Role of nitric oxide in optic nerve head blood flow regulation during isometric exercise in healthy humans

Abstract

Abstract Purpose Nitric oxide (NO) is an important regulator of optic nerve head (ONH) blood flow in humans. We have previously shown that NO is also involved in choroidal blood flow regulation during isometric exercise. Inhibition of NO synthase (NOS) has been reported to shift choroidal pressure–flow curves during squatting to the right. The hypothesis for the present study was that inhibition of NOS may also influence ONH blood flow during isometric exercise. Methods To test this hypothesis, a randomized, double‐masked, placebo‐controlled, three‐way crossover study was performed in 18 healthy volunteers. Subjects received on different study days intravenous infusions of NG‐monomethyl‐ L‐arginine (L‐NMMA), phenylephrine, or placebo. During these infusion periods, subjects were asked to squat for 6 minutes. ONH blood flow was assessed with laser Doppler flowmetry, and ocular perfusion pressure (OPP) was calculated from mean arterial pressure and intraocular pressure. Results L‐NMMA and phenylephrine increased resting OPP (p < 0.001 versus baseline), but only L‐NMMA reduced resting ONH blood flow (p = 0.02 versus baseline). The relative increase in OPP during isometric exercise was comparable with all drugs administered (p = 0.69). In addition, the change of ONH blood flow was comparable with all administered drugs (p = 0.43). Conclusion These data indicate that NO plays an important role in the regulation of ONH blood flow at baseline, but does not change the response of ONH blood flow during isometric exercise.