DAILY EXERCISE ATTENUATES THE SYMPATHETIC-NERVE RESPONSE TO EXERCISE BY ENHANCING CARDIAC AFFERENTS

''Central command'' may initiate the sympathoexcitatory responses at t he onset of exercise by shifting the operating point of the arterial b aroreflex toward higher pressures. Daily exercise (DE) attenuates the sympathoexcitatory responses to submaximal exercise. This DE-induced a daptation may be due, in part, to an enhanced inhibitory influence of cardiac afferents. This is suggested because cardiac afferents exert a tonic inhibitory influence on the arterial baroreflex which is enhanc ed by DE. Therefore, the influence of cardiac afferents on the regulat ion of renal sympathetic nerve activity (RSNA) during exercise was exa mined in a group of sedentary and age-matched DE rabbits. The rabbits were instrumented with a Silastic catheter inserted into the pericardi al sac, electrodes around the renal sympathetic nerves, and catheters in the femoral artery and vein. In the sedentary rabbits, treadmill ex ercise (12 m/min, 20% grade) significantly increased mean arterial pre ssure (Delta 18 +/- 3 mmHg), heart rate (Delta 36 +/- 3 beats/min), an d RSNA (Delta 295 +/- 23%). More importantly, cardiac afferent blockad e (2% intrapericardial procainamide) did not significantly alter the R SNA response to exercise in the sedentary rabbits. DE did not alter th e mean arterial pressure (Delta 15 +/- 1 mmHg) or heart rate (Delta 55 +/- 8 beats/min) response to exercise; however, RSNA (Delta 252 +/- 9 %) was significantly reduced. In contrast to the sedentary rabbits, ca rdiac afferent blockade in the DE rabbits significantly increased the RSNA response to exercise (Delta 417 +/- 30%). These results suggest t hat DE attenuates the RSNA response to dynamic exercise due, in part, to an enhanced inhibitory influence of cardiac afferents.