BETA-RECEPTOR SUBTYPES IN THE CANINE CORONARY CIRCULATION

The principal difficulty in determining the sub-type of coronary vascular beta-receptors in vivo is to avoid the local metabolic coronary vasodilation that occurs secondary to activation of myocardial beta-receptors. Therefore, a nonbeating cardiac preparation without chronotropic or inotropic effects is needed. In this study, the coronary circulation was perfused at constant pressure in closed-chest chloralose-anesthetized dogs. The increase in coronary blood flow due to intracoronary injections of the combined beta-1 and beta-2-agonist isoproterenol was determined during prolonged asystoles after the cessation of cardiac pacing in atrioventricular heart-blocked animals. Both beta-1-selective (practolol and L 650,744) and beta-2-selective (ICI 118,551) antagonists blocked isoproterenol-induced coronary vasodilation. In contrast, isoproterenol vasodilation in the femoral circulation was blocked by beta-2- but not by beta-1-selective antagonists. In conclusion, both beta-1- and beta-2-receptors in coronary resistance vessels are stimulated by isoproterenol to produce vasodilation during prolonged asystoles, when cardiac chronotropic and inotropic effects are absent.