Response of the frog skin to steady-state voltage clamping. I. The shunt pathway

Properties of the shunt pathway (a pathway in parallel to the Na transport system) in frog skin have been examined. The permeability of this shunt to urea increases markedly when the skin is depolarized to -100 mv (inside negative) but hyperpolarization to +100 mv produces no change in urea permeability compared to short-circuit conditions. The permeability increase at depolarizing potentials is dependent on the external solute concentration and is considerably reduced by the presence of external Ca. Neither urea permeability nor its response to changes in potential difference are affected by complete inhibition of Na transport by ouabain. In ouabain-poisoned skins, movements of Na, K, Cl, and mannitol through the shunt change in parallel with urea movements. Ion fluxes under these conditions and their response to potential can be described by the constant field equation. The selectivity of the shunt is in the order Cl > urea > K > Na > mannitol and this order does not appear to be affected by the absolute magnitude of the shunt permeability. Arguments are presented suggesting that the pathway is mainly between cells and that its permeability may be affected by cell swelling.