Carotid sinus reflex coronary vasoconstriction during controlled myocardial oxygen metabolism in the dog

We studied carotid baroreceptor reflex coronary vasoconstriction in closed-chest dogs with controlled aortic blood pressure and myocardial oxygen metabloism. The dogs were anesthetized with morphine and chloralose. Left coronary blood flow was measured by a cannula-tip flowmeter, and myocardial oxygen metabolism was calculated by measuring the arterial-coronary sinus oxygen difference. A bilateral vagotomy was performed in all animals and they were treated with propranolol. Reduction of carotid sinus pressure to 40 mm Hg caused an increase in aortic pressure that was limited to 15 mm Hg by means of a pressure-control reservoir. During carotid hypotension, heart rate and myocardial oxygen metabolism were unchanged but diastolic conronary vascular resistance increased by 21%. Intracoronary artery infusion of norepinephrine had similar effects. After interruption of the reflex are with the alpha-adrenergic receptor antagonist, dibozane, carotid sinus hypotension and the same increase in aortic pressure (15 mm Hg) resulted in only a 5% increase in diastolic coronary resistance. Dibozane also reduced the coronary responses to norepinephrine. It is concluded that carotid sinus hypotension results in reflex sympathetic alpha-receptor coronary vasoconstriction and that this reflex vasoconstriction is independent of changes in myocardial oxygen metabolism or changes in aortic pressure.