No abstract

Abstract-Nitric oxide (NO) plays an important role in the control of vascular tone. Traditionally, its vasorelaxant activity has been attributed to the free radical form of NO (NO ⅷ ), yet the reduced form of NO (NO Ϫ ) is also produced endogenously and is a potent vasodilator of large conduit arteries. The effects of NO Ϫ in the resistance vasculature remain unknown. This study examines the activity of NO Ϫ in rat small isolated mesenteric resistance-like arteries and characterizes its mechanism(s) of action. With the use of standard myographic techniques, the vasorelaxant properties of NO ⅷ (NO gas solution), NO Ϫ (Angeli's salt), and the NO donor sodium nitroprusside were compared. Relaxation responses to Angeli's salt (pEC 50 ϭ7.51Ϯ0.13, R max ϭ95.5Ϯ1.5%) were unchanged in the presence of carboxy-PTIO (NO ⅷ scavenger) but those to NO ⅷ and sodium nitroprusside were inhibited. L-Cysteine (NO Ϫ scavenger) decreased the sensitivity to Angeli's salt (PϽ0.01) and sodium nitroprusside (PϽ0.01) but not to NO ⅷ . The soluble guanylate cyclase inhibitor ODQ (3 and 10 mol/L) concentration-dependently inhibited relaxation responses to Angeli's salt (41.0Ϯ6.0% versus control 93.4Ϯ1.9% at 10 mol/L). The voltage-dependent K ϩ channel inhibitor 4-aminopyridine (1 mmol/L) caused a 9-fold (PϽ0.01) decrease in sensitivity to Angeli's salt, whereas glibenclamide, iberiotoxin, charybdotoxin, and apamin were without effect. In combination, ODQ and 4-aminopyridine abolished the response to Angeli's salt. In conclusion, NO Ϫ functions as a potent vasodilator of resistance arteries, mediating its response independently of NO , which mediates vascular smooth muscle relaxation predominantly through the activation of soluble guanylate cyclase and subsequent accumulation of cGMP. 1 NO, however, can also exist in the oxidized state as the nitrosonium cation (NO ϩ ) and in the reduced state as the nitroxyl anion (NO Ϫ ). Little attention has been afforded to the biological activity of these alternative redox forms of NO, yet recent findings that the nitroxyl anion is produced endogenously 2,3 and displays relaxant activity within the vasculature 4,5 highlights a possible physiological role for this nitrogen oxide species. NO Ϫ can be generated directly from the enzymatic activity of NO synthase (NOS) 2,3 such that NOS-catalyzed oxidation of L-arginine results in the production of NO Ϫ , which is further oxidized to NO ⅷ by superoxide dismutase. In addition, NO Ϫ can also be formed during oxidation of the decoupled NOS product N-hydroxy-L-arginine, 6,7 from NOS in the absence of tetrahydrobiopterin, 8 after the decomposition of S-nitrosothiols 9,10 and peroxynitrite 11 and from the reduction of NO ⅷ by mitochondrial cytochrome c. 12 The biological activity of NO Ϫ can be studied by using NO Ϫ donors such as Angeli's salt. Of particular interest is the identification of NO Ϫ as a potent vasodilator, mediating relaxation of large isolated conduit arteries, 4,5,13 exerting dilator activity in the intact pulmonary vascular bed, 14 and decreas...

Abstract-The nitroxyl anion (HNO) is emerging as a novel regulator of cardiovascular function with therapeutic potential in the treatment of diseases such as heart failure. It remains unknown whether tolerance develops to HNO donors, a limitation of currently used nitrovasodilators. The susceptibility of the HNO donor, Angeli's salt (AS), to the development of vascular tolerance was compared with the NO donors, glyceryl trinitrate (GTN) and diethylamine/NONOate (DEA/NO) in rat isolated aortae. Vasorelaxation to AS was attenuated (PϽ0.01) by the HNO scavenger L-cysteine, whereas the sensitivity to GTN and DEA/NO was decreased (

Background and purpose: Nitroxyl (HNO) is emerging as an important regulator of vascular tone as it is potentially produced endogenously and dilates conduit and resistance arteries. This study investigates the contribution of endogenous HNO to endothelium-dependent relaxation and hyperpolarization in resistance arteries. Experimental approach: Rat and mouse mesenteric arteries were mounted in small vessel myographs for isometric force and smooth muscle membrane potential recording. ) with hydroxocobalamin (HXC) or HNO with L-cysteine, or 4-AP decreased the sensitivity to ACh, and a combination of HXC and L-cysteine reduced ACh-mediated relaxation, as did L-NAME alone. ACh-induced hyperpolarizations were significantly attenuated by 4-AP alone and in combination with L-NAME. In rat mesenteric arteries, blocking the effects of endothelium-derived hyperpolarizing factor (EDHF) (charybdotoxin and apamin) decreased ACh-mediated relaxation 10-fold and unmasked a NO-dependent component, mediated equally by HNO and NO • , as HXC and L-cysteine in combination now abolished vasorelaxation to ACh. Furthermore, ACh-evoked hyperpolarizations, resistant to EDHF inhibition, were virtually abolished by 4-AP. Conclusions and implications:The factors contributing to vasorelaxation in mouse and rat mesenteric arteries are NO • = HNO > EDHF and EDHF > HNO = NO • respectively. This study identified HNO as an endothelium-derived relaxing and hyperpolarizing factor in resistance vessels.