Exercise training attenuates baroreflex regulation of nerve activity in rabbits.

Baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) during changes in arterial pressure were examined in conscious rabbits before and after 8 wk of treadmill endurance exercise training. Exercise-induced bradycardia (decreased HR at rest and at identical workloads) was used to document a cardiovascular training effect. At 29.4 m/min, 20% grade, posttraining HR was significantly lower than pretraining values (295 +/- 6 beats/min vs. 361 +/- 8 beats/min, P less than 0.05). Rabbits were instrumented with an occluder around the inferior vena cava (IVCO), arterial and venous catheters, and recording electrodes around renal sympathetic nerves. Baroreflex-mediated changes in HR and RSNA to a 25-mmHg decrease and increase in mean arterial pressure (MAP) were obtained by inflating the IVCO or progressive infusion of phenylephrine, respectively. After training, the range (maximum response minus minimum response) of baroreflex control of RSNA and HR was significantly attenuated (209 +/- 12 vs. 413 +/- 14% and 123 +/- 16 vs. 162 +/- 8 beats/min). In addition, the slope of baroreflex control of HR and RSNA was significantly attenuated after training (4.17 +/- 0.35 vs. 8.32 +/- 0.23%/mmHg and 2.57 +/- 0.4 vs. 4.14 +/- 0.4 beats.min-1.mmHg-1). These data demonstrate that endurance exercise training significantly attenuated baroreflex control of HR and RSNA in conscious rabbits.