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Abstract

Isotonic sodium transport, Ά_{Na iso}, and the transtubular concentration difference, Δc _Na, at equilibrium under conditions of zero net solvent and solute fluxes were measured in the proximal tubules and medullary collecting ducts of rats. Peritubular capillaries were simultaneously perfused, in the first group of experiments with a solution containing no Ca⁺⁺ or 3 mEq/l of Ca⁺⁺, and in the second group with a solution which contained either no ADH or 5 mU/l of ADH. Addition of ADH or omission of Ca⁺⁺ from the capillary perfusate did not affect isotonic sodium transport. The equilibrium concentration difference of sodium, Δc _Na, was, however, greatly reduced under the same conditions.
The results indicate that in the absence of Ca⁺⁺, or in the presence of ADH, the permeability for sodium is increased, but that in contradistinction to the behaviour of amphibian skins, active sodium transport remains unaffected.