoxybenzamine, i.e., increased norepinephrine release or decreased uptake leading to vasopressin potentiation as discussed above. Norberg and Palmer (7 6 ) measured blood flow in denervated areas of rat skin using the 133Xe-clearance technique. Clearance rate was decreased by phenylalanine2-lysinea-vasopressin (Octapressin) induced vasoconstric­ tion to the same extent both before and after a dose of phentolamine which blocked the effect of norepinephrine on skin blood flow. Thus phentolamine appeared to have no effect upon the vasoconstrictor action of vasopressin in the dog spleen or rat skin. Swan and Reynolds {8 9) have presented data from some limited observations involving the canine superior mesenteric arterial vasculature. They report that the vasoconstrictor response to single i.a. injections of vasopressin was unaltered by either the alpha antagonist phenoxybenzamine or the beta-adrenergic blocking agent propranolol.

In the present study the effect of alpha-adrenergic blocking agents on the response of the prehepatic splanchnic vasculature to vasopressin was Investigated rather extensively. Alpha blockade with dibenamine had no significant effect on the increase in vascular 122 resistance of completely isolated, acutely denervated, autoperfused segments of dog small intestine after 10 min i.v. infusion of vaso­ pressin in doses ranging from 95 to 761+ mU/min (see Figures 5,6 and Table 2). Observations were also made on the response of the in situ canine superior mesenteric arterial vascular bed to 10 min i.v. infusions of vasopressin in doses ranging from 95 to 1528 mU/min, before and after alpha blockade with phentolamine, A large share of the periarterial innervation was spared in these preparations. Except for the lowest dose used, the resistance response to vasopressin was decreased after phentolamine; however, this was statistically signifi­ cant only with the two highest vasopressin infusion rates, ?6U and

1528 mU/min (see Figures 2,3 and Table 1).

Changes in portal pressure somewhat reflected this result also.

Infusion of vasopressin at 7 6U mU/min decreased portal pressure 16 +_ 2% and 12 +_ 35, while infusion at 1528 mU/min decreased portal pressure

27 +, 3% and 21 +_ 1%, before and after phentolamine, respectively.

Thus the decrease in portal pressure seen was less after alpha-adrenergic blockade, but not to a statistically significant extent. Less decrease in portal venous pressure would be consistent with an attenuation of the constrictor effect of vasopressin on the prehepatic splanchnic

(superior mesenteric) vasculature after alpha-adrenergic blockade.

Role of vasopressin tachyphylaxis in the interpretation of

experimental findings -

The overall significance of the above findings, and similar findings of others, is not entirely clear, since in the present study it was found that a second 10 min infusion of vasopressin given at a 123 rate of 76k mU/min or more when repeated 60 min after conclusion of the first infusion would sometimes have less effect on intestinal vascular resistance than did the first infusion even in the absence of alpha blockade. This raises the question of vasopressin tachyphy­ laxis. The literature concerning this phenomenon has been extensively reviewed in the INTRODUCTION section of this dissertation. The possible role of vasopressin tachyphylaxis and its effect upon the interpretation of the results seen in the present study was also investigated. Two successive 10 min vasopressin infusions were given separated by a recovery period of 60 min without the administration of an alpha-adrenergic blocking agent during this time. Isolated small intestinal segments were used as the test system. Four different infusion rates were used; 95» 191* 382 and 76^ mU/min. Changes in mean systemic arterial pressure during the second infusion were not signifi­ cantly different from those seen during the first; thus no evidence of vasopressin tachyphylaxis. Changes in gut segment vascular resistance at the three lower doses were the same in the second infusion as in the first. However, after 10 min infusion at 76^ mU/min mean percent changes in resistance were 228 +_ 23 and 171 +, 23, respectively; less during the second infusion, but not to any highly significant extent

(p = 0.05)* Again no evidence of vasopressin tachyphylaxis except possibly at the 76U mU/min dose, and even that not particularly con­ vincing.

Interactions between catecholamines and neurohypophyseal hormones

at peripheral vascular sites - 12k

The findings discussed in the previous section suggest that since the vasoconstrictor action of high doses of vasopressin on the prehepatic splanchnic vasculature can sometimes he attenuated by alpha-adrenergic blockade, part of this response may in some way be related to interactions between vasopressin and adrenergic neurotrans­ mitters. There are a number of studies in the literature which speak to this point in one way or another.

There is evidence of certain poorly understood interrelationships between the cardiovascular responses to vasopressin and adrenergic mechanisms. Several studies have indicated that this occurs directly at a vascular site rather than by way of some central mechanism. It has been shown by Chenoweth, et al. (19) that the administration of certain pressor amines, including norepinephrine, sensitizes the in­ testinal and subcutaneous vascular beds of dogs, cats and monkeys such that they show a profound constriction in response to carotid occlusion

(19). The carotid occlusion supposedly caused the release of vasopressin in physiological amounts and this potentiated the constrictor capabilities of the pressor amines at certain peripheral vascular sites.

Hash, et al. (71) found that in the dog, vasopressin reversed isoproterenol vasodilation and potentiated the pressor response to epinephrine, an effect which might be explained on the basis of selective blockade of peripheral vasodilating beta receptors in certain resistance vessels without affecting beta receptors in the myocardium ajad hence causing no block of epinephrine induced cardiac stimulation.

However, under other circumstances Skivolocki and Thomford (81) advise, on the basis of their results in dogs, the combined administration 125 of vasopressin and the beta receptor agonist isoproterenol to the effect that the depressed cardiac output which usually occurs in human subjects during vasopressin therapy, would be overcome and the systemic pressor response would be attenuated. Certain findings of

Bartelstone and Nasmyth ( 8 ) are in line with the latter concept.

