The reduction of circulating
nitrite to
nitric oxide (NO) has emerged as an important physiological reaction aimed to increase vasodilation during tissue
hypoxia. Although
hemoglobin,
xanthine oxidase, endothelial
NO synthase, and the bc(1) complex of the mitochondria are known to reduce
nitrite anaerobically in vitro, their relative contribution to the hypoxic vasodilatory response has remained unsolved. Using a wire myograph, we have investigated how the
nitrite-dependent vasodilation in rat aortic rings is controlled by
oxygen tension,
norepinephrine concentration,
soluble guanylate cyclase (the target for vasoactive NO), and known
nitrite reductase activities under
hypoxia. Vasodilation followed overall first-order dependency on
nitrite concentration and, at low oxygenation and
norepinephrine levels, was induced by low-
nitrite concentrations, comparable to those found in vivo. The vasoactive effect of
nitrite during
hypoxia was abolished on inhibition of
soluble guanylate cyclase and was unaffected by removal of the endothelium or by inhibition of
xanthine oxidase and of the mitochondrial bc(1) complex. In the presence of
hemoglobin and
inositol hexaphosphate (which increases the fraction of deoxygenated
heme), the effect of
nitrite was not different from that observed with
inositol hexaphosphate alone, indicating that under the conditions investigated here deoxygenated
hemoglobin did not enhance
nitrite vasoactivity. Together, our results indicate that the mechanism for
nitrite vasorelaxation is largely intrinsic to the vessel and that under
hypoxia physiological
nitrite concentrations are sufficient to induce NO-mediated vasodilation independently of the
nitrite reductase activities investigated here. Possible reaction mechanisms for
nitrite vasoactivity, including formation of
S-nitrosothiols within the arterial smooth muscle, are discussed.