Residual NO modulates contractile responses and membrane potential in isolated rat mesenteric arteries.

Shear stress or vasocontriction causes endothelial nitric oxide (NO) release resulting in the regulation of vascular smooth muscle tone in small resistance arteries. Generation of NO is inhibited by nitric oxide synthase (NOS) inhibitors. In this study, we investigated the effect of residual NO, released even in the presence of NOS inhibitors, on the membrane depolarization and phenylephrine-induced contractions of smooth muscle. For this purpose, we used hydroxocobalamin (HC), an NO scavenger, in the presence of NOS inhibitiors, Nω-nitro- L-arginine (L-NA) or Nω-nitro-L-arginine methyl ester (L-NAME) in mesenteric arteries isolated from rats. Phenylephrine (0,01-10 µM), an α1-adrenoceptor agonist, led to depolarisation and concentration-dependent contraction in mesenteric arteries. The depolarisation and contractile responses were augmented by L-NA or L-NAME. Hydroxocobalamine (HC) or carboxy-PTIO (c-PTIO) also caused to further increase the membrane depolarization and contractions induced by phenylephrine in the presence of NOS inhibitors. Chemical removal of endothelium by saponin, tyrosin kinase inhibitor erbstatin A, but not calmodulin inhibitor calmidazolium inhibited the additional membrane depolarisation and contractile responses induced by L-NA or L-NAME and L-NA or L-NAME plus HC. These findings show that residual NO modulates the contractile responses in isolated rat mesenteric arteries by exerting a tonic inhibitor effect on the depolarization and vasoconstriction induced by phenylephrine.

Urocortin induces endothelium-dependent vasodilatation and hyperpolarization of rat mesenteric arteries by activating Ca2+-activated K+ channels.

Urocortin, a member of corticotropin releasing factor (CRF) peptide family, has positive chronotropic and inotropic effects on heart and also shows a vasodilatory effect. However, the mechanism underlying its vasodilatory effect has yet to be elucidated. Endothelium-dependent relaxation of resistance arteries is mainly achieved by activation of K+ channels. Therefore, we investigated possible role of K+ channels and hyperpolarization for the vasodilatory effect of urocortin using the isolated perfused rat mesenteric arteries. Urocortin (0.2 nM) produced a slow-onset decrease in the perfusion pressure of the mesenteric vascular bed, which was elevated by an alpha1-adrenoceptor agonist, phenylephrine (2-4 microM). Urocortin also hyperpolarized the main mesenteric artery. Removal of endothelium with saponin treatment considerably inhibited the relaxation and hyperpolarization induced by urocortin. In contrast, the hyperpolarization was not significantly changed by cyclooxygenase inhibitor, indomethacin (1 microM) and/or nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine (100 microM). Urocortin-induced relaxation was not affected by the combination of a guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 microM), indomethacin and N(omega)-nitro-L-arginine. However, the relaxation and hyperpolarization were abolished by high extracellular potassium concentration (40 mM) or by a large conductance Ca(2+)-activated K+ channel blocker, charybdotoxin (1 nM). Glibenclamide (1 microM), an ATP-dependent K+ channel inhibitor, did not affect the relaxation and hyperpolarization. These results suggest that urocortin causes endothelium-dependent relaxation and hyperpolarization of rat mesenteric arteries, probably through the activation of charybdotoxin sensitive Ca2+-activated K+ channels. These findings also indicate an essential role of the endothelium for the urocortin-elicited vascular relaxation and hyperpolarization.