These workers have made the statement that "in a limited number of experiments, the effect of vasopressin on the vascular response to isoproterenol (beta-adrenergic vasodilation), while not predictable, did produce increased depressor response in several runs/* Again this suggests potentiation by vasopressin of the peripheral vaso­ dilator effects of isoproterenol or attenuation of the peripheral vasoconstrictor effects of vasopressin by isoproterenol; whichever way one might care to look at it.

Finally, it should be added that Bartelstone and Nasmyth ( 8 ) have done studies which seem to definitely indicate interactions between vasopressin and the sympathetic system at the peripheral vascular level. Arginine vasopressin given in physiological amounts was found to modify pressor responses to catecholamines exogenously administered or endogenously released. Vasopressin in nonpressor doses potentiated the pressor response to administration of catechol­ amines in anesthetized dogs, pithed rats and spinal cats; potentiated the pressor response to bilateral carotid occlusion in anesthetized dogs; and potentiated the response of isolated rat aortic strips to norepinephrine or epinephrine. In conclusion, they postulate, "that vasopressin may modify an intermediate system with consequent accumula­ tion or depletion of secondary substances capable of altering the magnitude of smooth-muscle response to catecholamines". It is suggested that this might have something to do with modification of the cyclic adenosine 31, 5’ monophosphate system by vasopressin.

The present study includes some limited observations on the possible interaction between norepinephrine and vasopressin at peri­ pheral vascular sites. Isolated, autoperfused dog small intestine segment preparations were used. Norepinephrine and vasopressin were infused alone and in combination with each other. The sum of their individual effects on gut segment vascular resistance and mean systemic arterial pressure was compared with their combined effects (see

Figure ll). Even though their combined effects were somewhat greater, it did not prove to be statistically significant from the sum of their individual effects (p > 0.3). Thus the present study did demonstrate an additive pressor effect of vasopressin and norepinephrine but was unable to establish evidence of significant potentiation in spite of the fact that such potentiation has been strongly suggested by the results of several of the studies cited above.

Summary of findings in the present study -

Alpha-adrenergic blockade with either dibenamine or phentolamine appears to enhance the pressor response of mean systemic arterial pressure to pharmacological doses of vasopressin.

The mechanism involved is not understood; however, it may, in part, be due to the hypotension produced by the blocking agents, as well as decreased sympathetic tone with loss of the baroreceptor reflex mediated buffering action which normally attenuates the pressor action. The superior mesenteric arterial vasculature apparently does not participate 127 in the enhancement of the pressor response to vasopressin. The therapeutic efficacy of vasopressin in control of gastrointestinal bleeding and lowering of portal pressure would not be improved by pretreatment with alpha-adrenergic blocking agents. In fact, there is some evidence that portal pressure was decreased less by vasopressin after phentolamine than before and higher rates of vasopressin infusion tended to show less vasoconstrictor action on the superior mesenteric vasculature under these circumstances. It was postulated that at least a small part of the constrictor response of this vascular bed to vaso­ pressin might be due to the action of vasopressin on postganglionic vasoconstrictor nerve endings to cause the release of norepinephrine or by potentiating the action of catecholamines already present at the site. However, in the experimental test system which was used, the combination of vasopressin and norepinephrine resulted in a pressor response which appeared to be additive with little or no evidence of potentiation.

Response of the Hepatic Arterial Vasculature to the Administration of

Vasopressin

Introduction and significance -

The response of the hepatic artery to the administration of vasopressin has received considerable research attention. It was

at one time suggested that the infusion of vasopressin for the control of gastrointestinal hemorrhage might produce damaging ischemia of the

liver if the hepatic artery were to be constricted to the same extent as the pre-hepatic splanchnic (superior mesenteric) vasculature. Under

such circumstances the liver would be deprived of both hepatic arterial

and portal venous inflows to a large extent. Vasopressin, when given 128 in a variety of dosage regimens, has since been reported to produce a dilation of the hepatic artery; however, in some studies this was preceeded by a transient constriction (7,21,32,1*2,53,57). The type of response seen can depend upon the total amount given; whether it is administered as a single bolus or infused over a period of time at a certain rate; and also whether it is given by way of a systemic vein or directly into one of the major splanchnic arteries.

The mechanisms responsible for neither the transient vasoconstric­ tion, nor the subsequent dilation have, as yet, not been at all clearly elucidated. It has been suggested that the dilation is, at least in part, a manifestation of the reciprocal increase in hepatic arterial blood flow which is known to occur when portal venous flow is decreased

f1*1*). If this were the case, patients with reversed or even with very- low forward portal venous inflow due to liver disease or surgical portal-systemic venous anastomosis, would very likely not derive the benefit of a hepatic artery vasodilation phase during vasopressin therapy. Since portal venous perfusion of the liver would already be very low in these cases, one might assume that the hepatic artery would be in a stage of maximal reciprocal dilation even before the onset of vasopressin infusion. Hanson ( Uo ) has shown that the vaso­

dilator response of the hepatic artery to certain drugs such as papaverine,

to post occlusion reactive hyperemia and to other hemodynamic manipulations which cause its dilation is greatly attenuated by

decreased portal venous inflow.

It is at least apparent that the constrictor potential of the

hepatic artery with respect to vasopressin is limited when compared 129 with that of other vascular beds, such as the superior mesenteric, where little or no escape occurs from the intense sustained vasocon­ striction which attends vasopressin infusion (21,1+2). Thus, hepatic arterial dilation might be, in part, a passive consequence of the intense constriction which occurs elsewhere. This phenomenon is sometimes called the "steal mechanism."

Active vasomotor mechanisms of one sort or another might also be involved in the biphasic response of the hepatic artery. There has been much investigation into the possible interactions between vasopressin and adrenergic mechanisms upon the vasculature. Alpha- adrenergic blocking agents have been variously reported to enhance

(30,87), have no effect on (7 6 ,8 9) or even attenuate (9l) the pressor action of vasopressin. The latter finding has led to the suggestion that part of the pressor response is due to the accompanying release of catecholamines and their subsequent potentiating interaction upon the vascular smooth muscle (36,91). These points have been discussed in a previous section of this dissertation.

