Effect of saline loading on sodium reabsorption in the dog proximal tubule

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Abstract
The relationship between glomerular filtration rate (GFR) isotonic saline loading, and proximal tubule Na reabsorption was studied by determining the change in tubule fluid to plasma inulin concentration ratio as a function of distance along the proximal tubule in hydropenic and saline-loaded dogs with and without renal artery constriction. Using the equation log y = bx, the slope obtained from the values in each experimental group was calculated by the method of least squares, and the results compared by an analysis of covariance. Fractional reabsorption of sodium in the proximal tubule was rot altered by decreasing the GFR. Saline loading depressed fractional reabsorption of Na independent of reductions in GRF when compared with hydropenic controls. Proximal inhibition of Na reabsorption accounted for all the excess urinary Na excreted during saline loading. It is concluded that there exists a normal balance between GFR and proximal Na reabsorption. Saline loading inhibits proximal Na reabsorption by an undefined mechanism which is independent of vasopressin and mineralocorticoid, and is possibly the result of expansion of extracellular fluid volume.