Evidence for a transcellular component to the transepithelial sodium efflux in toad skin.

The transepithelial efflux of sodium, from the inner to the outer surface was measured across the isolated toad skin, mostly after abolition of the electrochemical gradient. The effects on this efflux of several agents and manipulations were studied in order to make a distinction between the paracellular component and a hypothetical transcellular one. Amiloride decreased the transepithelial efflux, while ouabain and cyanide increased it. From the known mode of action of those agents, it was inferred that part of the efflux occurred across the cell. Removal of sodium from the external solution interfered apparently with both components of the transepithelial efflux, while the action of external hypertonicity seemed to be restricted to the paracellular shunt pathway. Access of sodium from the internal solution to the active transport pool is thus suggested, with consequent increase in metabolic cost of transport. Yet, compared with the net influx, the amounts involved are very small; consequently, they escape detection by oxygen consumption measurements.