Nitric oxide and choroidal blood flow regulation.

ABSTRACT

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 micrograms/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.