This study was designed to evaluate the in vivo effect of acetylcholine on endothelial-damaged canine epicardial coronary arteries and the potential contribution of platelets to those acetylcholine-induced responses. Changes in left anterior descending artery cross-sectional area were determined by quantitative angiography in the closed chest anesthetized dog. Baseline cross-sectional area of the left anterior descending artery was not changed by removal of the endothelium by balloon-tipped catheter. Increased constrictor tone produced by prostaglandin Fα was comparable in endothelium-intact and endothelium-removed vessels, supporting an endothelium-independent mechanism for prostaglandin F2α in vivo. Acetylcholine produced anterior descending artery vasodilation with the endothelium intact; a comparable maximal dilator response was also obtained in the presence of increased constrictor tone (prostaglandin F2α). In contrast, acetylcholine produced vasoconstriction of the anterior descending artery when the endothelium was removed. To evaluate the mechanism of acetylcholine-induced vasoconstriction in endothelium-removed vessels, the same protocol was completed in the presence of the platelet inhibitor indomethacin. Indomethacin did not alter baseline cross-sectional area or the dilator response to acetylcholine in endothelium-intact vessels. In contrast, the constrictor response in endothelium-removed vessels was antagonized, and a dilator response comparable with that in endothelium-intact vessels was produced by acetylcholine. The results of this study provide an experimental basis for the observations in human studies in which apparently atherosclerotic vessels constrict in response to acetylcholine. Removal of the endothelium in vivo abolishes the dilator response to acetylcholine and converts the acetylcholine response to vasoconstriction or vasospasm. This vasoconstrictor response is reversed to a dilator response by indomethacin, suggesting that acetylcholine-induced vasoconstriction in endothelial-damaged arteries in vivo may, in part, be facilitated by platelet-released vasoconstricting substances.
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