Role of nitric oxide and carbon monoxide in Nω-Nitro-l-arginine methyl ester-resistant acetylcholine-induced relaxation in chicken carotid artery

The current study examined the hypothesis that acetylcholine-induced Nω-Nitro-l-arginine methyl ester (l-NAME)-resistant endothelium-dependent relaxations in the chicken carotid artery are mediated by nitric oxide and carbon monoxide. Acetylcholine (1 nM–3 µM) caused a concentration-dependent relaxation (pD2 6.81 ± 0.05, Rmax 115 ± 3%) of the artery segments precontracted with phenylephrine (3 µM). l-NAME (1 mM) decreased the sensitivity (pD2 6.44 ± 0.06), but not the efficacy (Rmax 108 ± 3%) of acetylcholine. It also partially decreased the acetylcholine (3 µM)-stimulated nitrite release. While treatment with Nω-Nitro-l-arginine (l-NNA; 1 mM) plus l-NAME (1 mM) decreased the acetylcholine-stimulated nitrite release to the basal level, it moderately inhibited (Rmax 77 ± 3%) the maximal relaxation elicited with the muscarinic agonist. 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO; 100 µM) a specific scavenger of nitric oxide (NO) plus the two NOS inhibitors further decreased the acetylcholine-evoked relaxation (Rmax 34 ± 2%). Although soluble guanylyl cyclase (sGC) inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 µM) markedly inhibited the acetylcholine-stimulated increase in tissue cGMP to less than the basal levels, it only decreased the sensitivity, but not the efficacy of the agonist either in the presence or absence of l-NAME (1 mM). Zinc Protoporphyrin-IX (ZnPP; 10 µM), a hemeoxygenase (HO) inhibitor, partially inhibited (Rmax 72 ± 3%) the l-NAME-resistant acetylcholine-induced relaxations. A combined treatment of the arterial rings with l-NAME, l-NNA, PTIO and ZnPP nearly abolished (Rmax 7 ± 0.9%) the vasodilator responses to acetylcholine. Endothelium removal abolished the relaxation response to acetylcholine. In conclusion, it is suggested that the acetylcholine-induced l-NAME-resistant relaxation is primarily, mediated by NO with a small but significant contribution from endothelium-derived carbon monoxide in the chicken carotid artery.