NITRIC-OXIDE AND CHOROIDAL BLOOD-FLOW REGULATION

Purpose. Nitric oxide (NO) has been found to be an endothelial-derived relaxing factor mediating the vasodilatation that results from the stimulation of muscarinic endothelial receptors. It also has been identified as a putative neurotransmitter of parasympathetic origin in choroidal perivascular autonomic fibers. The authors investigated a potential role of NO in choroidal blood flow (ChBF) regulation. Methods. Local ChBF in the tapetal region of 26 anesthetized cats was measured by laser Doppler flowmetry. Cats were infused through the femoral vein with increasing dosages of acetylcholine (ACh); N-omega-nitro-L-arginine (NNL-A), a specific inhibitor of NO synthesis; L-arginine; and D-arginine. ChBF and mean arterial pressure (MAP) were continuously recorded. Results. Infusion of 20 mu g/minute ACh induced a 68% increase in ChBF despite a 9% decrease in MAP. Infusion of 16 mg/minute NNL-A attenuated the ACh-induced increase in ChBF by 46% and increased MAP by 40%. Infusion of different dosages of NNL-A without prior administration of ACh caused ChBF to fall below and MAP to rise above baseline in a dose-dependent fashion. Infusion of L-arginine prior to ACh infusion enhanced by 27% the ACh induced increase in ChBF, whereas D-arginine had no effect on this increase. Conclusions. These findings suggest the presence of a local vasodilatory cholinergic mechanism in the choroid, inducing the release of NO. They also suggest that release of NO in the choroid may maintain basal blood flow to this tissue.