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Mechanism Behind Augmentation in Baroreflex Sensitivity After Acute 117 Hypertens Res Vol.29 (2006) No.2 p.117-122 Original Article Mechanism behind Augmentation in Baroreflex Sensitivity after Acute Exercise in Spontaneously Hypertensive Rats Naoyoshi MINAMI1), Nobuyoshi MORI1), Makoto NAGASAKA1), Osamu ITO1), Hajime KUROSAWA1), Masayuki KANAZAWA1), Ki KAKU1), Eigyoku LEE1), and Masahiro KOHZUKI1) A single bout of dynamic exercise increases baroreflex sensitivity (BRS) in spontaneously hypertensive rats (SHR). We examined whether change in hemodynamics (increases in blood pressure and heart rate) asso- ciated with dynamic exercise contribute to the post-exercise modulation of BRS. SHR aged 12 weeks were chronically instrumented with a carotid artery catheter and jugular vein catheter. They were then allocated to three groups submitted to 40 min of 1) running on a treadmill at 12 m/min (Run), 2) concomitant infusion of isoproterenol and a relatively high dose of phenylephrine (Iso+Phe(high)), or 3) concomitant infusion of isoproterenol and a relatively low dose of phenylephrine (Iso+Phe(low)). Arterial pressure and heart rate were continuously recorded throughout the experiments. BRS estimated by heart rate responses to phenyl- ephrine injection and systolic blood pressure–low frequency power amplitude (SBP-LFamp) evaluated by power spectral analysis of SBP, a marker of sympathetic activity, were examined before and after running (Run group), or administration of drugs (Iso+Phe(high) or Iso+Phe(low) groups). BRS increased signifi- cantly from 1.4 to 1.9 bpm/mmHg after running, but not after administration of Iso+Phe(high) or Iso+Phe(low). Blood pressure and SBP-LFamp significantly decreased in each of the Run, Iso+Phe(high) and Iso+Phe(low) groups. These results suggest that hemodynamic change alone does not contribute to post-exercise modulation of BRS, while hemodynamic change or sympathetic activation during exercise contributes to post-exercise hypotension associated with a reduction of sympathetic activity. (Hypertens Res 2006; 29: 117–122) Key Words: baroreflex sensitivity, blood pressure, exercise, heart rate, sympathetic activity known to induce hypotension in the post-exercise period in 3 Introduction hypertensive subjects ( ) and spontaneously hypertensive rats (SHR) (4–6). Post-exercise hypotension (PEH) is associated It has been well demonstrated that exercise training lowers with no change or a reduction of heart rate (HR) (7, 8) and a blood pressure (1, 2) and increases baroreflex sensitivity reduction in cardiac (9) and peripheral sympathetic nerve (BRS) (1) in hypertensive subjects. Apart from these chronic activity (10, 11). The reduction in blood pressure without a effects of exercise, a single bout of dynamic exercise is baroreflex-mediated compensatory tachycardia or sympa- From the 1)Department of Internal Medicine and Rehabilitation Science, Tohoku University Graduate School of Medicine, Sendai, Japan. This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 14570633). Address for Reprints: Naoyoshi Minami M.D., Ph.D., Department of Internal Medicine and Rehabilitation Science, Tohoku University Graduate School of Medicine, 1–1 Seiryo-cho, Aoba-ku, Sendai 980–8574, Japan. E-mail: [email protected] Received August 19, 2005; Accepted in revised form December 13, 2005. 118 Hypertens Res Vol. 29, No. 2 (2006) thetic excitation suggests that a single bout of dynamic exer- 20) were implanted into the left carotid artery and right jugu- cise resets the operating point of the arterial baroreflex to a lar vein, respectively. The free ends of these catheters were lower pressure (5). PEH is also associated with a reduction in brought subcutaneously to the back of the neck. The catheters cardiac vagal tone (5, 12). However, BRS in response to were then filled with heparinized saline (100 IU/ml), and their blood pressure elevation, which is predominantly determined ends were occluded. The rats were returned to individual by augmentation in cardiac vagal activity, increases after an cages and allowed to recover for 2 days after surgery. acute bout of exercise (13). These observations suggest that the tonic and phasic reflex activity in cardiac vagal nerves are Measurement of Arterial Pressure differently modulated in the post-exercise period. The arterial baroreceptor cardiovagal baroreflex gain is Arterial pressure was monitored from the arterial catheter determined by two components: mechanical gain, reflecting with a strain-gauge transducer (LIFE KIT DX-360; Nihon the transduction of pressure into arterial wall stretch, and neu- Kohden, Tokyo, Japan) and amplifier (MacLab Bridge Amp ral gain, reflecting the transduction of arterial diameter ADInstruments Pty Ltd., Castle Hill, Australia). Phasic pres- changes into HR responses via central integration. There are sure, mean arterial pressure (MAP) and HR were recorded at several possible mechanisms by which dynamic exercise a sampling rate of 200/s by a data acquisition system and