Prostacyclin synthesis elicited by cholinergic agonists is linked to activation of M 2α and M 2β muscarinic receptors in the rabbit aorta

Abstract

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-PGF 1α was assessed after exposing the aortic rings to different cholinergic agents. Acetylcholine (ACh) (M 1 and M 2 agonist) (1–10 μM) and arecaidine proparagyl ester (APE) (M 2 selective agonist) (1–10 μM) enhanced 6-keto-PGF 1α output in a concentration-dependent manner. A selective M 1 receptor agonist, McN-A-343, at 1 μM-1 mM did not alter 6-keto-PGF 1α output. ACH- and APE induced increases in 6-keto-PGF 1α output were attenuated by the M 1 M 2 antagonist atropine (0.1 μM), M 2α antagonist (AF-DX 116), (0.1–1.0 μM), and by selective M 2β antagonist, hexahydro-sila-difendiol (HHSiD) (0.1–10 μM), but not by the M 1 antagonist pirenzepine (1.0 μM). 6-Keto-PGF 1α output elicited by ACh- or APE was not altered by the adrenergic receptor antagonist phentolamine and propranolol or by the nicotinic receptor blocker hexamethonium. Similarly, the arachidonic acid- or norepinephrine induced 6-keto-PGF 1α accumulation was not altered by these muscarinic receptor antagonists. Indomethacin, a cyclooxygenase inhibitor, prevented arachidonic acid, ACh- or APE induced 6-keto-PGF 1α output. Removal of the endothelium abolished the production of 6-keto-PGF 1α elicited by ACH, APE, bradykinin, and calcium ionophore A 23187, but not that induced by angiotensin II, K + or norepinephrine. These data suggests that vascular prostaglandin generation elicited by cholinergic agonists is mediated via activation of M 2α and M 2β but not M 1 muscarinic receptors, which are most likely located on the endothelium.