Microelectrode study of voltage-dependent Ba2+ and Cs+ block of apical K+ channels in the skin of Rana temporaria

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Abstract
The blockage of the apical K+ channels in frog species Rana temporaria by Ba2+ and Cs+ is strongly voltage-dependent. The interaction of both blockers with the K+ channels was studied by recording relations between the K+ currents (I K) and the transepithelial and intracellular potential. Mucosal Ba2+ and Cs+ depress I K, hyperpolarize the cell and induce pronounced nonlinearities in the current/voltage (I/V) relations. The nonlinearities are caused by the voltage-dependent interaction of Ba2+ and Cs+ with the binding site. Consequently, the apical membrane resistance not only depends on the blocker concentration but also on the apical membrane potential. Also the fractional resistance, fR a, and the voltage divider ratio, fV a, will change with blocker concentration and voltage. Owing to this non-ohmic behaviour, measurements of fV a in the presence of Ba2+ deviate markedly from the expected fR a values. The inhibitory effect of Ba2+ and Cs+ was analysed at different transepithelial and apical membrane voltages. The relation between the Michaelis-Menten constants and the voltage could be fitted with equations based on Eyring rate theory with the assumption of a single binding site. With this model we calculated the relative electrical position of the binding site for the blocker (δ), referred to the extracellular side of the channel. We obtained for Ba2+, δ=0.34±0.05 and for Cs+, δ=0.81±0.01. Comparison of the results from apical and transepithelial I/V relations demonstrates that the analysis of the transepithelial data provides overestimated values of the Hill coefficient and results in an underestimation of δ.