Abstract

The effects of quinidine, an antiarrhythmic alkaloid, on the membrane conductances of Myxicola axonal membrane were investigated by using the voltage-clamp technique. In the presence of 0.2 mM quinidine sulfate, the resting membrane potential was not affected, whereas the repolarization phase of the action potential was prolonged. Under voltage-clamp conditions, potassium currents displayed a distinct peak before decaying exponentially to a plateau level. Both sodium and potassium conductances were suppressed. However, the potassium conductance was suppressed to a much greater extent and displayed a negative slope in the conductance-voltage curve for membrane potentials above +40 mV. The kinetics of sodium currents were not appreciably affected. The block of potassium channels by quinidine displayed both time- and voltage-dependent characteristics. The percentage of reduction in current depended upon the fraction of the potassium channels open under control conditions. Quinidine blocked both outward and inward potassium currents. The block was less in the presence of high external potassium ions. Two-pulse experiments showed that the number of "inactivated" potassium channels was dependent upon the prepulse potential. Recovery from inactivation was enhanced by hyperpolarization. The primary effect of quinidine in Myxicola was attributed to a quinidine-induced block of potassium conductance.