title: Hyperoxia reduces basal release of nitric oxide and contracts porcine coronary arteries.
abstract: OBJECTIVE
  The purpose of the present study was to investigate whether changes in nitric oxide (NO) concentration is involved in hyperoxia-induced vasoconstriction in porcine conduit coronary arteries.
  
  
  METHODS
  The effect of hyperoxia on NO release and vasoconstriction was evaluated by tension recording, microsensor measurements, and immunoblotting in porcine conduit coronary arteries contracted with U46619 or 5-hydroxytryptamine.
  
  
  RESULTS
  In endothelium-intact segments exchanging 20% O2, 5% CO2, 75% N2 (normoxia) for 95% O2, 5% CO2 (hyperoxia) increased contraction. In segments without endothelium hyperoxia-evoked contraction was abolished, but restored by an encircling donor segment with endothelium. An inhibitor of NOS, asymmetric dimethylarginine (ADMA, 300 mum), reduced hyperoxic contraction and basal NO concentration by, respectively, 38 +/- 12% and 46 +/- 3% (P < 0.05, n = 9). A NO donor, S-nitroso-N-acetylpenicillamine (SNAP), increased NO concentration and evoked relaxation to the same levels in normoxic and hyperoxic conditions. beta-actin and endothelial NO synthase (eNOS) protein expression was similar in normoxic and hyperoxic arterial segments. Phosphorylation of eNOS was unaltered in normoxia vs. hyperoxia, but phosphorylation of eNOS-Ser(1177) was increased and phosphorylation of eNOS-Thr(495) decreased by U46619. Blockers of ATP-sensitive, voltage-dependent and calcium-activated K+ channels did not change hyperoxic contraction. However, high extracellular K+ concentration or a second and third exposure to hyperoxia decreased contraction.
  
  
  CONCLUSIONS
  The present study provides direct evidence that hyperoxia reduces basal release of NO leading to depletable endothelium-dependent vasoconstriction in porcine coronary arteries independent of changes in eNOS phosphorylation.
authors.affiliation: Department of Pharmacology, University of Aarhus, Aarhus C, Denmark.
authors.forename: T
authors.initials: T
authors.lastname: Pasgaard
authors.forename: E
authors.initials: E
authors.lastname: Stankevicius
authors.forename: M M
authors.initials: MM
authors.lastname: Jørgensen
authors.forename: L
authors.initials: L
authors.lastname: Ostergaard
authors.forename: U
authors.initials: U
authors.lastname: Simonsen
authors.forename: O
authors.initials: O
authors.lastname: Frøbert
mesh_terms: D000818:Animals; D003331:Coronary Vessels / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000378:metabolism*; D018496:Hyperoxia / Q000378:metabolism*; D066298:In Vitro Techniques; D009569:Nitric Oxide / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D013552:Swine