Modulation of neuroeffector transmission by endogenous nitric oxide: a role for acetylcholine receptor-activated nitric oxide formation, as indicated by measurements of nitric oxide/ nitrite release

Nitric oxide (NO) synthase inhibitors enhanced nerve-mediated contractile responses in guinea pig ileum longitudinal muscle, likely via a prejunctional effect on substance P-like neuroeffector transmission. Supporting a modulatory role for NO, application of NO through administration of acid sodium nitrite evoked marked inhibitory effects on responses to transmural nerve stimulation. Substance P-like responses to nerve stimulation were abolished by substance P receptor antagonists and were enhanced by atropine, indicating a cholinergic influence on substance P-like neuroeffector transmission. Since acetylcholine can evoke release of NO from endothelium, the possible role of acetylcholine in NO release in ileum was examined. The release of NO/nitrite, determined by chemiluminescence, was inhibited by N G-monomethyl-L-arginine (L-NMMA), by calcium removal, by tetrodotoxin or by atropine, indicating a nerve-mediated control of NO production. A basis for the NO release is likely to be spontaneous neuronal activity, where release of acetylcholine, with subsequent muscarinic receptor activation, contributes to stimulation of NO formation.