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

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Abstract
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 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.

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