Acute shifts of baroreflex control of renal sympathetic nerve activity induced by treadmill exercise in rats

Abstract

The present study aimed to investigate whether there was a resetting of the baroreflex control of renal sympathetic nerve activity (RSNA) and heart rate (HR) during exercise. Wistar female rats (n = 11) were chronically implanted with catheters for the measurement of systemic arterial (Pa) and central venous pressures and with electrodes for measurement of RSNA and electrocardiogram (ECG) at least 3 days before study. The baroreflex curve for RSNA was determined by changing Pa using rapid intravenous infusions of phenylephrine and nitroprusside. The baroreflex response curves for RSNA and HR were characterized by an inverse sigmoid function curve from which the response range, gain, centering point and minimum response were estimated. Exercise shifted the Pa-RSNA baroreflex curve upward and to the right and was associated with increases in response range of 122 +/- 44 % (P < 0.05), maximum response of 173 +/- 40 % (P < 0.05), maximum gain of 149 +/- 66 % (P < 0.05) and midpoint pressure of 15 +/- 5 mmHg (P < 0.05) compared with the pre-exercise level. After cessation of exercise, the Pa-RSNA baroreflex curve was suppressed vertically with a significant decrease in maximum response of 57 +/- 14 % (P < 0.05) compared with the pre-exercise level. These data suggest that the right-upward shift of baroreflex control of sympathetic nerve activity may play a critical role in raising and stabilizing Pa during exercise. The suppression of the baroreflex control of sympathetic nerve activity may partly explain the post-exercise inhibition of sympathetic nerve activity and contribute to the post-exercise hypotension.

Figure 5. First derivative of the P_a-RSNA baroreflex curve shown in Fig. 2

Symbols indicate steady state levels of P_a in Table 1, which were obtained before the pharmacological manipulations of P_a.

Figure 1. Typical recordings from an individual rat of electrocardiogram (ECG), systemic arterial pressure (P_a), central venous pressure (P_cv), heart rate (HR), renal sympathetic nerve activity (RSNA) and integrated RSNA during pharmacological manipulation of P_a during treadmill exercise

A bolus intravenous infusion of phenylephrine (10 μg) and nitroprusside (10 μg) was given to generate the stimulus-response curves for RSNA and HR. Data are presented at two different recording speeds.

Figure 2. Shifts in the baroreflex curves for RSNA obtained during pre-exercise, treadmill exercise and the post-exercise periods

Curves reflect data averaged from 11 animals and symbols and bars indicate means ±s.e.m., respectively, estimated over each 2.5 mmHg bin of P_a.

Figure 3. Shifts in the baroreflex curves for HR obtained during pre-exercise, treadmill exercise and the post-exercise periods

Curves reflect data averaged from 11 animals and symbols and bars indicate means ±s.e.m., respectively, estimated over each 2.5 mmHg bin of P_a.

Figure 4. Shifts in the baroreflex curves for RSNA and HR observed during pre-sham exercise, sham exercise and post-sham exercise periods

Sham exercise was carried out by placing a plate above the treadmill lane such that rats were exposed to exactly the same conditions of treadmill exercise except that there was no running. Curves reflect data averaged from 11 animals and symbols and bars indicate means ±s.e.m., respectively, which were averaged over each 2.5 mmHg bin of P_a.