title: In vivo reduction of cell-free methemoglobin to oxyhemoglobin results in vasoconstriction in canines.
abstract: BACKGROUND
  Cell-free hemoglobin (Hb) in the vasculature leads to vasoconstriction and injury. Proposed mechanisms have been based on nitric oxide (NO) scavenging by oxyhemoglobin (oxyHb) or processes mediated by oxidative reactions of methemoglobin (metHb). To clarify this, we tested the vascular effect and fate of oxyHb or metHb infusions.
  
  
  METHODS
  Twenty beagles were challenged with 1-hour similar infusions of (200 μmol/L) metHb (n = 5), oxyHb (n = 5), albumin (n = 5), or saline (n = 5). Measurements were taken over 3 hours.
  
  
  RESULTS
  Infusions of the two pure Hb species resulted in increases in mean arterial blood pressure (MAP), systemic vascular resistance index, and NO consumption capacity of plasma (all p < 0.05) with the effects of oxyHb being greater than that from metHb (MAP; increase 0 to 3 hr; 27 ± 6% vs. 7 ± 2%, respectively; all p < 0.05). The significant vasoconstrictive response of metHb (vs. albumin and saline controls) was related to in vivo autoreduction of metHb to oxyHb, and the vasoactive Hb species that significantly correlated with MAP was always oxyHb, either from direct infusion or after in vivo reduction from metHb. Clearance of total Hb from plasma was faster after metHb than oxyHb infusion (p < 0.0001).
  
  
  CONCLUSIONS
  These findings indicate that greater NO consumption capacity makes oxyHb more vasoactive than metHb. Additionally, metHb is reduced to oxyHb after infusion and cleared faster or is less stable than oxyHb. Although we found no direct evidence that metHb itself is involved in acute vascular effects, in aggregate, these studies suggest that metHb is not inert and its mechanism of vasoconstriction is due to its delayed conversion to oxyHb by plasma-reducing agents.
authors.affiliation: Critical Care Medicine Department, Clinical Center, National Institutes of Health, Bethesda, Maryland; Molecular Medicine Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland; Division of Pulmonary, Allergy and Critical Care Medicine and the Vascular Medicine Institute, University of Pittsburgh, Pittsburgh, Pennsylvania; Department of Physics and the Translational Science Center, Wake Forest University, Winston-Salem, North Carolina; Anesthesia and Critical Care Medicine Department, West China Hospital of Sichuan University, Cheng Du, China; Critical Care Medicine Department, Hospital Universitario de Getafe, Getafe, Madrid, Spain.
authors.forename: Dong
authors.initials: D
authors.lastname: Wang
authors.forename: Barbora
authors.initials: B
authors.lastname: Piknova
authors.forename: Steven B
authors.initials: SB
authors.lastname: Solomon
authors.forename: Irene
authors.initials: I
authors.lastname: Cortes-Puch
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Kern
authors.forename: Junfeng
authors.initials: J
authors.lastname: Sun
authors.forename: Tamir
authors.initials: T
authors.lastname: Kanias
authors.forename: Mark T
authors.initials: MT
authors.lastname: Gladwin
authors.forename: Christine
authors.initials: C
authors.lastname: Helms
authors.forename: Daniel B
authors.initials: DB
authors.lastname: Kim-Shapiro
authors.forename: Alan N
authors.initials: AN
authors.lastname: Schechter
authors.forename: Charles
authors.initials: C
authors.lastname: Natanson
mesh_terms: D000418:Albumins / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D004285:Dogs; D008706:Methemoglobin / Q000378:metabolism / Q000494:pharmacology*; D009569:Nitric Oxide / Q000378:metabolism; D010108:Oxyhemoglobins / Q000378:metabolism / Q000494:pharmacology*; D011897:Random Allocation; D014661:Vasoconstriction / Q000187:drug effects*
chemical_list: D000418:Albumins; D010108:Oxyhemoglobins; D009569:Nitric Oxide; D008706:Methemoglobin