Muscarinic stimulation of prostacyclin synthesis by the rat penis

An in vitro model for the investigation of PGI₂ synthesis by the rat penis is described. Acetyl-β-methylcholine (methacholine; MeCh) and carbamyl choline (carbachol) stimulated PGI₂ synthesis in a dose-dependent manner (ED₅₀ = 1.5 × 10^-6; 2.5 × 10^-6, respectively), whereas adrenaline, noradrenaline, histamine, serotonin, vasoactive intestinal polypeptide (VIP), nicotine, dimethyl phenylpiperazinium (DMPP) and adenosine triphosphate (ATP) were without effect. MeCh (10^-6 mol/1)- and carbachol (10^-6 mol/1)-stimulated PGI₂ synthesis was inhibited by atropine in a dose-dependent manner (ID₅₀ = 2.5 × 10^-7mol/1; 7 × 10^-6mol/1, respectively). The inhibition by atropine of MeCh-stimulated PGI₂ synthesis was competitive. MeCh (10^-6 mol/1)-stimulated PGI₂ synthesis was inhibited by verapamil (ID₅₀ = 6 × 10^-5) and calcium-free incubation media. It is concluded that in the rat penis PGI₂ synthesis is principally under muscarinic control and is calcium transport-dependent. Since PGI₂ is a vasodilator, these findings may be relevant to the increased blood flow into the penis associated with penile erection. They may also be relevant to the protection of the penile vasculature from thrombosis, since PGI₂ is also a potent inhibitor of platelet aggregation.