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The proximal and distal tubular sodium transport systems were studied under conditions of an increased sodium load. Fluid was collected by micropuncture from various sites along the nephron of rats receiving a 4% sodium chloride infusion. Samples were analyzed for sodium and inulin-C 14 , and compared with those obtained from a nondiuretic control group. Inulin clearances were higher in the sodium chloride-loaded rats while their over-all fractional sodium reabsorption was significantly reduced. In animals in which the urinary excretion rate of sodium remained below 12% of the filtered load, fractional sodium and water reabsorption within the proximal tubule were not different from the control group. Under these conditions, no proximal maximal tubular transfer rate for sodium could be observed. Only when urinary excretion rates of sodium exceeded 12% of the filtered load was fractional proximal tubular sodium reabsorption depressed. During sodium chloride loading fractional sodium reabsorption was always reduced in the distal tubule and the collecting duct when compared with the control group. Since this distal tubular transfer capacity does not substantially increase, the natriuretic and diuretic response to an elevation of the plasma sodium concentration is effected by surpassing a distal tubular transport maximum for sodium.
Giebisch et al. (Wed,) studied this question.