Nitric oxide inhibits α2-adrenoceptor-mediated endothelium-dependent vasodilation

This study was designed to investigate the interaction between the NO/L-arginine pathway and the alpha(2)-adrenoceptor-mediated endothelium-dependent vasorelaxation, Reactivity of isolated resistance mesenteric arterial segments from mice lacking the gene for constitutive endothelial NO synthase (eNOS(-) mice, n=14) and from their wild-type controls (WT mice, n=46) was studied in isometric conditions in the presence of indomethacin (blocker of cyclooxygenase). Oxymetazoline (OXY, 0.01 to 30 mu mol/L; a selective alpha(2)-adrenoceptor agonist) induced an endothelium-dependent relaxation of eNOS- but not WT arteries preconstricted either with phenylephrine or serotonin, In the presence of N-omega-nitro-L-arginine (1-NNA, 100 mu mol/L), an inhibitor of NOS, OXY induced an endothelium-dependent relaxation of WT mesenteric arteries. 1-NNA had no effect on the relaxation caused by OXY in eNOS(-) arterial rings, Therefore, the relaxation caused by OXY was independent of NO formation. To demonstrate the inhibitory role of NO on the alpha(2)-adrenoceptor-mediated relaxation, subthreshold (0.1 nmol/L) to threshold (1 nmol/L) concentrations of sodium nitroprusside (donor of NO) were added to 1-NNA-treated arteries before OXY challenges: in these conditions, the alpha(2)-adrenoceptor-mediated relaxation of eNOS(-) and WT arteries was inhibited. OXY-induced relaxation was restored on readdition of methylene blue (1 mu mol/L, inhibitor of guanylate cyclase), suggesting that cGMP may be the mechanism of inhibition of the alpha(2)-adrenergic pathway in the presence of NO, Finally, OXY-mediated relaxation was blocked by tetraethylammonium (1 mmol/L) but nor glibenclamide (1 mu mol/L), suggesting the involvement of an endothelium-derived hyperpolarizing factor that activates Ca2+-activated K+ channels. In conclusion, alpha(2)-adrenoceptor activation caused relaxation of isolated murine mesenteric arteries that was functionally blocked by NO through a mechanism that may involve activation of the soluble guanylate cyclase and cGMP formation. The endothelium-dependent alpha(2)-adrenoceptor-mediated relaxation is likely to be due to an endothelium-derived hyperpolarizing factor, whose release and/or production is reduced by concurrent NO formation.