STUDIES ON THE CHARACTERIZATION OF THE SUBTYPE(S) OF MUSCARINIC RECEPTOR INVOLVED IN PROSTACYCLIN SYNTHESIS IN RABBIT CARDIOMYOCYTES

The present study was conducted to localize and characterize the subty pe(s) of muscarinic receptor involved in prostacyclin (PGI(2)) product ion elicited by the cholinergic transmitter acetylcholine (ACh) in var ious cell types in the rabbit heart. ACh increased PGI(2) synthesis me asured as 6-keto-PGF1 alpha, in cultured coronary endothelial cells an d freshly dissociated ventricular myocytes in a dose dependent manner but not in cultured coronary smooth muscle cells of rabbit heart. McN- A-343, a partially selective M(1) muscarinic ACh receptor (mAChR) agon ist, did not alter 6-keto-PGF1 alpha synthesis in these cell types. AC h induced 6-keto-PGF1 alpha synthesis in coronary endothelial cells an d ventricular myocytes was not altered by a low concentration (10(-8) M) of pirenzipine, an M(1) mAChR antagonist but was reduced by a highe r concentration (10(-6) M). In coronary endothelial cells ACh induced 6-keto-PGF1 alpha production was reduced by hexahydro-situ-difendial ( HHSiD), an M(3) mAChR antagonist, and in ventricular myocytes by both l-5,11-dihydro-6-H-pyrido-[2,3-b]-benzodiazepine-6 one} (AF-DX 116), a n M(2) receptor antagonist, and HHSiD. The decrease by ACh of isoporte renol stimulated cAMP accumulation was minimized by AF-DX 116 but not by HHSiD or pirenzipine. Pertussis toxin treatment minimized ACh induc ed decrease in isoproterenol stimulated rise in cAMP and ATP release, but not ACh induced 6-keto-PGF1 alpha synthesis. These data suggest th at ACh stimulates prostacyclin production in coronary endothelial cell s via M(3) mAChR and in ventricular myocytes M(2) and M(3) mAChR. More over, ACh induced decrease in cAMP, but not the increase in 6-keto-PGF 1 alpha production, is mediated by pertussis toxin sensitive G alpha i proteins in these cells.