Abstract
The relationship between transepithelial Na^+^ and Cl^−^ active transport from the mucosal toward the serosal surface was studied by ion substitution experiments in the isolated, perfused urinary bladder of the winter flounder, Pseudopleuronectes americanus. The tissue behaves as a simple ohmic resistance of about 1000 ohm·cm^2^ which was not significantly changed by perfusion of the lumen (mucosal surface) with Na‐free or Cl‐free solutions. Transepithelial electrical potential difference and short‐circuit current were reversed (mucosa positive to negative) by Cl‐free perfusion but unaltered by Na‐free medium. Short‐circuit current did not agree with isotopically measured net Na^+^ and Cl^−^ flux in either control or ion‐substituted medium. Cl‐free perfusion depressed net Na^+^ flux more than 50% and a similar effect was seen on net Clflux upon Na‐free perfusion. A kinetic analysis of the relationship of Na^+^ or Clflux to the concentration of Na^+^ or Cl^−^ in the perfusate in the presence and absence of Cl^−^ or Na^+^, respectively, indicated that Cl‐free medium acted as a non‐competitive inhibitor of Na^+^ transport, and the effect of Na‐free medium on Cl^−^ transport could be interpreted similarly. This inter‐dependence of Na^+^ and Cl^−^ transport was taken as an indication of co‐transport of these two ions. Amiloride (10^−4^ M) seemed to inhibit a component of Na flux distinct from the Cl‐dependent flux. It had no effect on Cl^−^ transport.