lab- modulates these components. Arterial pressure and HR oratory computer (MacLab 8 analog-to-digital converter and increase during dynamic exercise. These hemodynamic Macintosh computer). changes could influence the viscoelastic properties of the arterial wall where arterial baroreceptors are located (14). Wavelet Transformation (WT) Actually, Studinger et al. (15) have demonstrated that an index of carotid arterial distensibility is altered in parallel Arterial pressure data were also stored on a magneto-optical with BRS during the post-exercise period. Hemodynamic disk (LX-10 RECORDING UNIT; TEAC, Tokyo, Japan), changes associated with dynamic exercise could also lead to and the WT was computed using software running on a per- alternations in central baroreflex processing by arterial sonal computer (Fluclet; Dai-nippon Pharmaceutical, Osaka, baroreceptors (5) or cardiac mechanoreceptor stimulation (4). Japan). This software performs the spectral analysis of sys- Alternatively, dynamic exercise may modulate central tolic blood pressure (SBP) fluctuation based on WT and pro- baroreflex integration by triggering increased discharge from vides a description of the spectral parameters every s. The mechanosensitive afferent nerve fibers arising from contract- reliability of this analytical method has been reported else- ing skeletal muscle (16). It is not known, however, what where (17). In the present study, the spectral component, the changes associated with dynamic exercise contribute to the low frequency (LF; 0.26–0.74 Hz) of SBP, was calculated at post-exercise modulation of BRS. To address this question, in every 1-s interval with continuous wavelet transform. The the present study BRS was measured in SHR before and after magnitude of the LF component was expressed as amplitude running on a treadmill or concomitant infusion of isoprotere- rather than power (area under the curve obtained by spectral nol and phenylephrine, a combination known to increase analysis). The mean amplitude was obtained as follows: mean blood pressure and HR. amplitude = (2 × power)1/2. A secondary infinite impulse response Butterworth digital filter was used for smoothing of Methods the amplitudes. The cut-off frequency was 0.05 Hz. The LF amplitude of SBP (SBP-LFamp) was determined as the mean All experimental procedures were performed in accordance of the filtered LFamp values obtained over each successive 5 with institutional guidelines. s. Of these data, mean of three successive SBP-LFamp before and 20 to 30 min after running or concomitant infusion of iso- proterenol and phenylephrine, at which the rat was quiet, was Animal Care used as an index of sympathetic activity (18). Male SHR were obtained from Charles River, Astugi, Japan. The rats were fed standard laboratory chow and water ad libi- Experimental Protocols tum while housed at a controlled temperature (23°C) with a 12-h light-dark cycle. At the age of 11 weeks, all rats were The rats were assigned to three groups that were subjected to submitted to 10-min periods of exercise on the motor tread- 40 min of 1) running on a motor-driven treadmill at 12 m/min mill at 10 m/min on a 0 grade incline every day for 1 week to at a 0 grade incline (Run) (n=6), 2) concomitant infusion of become accustomed to the experimental procedures. isoproterenol and a relatively high dose of phenylephrine (Iso+Phe(high)) (n=8), or 3) concomitant infusion of isopro- terenol and a relatively low dose of phenylephrine Surgical Procedures (Iso+Phe(low)) (n=5). All rats were moved from individual At the age of 12 weeks, under ether anesthesia, an arterial cages to the treadmill and allowed to habituate to the experi- catheter (tapered PE 100 on one end) and venous catheter (PE mental conditions for at least 1 h, while the catheters were Minami et al: Baroreflex Sensitivity after Exercise 119 (A) infusion as the rats in the Iso+Phe(high) group during the first (mmHg) Run 10 min, and then the infusion rate of phenylephrine was Iso+Phe(high) decreased to ~20 μg/kg/min so that the blood pressure 200 Iso+Phe(low) changes resembled those in the rats during exercise. The rats in the Run group received a saline infusion at the same infu- 180 sion rate. Thirty min after exercise or administration of Iso+Phe(high) or Iso+Phe(low), and an additional 60 min 160 after administration of Iso+Phe(low), BRS was evaluated as described above. MAP * * 140 * * Data Analysis * * 120 All results were expressed as the means±SEM. Values of MAP and HR during and after exercise or drug administration Run or Iso+Phe were compared with baseline data (time 0) in each group 100 -30 -20 -10 0 10 20 30 40 +10 +20 +30 using a paired t-test. Values of BRS and SBP-LFamp before and after exercise or drug administration were also compared (B) using a paired t-test. Differences were considered significant (bpm) when p<0.05. 550 500 Results 450 MAP and HR Figure 1 shows the values of MAP and HR before, during, HR 400 and after the Run, Iso+Phe(high) or Iso+Phe(low) period.
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