Alpha-adrenergic regulation of the coronary circulation in the conscious dog

Abstract

To examine the role of neural and humoral factors in the control of coronary vasoactivity in conscious animals in which the complicating effects of general anesthesia and recent surgery were absent, dogs were instrumented with miniature pressure gauges in the aorta and left ventricle to measure aortic and left ventricular pressures and with flow probes in the left circumflex coronary artery to measure coronary blood flow. Several weeks after recovery from operation, experiments were conducted in the conscious dogs. Stimulation of the carotid chemoreceptor reflex elicted a biphasic response; initially striking reflex coronary vasodilation was observed: Coronary blood flow tripled even after changes in metabolic factors were minimized by pretreatment with propranolol. The chemoreceptor reflex coronary vasodilation was mediated both by cholinergic activation and withdrawal of alpha-adrenergic tone. The second phase of the response involved an increase in coronary vascular resistance, associated with elevated arterial pressure and an absolute reduction in coronary blood flow and coronary sinus oxygen content. The later period of coronary vasoconstriction was abolished by alpha-adrenergic blockade. Intravenous infusion of the alpha-adrenergic agonist methoxamine increased mean arterial pressure by 65 ± 5% and total coronary vascular resistance by 92 ± 14%. To determine whether the large coronary arteries were also under autonomic control, the dogs were instrumented with a pair of ultrasonic crystals on opposing surfaces of the left circumflex coronary artery to measure its diameter directly and continuously. Methoxamine reduced coronary cross-sectional area substantially, thereby constricting large coronary arteries and resistance coronary vessels. Thus, while the coronary arteries are regulated by changes in myocardial metabolic demands, they are also responsive to changes in neural stimuli. The most striking changes were observed with augmentation and withdrawal of alpha-adrenergic constrictor tone, which appear to be important in the control of large as well as small coronary vessels.