Selective hypercontraction caused by ergonovine in the canine coronary artery under conditions of induced atherosclerosis

Spasm of the coronary arteries: causes and consequences (the scientist's viewpoint)

Both beta 1- and alpha 1-adrenoceptors are present on canine coronary arteries, and they are accessible to norepinephrine released from the sympathetic nerves. Under normal conditions, these arteries relax because of the predominance of the beta 1-adrenoceptors, whereas constriction prevails in the presence of beta 1-adrenoceptor antagonists. The coronary arteries also have cholinergic nerves. When activated, these nerves release acetylcholine, which acts on muscarinic receptors on the sympathetic nerve terminals to reduce the output of norepinephrine and thereby lessen the relaxation mediated by beta 1-adrenoceptors. Thus, muscarinic agonists can precipitate coronary artery spasm. If the smooth muscle cells of the coronary arteries become hypoxic, their responsiveness to beta-adrenergic stimulation is lost and constrictor responses are exaggerated. Cardiac glycosides prevent the predominance of the beta-adrenergic effects of norepinephrine. Therefore, after treatment with ouabain, release of norepinephrine from the sympathetic nerves leads not to relaxation but to further contraction of coronary arteries. The endothelium of the coronary arteries inhibits platelet aggregation by the formation and release of prostacyclin, and it reacts to platelet products by causing relaxation of the underlying smooth muscle. In addition, if any thrombin is formed, it also causes endothelium-mediated relaxation. If the endothelium is damaged, these protective mechanisms are lost. Patients with coronary artery spasm usually have morphologic changes in the artery at the site of the spasm. Platelets can aggregate at this site and release vasoactive substances, which--aided by formation of thrombin--cause contraction. Thus, the blood supply to the myocardium is reduced; the ensuing hypoxia augments the constriction. Acute myocardial ischemia caused by coronary vasospasm may precipitate acute cardiac rhythm disturbances and sudden death by ventricular tachycardia or fibrillation.

Increased vasoconstrictor activity of proximal coronary arteries with endothelial damage in intact dogs

In this study we examined the hypothesis that endothelial damage increases proximal coronary arterial vasomotor tone and sensitivity to vasoconstrictor stimulation. The response of the left anterior descending coronary artery (LAD) (% area change) to serotonin and nitroglycerin were examined in eight anesthetized (Innovar + nitrous oxide), closed-chest dogs by means of quantitative coronary angiography. Dose-response curves of percent change in arterial cross-sectional area for three doses of intracoronary serotonin were examined before and after endothelial damage produced by a balloon catheter in the LAD. Endothelial damage was verified by postmortem scanning electron microscopic examination. Intracoronary injection of 133Xe provided coronary flow data. The damaged segment of LAD showed spontaneous vasoconstriction and further constriction in response to serotonin (33 +/- 5% before and 52 +/- 6% area reduction after damage; p less than .05). Nitroglycerin reversed serotonin-induced vasoconstriction in LAD segments without damage but not in the LAD segment with endothelial damage. No significant changes were observed in aortic pressure, and heart rate was kept constant by pacing. Blood flow in the LAD was not affected by endothelial damage itself (control, 2.44 +/- 0.09 ml/min/g; damage, 2.53 +/- 0.22 ml/min/g). Endothelial damage induced spontaneous proximal coronary constriction and diminished the relaxant response to nitroglycerin in the presence of serotonin. These results suggest that focal coronary narrowing that occurs in some patients after provocation with vasoconstrictor agents may be caused by local areas of damaged endothelium.