Current-voltage relations of Cs⁺-inhibited K⁺ currents through the apical membrane of frog skin

Abstract

The voltage-dependence of the inhibitory effect of mucosal Cs⁺ on the inward K⁺ current through the apical membrane of frog skin (Rana temporaria) was studied by recording transepithelial current-voltage relations. Experiments were performed with skins exposed to NaCl and KCl Ringer solutions on the serosal and mucosal side respectively (contron skins), as well as with tissues incubated with K₂SO₄ Ringer solutions on both sides (depolarized skins). Studies of the dose-depedence of the Cs⁺ block showed that under both experimental conditions the apparent affinity of Cs⁺ increased as the transepithelial potential was clamped at higher mucosal positive voltages. Under control conditions, the concentration of Cs⁺ required to block 50% of the K⁺ current (K_Cs) recorded while the transepithelial voltage was clamped at zero mV was 16 mmol/1. K_Cs decreased exponentially with muscosal positive voltages. The dependence of K_Cs on the membrane potential was analyzed with Eyring rate theory in which Cs⁺ was assumed to block the K⁺ transport by binding to a site within the channel. The analysis showed that this site is located at a relative electrical distance δ=0.32 of the voltage drop across the apical membrane, measured from the cytosolic side. The Hill coefficient obtained from this analysis wasn=3.1. Experiments with K⁺-depolarized tissues showed that only inward K⁺ currents recorded with positive transepithelial voltages were depressed by external Cs⁺. Also under these conditions K_Cs showed an exponential dependence on the transepithelial potential. The analysis of these data with the rate theory revealed δ=0.09 andn=1.7. The difference in δ found in control and depolarized tissues can be explained by the influence of the basolateral membrane resistance on theI–V relations.