Increased renal tubular sodium reabsorption during exercise-induced hypervolemia in humans

We tested the hypothesis that renal tubular Na+ reabsorption increased duri ng the first 24 h of exercise-induced plasma volume expansion. Renal functi on was assessed 1 day after no-exercise control (C) or intermittent cycle e rgometer exercise (Ex, 85% of peak O-2 uptake) for 2 h before and 3 h after saline loading (12.5 ml/kg over 30 min) in seven subjects. Ex reduced rena l blood flow (p-aminohippurate clearance) compared with C (0.83 +/- 0.12 vs . 1.49 +/- 0.24 l/min, P< 0.05) but did not influence glomerular filtration rates (97 +/- 10 ml/min, inulin clearance). Fractional tubular reabsorptio n of Na+ in the proximal tubules was higher in Ex than in C (P< 0.05). Sali ne loading decreased fractional tubular reabsorption of Na+ from 99.1 +/- 0 .1 to 98.7 +/- 0.1% (P< 0.05) in C but not in Ex (99.3 +/- 0.1 to 99.4 +/- 0.1%). Saline loading reduced plasma renin activity and plasma arginine vas opressin levels in C and Ex, although the magnitude of decrease was greater in C (P< 0.05). These results indicate that, during the acute phase of exe rcise-induced plasma volume expansion, increased tubular Na+ reabsorption i s directed primarily to the proximal tubules and is associated with a decre ase in renal blood flow. In addition, saline infusion caused a smaller redu ction in fluid-regulating hormones in Ex. The attenuated volume-regulatory response acts to preserve distal tubular Na+ reabsorption during saline inf usion 24 h after exercise.