Treatment of the apical surface of the isolated, ouabain-inhibited turtle colon with the polyene antibiotic amphotericin B permitted the properties of a barium-sensitive potassium conductance in the basolateral membrane to be discerned from the measurements of transepithelial fluxes and electrical currents. Simultaneous measurements of potassium currents and 42K fluxes showed that the movement of potassium was not in accord with simple diffusion. Two other cations, thallium and rubidium, were also permeable and, in addition, exhibited strong interactions with the potassium tracer fluxes. The results indicate that permeant cations exhibit positive coupling, which is consistent with a single-file mechanism of ion translocation through a membrane channel.
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