title: Hypoxia, red blood cells, and nitrite regulate NO-dependent hypoxic vasodilation.
abstract: 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.
authors.affiliation: Department of Pathology, Center for Free Radical Biology, University of Alabama at Birmingham, 35294, USA.
authors.forename: Jack H
authors.initials: JH
authors.lastname: Crawford
authors.forename: T Scott
authors.initials: TS
authors.lastname: Isbell
authors.forename: Zhi
authors.initials: Z
authors.lastname: Huang
authors.forename: Sruti
authors.initials: S
authors.lastname: Shiva
authors.forename: Balu K
authors.initials: BK
authors.lastname: Chacko
authors.forename: Alan N
authors.initials: AN
authors.lastname: Schechter
authors.forename: Victor M
authors.initials: VM
authors.lastname: Darley-Usmar
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Kerby
authors.forename: John D
authors.initials: JD
authors.lastname: Lang
authors.forename: David
authors.initials: D
authors.lastname: Kraus
authors.forename: Chien
authors.initials: C
authors.lastname: Ho
authors.forename: Mark T
authors.initials: MT
authors.lastname: Gladwin
authors.forename: Rakesh P
authors.initials: RP
authors.lastname: Patel
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000494:Allosteric Regulation; D015687:Cell Hypoxia; D006152:Cyclic GMP / Q000378:metabolism; D004912:Erythrocytes / Q000378:metabolism*; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D008928:Mitochondria / Q000378:metabolism; D009154:Mutation; D009569:Nitric Oxide / Q000378:metabolism*; D009572:Nitrite Reductases / Q000378:metabolism; D009573:Nitrites / Q000494:pharmacology*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D012119:Respiration; D014664:Vasodilation
chemical_list: D006454:Hemoglobins; D009573:Nitrites; C103574:S-nitrosohemoglobin; D009569:Nitric Oxide; D006418:Heme; D000255:Adenosine Triphosphate; D009572:Nitrite Reductases; D006152:Cyclic GMP; D010100:Oxygen