Verapamil-induced contracture in an excised, blood-perfused skeletal muscle preparation from the dog

The ability of verapamil to induce a contractile response in skeletal muscle was investigated using the canine isolated, blood-perfused diaphragm preparation. Verapamil-induced contracture was characterized by a biphasic increase in muscle tension consisting of an initial phasic and subsequent tonic contracture. The second tonic component of the verapamil-induced contracture was inhibited by procaine, but procaine had no effect on the initial phasic contracture. Verapamil potentiated both KCl- and caffeine-induced contractures during the second tonic stage. This action might be explained by enhancement by the drug of availability, or by an increased sensitivity of the contractile machinery to calcium liberated by membrane depolarization or by the calcium-induced calcium release mechanism. The second tonic component of verapamil-induced contracture was much less than the initial phasic component. It seems, therefore, that the calcium release induced by the procaine-resistant process is the principal action of verapamil in inducing contracture in skeletal muscle.