title: Nitrite and S-Nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation: Relationship to Basal and Exercise Blood Flow Responses to Hypoxia.
abstract: BACKGROUND
  The mechanisms underlying red blood cell (RBC)-mediated hypoxic vasodilation remain controversial, with separate roles for nitrite () and S-nitrosohemoglobin (SNO-Hb) widely contested given their ability to transduce nitric oxide bioactivity within the microcirculation. To establish their relative contribution in vivo, we quantified arterial-venous concentration gradients across the human cerebral and femoral circulation at rest and during exercise, an ideal model system characterized by physiological extremes of O2tension and blood flow.
  
  
  METHODS
  Ten healthy participants (5 men, 5 women) aged 24±4 (mean±SD) years old were randomly assigned to a normoxic (21% O2) and hypoxic (10% O2) trial with measurements performed at rest and after 30 minutes of cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled simultaneously from the brachial artery and internal jugular and femoral veins with plasma and RBC nitric oxide metabolites measured by tri-iodide reductive chemiluminescence. Blood flow was determined by transcranial Doppler ultrasound (cerebral blood flow) and constant infusion thermodilution (femoral blood flow) with net exchange calculated via the Fick principle.
  
  
  RESULTS
  Hypoxia was associated with a mild increase in both cerebral blood flow and femoral blood flow (P<0.05 versus normoxia) with further, more pronounced increases observed in femoral blood flow during exercise (P<0.05 versus rest) in proportion to the reduction in RBC oxygenation (r=0.680-0.769, P<0.001). Plasma gradients reflecting consumption (arterial>venous; P<0.05) were accompanied by RBC iron nitrosylhemoglobin formation (venous>arterial; P<0.05) at rest in normoxia, during hypoxia (P<0.05 versus normoxia), and especially during exercise (P<0.05 versus rest), with the most pronounced gradients observed across the bioenergetically more active, hypoxemic, and acidotic femoral circulation (P<0.05 versus cerebral). In contrast, we failed to observe any gradients consistent with RBC SNO-Hb consumption and corresponding delivery of plasma S-nitrosothiols (P>0.05).
  
  
  CONCLUSIONS
  These findings suggest that hypoxia and, to a far greater extent, exercise independently promote arterial-venous delivery gradients of intravascular nitric oxide, with deoxyhemoglobin-mediated reduction identified as the dominant mechanism underlying hypoxic vasodilation.
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.). damian.bailey@southwales.ac.uk.
authors.forename: Damian M
authors.initials: DM
authors.lastname: Bailey
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Peter
authors.initials: P
authors.lastname: Rasmussen
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Morten
authors.initials: M
authors.lastname: Overgaard
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Kevin A
authors.initials: KA
authors.lastname: Evans
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Aske M
authors.initials: AM
authors.lastname: Bohm
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Thomas
authors.initials: T
authors.lastname: Seifert
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Patrice
authors.initials: P
authors.lastname: Brassard
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Morten
authors.initials: M
authors.lastname: Zaar
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Henning B
authors.initials: HB
authors.lastname: Nielsen
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Peter B
authors.initials: PB
authors.lastname: Raven
authors.affiliation: From Faculty of Medicine, Reichwald Health Sciences Centre, University of British Columbia-Okanagan, Kelowna, BC, Canada (D.M.B.); Neurovascular Research Laboratory, Faculty of Life Sciences and Education, University of South Wales, Glamorgan, UK (K.A.E.); Department of Anesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark (P.R., M.D., A.M.B., T.S., P.B., M.Z., H.B.N., N.H.S.); Université Laval, Québec, QC, Canada (P.B.); and Department of Integrative Physiology and Anatomy, University of North Texas, Health Science Center, Ft Worth (P.B.R.).
authors.forename: Niels H
authors.initials: NH
authors.lastname: Secher
mesh_terms: D000328:Adult; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004912:Erythrocytes / Q000378:metabolism; D015444:Exercise / Q000502:physiology*; D005260:Female; D006454:Hemoglobins / Q000032:analysis* / Q000378:metabolism; D006801:Humans; D000860:Hypoxia / Q000378:metabolism*; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply; D009569:Nitric Oxide / Q000378:metabolism*; D009573:Nitrites / Q000097:blood*; D010100:Oxygen / Q000097:blood
chemical_list: D006454:Hemoglobins; D009573:Nitrites; C103574:S-nitrosohemoglobin; D009569:Nitric Oxide; D010100:Oxygen
keywords: brain; hypoxia, exercise; muscles; nitric oxide; physiology