Role of nitric oxide in optic nerve head blood flow regulation during experimental increase of intraocular pressure in healthy humans

Abstract

AbstractPurpose Several studies have indicated that the choroid shows some regulatory potential during changes in ocular perfusion pressure (OPP). For the optic nerve head (ONH) only few data are available. The present study set out to investigate the behavior of ONH blood flow (ONHBF) during an experimental decrease in OPP and to explore whether inhibition of Nitric Oxide Synthase (NOS) alters this response.Methods Twelve healthy subjects participated in this randomized, double‐masked, placebo‐controlled three‐way crossover study. For each subject, three study days were scheduled, on which they either received intravenous infusions of NG‐monomethyl‐ L‐arginine (L‐NMMA), phenylephrine, or placebo. OPP was increased stepwise by the suction cup method. ONHBF was assessed continuously with laser Doppler flowmetry and OPP was calculated as 2/3*mean arterial pressure‐intraocular pressure.Results Administration of L‐NMMA and phenylephrine significantly increased resting OPP compared to placebo (p<0.001 and p=0.016, respectively). As expected, L‐NMMA decreased resting ONHBF compared to phenylephrine (p=0.04). The relative decrease in OPP during suction cup application was comparable with all drugs administered (between ‐69% and ‐72%, p=0.19). In all three groups, the decrease in ONHBF was less pronounced than the decrease in OPP, but not significantly different between groups.Conclusion The present data indicate that NO plays an important role in the regulation of basal ONHBF, but not in ONHBF autoregulation.