Coronary artery reactivity in human vessels: some questions and some answers.

Spasm of a conduit coronary artery, converting it into a major resistance vessel impeding myocardial blood flow, may have severe short- or long-term effects on cardiac rhythm and systolic ejection of blood. It is now clear that human coronary arteries in vitro contract to acetylcholine but that relaxation is the only response observed in dog coronary vessels. Acetylcholine is as powerful a constrictor of human coronary arteries, in terms of tension induced, as 5-hydroxytryptamine (5-HT) or histamine and is a substantially more powerful constrictor than norepinephrine. Field stimulation of coronary artery strips caused a vasoconstriction that was partially antagonized by atropine (3.45 X 10(-6) M). An enhanced reactivity of the epicardial arteries of cardiac and older patients to several agonists was also observed and appears to provide a background against which a number of vasoactive agents might induce spasm. Coronary tissue from cardiac patients also contains stores of 5-HT and histamine, and the histamine levels are substantially increased above the values in vessels from noncardiac patients. Coronary artery spasm or contraction probably can be initiated by diverse intrinsic and extrinsic influences, including autonomic discharge from either the parasympathetic or sympathetic nervous system or from histamine or 5-HT, and probably no one agent or entity is causative in all cases.