Renal sodium handling after exercise induced plasma volume expansion

The purpose of this investigation was to examine the effects of exercise induced plasma volume expansion on sodium balance following a bolus infusion of isotonic saline. Five subjects rested in upright posture for 6.5 h in two separate condition: 24 h after exercise (EX) and after no exercise (CON). The exercise was consisted of eight 4-min bouts of exercise at 85% of peak O₂ uptake with 5-min recovery between bouts that induced a 6.l±3.4% (p<0.05, 2.43±1.21 ml/kg body wt) plasma volume expansion. After 3 h of rest 15 ml/kg of isotonic saline was infused over a 30-min period that expanded plasma volume by 3.56±0.59 and 3.79±0.36 ml/kg body wt (p<0.05) in CON and EX and remained elevated until the end (p<0.05, 2.86±0.48 and 2.35±0.46 ml/kg body wt). Glomerular filtration rate (GFR, creatinine clearance), filter load of sodium, lithium clearance, and proximal tubular sodium output during the recovery were greater for the CON than the EX trial (p<0.05). No differences were found in distal tubular sodium handling. Urinary sodium excretion and sodium clearance tended to be higher during the CON trial following the infusion (p>0.05). Plasma renin activity and aldosterone levels were decreased after the infusion and remained lower (p<0.05), but no differences between the CON and EX trials were found. Atrial natriuretic peptides level remained unchanged during the experiment. These data indicate that during exercise induced hypervolemia sodium sparing occured following saline loading and is mediated by a decrease in GFR and proximal tubular sodium.