Effect of transepithelial concentration gradients on the passive fluxes of sodium across toad bladder

Summary Bidirectional sodium fluxes across toad bladder were measured after eliminating active transport with ouabain. Mucosal sodium concentration,C_m, was progressively reduced (from 114 to 3mm) while serosal sodium remained constant. Potential difference was maintained at zero by current passage. The ratio,Q, of the bulk permeability coefficient for sodium,P, to the tracer sodium permeability coefficientP^*, was found to remain constant asC_m decreased. Equations were derived on this basis for bidirectional fluxes and forP andP^* as functions ofC_m, which corresponded closely to the observed data. The explanation for the observed value ofQ and its constancy under these conditions is uncertain.