The present study was initiated in an effort to examine some of the above mentioned suggestions concerning the behavior of the hepatic artery. Hepatic artery blood flow perfusion pressure (Pj^) and calculated vascular resistance (Rjia) were followed during 10 min intravenous infusions of vasopressin at two different dose rates.

Changes in mean systemic arterial pressure (P^) and portal venous pressure (Ppy) were also followed. The response under control con­ ditions was compared with that seen following alpha-adrenergic blockade with dibenamine, or during total shunting of portal venous flow into 130 the systemic venous system, or a combination of the two situations.

The findings, along with those of another recent report from other workers (16), offer some insight into the mechanisms involved in the biphasic response of the hepatic artery to the infusion of vasopressin.

Responses of the hepatic artery to various modes of vasopressin

infusion -

In the present study the hepatic arterial vasculature of anesthetized dogs was found to undergo a biphasic response, an initial transient vasoconstriction followed by a dilation, during the

10 min intravenous infusion of vasopressin at two different dose rates.

Doses were 382 and 76U mU/min, or about 20 and Uo mU/min*kg. At the higher dose, hepatic artery resistance was increased 31 ±_ 8% after 3 min of infusion but then declined until it was lU +_ 3>% below its initial level by the end of the infusion, that is, after 10 minutes (see

Figures lU, 15,18and Tables 9,10 ). Portal venous pressure declined throughout the entire infusion period and was decreased by about 25% after 10 minutes (see Table 12). Mean systemic arterial pressure increased throughout the entire infusion period and was increased by about 1&% after 10 minutes (see Figure 20 ).

Many of the studies in which vasopressin has been given as a continuous infusion of several minutes duration have utilized the celiac or hepatic artery as a route of administration (7,16,32,53,83 ).

This will produce a characteristic biphasic hepatic artery response of the magnitude which we have seen with i.v. infusion, however, a much lower dose is required when it is given by the intra-arterial route. Kerr, et al. (53), using anesthetized dogs, made 10 min 131 infusions of vasopressin into the hepatic artery by way of its right gastric branch in doses of 0.05 - 50 mU/min*kg. A dose of only 5 mU/min-kg increased hepatic artery resistance by about 31%> after 1 min infusion and subsequently resistance declined such that by the end of the 10 min infusion it had essentially returned to its pre-infusion level. In order to produce a comparable response by i.v. administration the infusion rate must be seven times or more greater as evidenced by our experiments and in other studies as well (42).

Intravenous infusions of vasopressin at rates slower than those used in our experiments will not produce a biphasic hepatic artery response, however, the secondary vasodilator component does occur.

Simmons, et al. (83), also using anesthetized dogs, infused vasopressin at a rate of about 3 mtl/min*kg for 60 min using three different routes of administration; hepatic artery, systemic vein and superior mesenteric artery. The biphasic hepatic artery response was seen only when infusion was into that artery. In all three cases hepatic artery dilation intimately took place. Its magnitude and time of onset appeared to follow the onset and course of the accompanying reduction of superior mesenteric artery flow. This, and similar observations by others (7,21,32,42,83 ), has led to the following explanation for

the biphasic response. Vasopressin has a primary constrictor effect

on the hepatic artery. However, it is opposed by a dilator component;

an intrinsic hepatic vasomotor mechanism which is said to occur as a

consequence of decreased portal venous flow. A reciprocal hepatic

artery vasodilation in response to decreased portal venous flow has

been well documented by Hanson and Johnson (44) and other workers. 132

According to this concept, the reciprocal dilator component would have to exert dominance to the extent that the primary constrictor component is seen only when vasopressin infusions are made directly into the hepatic artery or by another route at a rate fast enough to allow an adequate drug concentration to reach the hepatic arterial site such that transient constriction occurs before dilation super­ venes. Cohen, et al. (21) have reported data on the response of anesthetized cats to 5 min i.v. infusions of vasopressin which would also appear to support this concept. Doses ranged from 0.5 to 50 mU/min*kg. Optimum hepatic arterial vasodilation was seen with a dose of 10 mU/min*kg, while after 5 min infusion at higher rates vasoconstriction became more in evidence and the net effect was only a slight dilation.

Single i.v. bolus injections of 100 mU/kg vasopressin in dogs were found by Lechln and coworkers (57) to also produce a somewhat biphasic response of hepatic artery flow. It was noted that elevated hepatic artery flow persisted even after full recovery from the reduction in portal flow. They conclude that some factor other than decreased portal flow must, at least in part, be responsible for the secondary increase in hepatic artery flow. Accumulation of vaso­ dilator metabolites as a consequence of reduced liver blood flow (^2) or other "autoregulatory escape" mechanisms (53,57) have been suggested as mediators of the dilator phase.

Effect of alpha-adrenergic blockade with dibenamine on the

response of hepatic hemodynamics to vasopressin infusion with

intact portal venous flow - 133

There has been much interest in the possible interactions between vasopressin and autonomic mechanisms upon the peripheral vasculature. Generally, alpha-adrenergic blocking agents have en­ hanced the pressor effect of vasopressin on systemic arterial pres­ sure (30,87) and that proved to be the case in our experiments in which dibenamine was given. This has not been the result in every study, however. Traber, Gary and Gardier (91) found that in anesthetized dogs which had been subjected to carotid sinus isolation and vagotomy, alpha-adrenergic blockade reduced the pressor response to vasopressin.

It has been suggested, on the basis of this as well as other experi­ mental findings, that part of the pressor response to vasopressin might be the result of released catecholamines which add to, or even potentfate, the action of vasopressin (36,91). This concept has been discussed in detail in a previous section of this dissertation.

In our series of liver experiments we were able to abolish the initial transient constrictor response and enhance the dilation of the hepatic artery seen during i.v. infusion of vasopressin. This was accomplished by alpha-adrenergic blockade with dibenamine.

