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

Purpose. We determined whether administration of a nitric oxide synthase (NOS) inhibitor alters optic nerve head blood flow (ONHBF) regulation during isometric exercise in healthy subjects. Methods. Our study was done in a randomized, placebocontrolled, double-masked, three-way crossover design. A total of 18 healthy subjects was randomized to receive either placebo, phenylephrine, or an inhibitor of NOS (L-NMMA) on three different study days. ONHBF was measured with laser Doppler flowmetry while the study participants performed isometric exercise (squatting). This was done before drug administration and during infusion of the study drugs. Mean arterial pressure (MAP) and IOP were measured noninvasively, and ocular perfusion pressure (OPP) was calculated as 2/3 MAP - IOP. Results. The response in ONHBF to isometric exercise was less pronounced than the response in OPP, indicating for some autoregulatory capacity in the ONH. Administration of LNMMA significantly decreased ONHBF at rest (P < 0.01). In contrast, inhibition of NOS did not alter the pressure-flow relationship in the ONH during an experimental increase in OPP compared to phenylephrine and placebo (P = 0.37 between groups). Conclusions. The data of our study support previous findings that ONHBF is autoregulated during an experimental increase in OPP. Nitric oxide has an important role in basal ONHBF regulation, but seems not to be involved in the autoregulatory response during an increase in OPP induced by isometric exercise.