Effects of chronic sodium depletion on renal tubular sodium and water reabsorption in man.

In order to assess the effects of sodium depletion on tubular Na and H2O transport in man, renal clearance studies were performed under control conditions and after sodium depletion in 7 normal subjects. The control diet contained 250 mEq of Na daily. Sodium depletion was induced by placing subjects on a 20-mEq/day na diet and administering furosemide (40 mg) dialy for 3 days. Balance studies revealed that the subjects were in Na balance under control conditions, and that after sodium depletion had been established they were in a sodium-retaining state. Subjects were studied under both conditions, after H2O hydration and during a subsequent hypotonic mannitol infusion. After H2O hydration the minimal urinary osmolality averaged 61 and 67 mosm/kg in the sodium depletion and control states, respectively. Sodium depletion was associated with a significant reduction in C inulin, sodium clearance (CNa), urine flow (V), fractional urine flow (V/GFR), free water (CH2O), and fractional free water clearance (CH2O/GFR). During hypotonic mannitol infusion indices of absolute and fractional delivery of Na from the proximal tubule (CH2O + CNa; CH2O/GFR + CNa/GFR) were consistently reduced after sodium depletion and rose more slowly as absolute and fractional distal sodium chloride supply (CH2O + CNa; CH2O/GFR + CNa/GFR) were increased. These data are consistent with the view that modest sodium depletion in man provokes an increase in fractional sodium chloride reabsorption in both the proximal and distal tubules.