A role for nitroxyl (HNO) as an endothelium-derived relaxing and hyperpolarizing factor in resistance arteries

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. Rat and mouse mesenteric arteries were mounted in small vessel myographs for isometric force and smooth muscle membrane potential recording. Vasorelaxation to the HNO donor, Angeli's salt, was attenuated in both species by the soluble guanylate cyclase inhibitor (ODQ, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one), the voltage-dependent K+ channel inhibitor, 4-aminopyridine (4-AP) and the HNO scavenger, l-cysteine. In mouse mesenteric arteries, nitric oxide (NO) synthase inhibition (with l-NAME, N-omega-Nitro-L-arginine methyl ester) markedly attenuated acetylcholine (ACh)-mediated relaxation. Scavenging the uncharged form of NO (NO center dot) 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 center dot, 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. The factors contributing to vasorelaxation in mouse and rat mesenteric arteries are NO center dot = HNO > EDHF and EDHF > HNO = NO center dot respectively. This study identified HNO as an endothelium-derived relaxing and hyperpolarizing factor in resistance vessels. British Journal of Pharmacology (2009) 157, 540-550; doi:10.1111/j.1476-5381.2009.00150.x; published online 26 March 2009 This article is commented on by Martin, pp. 537-539 of this issue and is part of a themed section on Endothelium in Pharmacology. For a list of all articles in this section see the end of this paper, or visit: http://www3.interscience.wiley.com/journal/121548564/issueyear?year=2009.