Electrochemical potentials in frog skin: inferences for electrical and mechanistic models

To evaluate possible mechanisms of transport at apical and basolateral barriers of Na transporting cells of epithelia, it is necessary to know the difference of electrochemical potentials at each barrier. A reevaluation in light of new data of intracellular voltages of frog skin leads to fundamental questions concerning the origin of the voltages at both inner and outer barriers of this tissue. Whereas the inner barrier is highly selective for K, confirming the observations of Koefoed-Johnsen and Ussing, the voltage across the inner barrier, Vi, especially in the absence of transepithelial Na transport, may be greater than the Nernst equilibrium potential for K estimated from the maximum values of intracellular [K] reported in the literature. Consequently, it is proposed that the Na:K pumps may, under some conditions, behave not only as a Na:K exchange pump but also as a cation extrusion pump for K especially when intracellular [Na] falls to low levels. In order to explain the relationship between Na entry and the voltage at the outer barrier, it is proposed that the conductance of the outer barrier is voltage dependent, in line with previous observations of the nonlinear electrical behavior of the apical barrier of Na transporting cells. Thus, the outer barrier may behave as a simple voltage independent resistor with a Thévenin electromotive force of zero at negative intracellular voltages despite the existence of a chemical potential for Na at this barrier.