Responses of Coronary Vessels to Adrenergic Stimuli

Experiments were done on anesthetized dogs (chloralose-urethane) to characterize the pattern of responsisveness to adrenergic stimuli. The left circumfles coronary artery was perfused with blood at constant rate. Changes in perfusion pressure of the coronary vessels reflected changes in coronary vascular resistance. Responses to direct electrical nerve stimulation of the cardiac sympathetic nerves, to intra-coronary injections of norepinephrine, isoproterenol, epinephrine; to electrical stimulation of carotid sinus nerves; and to stimulation of carotid chemoreceptors with nicotine and cyanide were tested. The intravenous administration of Practolol (1–2 mg/kg) eliminated changes in contractility which could result from these interventions and thus minimized or eliminated changes in coronary vascular resistance in response to changes in myocardial metabolism. The results indicated that 1) there is a paucity of alpha adrenergic vasoconstrictor receptors in the coronary vessels as compared to other vascular beds; 2) the coronary vascular beta receptors are not homologous to the cardiac beta receptors and are responsive to isoproterenol and epinephrine but not to norepinephrine nor to mose sympathetic nerve stimulation; 3) most of the dilator effect of isoproterenol represents activation of coronary and not cardiac beta receptors; 4) stimulation of baroreceptor nerves causes withdrawal of the sympathetic vasoconstrictor influence and minimal activation of cholinergic vagal vasodilator fibers; and 5) stimulation of chemoreceptors caused significant activation of cholinergic vagal vasodilator fibers.