Depolarization of Rhodnius oocytes evokes action potentials (APs) whose rising phase is calcium-dependent. The ionic basis for the repolarizing (i.e. falling) phase of the AP was examined.

Addition of potassium channel blockers (tetraethylammonium, tetrabutylammonium, 4-aminopyridine, atropine) to the bathing saline increased the duration and overshoot of APs. Intracellular injection of tetraethyl ammonium had similar effects. These results suggest that a voltage-dependent potassium conductance normally contributes to repolarization. Repolarization does not require a chloride influx, because substitution of impermeant anions for chloride did not increase AP duration. AP duration and overshoot actually decreased progressively when chloride levels were reduced.

Current/voltage curves show inward and outward rectification, properties often associated with potassium conductances. Outward rectification was largely blocked by external tetraethylammonium. Possible functions of the rectifying properties of the oocyte membrane are discussed.

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