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

Abstract

The reaction of abdominal skins of the frog species Rana 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 NH4+ 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--200 mM), and the maximal current of the saturable SCC (6--50 microa . 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 and G 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 6.6 mM for the inhibitory action of Cs+. Proton titration resulted in a decrease of K+ current at pH less than 3. An acidic membrane component (apparent dissociation constant 2.5 x 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.