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Recent studies from this laboratory have indicated that sympathetic α-receptor-mediated coronary vasoconstriction can compete with local metabolic vasodilation to reduce the oxygen supply to the myocardium. In vitro studies from other laboratories on isolated coronary vessel strips suggest that large epicardial vessels are the dominant site of adrenergic a-receptor activity. In this study we used anesthetized, open-chest dogs to test the hypothesis that α-receptor-mediated vasoconstriction occurs predominantly in epicardial vessels, which are partially removed from the metabolic milieu in the myocardium. Adrenergic α-receptor blockade was achieved by propranolol (3 mg/kg, iv). The circumflex coronary artery was pump-perfused at constant pressure to minimize passive changes in large vessel resistance. Pressure was measured at the tip of the perfusion cannula sealed in the circumflex artery, and in an apical branch of the circumflex artery. Large vessel resistance was calculated as the pressure gradient along the vessel segment divided by the coronary flow. Intracoronary injections of nitrogly cerin were used as an independent measure of the vasomotor responsiveness of the large vessel segment. Adrenergic β-receptor activation was produced by intracoronary bolus injections of norepinephrine and by electrical stimulation of the left stellate ganglion. Alpha-receptor stimulation caused an increase in total coronary vascular resistance; however, the relative increase in the resistance of the large vessel was only about 60% of that seen for the entire coronary bed. These data suggest that, contrary to the proposed hypothesis, adrenergic a-receptor-mediated vasoconstriction in the large coronary vessels is not proportionally greater than that observed in the total coronary vascular bed.