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Role of NO in the Control of Choroidal Blood Flow during a Decrease in Ocular Perfusion Pressure

¹From the Department of Clinical Pharmacology, Division of Ophthalmo-Pharmacology, the ²Department of Ophthalmology, and the ³Center for Biomedical Engineering and Physics, Institute of Medical Physics, Medical University of Vienna, Vienna, Austria.

PURPOSE. The study was conducted to investigate whether the L-arginine/nitric oxide system plays a role in choroidal blood flow (ChBF) regulation during a decrease in ocular perfusion pressure (OPP).

METHODS. Experiments were performed on 3 days in a randomized double-masked, placebo-controlled, three-way crossover design. On different study days, subjects received intravenous infusions of N^G-monomethyl-L-arginine (L-NMMA), phenylephrine, or placebo. Intraocular pressure was raised in stepwise increments using the suction cup method. Choroidal blood flow (ChBF, laser Doppler flowmetry), mean arterial blood pressure (MAP), and IOP were assessed. Ocular perfusion pressure was calculated as OPP = (MAP – IOP). For correlation analysis all OPP/ChBF data pairs from all subjects were pooled independent of time point of measurement. Then, the pooled data were sorted according to OPP, and correlation analyses were performed.

RESULTS. L-NMMA and phenylephrine increased resting OPP by +17% ± 18% and +14% ± 21%, respectively (P < 0.05). L-NMMA reduced resting ChBF by –21% ± 17% (P < 0.05). The relative decrease in OPP during suction cup application was comparable with all drugs administered. The decrease in OPP was paralleled by a significant decrease in ChBF (maximum between –39% and –47%), which was less pronounced, however, than the decrease in OPP (maximum between –69% and –74%). Neither placebo nor L-NMMA, nor phenylephrine, influenced the OPP/ChBF relationship.

CONCLUSIONS. The data confirm previously published observations that the choroid shows some regulatory capacity during reduced OPP. The L-arginine/nitric oxide-system plays a role in the maintenance of basal vascular tone but seems not to be involved in the choroidal vasodilator response when IOP is increased.