The renal response to blood volume expansion in the rat: proximal tubular function and urinary excretion.

Abstract

Chronically salt- and deoxycorticosterone-acetate-loaded (DOCA) rats and chronically salt-deficient (NaD) rats were infused with blood, and consequent changes in renal excretion of water and electrolytes were compared with simultaneous alterations in proximal tubular function. During the period of intravascular expansion maintained by reinfusion of urine, diuresis and natriuresis, significant compared to control excretion, were seen in both groups. Although absolute levels of diuresis were not statistically different, DOCA rats excreted more sodium and less potassium than NaD rats; total cation excretion (Na+K) was comparable. A 10% reduction in proximal fractional reabsorption occurred in both series, associated, however, with large similar increases in single nephron filtration rate, so that absolute rates of proximal fluid reabsorption were actually increased. No correlation was found between changes in fractional or absolute reabsorption and renal excretion. Since vascular expansion results in similar alterations of filtration rate and proximal transport in both groups, the different ratios of excretion of sodium and potassium must be due to differences in tubular function at sites distal to the proximal convolution. The lack of direct correlation between proximal function and magnitude of renal excretion in these experiments further suggests that the excretory response to blood volume increase is determined ultimately by tubular mechanisms located in the distal part of the nephron.