Resistance progressively decreased throughout the entire 10 min period of vasopressin infusion after dibenamine pretreatment (see Figure 18 and Table 10 ). This would suggest that, at least in this vascular bed, when a constrictor response to vasopressin is seen, activation of alpha—adrenergic receptor sites is involved to a very significant

extent. This would be in contrast to our findings with the prehepatic

splanchnic (superior mesenteric) vasculature in which vasopressin

infused at the same dose produced an intense and sustained 13** vasoconstriction which was only modestly attenuated by alpha-adrenergic blockade. This is reflected in comparison of the changes in portal venous pressure seen during vasopressin infusion before and after

dibenamine in the series of liver experiments. After 10 min infusion

at 7 mU/min it was decreased 25# under control conditions and by

only 16# after 10 min vasopressin infusion at mU/min after

dibenamine (see Table 12). This might be taken as evidence for a

decrease in the constrictor action of vasopressin upon the prehepatic

splanchnic (superior mesenteric) vasculature after alpha-adrenergic blockade.

The vasodilator component of the response of the hepatic artery

to vasopressin infusion, on the other hand, is very likely mediated by more than one mechanism. It could, in part, be the passive result

of increased blood flow shifted from other areas which are more

intensely vasoconstricted by a direct and sustained action of vaso­

pressin. Both the relative and absolute increase in mean systemic

arterial pressure during vasopressin infusion was greatly enhanced

by dibenamine in this series of experiments (see Figure 20 )• After

10 min Infusion at 76*+ mU/min it was Increased by only about 18#

under control conditions as compared to about 75# after dibenamine

pretreatment. This was also the case with respect to the increase

in hepatic artery flow (see Figure 17). It was increased by about

33# after 10 min infusion at 76U mU/min under control conditions as

compared to 125# after 10 min infusion of vasopressin at 76U mU/min

following dibenamine. This enhanced hyperemia of the hepatic artery

along with an enhanced systemic arterial hypertension seen during 135 vasopressin infusion following dibenamine could conceivably be a reflection of a marked generalized vasoconstriction in which the hepatic arterial vascular bed does not participate.

It should be noted that the 20 mg/kg dose of dibenamine actually did not enhance the absolute value of hepatic artery flow attained during vasopressin infusion (see Figure 15). Dibenamine itself resulted in a U6j£ decrease in blood flow such that even though it was then more than doubled by vasopressin infusion it did not reach the levels attained after only a increase seen during vasopressin infusion before dibenamine. This is not to say that hepatic arterial hyperemia during vasopressin infusion might be absolutely enhanced by some other dosage combination of the two drugs.

Effect of alpha-adrenergic blockade with dibenamine on the

response of hepatic hemodynamics to vasopressin infusion during

shunting of portal inflow Into the systemic venous system -

There is some evidence, however, that the hepatic artery vasodilation in response to vasopressin results, at least in part, from some active mechanism. The reports of a close relationship between the secondary rise in hepatic artery flow and the fall in portal venous flow seen during vasopressin infusion, and the well documented reciprocal decrease in hepatic arterial resistance which accompanies a decrease in portal venous perfusion of the liver, have been discussed above. Insight into this might be gained by observing the be- havior of the hepatic artery during vasopressin infusion in the absence of portal venous inflow to the liver.

We ran a series of liver experiments in which the portal flow 136 was shunted into the systemic venous system by way of an extracorporeal circuit. The characteristic biphasic response of hepatic artery resistance during vasopressin infusion was still seen under these circumstances. The relative (percent change) increases seen at the peak after 3 min infusion were not significantly different from those seen with portal flow intact. However, in the absence of portal venous inflow there was significantly less secondary decline in hepatic artery resistance following its peak increase after 3 min infusion (see Figure 19).

After 10 min vasopressin infusion at T6U mU/min with portal flow intact resistance was about ±3% below its initial level while with portal venous flow shunted it was about 6% above its initial level at the end of the

10 min vasopressin infusion. Thus, part of the vasodilator phase of the hepatic arterial response to vasopressin could be the above discussed reciprocal response to decreased portal flow. With portal flow already ablated vasopressin infusion could result in no further reduction of this parameter. Hence, the part of the vasodilator phase resulting from the usual vasopressin induced reduction in portal flow was lost under these circumstances.

Finally, the effect of vasopressin infusion on the hepatic artery following alpha adrenergic blockade with dibenamine along with the ab­ sence of portal venous flow was observed. Again, as was the case following dibenamine with portal venous inflow intact, no transient in­ crease in hepatic artery resistance was seen during vasopressin infusion.

Resistance declined throughout the entire infusion period, but not to the extent seen after dibenamine with portal venous flow intact; - 1U£ as compared to - 28$ in the former case. One might conclude from this that part of the hepatic arterial vasodilator phase seen during 13T vasopressin infusion in livers with intact portal venous inflow is due to some other factor in addition to the reciprocal dilator response of the hepatic artery to the vasopressin induced decrease in portal venous inflow.

We have no experimental evidence to indicate what this other factor might be. However, Bynum and Fara (l6) have recently presented a pre­ liminary report of some findings which may add a great deal toward com­ pletion of the picture. They found that the i.a. infusion of vasopressin in dogs produced the same magnitude of transient increase in hepatic arterial resistance, followed by an identical decline in resistance, whether portal inflow to the liver was in its natural intact state, pump perfused at constant 30 ml/rnin, or completely absent. Hence, con­ trary to our findings, there was no evidence at all of a relationship * between vasopressin induced arterial dilation and decreased portal flow.

The administration of the beta adrenergic antagonist propranolol in­ creased the initial constriction and abolished the secondary dilation.

They conclude that the hepatic artery has relatively few vasopressin constrictor receptors and their effect is overcame by a beta adrenergic dilation, possibly mediated by way of released epinephrine.

