Passive Transepithelial Cationic Fluxes across the Frog Skin under Short-Circuit Conditions

Abstract

The short-circuit current (I_sc) 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 I_sc. It has been suggested that this slow ouabain effect on I_sc 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 I_sc even under the condition of active Na⁺ transport arrest. However, this hypothesis on the transient I_sc has not been studied experimentally. In the present study, transient inward and outward I_sc 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 I_sc decay was 20-30min. The I_sc responses were quicker and stronger on the isolated epithelia than on the whole skin. Both I_sc 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 I_sc under the ouabain-treated condition of frog skin.