Free-flow potential profile along rat kidney proximal tubule - Comments (Reprinted from Pflugers. Arch. vol 351,pg 69-83, 1974)

The transepithelial electrical potential difference across rat renal proxim al tubule was reinvestigated, using improved techniques. To diminish tip po tential artefacts the microelectrodes were filled with HCO3- Ringer's solut ion instead of 3 molar KCI. The error of the potential measurements with HC O3- Ringer's microelectrodes was tested and was found to be less than or eq ual to0.5 mV A significant electrical potential profile was detected along the proximal tubular lumen under free flow. From near zero at the glomerulu m the potential difference rose to -1.5 mV, lumen negative, in the first tu bular loop at approximately 0.1 to 0.3 mm of tubular length. It decreased t hen rapidly, changed sign and attained a maximum of ca.2.0 mV, lumen positi ve, at I 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-negativ e potential difference across early loops can be explained as an active tra nsport potential. It is caused by the presence of glucose and amino acids i n the glomerular filtrate, which increase the rate of active Na absorption over that of active HCO3- absorption, The lumen-positive potential differen ce in intermediate and late loops is explained as the sum of a membrane dif fusion potential arising from the shift in intratubular Cl- and HCO3- conce ntrations and a small lumen-positive active transport potential from H+ sec retion/HCO3- absorption.