title: Role of inorganic nitrate and nitrite in driving nitric oxide-cGMP-mediated inhibition of platelet aggregation in vitro and in vivo.
abstract: BACKGROUND
  Nitric oxide (NO) is a critical negative regulator of platelets that is implicated in the pathology of thrombotic diseases. Platelets generate NO, but the presence and functional significance of NO synthase (NOS) in platelets is unclear. Inorganic nitrate/nitrite is increasingly being recognized as a source of bioactive NO, although its role in modulating platelets during health and vascular dysfunction is incompletely understood.
  
  
  METHODS
  We investigated the functional significance and upstream sources of NO-cGMP signaling events in platelets by using established methods for assessing in vitro and in vivo platelet aggregation, and assessed the bioconversion of inorganic nitrate to nitrite during deficiency of endothelial NOS (eNOS).
  
  
  RESULTS
  The phosphodiesterase 5 (PDE5) inhibitor sildenafil inhibited human platelet aggregation in vitro. This inhibitory effect was abolished by a guanylyl cyclase inhibitor and NO scavengers, but unaffected by NOS inhibition. Inorganic nitrite drove cGMP-mediated inhibition of human platelet aggregation in vitro and nitrate inhibited platelet function in eNOS(-/-) mice in vivo in a model of thromboembolic radiolabeled platelet aggregation associated with an enhanced plasma nitrite concentration as compared with wild-type mice.
  
  
  CONCLUSIONS
  Platelets generate transient, endogenous cGMP signals downstream of NO that are primarily independent of NOS and may be enhanced by inhibition of PDE5. Furthermore, nitrite can generate transient NO-cGMP signals in platelets. The absence of eNOS leads to enhanced plasma nitrite levels following nitrate administration in vivo, which negatively impacts on platelet function. Our data suggest that inorganic nitrate exerts an antiplatelet effect during eNOS deficiency, and, potentially, that dietary nitrate may reduce platelet hyperactivity during endothelial dysfunction.
authors.affiliation: Platelet Biology Group, Molecular Medicine Section, National Heart and Lung Institute, Imperial College London, London, UK.
authors.forename: G L
authors.initials: GL
authors.lastname: Apostoli
authors.forename: A
authors.initials: A
authors.lastname: Solomon
authors.forename: M J
authors.initials: MJ
authors.lastname: Smallwood
authors.forename: P G
authors.initials: PG
authors.lastname: Winyard
authors.forename: M
authors.initials: M
authors.lastname: Emerson
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D006152:Cyclic GMP / Q000097:blood*; D006801:Humans; D008297:Male; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009566:Nitrates / Q000494:pharmacology*; D009569:Nitric Oxide / Q000097:blood*; D052250:Nitric Oxide Synthase Type III / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D058986:Phosphodiesterase 5 Inhibitors / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology*; D015290:Second Messenger Systems / Q000187:drug effects; D012977:Sodium Nitrite / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D009566:Nitrates; D058986:Phosphodiesterase 5 Inhibitors; D010975:Platelet Aggregation Inhibitors; D009569:Nitric Oxide; C031618:sodium nitrate; D052250:Nitric Oxide Synthase Type III; C496315:Nos3 protein, mouse; D006152:Cyclic GMP; D012977:Sodium Nitrite
keywords: nitric oxide; nitrites; pharmacology; platelets; thrombosis