Effects of l-penbutolol (Hoe 893d) on the transmembrane potentials of canine purkinje fibers and ventricular muscle fibers.

Intrinsic membrane effects (non-specific effects) and beta-adrenergic blocking action (specific effects) of l-penbutolol were studied using intracellular microelectrode techniques. l-Penbutolol in concentrations above 0.1 mg/L reduced the maximum rate of depolarization at phase O of both Purkinje fibers and ventricular muscle fibers without affecting their maximum diastolic potential. The action potential duration of Purkinje fibers were also shortened. The shortening of action potential duration and enhancement of automaticity in Purkinje fibers, normally induced by isoproterenol (0.2 mg/L), were antagonized by l-penbutolol in much lower concentraions (0.1--1.0 microgram/L) than those at which nonspecific effects are observed. The difference between these two concentration ranges is sufficient to suggest that l-penbutolol might be a highly specific beta-adrenergic blocking agent.