Components of sodium and chloride flux across toad bladder.

The effect of transepithelial potential difference (psi) on Na and Cl flux across toad bladder was assessed by measuring isotopic flux between identical media at various values of psi. The contribution of edge damage to ionic permeability was eliminated, resulting in relatively high spontaneous psi (-97 +/-4 mv) and low electrical conductance g. Bidirectional Na fluxes were measured simultaneously. Unidirectional Cl fluxes were measured in paired hemibladders at psi = 0 mv or -97 mv. Net Na flux J(Na), at psi = 0 mv, was slightly less than short-circuit current (SCC). At psi = -97 mv, J(Na) averaged 17% of SCC, and was sometimes zero. DeltaJ(Na)/Deltapsi (= g(+)) averaged 60% of g between -97 mv and +75 mv; at -150 mv, g(+) fell, indicating rectification. Analysis of unidirectional Na fluxes indicates low passive conductance (1.5 mumho/mg wet weight), a bidirectional, electrically neutral flux of approximately 0.13 mua/mg, and relatively large conductance of the active transport path at psi >/= -97 mv. The absence of appreciable transstimulation of serosal (S)-to-mucosal (M) Na flux (in response to increasing mucosal Na concentration) indicates that the electrically neutral flux is not exchange diffusion in the usual sense. Analysis of Cl fluxes indicates similar values for passive conductance and neutral flux, suggesting linked neutral flux of Na and Cl. Either the electromotive force of the Na pump E, its conductance g(a), or both are strong functions of psi. The product of these two quantities, Eg(a), is a measure of the "transport capacity" at any given value of psi, independent of the direct effect of psi on J(Na) through the pump path. Eg(a) varies with psi. Hence estimation of the net Na flux or current at any one value of psi, including psi = 0, fails to reveal the maximal transport capacity of the pump, its resting electromotive force (when J(Na) = 0 through the pump), or the dependence of transport capacity on potential.