title: Autoregulation of retinal blood flow in response to decreased ocular perfusion pressure in cats: comparison of the effects of increased intraocular pressure and systemic hypotension.
abstract: OBJECTIVE
  To investigate the regulatory mechanisms responsible for autoregulation of retinal blood flow (RBF) during periods of decreased ocular perfusion pressure (OPP).
  
  
  METHODS
  The effects of acute reductions in OPP on RBF were assessed using laser Doppler velocimetry in cats. The OPP decreased from 90 to 40 mm Hg by increasing the IOP (elevated IOP) or by decreasing the systemic blood pressure via exsanguination (systemic hypotension). The contributions of nitric oxide (NO), adenosine, and/or N-methyl-D-aspartic acid (NMDA) in regulation of the retinal arteriolar hemodynamics during decreased OPP was determined at 120 minutes after intravitreal injection of various inhibitors or PBS.
  
  
  RESULTS
  Following PBS injection, the flow velocity decreased in proportion to the decrease in OPP; however, the retinal arteriolar diameter gradually increased. Consequently, the RBF was maintained near baseline levels when the OPP exceeded 70 mm Hg but decreased significantly (P < 0.01) when the OPP fell to less than or equal to 60 mm Hg due to elevated IOP or systemic hypotension. Adenosine receptor blocker 8-(p-sulfophenyl)theophylline, significantly (P < 0.01) enhanced decreases in RBF induced by elevated IOP and systemic hypotension at OPP from 80 to 40 mm Hg, whereas NO synthase inhibitor N(G)-nitro-L-arginine-methyl ester and NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid only significantly (P < 0.01) enhanced reductions in RBF induced by elevated IOP.
  
  
  CONCLUSIONS
  These results indicate that adenosine contributes to autoregulation of RBF during systemic hypotension, whereas adenosine, NO, and NMDA receptors autoregulate the RBF after elevated IOP. Different vasoregulatory factors might contribute to autoregulation of RBF after decreases in OPP induced by elevated IOP and systemic hypotension.
authors.affiliation: Department of Ophthalmology, Asahikawa Medical University, Asahikawa, Japan.
authors.forename: Tomofumi
authors.initials: T
authors.lastname: Tani
authors.forename: Taiji
authors.initials: T
authors.lastname: Nagaoka
authors.forename: Seigo
authors.initials: S
authors.lastname: Nakabayashi
authors.forename: Takafumi
authors.initials: T
authors.lastname: Yoshioka
authors.forename: Akitoshi
authors.initials: A
authors.lastname: Yoshida
mesh_terms: D000241:Adenosine / Q000008:administration & dosage; D000818:Animals; D002415:Cats; D004195:Disease Models, Animal; D045462:Endothelium-Dependent Relaxing Factors / Q000008:administration & dosage; D018690:Excitatory Amino Acid Agonists / Q000008:administration & dosage; D005260:Female; D006706:Homeostasis; D007022:Hypotension / Q000503:physiopathology*; D007429:Intraocular Pressure / Q000187:drug effects / Q000502:physiology*; D058449:Intravitreal Injections; D017078:Laser-Doppler Flowmetry; D008297:Male; D016202:N-Methylaspartate / Q000008:administration & dosage; D009569:Nitric Oxide / Q000008:administration & dosage; D009798:Ocular Hypertension / Q000503:physiopathology*; D012039:Regional Blood Flow / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000187:drug effects / Q000503:physiopathology*; D014665:Vasodilator Agents / Q000008:administration & dosage
chemical_list: D045462:Endothelium-Dependent Relaxing Factors; D018690:Excitatory Amino Acid Agonists; D014665:Vasodilator Agents; D009569:Nitric Oxide; D016202:N-Methylaspartate; D000241:Adenosine
keywords: adenosine; autoregulation; intraocular pressure; nitric oxide; retinal blood flow