HORMONAL, ELECTROLYTE, AND RENAL RESPONSES TO EXERCISE ARE INTENSITY DEPENDENT

Previous work indicates that the magnitude and direction of renal responses to exercise depend on the exercise intensity. To examine mechanisms responsible for these findings, renal and hormonal responses were studied in eight healthy male subjects (29.6 +/- 1.9 yr) before and immediately after four 20-min bouts of submaximal exercise (cycle ergometry) at work loads representing 25, 40, 60, and 80% of maximal oxygen consumption. Urine flow, osmotic clearance, glomerular filtration rate, and sodium excretion (U(Na)+V) all tended to rise at the 25% work load but were markedly reduced at the higher work intensities. Changes in urine flow paralleled changes in glomerular filtration rate (r = 0.91). Plasma vasopressin (ADH), aldosterone, and plasma renin activity tended to increase progressively with increases in work load, with the increases for all hormones reaching statistical significance when the level of exercise reached greater-than-or-equal-to 60% of maximal oxygen consumption. However, atrial natriuretic peptide was elevated (P < 0.05) at all work loads from > 1.6-fold of control levels at the 25% work load to > 7-fold at the 80% work load. The increase in urine flow (6 of 8 subjects) and U(Na)+V (7 of 8 subjects) may be due to the increase in atrial natriuretic peptide and/or a 10% suppression (P < 0.05) of ADH at the 25% work load. Despite ADH being elevated (P < 0.05) 1.8- and 4.5-fold at the 60 and 80% work loads, average free water clearance increased (P < 0.05) by 0.5 and 1.2 ml/min and urinary osmolality decreased (P < 0.05) by 29.7 and 24.3 mosmol/kg, respectively, indicating that during high-intensity exercise the concentrating capability of the kidney may be impaired.