Item

Abstract

The transepithelial electrical potential difference across rat rena proximal tubule was reinvestigated, using improved techniques. To diminish tip potential artefacts the microelectrodes were filled with HCO₃-Ringer's solution instead of 3 molar KCl. The error of the potential measurements with HCO₃-Ringer's microelectrodes was tested and was found to be ≤0.5 mV. A significant electrical potential profile was detected along the proximal tubular lumen under free flow. From near zero at the glomerulum the potential difference rose to −1.5 mV, lumen negative, in the first tubular loop at approximately 0.1 to 0.3 mm of tubular length. It decreased then rapidly, changed sign and attained a maximum of ca. 2.0 mV, lumen positive, at 1 mm of tubular length, after which it declined gradually to +1.6 mV in the last accessible loop. The mean of 85 punctures in intermediate and late loops was+1.8, S.D.±0.33 mV, range+1.0 to+3.2 mV. On the basis of perfusion experiments described in the subsequent paper, the lumen-negative potential difference across early loops can be explained as an active transport potential. It is caused by the presence of glucose and amino acids in the glomerular filtrate, which increase the rate of active Na⁺ absorption over that of active HCO₃ − absorption. The lumen-positive potential difference in intermediate and late loops is explained as the sum of a membrane diffusion potential arising from the shift in intratubular Cl⁻ and HCO₃ − concentrations and a small lumen-positive active transport potential from H⁺ secretion/HCO₃ − absorption.