Adrenergic coronary vasoconstriction in the presence of coronary stenosis in the dog.

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Abstract
Previous studies have demonstrated competition between sympathetic alpha receptor-mediated coronary vasoconstriction and local metabolic vasodilation during sympathetic activation. The present study tested if this competition also occurs in the presence of coronary stenosis. In closed-chest dogs, the left coronary artery was cannulated, and blood flow from the aorta was restricted by a moderate stenosis (70% area reduction). Intracoronary norepinephrine infusion produced coronary vascular alpha receptor and myocardial beta receptor activation. Norepinephrine infusion was repeated following alpha receptor blockade with phenoxybenzamine (0.25 mg/kg, injected into the coronary artery). Myocardial oxygen and lactate extraction, coronary sinus blood oxygen tension, and coronary resistance were compared at equal levels of myocardial oxygen consumption before and after coronary alpha receptor blockade. In the presence of coronary stenosis, intracoronary norepinephrine infusion decreased coronary sinus oxygen content and increased myocardial oxygen extraction. At comparable myocardial oxygen consumptions coronary vascular resistance was greater with alpha receptors intact than after alpha receptor blockade. The increase in myocardial oxygen extraction was prevented by alpha receptor blockade. We conclude that a sympathetic alpha receptor-mediated coronary vasoconstrictor influence operates, even in the presence of coronary stenosis, to limit oxygen delivery to the heart and increase myocardial oxygen extraction up to the point of cardiac failure, but that this vasoconstrictor effect does not result in net myocardial lactate production.