[Effects of labetalol on the circulatory system (author's transl)]

Effects of labetalol on the cardiovascular system were studied in dogs, guinea pigs, rats, rabbits, and bullfrogs. In dogs anesthetized with pentobarbital, labetalol decreased heart rate, arterial blood pressure, and myocardial contractile force, and inhibited carotid sinus reflex, without affecting pO2 and pH of circulating blood, urine volume, and urinary excretion of sodium and potassium. Alpha- and beta-adrenergic blocking action of labetalol was demonstrated in dog hearts, arterial preparations of guinea pigs and rats, isolated rabbit hearts, and bullfrog hind limb and rabbit ear preparations. In bullfrog hind limb and rabbit ear preparations, labetalol dilated blood vessels in concentrations larger than those which produced alpha- and beta-adrenergic blockade. The dilatation of blood vessels was observed only when labetalol was added to the perfusate, whereas alpha- and beta-adrenergic blockade continued even after removal of labetalol from the perfusate. It is concluded that labetalol blocks alpha- and beta-adrenergic receptors in the heart and blood vessels of the species used herein, and that this compound inhibits carotid sinus reflex in dogs. In addition, the dilatation of the blood vessels of the bullfrog hind limb and rabbit ear as evoked by labetalol probably involves a mechanism different from that of adrenergic blockade.