Ca(2+)-blockable, poorly selective cation channels in the apical membrane of amphibian epithelia. Tetracaine blocks the UO2(2+)-insensitive pathway.

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Abstract
We examined the effect of the local anesthetic tetracaine on the Ca(2+)-blockable, poorly selective cation channels in the isolated skin of Rana temporaria and the urinary bladder of Bufo marinus using noise analysis and microelectrode impalements. Experiments with frog skin demonstrated that mucosal concentrations of the compound up to 100 microM did not affect the Na+ current through type S channels (slowly fluctuating, UO2(2+)-blockable channels) and the associated noise. On the other hand, 20 microM mucosal tetracaine already suffices to inhibit approximately 50% of the current carried by Cs+ and Na+ through channel type F (fast fluctuating, UO2(2+)-insensitive channel) and So of the associated Lorentzian component. With 100 microM of the inhibitor the current and So values were reduced by at least 70-80%. The time course of the response to serosal tetracaine was markedly slower and the effects on the current and So were smaller. Possible effects on the basolateral K+ conductance were excluded on the basis of the lack of response of transepithelial K+ movements to 100 microM tetracaine. UO2(2+) and tetracaine together blocked the poorly selective cation pathways almost completely. Moreover, both agents retain their inhibitory effect in the presence of the other. In toad urinary bladder, the Ca(2+)-blockable channel is also tetracaine blockable. The concentration required for half-maximal inhibition is approximately 100 microM in SO4(2-) and approximately 20 microM in Cl-. The data with tetracaine complement those obtained with UO2(2+) and support the idea that the Ca(2+)-blockable current proceeds through two distinct classes of cation channels. Using tetracaine and UO2(2+) as channel-specific compounds, we demonstrated with microelectrode measurements that both channel types are located in the granulosum cells.