Renal tubular reabsorptive response to hypernatremia.

Renal tubular reabsorptive response to rapid infusions of isotonic saline and 5% NaCl solutions were measured during brisk ethacrynic acid diuresis in anesthetized dogs. When adjustments were made for effects of variations in volume expansion, as indexed by plasma protein concentration ([Pprot]), tubular reabsorption of sodium per unit filtrate volume (TNa/GFR) was found to be significantly and positively correlated with plasma sodium concentration ([PNa]) despite hypernatremia and total body surfeit of sodium. The proportions of sodium and water reabsorbed were also homeostatically inappropriate, since the sodium concentration in the reabsorbate was somewhat in excess of that in contemporary plasma ultrafiltrate. These findings signify that glomerulotubular balance holds when the filtered load of sodium is increased by an increment in [pNa] as well as GFR. It is proposed that the moiety of tubular reabsorption (some 75% of GFR) studied here is more closely related to regulation of volume than of osmolality of sodium concentration, and the primary regulation exerted is on tubular volume reabsorption (bulk fluid reabsorption) rather than on the amount of sodium reabsorbed.