N(G)-NITRO L-ARGININE METHYL-ESTER AND OTHER ALKYL ESTERS OF ARGININE ARE MUSCARINIC RECEPTOR ANTAGONISTS

Analogues Of L-arginine with modifications at the terminal guanidino nitrogen and/or the carboxyl terminus of the molecule have been widely used for their ability to inhibit the production of nitric oxide and are thought to be competitive antagonists of nitric oxide synthase. The present studies were designed to test the possibility that these agents are also muscarinic receptor antagonists. Acetylcholine produced concentration-dependent contraction of endothelium-denuded rabbit coronary artery as well as isolated strips of canine colonic smooth muscle. The arginine analogue N(G)-nitro L-arginine methyl ester (L-NAME, 100 muM) but not N(G)-monomethyl L-arginine (L-NMMA, 100 muM) significantly shifted these contractile relations to the right, an effect that was not reversed by addition of 1 mM L-arginine. In radioligand binding studies using the muscarinic radioligand [H-3]quinuclidinyl benzilate and tissues known to contain differing contributions of M1, M2, and M3 muscarinic receptors, addition of increasing concentrations of L-NAME resulted in a monophasic competition of binding with affinities (K(i)) ranging from 68 muM in endothelium to 317 muM in whole aorta. Addition of the hydrolysis-resistant guanosine 5'-triphosphate analogue GTPgammaS (100 muM) had no effect on L-NAME competition of [H-3]quinuclidinyl benzilate binding. Addition of L-NAME in radioligand binding competition studies using the agonist carbachol did not result in an alteration of the receptor's affinity for agonist, confirming the competitive nature of the interaction of L-NAME with the muscarinic receptor. Several L-arginine analogues with alkyl ester modifications at the carboxyl end of the molecule as well as those without this modification were evaluated as muscarinic antagonists in radioligand binding experiments. Only those arginine compounds with a modified carboxyl group were able to compete for radioligand binding to the muscarinic receptor. Our results indicate that alkyl esters Of L-arginine are muscarinic antagonists and suggest that these compounds are poor choices as nitric oxide synthase inhibitors in studies in which muscarinic receptors are not blocked.