Passive transepithelial cationic fluxes across the frog skin under short-circuit conditions

The short-circuit current (Isc) across the frog skin in the steady state reflects the active Na+ transport. Inhibition of the Na+-K+ pump by ouabain causes slow decay of Isc. It has been suggested that this slow ouabain effect on Isc could be due to the asymmetric ionic permeations of frog skin. That is, Na+ flux at the apical membrane and K+ flux at the basolateral membrane transiently generate the transepithelial cationic fluxes and are measured as the Isc even under the condition of active Na+ transport arrest. However, this hypothesis on the transient Isc has not been studied experimentally. In the present study, transient inward and outward Isc were observed alternately even after pump arrest by changing the ionic compositions of the bathing solutions in a Ussing's chamber. The time constant of Isc decay was 20-30 min. The Isc responses were quicker and stronger on the isolated epithelia than on the whole skin. Both Isc responses were blocked by amiloride, a Na+ channel blocker. Measurements of the ionic composition of isolated epithelia under experimental conditions indicate that the passive Na+ flux across the apical membrane and the passive K+ flux across the basolateral membrane cause both transient inward and outward Isc under the ouabain-treated condition of frog skin.