title: Role of nitric oxide in choroidal blood flow regulation during light/dark transitions.
abstract: OBJECTIVE
  Several studies have recently shown that a transition from light to dark is associated with a reduction in choroidal blood flow. The mechanism underlying this effect is unclear but may be related to changes in neural input. In the present study, the authors hypothesized that either the alpha-receptor agonist phenylephrine or the nitric oxide synthase (NOS) inhibitor L-NMMA may alter the choroidal blood flow response during a transition from light to dark.
  
  
  METHODS
  In 15 healthy male nonsmoking subjects, the response of choroidal perfusion was studied in a randomized placebo-controlled three-way crossover study. Phenylephrine, L-NMMA or placebo was administered on different study days, and the effect of a light/dark transition on choroidal perfusion parameters was studied. Subfoveal choroidal blood flow and fundus pulsation amplitude were assessed with laser Doppler flowmetry and laser interferometry, respectively.
  
  
  RESULTS
  Before drug administration, a transition from light to dark reduced both choroidal hemodynamic parameters by 11% to 20%. Neither phenylephrine nor placebo altered basal choroidal blood flow or choroidal blood flow responses to the light/dark transitions. By contrast, the NOS inhibitor L-NMMA significantly reduced basal choroidal blood flow by 20.5% +/- 5.9% (P < 0.001) and basal fundus pulsation amplitude by 21.5% +/- 4.8% (P < 0.001). In addition, the response of subfoveal choroidal blood flow (-6.2% +/- 3.2%; P = 0.008) and fundus pulsation amplitude (-4.2% +/- 2.4%; P < 0.001) to the light/dark transition was significantly diminished.
  
  
  CONCLUSIONS
  The present study indicates that NO plays a role in the choroidal blood flow decrease during a transition from light to dark. Given that L-NMMA is a nonspecific inhibitor of NOS, the present study does not clarify whether this NO is from endothelial or neural sources.
authors.affiliation: Department of Clinical Pharmacology, Medical University of Vienna, Vienna, USA.
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Huemer
authors.forename: Gerhard
authors.initials: G
authors.lastname: Garhofer
authors.forename: Tina
authors.initials: T
authors.lastname: Aggermann
authors.forename: Julia
authors.initials: J
authors.lastname: Kolodjaschna
authors.forename: Leopold
authors.initials: L
authors.lastname: Schmetterer
authors.forename: Gabriele
authors.initials: G
authors.lastname: Fuchsjäger-Mayrl
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000528:radiation effects; D001794:Blood Pressure; D002829:Choroid / Q000098:blood supply*; D018592:Cross-Over Studies; D003623:Dark Adaptation; D004791:Enzyme Inhibitors / Q000008:administration & dosage; D006339:Heart Rate; D006801:Humans; D007368:Interferometry; D007429:Intraocular Pressure; D017078:Laser-Doppler Flowmetry; D007834:Lasers; D008027:Light; D008297:Male; D009569:Nitric Oxide / Q000502:physiology*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D010656:Phenylephrine / Q000008:administration & dosage; D012039:Regional Blood Flow / Q000528:radiation effects; D019323:omega-N-Methylarginine / Q000008:administration & dosage
chemical_list: D004791:Enzyme Inhibitors; D010656:Phenylephrine; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase