Tubular reabsorption of sodium during acute and chronic volume expansion in man

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Abstract
Renal hemodynamics and tubular fractional sodium reabsorption (FSR) were evaluated by clearance techniques during acute and chronic extracellular volume expansion in man. (1 − V/GFR) × 100 was used as an index of proximal and (CH2O/V) × 100 as an estimate of distal fractional reabsorption. After acute loading with isotonic saline 37 ml/kg body wt, proximal FSR decreased by 4.8% and distal FSR decreased by 4.4%. After comparable chronic expansion by mineralocorticoids (“escape”), proximal FSR also decreased by 3.9%, but distal reabsorption was not altered. In separate studies, subjects were progressively infused with saline to 57 (E1) and to 80 (E2) ml/kg body wt, and appeared to divide into “excreters” (maximum UNaV > 1000 μEq/min) and “nonexcreters” (maximum UNaV < 550 μEq/min). In the excreters, GFR rose, proximal FSR decreased by 7.1% after E1 and only 0.9% further after E2. Distal FSR fell by 14.8% after E1 and by an additional 4.9% after E2. In the nonexcreters, GFR was stable and proximal FSR did not fall significantly after E1 or E2. Distal FSR decreased 4.5% after E1 and 1.3% further after E2. It is concluded that both acute and chronic extracellular expansion decrease proximal FSR in man, but only acute loading depresses distal FSR. Ability of some men to excrete sodium rapidly after acute infusion is related to larger increases in GFR and greater decreases in both proximal and distal FSR than occur in men in whom natriuresis is more limited.