Summary and conclusions from present study -

In summary, the i.v. infusion of vasopressin results in a bi- phasic response of hepatic artery resistance; a transient increase

followed by a decrease. Therapeutic doses of vasopressin do not pro­ duce a threatening sustained decrease in hepatic artery flow, but can even result in an increased blood flew through this vascular bed. This will occur to a significant extent even in the face of a portal-caval 138

shunt. Alpha adrenergic blockade -with dibenamine will enhance the pressor response of systemic arterial pressure to vasopressin and, on the other hand, abolish the transient vasoconstrictor response and

enhance dilation of the hepatic artery. Action of released catechol­

amines upon alpha receptors may be responsible for the transient constric­ tion of the hepatic artery. The secondary hepatic arterial vasodilation during vasopressin infusion is only partially abolished by diversion of portal venous inflow away from the liver. The dilator response may be the

result of several factors, including: a reciprocal dilation of the hepatic

arterial vessels due to the decrease in portal venous pressure and flow which occurs during vasopressin infusion; a passive increase in hepatic

artery flow due to diversion of blood flow from those vascular beds which

undergo a marked and sustained contriction during vasopressin infusion

into one upon which vasopressin exerts a very limited direct constrictor

effect; and possibly other vasodilator mechanisms, such as accumulation

of metabolites or activation of beta adrenergic receptors.

The Cardiovascular Effects of Oxytocin

Introduction and significance -

There has been a considerable volume of literature published

concerning the cardiovascular effects of oxytocin. This has been reviewed

in detail in the INTRODUCTION section of this dissertation. Most of

these studies have dealt with the effects of oxytocin on central

parameters such as heart rate, mean systemic arterial pressure, etc.

Relatively few studies have been done which provide information on

the effects of oxytocin on specific vascular beds.

The results concerning the cardiovascular effects of oxytocin

are also very inconsistent and confusing. Its effects seem to be 139 modified "by a variety of factors including dose, route of administra­ tion, sex, reproductive status of the animal, preservatives added to commercial pharmaceutical preparations and also the species in question.

Oxytocin is routinely given in pharmacological amounts in obstetrics because of its ability to contract the uterus. Indications for Its use include; induction of labor at term, to control post partum hemorrhage, to correct post partum uterine atony, to induce uterine contraction after uterine surgery and to induce second tri­ mester therapeutic abortion. It is given in amounts of 10 to 30

Units either as a continuous i.v. drip, as buccal tablets, nasal spray, as an intramuscular injection or even in emergency situations as a single i.v. bolus. Recent concern has developed over reports of sudden unexplained cardiovascular deaths when oxytocin was given post partum as a single i.v. bolus (86).

Effects of i.v. oxytocin on mean systemic arterial pressure -

Oxytocin has been variously reported to increase, decrease or have no effect on mean systemic arterial pressure in dogs.

Assali, Holm and Parker (U) administered synthetic oxytocin (Syn- tocinon, Sandoz) to bitches and ewes in amounts ranging from 3 to

60 mU/kg by single i.v. bolus or continuous i.v. drip. Animals were tested at various times throughout pregnancy, at term and during labor. At no time did they find any significant changes in either cardiac output or mean systemic arterial pressure which could be attributed to oxytocin. Others (12,23,31,62,7^*93) have also reported equivocal effects of pure synthetic oxytocin on blood pressure in the canine. iko

Woodbury and Abreu (93) have found oxytocin (Pitocin, Parke-

Davis) to increase or have no effect on blood pressure in dogs. It was given as a single i.v. bolus of 10 U/kg. They attribute the pressor effect to vasopressin material in the oxytocic posterior pituitary preparations which were in use at that time. There are few reports of pure synthetic oxytocin producing any significant rise in mean systemic arterial pressure in laboratory animals or in humans under ordinary experimental conditions. Nakano (6 9) has reported that i.v. doses of Syntocinon (ca. 3.2 U/kg) produced

10-30 mmHg rises in mean systemic arterial pressure in rats, guinea pigs, rabbits and opossums. Spurgeon (86) found 10-25 mmHg rises in arterial pressure in rats and rabbits receiving i.v. doses of

100-800 mU/kg.

Nakano and Fisher (71), on the other hand, gave pure synthetic oxytocin (Syntocinon) to male and estrous female dogs in single i.v. doses of 0.5 U/kg and found significant (ca. 31 mmHg) decreases in mean arterial pressure. Pure synthetic oxytocin has usually been found to decrease systemic arterial pressure in human subjects also. Weis,

et al. (92) found that a single 0.1 U/kg i.v. bolus of oxytocin also caused a 31# decrease in mean systemic arterial pressure in women in their first trimester of pregnancy. Mazhar, et al. (66) found that a

single i.v. bolus of Syntocinon caused a modest drop in blood pres­

sure in female human subjects; 10-25 mmHg drop in systolic and a 10-15 mmHg drop in diastolic pressure. Kitchen, Lloyd and Pickford (5*0

gave i.v. infusions of Syntocinon to male and female human subjects

at 500 to 1000 mU/min. After 300 to 500 mU had been given mean blood lUl pressure had decreased an average of 21 percent. Spurgeon (86) reports that in three Rhesus monkeys 2.5 U oxytocin i.v. caused a

505 drop in mean systemic arterial pressure. He concludes that lover primates are the only suitable animal model for study of the cardiovascular effects of oxytocin in man and that the oxytocin- induced hypotension seen in monkeys and man is probably due to peripheral vasodilation and not direct myocardial depression.

