Passive sodium fluxes across toad bladder in the presence of simultaneous transepithellal gradients of concentration and potential.

Bidirectional sodium fluxes were measured across toad bladder sacs after eliminating active transport with ouabain. Transepithelial potential was clamped to 100 mV or the Nernst potential, psieq, at varying sodium concentrations, Cm, in the mucosal medium. Serosal sodium concentration, Cs, was held constant. Equations were derived for permeability, partial ionic conductance, and unidirectional fluxes as functions of Cm and Cs, based in part on the assumption that the ratio, Q, of bulk sodium permeability to tracer sodium permeability is a constant, independent of concentration and potential. The results conformed closely to these equations.