Prostacyclin synthesis elicited by cholinergic agonists is linked to activation of M2 alpha and M2 beta muscarinic receptors in the rabbit aorta.

This study was conducted to investigate the subtypes of muscarinic receptors involved in the action of cholinergic agents on prostacyclin synthesis in the rabbit aorta. Prostacyclin production measured as 6-keto-PGF1 alpha was assessed after exposing the aortic rings to different cholinergic agents. Acetylcholine (ACh) (M1 and M2 agonist) (1-10 microM) and arecaidine proparagyl ester (APE) (M2 selective agonist) (1-10 microM) enhanced 6-keto-PGF1 alpha output in a concentration-dependent manner. A selective M1 receptor agonist, McN-A-343, at 1 microM-1 mM did not alter 6-keto-PGF1 alpha output. ACh- and APE induced increases in 6-keto-PGF1 alpha output were attenuated by the M1/M2 antagonist atropine (0.1 microM), M2 alpha antagonist (AF-DX 116), (0.1-1.0 microM), and by selective M2 beta antagonist, hexahydro-sila-difendiol (HHSiD) (0.1-1.0 microM), but not by the M1 antagonist pirenzepine (1.0 microM). 6-Keto-PGF1 alpha output elicited by ACh- or APE was not altered by the adrenergic receptor antagonists phentolamine and propranolol or by the nicotinic receptor blocker hexamethonium. Similarly, the arachidonic acid- or norepinephrine induced 6-keto-PGF1 alpha accumulation was not altered by these muscarinic receptor antagonists. Indomethacin, a cyclooxygenase inhibitor, prevented arachidonic acid, ACh- or APE induced 6-keto-PGF1 alpha output. Removal of the endothelium abolished the production of 6-keto-PGF1 alpha elicited by ACh, APE, bradykinin, and calcium ionophore A 23187, but not that induced by angiotensin II, K+ or norepinephrine. These data suggest that vascular prostaglandin generation elicited by cholinergic agonists is mediated via activation of M2 alpha and M2 beta but not M1 muscarinic receptors, which are most likely located on the endothelium.