Katz (51) found that in human subjects 10 Units (ca. ll+0 mU/kg) of Syntocinon i.v. produced a biphasic response in mean systemic arterial pressure. There was a transient 20-Uo mmHg fall in pres­ sure 30 to 60 sec after the injection followed by a 5-15 mmHg rise in pressure which lasted from 2 to 5 minutes. In the present study mean systemic arterial pressure also showed a biphasic response during 10 min I.v. infusions of oxytocin. During infusion pressure reached a minimum after about 3 min and then began to rise back to­ ward its preinfusion level during the remainder of the 10 min infusion period. Thus during infusion at a rate of 1.9 U/min after 3 min about 285 mU/kg had been given and systemic pressure had been decreased by about 12 mmHg (95), while by the end of the infusion when about

950 mU/kg had been given it was only 3 mmHg below its initial value

(ca - 35). During infusion at a rate of 7.6 U/min after 3 min about lU^O mU/kg had been given and systemic pressure had decreased by about

23 mmHg (2^5), while by the end of the infusion when about 3300 mU/kg had been given it had increased 3 mmHg above (+^5) its initial level.

The lower dose (1.9 U/min for 10 min) even exceeds the maximum thera­ peutic doses of 30 Units (ca. U20 mU/kg) which are given to humans. 1U2

After a recovery period of about 60 min the oxytocin infusions were repeated. No significant transient decrease in mean systemic arterial pressure was seen during the second infusion at any dose rate, but rather it continued to show a modest rise throughout the second infusion (see Figures 21,22 ). During infusion at a rate of J.6

U/min, after 10 min when about 3800 mU/kg had been given pressure had risen by about only 9 mmHg (9%) above its initial value. Thus pharmacologic doses of synthetic oxytocin in the canine produce only a modest and transient decrease in mean systemic arterial pressure which is followed by little or no tendency for systemic pressure to increase above its initial level even though additional oxytocin is administered. At the present time there is no cogent explanation as to why there is an apparent tachyphylaxis to the transient decrease in systemic pressure.

Effect of oxytocin on heart rate -

In most experiments oxytocin has been reported to cause an increase in heart rate in the dog. Nakano and Fisher (71) found that

0.5 U/kg i.v. doses of synthetic oxytocin increased heart rate by about lk beats/min in both male and female dogs. The tachycardia might be attributed to reflex baroreceptor activity in response to the fall in mean systemic arterial pressure. Kitchen, Lloyd and Pickford (5M report tachycardia in male and female human subjects to be associated with the decrease in systemic arterial pressure. Some workers have suggested that oxytocin may exert a direct positive chronotropic effect.

Spurgeon (85) reports a profound tachycardia following 2.5 Units oxytocin i.v. in monkeys. Beta-adrenergic blocking agents eliminated 1U3 the tachycardia but had no effect on the depressor action of oxytocin.

Also,they found oxytocin to have no effect on isolated papillary muscle in vitro. Thus,they conclude that the tachycardia produced by oxytocin in monkeys and humans is primarily reflex in origin, that is, due to the fall in blood pressure. Covino (23), on the other hand did experiments with in vitro isolated atria and papillary muscles from cats. He found no change in atrial rate or amplitude of contraction until

Syntocinon concentration in the bathing fluid was raised to 90 mU/ml.

This caused a significant decrease in atrial contraction rate. In the papillary muscle preparation the amplitude of contraction was increased (positive inotropic effect) when the concentration of

Syntocinon in the bathing fluid reached 15 mU/ml.

The present study found that pharmacological doses of oxytocin had no significant effect on heart rate in the anesthetized canine.

However, it should be pointed out that heart rate was determined at the beginning and end of the 10 min oxytocin infusion when a slight bradycardia was seen (see Figure 33 ). No heart rate values were taken at the 3 min point in time when mean systemic arterial pressure was going through its transient minimum.

Effect of oxytocin on the peripheral vasculature -

Oxytocin has been variously reported to increase or decrease peripheral vascular resistance. Nakano (69) has reported increased peripheral resistance in rodents and marsupials given intraarterial injections of synthetic oxytocin (0.1 U/kg). Weis (921 has postulated from his observations on the effects of oxytocin on pregnant women that it acts to stimulate all beta receptors leading to decreased 1UU peripheral resistance accompanied by a positive inotropic and positive chronotropic effect on the heart such that there is little or no change in mean systemic arterial pressure during, continuous infusions even though single bolus injections caused a decrease in mean systemic arterial pressure (see above}.

Barnes has studied the effect of Syntocinon on renal blood flow in the dog using the ®sKrypton method. In 1970 (6) she reported a dose dependent increase in canine (unanesthetized trained females) renal blood flew when oxytocin was given (lto 100 mU/min infused into the renal artery). Since the response was not affected by autonomic blocking agents it was implied that this was a direct action on the kidney vasculature and not a reflex nor something involving autonomic receptor

sites. In 1973 (6} she reported that Syntocinon had no consistent

effect on total renal blood flow in dogs but did cause a dilation of

the postglomerular vasculature particularly the vasa rectae. Assali,

et al. (U) using chronically implanted electromagnetic flow transducers

report that Syntocinon had no effect on total renal blood flow in

pregnant ewes.

Oxytocin, has a contractile effect upon uterine smooth muscle.

This action is particularly intense and sustained at term of gestation

Assali, et al. (U) noted that in pregnant ewes followed throughout

gestation it was only at term that 500 mU/kg doses of oxytocin caused

significant decreases in uterine blood flow. At this time rhythmic

uterine contractions attained pressures of 25-50 mmHg. Oxytocin

increased the intensity of these contractions and decreased uterine

blood flow (about - 2556) by virtue of decreased transmural pressure 1U5

in the vasculature, a simply passive effect. Klingenberg (56)

observed a similar decrease in uterine blood flow in non-pregnant women undergoing pelvic laparotomy. Doses of oxytocin used were 5

to 10 Units. It is suggested that at least part of the decrease in

uterine blood flow was due to active constriction of its vasculature.

Davies and Withrington (25) made some observations on the actions

of oxytocin on the isolated blood perfused dog spleen. A dose of

0.5 U was given by close intraarterial injection. They conclude

that both the capsular and vascular smooth were contracted by oxytocin.

