Comparison of Ca2+ channel inhibitory effects of cibenzoline with verapamil on guinea-pig heart

1. Cibenzoline, a class 1 antiarrhythmic drug, was studied on the effects upon the myocardial slow inward Ca2+ current (ICa) and the contractile force, and was compared with verapamil. 2. In voltage-clamp experiments with guinea-pig ventricular myocytes, cibenzoline caused a concentration-related inhibition of ICa (IC50 = 30 microM), whereas verapamil exerts a stronger effect (IC50 = 0.6 microM). 3. Cibenzoline also produced a concentration-related decrease in contractile force of isolated papillary muscle preparation (IC50 = 30 microM), while IC50 of verapamil was 0.8 microM. 4. Although the myocardial Ca2+ channel blocking effect of cibenzoline is about 1/50 compared to verapamil, cibenzoline possesses an inhibitory action on Ca2+ channel as well as on Na+ channel.