The nitrite anion is reduced to nitric oxide (NO ^• ) as oxygen tension decreases. Whereas this pathway modulates hypoxic NO ^• signaling and mitochondrial respiration and limits myocardial infarction in mammalian species, the pathways to nitrite bioactivation remain uncertain. Studies suggest that hemoglobin and myoglobin may subserve a fundamental physiological function as hypoxia dependent nitrite reductases. Using myoglobin wild-type ( ^+/+ ) and knockout ( ^−/− ) mice, we here test the central role of myoglobin as a functional nitrite reductase that regulates hypoxic NO ^• generation, controls cellular respiration, and therefore confirms a cytoprotective response to cardiac ischemia-reperfusion (I/R) injury. We find that myoglobin is responsible for nitrite-dependent NO ^• generation and cardiomyocyte protein iron-nitrosylation. Nitrite reduction to NO ^• by myoglobin dynamically inhibits cellular respiration and limits reactive oxygen species generation and mitochondrial enzyme oxidative inactivation after I/R injury. In isolated myoglobin ^+/+ but not in myoglobin ^−/− hearts, nitrite treatment resulted in an improved recovery of postischemic left ventricular developed pressure of 29%. In vivo administration of nitrite reduced myocardial infarction by 61% in myoglobin ^+/+ mice, whereas in myoglobin ^−/− mice nitrite had no protective effects. These data support an emerging paradigm that myoglobin and the heme globin family subserve a critical function as an intrinsic nitrite reductase that regulates responses to cellular hypoxia and reoxygenation. myoglobin knockout mice

Background—: Hypoxic vasodilation is a physiological response to low oxygen tension that increases blood supply to match metabolic demands. Although this response has been characterized for >100 years, the underlying hypoxic sensing and effector signaling mechanisms remain uncertain. We have shown that deoxygenated myoglobin in the heart can reduce nitrite to nitric oxide (NO·) and thereby contribute to cardiomyocyte NO· signaling during ischemia. On the basis of recent observations that myoglobin is expressed in the vasculature of hypoxia-tolerant fish, we hypothesized that endogenous nitrite may contribute to physiological hypoxic vasodilation via reactions with vascular myoglobin to form NO·.Methods and Results—: We show in the present study that myoglobin is expressed in vascular smooth muscle and contributes significantly to nitrite-dependent hypoxic vasodilation in vivo and ex vivo. The generation of NO· from nitrite reduction by deoxygenated myoglobin activates canonical soluble guanylate cyclase/cGMP signaling pathways. In vivo and ex vivo vasodilation responses, the reduction of nitrite to NO·, and the subsequent signal transduction mechanisms were all significantly impaired in mice without myoglobin. Hypoxic vasodilation studies in myoglobin and endothelial and inducible NO synthase knockout models suggest that only myoglobin contributes to systemic hypoxic vasodilatory responses in mice.Conclusions—: Endogenous nitrite is a physiological effector of hypoxic vasodilation. Its reduction to NO· via the heme globin myoglobin enhances blood flow and matches O₂supply to increased metabolic demands under hypoxic conditions.