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Abstract

Proximal tubules of the rat kidney were perfused in vivo with NaCl-NaHCO₃ Ringer's solution and the net rates of fluid absorption from Gertz shrinking drops were measured as well as the stationary electro-chemical potential differences for Na⁺ and Cl⁻ that develop across the tubular wall during constant fluid absorption. By altering the rate of fluid absorption through addition of raffinose to the peritubular perfusate or to the lumen fluid, the relations between the net ion fluxes and the electrochemical potential differences were obtained for Na⁺, Cl⁻ and HCO⁻₃. From these relations which were reasonably linear for Na⁺ and Cl⁻ over small deviations from equilibrium, single ion reflection coefficients and active transport rates were calculated. Since the calculations required a knowledge of the permeability coefficients of the tubular wall for Na⁺ and Cl⁻, in a separate series of experiments these coefficients were determined from tracer flux experiments. The calculations yield _Na=0.7, and σ_σCl=0.5 σ_HCO2 can be estimated to be substantially greater than σ_Cl. Comparing the active transport rates to the net fluid absorption under conditions similar to free flow in the normal kidney, the following conclusions can be drawn: approximately one third of the sodium is resorbed by active transport, one third by electrical transference and one third by solvent drag. Chloride transport is entirely passive. One half of the chloride is resorbed by diffusion and one half by solvent drag. Bicarbonate transport appears to be entirely active, and the active transport rate is greater than the net transport pointing to passive bicarbonate back flux.