Local vasodilation in response to
hypoxia is a fundamental physiologic response ensuring
oxygen delivery to tissues under metabolic stress. Recent studies identify a role for the red blood cell (RBC), with
hemoglobin the hypoxic sensor. Herein, we investigate the mechanisms regulating this process and explore the relative roles of
adenosine triphosphate,
S-nitrosohemoglobin, and
nitrite as effectors. We provide evidence that hypoxic RBCs mediate vasodilation by reducing
nitrite to
nitric oxide (NO) and
ATP release. NO dependence for
nitrite-mediated vasodilation was evidenced by NO gas formation, stimulation of cGMP production, and inhibition of mitochondrial respiration in a process sensitive to the NO scavenger C-
PTIO. The
nitrite reductase activity of
hemoglobin is modulated by
heme deoxygenation and
heme redox potential, with maximal activity observed at 50%
hemoglobin oxygenation (P(50)). Concomitantly, vasodilation is initiated at the P(50), suggesting that
oxygen sensing by
hemoglobin is mechanistically linked to
nitrite reduction and stimulation of vasodilation. Mutation of the conserved beta93cys residue decreases the
heme redox potential (ie, decreases E(1/2)), an effect that increases
nitrite reductase activity and vasodilation at any given
hemoglobin saturation. These data support a function for RBC
hemoglobin as an allosterically and redox-regulated
nitrite reductase whose "
enzyme activity" couples
hypoxia to increased NO-dependent blood flow.