Carotid baroreceptor reflex coronary vasodilation in the dog

The hypothesis that neurally mediated coronary vasodilation occurs as part of the carotid baroreceptor reflex was investigated. The left main coronary artery was cannulated and perfused at constant pressure (100 mm Hg) in closed-chest, chloralose-anesthetized dogs. The heart was paced at a constant rate between 60 and 70 beats/min after atrioventricular heart block. Propranolol (1 mg/kg) was given to prevent beta-receptor-mediated alterations in contractility. Aortic blood pressure was stabilized with a blood reservoir. The aortic depressor nerves were cut bilaterally to prevent the buffering influence of aortic arch baroreceptors on the carotid baroreceptor reflex. The carotid sinuses were vascularly isolated and perfused with arterial blood at controlled pressures. Under these conditions, a step change in carotid sinus pressure from 70 to 210 mm Hg produced a 0.29 ml/min per g increase in coronary flow above control and a 10 mm Hg increase in coronary sinus blood oxygen tension. A step in carotid sinus pressure from 70 to 150 mm Hg resulted in a flow increase of 0.13 ml/min per g and a coronary sinus oxygen tension increase of 5.3 mm Hg relative to prestimulation values. Atropine (0.5 mg/kg, iv) blocked most of the reflex coronary vasodilation, indicating a parasympathetic component, and the addition of adrenergic alpha-receptor blockade with phenoxybenzamine (0.25 mg/kg, ic) abolished the remaining response, demonstrating sympathetic participation. The reflex nature of the coronary response was confirmed with carotid sinus denervation and vagotomy. It is concluded that carotid sinus hypertension results in a graded reflex neural coronary vasodilation independent of myocardial metabolic factors. The major component is due to activation of parasympathetic coronary vasodilator fibers, but there is also inhibition of sympathetic vasoconstrictor fibers.