A single-gene mutant of Paramecium aurelia is analysed electrophysiologically. (a) The regenerative Ca-response, triggered by small or moderate current, was smaller and slower in the mutant than in wild type. (b) Input resistance of the mutant membrane is about half of that of wild type bathed in various solutions. This is true for the zero-current input resistance and the chord resistance measured with high depolarizing current. (c) Membrane resistance of the mutant measure with hyperpolarizing currents is smaller than that of wild type only when K+ is the major external cation. (d) Internally applied TEA+ or externally applied Ba+ increases the membrane resistance of the mutant to that of wild type similarly treated. We conclude that the mutant has an increased K conductance.

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