Splenic vascular resistance was increased by 1000 percent with only a

10 ml decrease in splenic volume. Ayers, et al. (5) using isolated

perfused human spleens found oxytocin to cause a slight vasodilation

with no effect on the splenic capsule. They attributed the slight

vasodilation to the presence of the preservative chlorobutanol in the

commercial oxytocin preparation used.

Nakano and Fisher (71) have studied the effect of Syntocinon

infused i.a. on vascular resistance in several vascular beds in

anesthetized male, estrous female and pregnant dogs. The dose given

was 5 mU/kg. The vascular beds under study were pump perfused. Only

limited observations were made; however, vascular resistance appeared

to be decreased by Syntocinon in the coronary, carotid, brachial,

femoral and superior mesenteric vasculatures. Kitchen, et. al. (5 M

have reported significant increases in human forearm blood flow

measured by plethysmography following i.v. doses of Syntocinon in the

range of 200-800 mU. In another study (55) this same group found that

oxytocin caused a fall in human hand blood flow following local nerve lU6 block. Again this raises questions as to the differences between

direct and reflex actions of oxytocin and the influence of the

autonomic nervous system upon its direct peripheral vascular actions.

Fortner, et al. (33) give results on the effects of oxytocin on

canine coronary blood flow which again raise the question concerning

the influence of ethanol and chlorobutanol used as preservatives in

Syntocinon and other commercial preparations. They concluded that pure (preservative-free) oxytocin had a direct vasoconstrictor action

on the coronary vessels while a mixture of pure oxytocin and chloro­

butanol acted like commercial Syntocinon. This gave a biphasic

response of coronary blood flow, an increase followed by a decrease.

Goldman (38) using the S6Rubidium technique studied the effect of

preservative-free oxytocin and commercial Syntocinon on several vascular

beds in male Wistar rats. They were infused at a rate of 90 mU/kg*min.

Seminal vesicle, prostate and kidney blood flows were significantly

increased by both preparations, while cardiac output was not affected.

This suggests a physiological role for oxytocin in the male. The male

rat neurohypophysis contains significant amounts of oxytocin which can

be released by various stimuli including mechanical stimulation of the

genital tract. The observation of significance to the present dis­

cussion which emerged from these rat experiments was that intestine

and liver blood flows were only increased when preservative-free

oxytocin was used.

Effects of i.v. oxytocin infusion upon the canine splanchnic

vasculature as observed in the present study - iVr

The present study utilized preservative-free pure synthetic oxytocin made up in a saline solution. The test system used was

isolated, autoperfused, acutely denervated segments of dog small

intestine. The oxytocin was infused i.v. for 10 min at different

rates. Thus it can he safely assumed that any observed effects on

gut segment vascular resistance would be a direct effect of oxytocin

on its vasculature. Resistance in the vasculature of the isolated

gut segments showed a biphasic response to i.v. oxytocin infusion.

There was an initial transient decrease in resistance which reached

a minimum after about 3 min of infusion. As the infusion was continued

throughout the remainder of the 10 min infusion period, vascular

resistance increased to levels above it preinfusion values (see Figures

29,30). The initial transient vasodilator response did not appear

to be dose dependent in nature but was about the same after 3 min

infusion at any dose rate (see Figure 30) which would indicate that

oxytocin might have a direct vasodilator action upon this vascular

bed rather than the alternate conclusion that low doses of oxytocin

dilate while higher doses constrict. It is also possible that oxytocin

causes the release of something which is responsible for the initial

transient vasodilation.

During 10 min i.v. infusion of oxytocin at 1.9 U/min resistance

decreased from 3.9 0.5 to 2-9 +. 0.6 mmHg/ml-min*100g (-26%) after

3 min infusion (see Table 16). After 10 min infusion resistance was

5.0 +_ 0.8 mmHg/ml*min*100g (+28#). During infusion at a rate of 7.6

U/min resistance decreased from 3.5 +_ 0.8 to 2.7 +_ 0.8 mmHg/ml*min*lOQg

(—23#) after 3 min infusion and then increased throughout the remainder 1U3 of the Infusion to a value of 5*6 +_ 0.9 mmHg/ml*min'100g after 10 min infusion (*60%) (see Figure 30 and Table 16), Thus even though the initial transient vasodilation did not appear to be dose related the secondary vasoconstrictor response clearly did (see Figure 30 and

Table 16 ).

After a 60 min recovery period the oxytocin infusions were repeated. No initial transient vasodilator response was seen with any dose rate. During infusion at 1.9 U/min resistance increased from 5.3 +, 0.8 to 7.1 +_ 1.0 mmHg/ml*min*100g after 10 min infusion

(+3^). During infusion at 7.6 U/min resistance increased from 5-1

+_ 1.0 to 8.0 +, 2.0 mmHg/ml*min*lOOg after 10 min infusion (+5T^)

(see Figure 31 and Table l£ ). After 60 min gut segment vascular resistance remained elevated at values about the same as those existing at the end of the first infusion. Thus, even though mean systemic arterial pressure appeared to have recovered from the effects of the first oxytocin infusion (see Figure 2U ) gut segment resistance had not but rather continued to increase as more oxytocin was administered.

One must conclude from this that the direct effect of pure oxytocin on the canine small intestine vasculature i3 a dose related vasoconstric­ tion. There is at present no cogent explanation for the initial tran­ sient vasodilation which was rather consistent in magnitude at the three higher oxytocin infusion rates used and was not seen during second oxytocin infusions given 60 min after the conclusion of the first infusion at the three higher dose rates used.

