UNIDIRECTIONAL K+ FLUXES THROUGH RECOMBINANT SHAKER POTASSIUM CHANNELS EXPRESSED IN SINGLE XENOPUS OOCYTES

We describe a method to evaluate the ratio of ionic fluxes through rec ombinant channels expressed in a single Xenopus oocyte. A potassium ch annel encoded by the Drosophila Shaker gene tested by this method exhi bited flux ratios far from those expected for independent ion movement . At a fixed extracellular concentration of 25 mM K+, this channel sho wed single-file diffusion with an Ussing flux-ratio exponent, n', of 3 .4 at a membrane potential of -30 mV. There was an apparent, small vol tage dependence of this parameter with n' values of 2.4 at -15 and -5 mV. These results indicate that the pore in these channels can simulta neously accommodate at least four K+ ions. If each of these K+ ions is in contact with two water molecules, the minimum length of tile pore is 24 Angstrom.