Quinidine-induced inhibition of transient outward current in cardiac muscle

Quinidine is frequently used as a class I antiarrhythmic agent in the management of cardiac rhythm disturbances. It depresses the rapid initial depolarization of the action potential by blocking the sodium current, INa. In addition, quinidine increases the duration of the action potential and lengthens the refractory period. We have used a whole cell voltage-clamp technique to study the ionic mechanism underlying the lengthening of the action potential in single cells from the atrium and ventricle of the rabbit heart. Our data show that quinidine at therapeutic doses (3-10 microM) is a potent and selective inhibitor of a transient outward current, which controls the early repolarization of the action potential. In contrast, neither the calcium current, ICa, nor the time-independent background K+ current, IK1, is changed significantly by 10 microM quinidine. The reduction in the transient outward current can explain the lengthening of action potential and provides new insight into the mechanism of action of quinidine as an antiarrhythmic agent.