Saturable K⁺ pathway across the outer border of frog skin (Rana temporaria): Kinetics and inhibition by Cs⁺ and other cations

Summary

The reaction of abdominal skins of the frog speciesRana temporaria on mucosal K⁺-containing solutions was studied in an Ussing-type chamber by recording transepithelial potential difference (PD), short-circuit current (SCC) and conductance (G). With Na-Ringer's as serosal medium, a linear correlation between PD and the logarithm of the mucosal K⁺-concentration ([K] _o ) was obtained. The K⁺-dependent SCC saturated with increasing [K] _o , and could quickly and reversibly be depressed by addition of Rb⁺, Cs⁺, and H⁺, Li⁺, Na⁺, and NH ⁺₄ did not influence K⁺ current. A large scatter was obtained for kinetic parameters like the slope of the PD-log [K] _o -line (18–36.5 mV/decade), the apparent Michaelis constant (13–200mm), and the maximal current of the saturable SCC (6–50 μA·cm⁻²), as well as for the degree of inhibition by Cs⁺ ions. This seemed to be caused by a time-dependent change during long time exposure to high [K] _o (more than 30 sec), thereby inducing a selectivity loss of K⁺-transporting structures, together with an increase in SCC andG and a decrease in PD. Short time exposure to K⁺-containing solutions showed a competitive inhibition of K⁺ current by Cs⁺ ions, and a Michaelis constant of 6.6mm for the inhibitory action of Cs⁺. Proton titration resulted in a decrease of K⁺ current at pH<3. An acidic membrane component (apparent dissociation constant 2.5×10⁻³ m) is virtually controlling K⁺ transfer. Reducing the transepithelial K⁺-concentration gradient by raising the serosal potassium concentration was accompanied by the disappearance of SCC and PD.