Depression of fractional sodium reabsorption by the proximal tubule of the dog without sodium diuresis.

The effect of infusions of hyperoncotic solutions on fractional sodium reabsorption by the proximal tubule of the dog was studied by the recollection micropuncture method. Tubule fluid to plasma inulin concentration ratios were measured for identified proximal tubule segments before and after infusion of 25% albumin or dextran solutions. Results were compared with changes in fractional reabsorption during saline diuresis. Plasma volume increased 66% +/- SE 5.8 after infusion of albumin solution and 94% +/- SE 8.2 after infusion of dextran solution. Fractional sodium reabosorption by the proximal tubule was depressed after infusion of both of these hyperoncotic solutions. Nevertheless, changes in sodium excretion after infusion of albumin and dextran were small. In contrast, after infusions of isotonic sodium chloride solution, which increased plasma volume 61% +/- SE 5.8, a decrease in fractional reabsorption of 50.7% +/- SE 7.2 was associated with large changes in sodium excretion.