Inhibition of nitrergic relaxations by the M-3-selective antagonist 4-DAMP

The inhibitory effect of the selective M-3 musarinic acetylcholine receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP, 0.1 -10 mu M) on nicotine (100 mu M)-induced nitrergic relaxation was investiga ted in comparison to d-tubocurarine (0.1-10 mu M) and hexamethonium (0.1-10 mu M) by using phenylephrine (1 mu M)-precontracted rat anococcygeus muscl es in vitro. Nicotine produced a 60.1 +/- 2.4% (n = 40) inhibition of pheny lephrine precontractions. But this relaxant response was at a significantly lower magnitude of 20.2 +/- 4.6% (n = 18, P < 0.01 vs. control) in the pre sence of the nitric oxide synthase (NOS) blocker N-G-nitro-L-arginine methy l ester (L-NAME, 30 mu M), and it was 26.5 +/- 5.5% (n = 18, P < 0.01 vs, c ontrol) in the presence of the soluble guanylate cyclase inhibitor methylen e blue (30 mu M). However, aminoguanidine (100 mu M), a relatively selectiv e blocker of the inducible nitric oxide synthase (iNOS), had no significant effect. Similarly, other iNOS inhibitors such as dexamethasone (5 mg/kg) o r L-canavanine (100 mg/kg) did not modify contractile nor relaxant response s when they were given in vivo, concomitantly with Escherichia coli endotox in (1 mg/kg, ip) 4 h before the isolation of the tissues. 4-DAMP, hexametho nium, and d-tubocurarine inhibited nicotine-induced relaxation in a concent ration-dependent manner with the following order of potency: 4-DAMP > hexam ethonium > d-tubocurarine with IC50 values being 0.47 +/- 0.04 mu M, 0.75 /- 0.06 mu M, and 1.02 +/- 0.05 mu M, respectively Therefore, it was conclu ded that the selective Mg muscarinic acetylcholine receptor antagonist 4-DA MP also possesses po tent antagonistic action on nicotinic receptors of per ipheral nitrergic neurons that innervate the rat anococcygeus muscle. (C) 1 999 Wiley-Liss, Inc.