It is possible that the vasodilator phase might have been a more sustained and prominent part of the entire response, even dose 11*9 dependent in itself, if some experiments had been done in which oxytocin was infused at a slower rate. Some limited pilot trials failed to encourage pursuit of this idea. The possibility was raised by looking at the effects of the lowest dose rate used (0.95 U/min) on gut segment vascular resistance (see Figure 29 and Table 16 ). During the first infusion at 0.95 U/min resistance decreased from 3.6 +_ 0.5 to 3.3 1 0.5 mmHg/ml*min*100g after 3 min infusion (-8%, a statistically non-significant decrease) (see Figure 30 and Table 16). After 10 min infusion at 0.95

U/min gut segment resistance had increased to U.l +_ 0.U mmHg/ml•min*lOOg

(+ll*%, a statistically significant increase),

A group of similar experiments were done using the denervated autoperfused canine hepatic arterial vascular bed as a test system for the direct peripheral vascular effects of pure preservative-free synthetic oxytocin. Doses were 1.91 and 3.82 U/min i.v. for 10 min.

After a 60 min recovery period the infusions were repeated (see

Tables 20, 21, 22, 23 ). During infusion at 1.91 U/min hepatic artery resistance decreased from U.5 + 0.7 to 3.6 +0.5 mmHg/ml’min■lOOg after 3 min infusion (-20?!, a statistically significant decrease).

After 10 min infusion resistance was U.l + 0.7 mmHg/ml*min*100g (-9#, not significantly below preinfusion levels). During infusion at 3.82

U/min hepatic artery resistance decreased from 5.2 +_ 0.9 to 3.6 +_ 0.6 mmHg/ml*min*lOOg after 3 min infusion (-30%, a statistically significant de­ crease). After 10 min infusion resistance was U.2 mmHg/ml*min*100g (-20%), still significantly below control level but not to the same extent as seen after only 3 min infusion. During the second 10 min infusions hepatic artery resistance was decreased by only 7% (not statistically 150 significant) with both dose rates. It would appear that under these experimental conditions the transient vasodilator response seen during oxytocin infusion is much more pronounced in the hepatic arterial than in the small intestinal vasculature and it is not superceded "by a vasoconstrictor response even during the second 10 min infusion. The small intestinal vasculature is primarily vasoconstrictor in response to "both vasopressin and oxytocin as compared to the hepatic arterial vasculature.

The effect of i.v. oxytocin infusion upon portal venous pressure as recorded in this series of hepatic artery studies served to further hear out the findings in the series of isolated small intestinal segment studies* that is, the primary action of oxytocin on the small intestinal vasculature is vasoconstriction. For example when oxytocin was infused i.v. at a rate of 3.8 U/min portal venous pressure decreased from 5-8 +_ 0.5 to 5-^iO.U mmHg after 3 min infusion (-7%, a statistically non-significant decrease). After 10 min infusion portal venous pres­ sure had decreased to U.9 + 0.5 mmHg (-157*» a statistically significant decrease). During the second oxytocin infusion at 3.8 U/min portal pressure decreased from +_ 0.3 to k.5 +, 0*3 mmHg after 3 min infusion

(-17^, a statistically significant decrease). After 10 min infusion portal venous pressure had decreased no further. The relative amount of decrease in portal venous pressure of course reflects the degree of constriction in the prehepatic splanchnic (small intestinal vasculature).

Effect of estrogens on the cardiovascular response to oxytocin -

There have been several reports in the literature In which the pressor response to oxytocin has been enhanced or even converted from vasodilator to vasoconstrictor by some interaction with female

sex hormones. This literature is reviewed in detail in the INTRODUCTION

section of this dissertation. Lloyd and Pickford (62) measured hind

limb blood flow in both male and female dogs and found that oxytocin

(250-500 mU) increased blood flow, while after estrogen administration,

sympathetic blocking agents or acute sympathectomy, oxytocin usually caused a decrease in hind limb blood flow. Haigh, Kitchen and Pickford

(55) found that intraarterial oxytocin caused an increase in hand blood flow in normal men and women while after estrogen administration oxytocin caused a decrease in hand blood flow. Altura (2) has made direct observation on the changes in size of rat mesenteric arterioles under various circumstances using the image splitting television microscope recording system. Under control conditions in male

rats the topical application to the mesentery of 10 mU oxytocin

caused about a 10% decrease in the lumen diameter of terminal mesenteric

arterioles while the same treatment in female rats caused a 35%

decrease. He found that injection of male rats with estrogenic

hormones also increased the constrictor action of oxytocin.

Some limited observations were made in the present study to deter­ mine if pretreatment of male dogs with estrone would have any effect

on the response of their mean systemic arterial pressure or on the

response of isolated, acutely denervated, autoperfused segments of

dog small intestine to the i.v. infusion of oxytocin. The dogs were

treated from 5 to 7 days with i.m. estrone aqueous suspension (U mg/day)

and also given an inJ ection on the day of the acute experiment. This

treatment raised their plasma levels of estrone to about Ul+00 picograms/ml 152 and plasma levels of estradiol to about 21+00 picograms/ml {see

Table 17). When these animals were given 10 min i.v. infusions of oxytocin at a rate of 0.95 U/min there was little or no change in mean systemic arterial pressure Just as was the case in the control group of male dogs given the same oxytocin dose (see Table 18 ).

The effect of i.v. oxytocin infusion at 0.95 U/min was also not significantly affected by estrone pretreatment. There was a difference in the response of resistance to oxytocin after estrone pretreatmentj however, even though it was not significantly different from the response seen before estrone pretreatment. In the control group gut segment vascular resistance increased from 3.6 +,0.5 to 4.1 +_ 0.4 mmHg/ml• min-10Og (+12%, statistically significant increase) after 10 min during the first i.v. oxytocin infusion at 0.95 U/min. In the estrone pretreated group gut segment vascular resistance decreased from 3*9 +. 0.5 to 3.8 +_ 0.4 mmHg/ml*min-lOOg (-3%, statistically non-significant decrease) after 10 min i.v. oxytocin infusion at

0.95 U/min. Thus in this particular experimental protocol there was no evidence that estrogen pretreatment increased the vasoconstrictor action of oxytocin on the canine small intestinal vasculature (see

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