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A mechanism by which acetylcholine (ACh) may elicit vasoconstrictor response in coronary vessels was studied in rat hearts perfused at a constant flow rate. In spontaneously beating hearts, bolus injections of ACh and carbachol (CCh) produced biphasic changes in coronary perfusion pressure (CPP): a transient increase at the initial period followed by a sustained decrease. In KCl-arrested hearts, ACh and CCh produced a monophasic increase in CPP, which was attenuated by either removal of endothelial cells by saponin or cyclooxygenase inhibition by diclofenac sodium. In the spontaneously beating heart, ACh-induced vasoconstriction was almost abolished by atropine (0.1 microM) and was markedly attenuated by an M2 antagonist, methoctramine (0.1 microM), but not by an M1 antagonist, pirenzepine (1 microM). Arecaidine propargyl ester (APE), an M2 agonist, produced coronary artery constriction which was attenuated by methoctramine (0.1 microM) but not by pirenzepine (0.1 microM) in both spontaneously beating and KCl-arrested hearts. McN-A-343, an M1 agonist, increased CPP in both beating and KCl-arrested hearts, but to a lesser degree than APE. These results suggest that the release of vasoconstrictor prostaglandins from endothelial cells contributes to the vasoconstrictor response to ACh in perfused rat coronary vessels, and the response to ACh appears to be mediated, in part, via the M2 subtype of muscarinic receptors.