Influence of Mucosal Sodium Concentration on Electrical Resistances and Potentials of Toad Bladder 1

Summary

This study was undertaken to delineate the upper limits of paracellular resistance in toad bladder, and the effect of mucosal [Na⁺] <10 mM on cellular resistance and epithelial, apical, and basallateral potentials. These parameters were determined at mucosal Na⁺ of 100, 20, 10, 7, 3, and 1 mM. There was an enormous increase in paracellular or junctional resistance which, at 1 mM Na⁺, was over fourfold that determined at 100 mM. There was no effect on cellular resistance. A primary effect of low mucosal Na⁺ on apical potential independent of effect on basal-lateral potential was not demonstrable. Although apical membrane potential responds to mucosal [Na⁺], the response is less than that seen with pure Na⁺ sensitive electrode. The response of basal-lateral potential to mucosal [Na⁺] suggest that, to some considerable extent, ψα_b may be the result of Na⁺ pump activity. However, basal-lateral K⁺ diffusion potentials may contribute to ψα_b and both membrane potentials may be influenced by shunt current flow. The data also support the concept that Na⁺ penetration of apical and basal-lateral membranes is unidirectional, from mucosal solution to cell to serosal solution. Shunt resistance varies inversely with epithelial potential and mucosal Na⁺. This is probably required to maintain large transepithelial concentration gradients.