Evidence for a class 4 effect of cibenzoline "in vivo"

In anesthetized dogs, it has already been shown that cibenzoline (4 mg/kg i.v.) possesses class 1 anti-arrhythmic properties. In this work, the cardiac electrophysiologic effects of cibenzoline (1 mg/kg i.v.) were studied before and after propranolol (0.2 mg/kg i.v.) treatment. Cibenzoline caused a slight tachycardia, a reduction of conduction velocity in the His-Purkinje system and in the ventricle, but no significant effects in the atria and the atrioventricular node were detected. On the contrary, when dogs were given the beta-adrenoceptor blocking agent propranolol, cibenzoline produced major effects in the slow response structures, especially in the sinus node and the atrioventricular node (bradycardia and depression of the atrioventricular nodal conduction). No further effects were observed on the His-Purkinje system and ventricle as compared to administration of cibenzoline alone. In dogs, cibenzoline given i.v., had no effects on the slow response systems, probably because of sympathetic nervous system intervention since the class 4 effects of cibenzoline appeared after beta-adrenoceptor blockade.