CARDIOVASCULAR AND RENAL-FUNCTION DURING EXERCISE-INDUCED BLOOD-VOLUME EXPANSION IN MEN

To test the hypothesis that reduced baroreflex sensitivity is a direct result of exercise, we measured forearm vascular conductance (FVC) re sponses to graded lower body negative pressure (LBNP) 2, 20, and 44 h after intense exercise. Eight 4-min bouts of exercise at 85% of maximu m oxygen uptake produced 3.5 +/- 0.7 and 3.9 +/- 1.0% blood volume (BV ) expansions at 20 and 44 h of recovery, respectively. BV was unchange d from control values 2 h after exercise. The reduction in FVC was sig nificantly less than control values during 30 and 40 mmHg of LBNP at 2 and 20 h of recovery, respectively, whereas heart rate and cardiac st roke volume responses were unchanged. Thus, a reduced FVC response to LBNP preceded BV expansion, demonstrating that exercise itself can eli cit an attenuation of baroreflex function. To test the hypothesis that volume sensitivity of renal function is attenuated by intense exercis e, we measured cardiovascular variables, plasma hormone concentrations , and renal output. At 20 h of recovery, resting mean arterial blood p ressure and cardiac output were increased by 6 +/- 1 mmHg and 0.6 +/- 0.2 l/min, respectively, but resting plasma aldosterone and overnight Na+ excretion rate were unchanged. At 44 h of recovery, plasma aldoste rone was decreased by 26 +/- 9% and overnight Na+ excretion rate was i ncreased by 51 +/- 26%. Thus, appropriate endocrine and renal response s to increased BV were delayed until 44 h of recovery. Our findings su ggest that a postexercise attenuation of baroreflex function participa tes in the induction of BV expansion by intense exercise.