The influence of potassium concentration on the action of quinidine and of some antimalarial substances on cardiac muscle.

The action of quinidine and some other antimalarial substances on cardiac muscle has been shown to be closely related to the K(+) concentration of the surrounding medium. The depression of the amplitude and rate of the contractions of the isolated perfused rabbit heart and of the isolated rabbit atria which the antimalarial substances produced seems to be due to a diminution of the permeability of the membrane to K(+), since it can be reversed by lowering the external K(+) concentration. During exposure to any of the antimalarial compounds tested, the normal inhibitory action of acetylcholine was converted to a stimulant action. This stimulant action of acetylcholine is probably due to its effect in increasing the permeability of the membrane to K(+). There were slight differences in behaviour between proguanil and quinidine on the one hand, and chloroquine, mepacrine and pyrimethamine on the other. The observations may explain the action of quinidine-like substances in abolishing fibrillation.