Effects of Propranolol and Its Stereoisomers upon Coronary Vascular Resistance

Abstract

Responses of the coronary vascular bed to propranolol and its stereoisomers were studied in dogs anesthetized with allobarbital and urethane. The circumflex coronary artery was cannulated and perfused at a constant rate with blood from a femoral artery, coronary perfusion pressure being monitored as an index of coronary vascular resistance. dl-Propranolol (0.5 mg/kg iv) increased coronary vascular resistance and simultaneously reduced myocardial contractile force, heart rate, and systemic blood pressure. Approximately two thirds of the increase in coronary resistance was associated with the negative chronotropic action of propranolol. Intracoronary injections of dl-propranolol and l-propranolol increased coronary vascular resistance; d-propranolol increased resistance after an initial transient coronary dilatation. Intracoronary injections reduced contractile force in the pump-perfused area, but were without effects on heart rate and blood pressure. Reserpine pretreatment reduced, but did not abolish, the coronary vascular response to systemic propranolol; the chronotropic and inotropic responses were abolished. The responses to intracoronary d-propranolol were not altered. It was concluded that the increase in coronary vascular resistance following propranolol is due largely to the negative chronotropic effect of the drug and to an action of the drug which is unrelated to specific β-adrenergic receptor blockade.