Optic Nerve Head Blood Flow Regulation during an Experimental Increase in Ocular Perfusion Pressure: Role of Nitric Oxide

Abstract

Purpose: : Previous studies have shown that nitric oxide (NO) is involved in choroidal blood flow regulation during an experimental increase in ocular perfusion pressure (OPP). Furthermore, inhibition of NO synthase (NOS) shifts choroidal pressure - flow curves during isometric exercise to the right. In the present study, the hypothesis that inhibition of NOS may also influence optic nerve head (ONH) blood flow during isometric exercise was tested.

Methods: : The present study was carried out in a randomized, double-masked, placebo-controlled, three - way crossover design. 18 subjects performed squatting for 6 minutes to increase systemic perfusion pressure during administration of either a NOS inhibitor (L-NMMA), an α-receptor agonist (phenylephrine) or placebo. ONH blood flow was measured continuously during the procedure using laser Doppler flowmetry (LDF). OPP was calculated as 2/3 mean arterial pressure minus intraocular pressure (IOP).

Results: : OPP at rest was significantly increased by L-NMMA and phenylephrine ( p < 0.001 versus baseline). A significant decrease in resting ONH blood flow was, however, only seen after administration of L-NMMA (p = 0.02 versus baseline). During isometric exercise, there was no difference in the response of ONH blood flow as well as of OPP with all three administered drugs (p=0.43 and p=0.69, respectively).

Conclusions: : The data from the present study indicate that NO is not involved in the regulatory mechanisms of the ONH during isometric exercise. However, NO seems to play an important role in basal ONH blood flow regulation.

Clinical Trial: : http://www.clinicaltrials.gov NCT00806741

Keywords: optic disc • nitric oxide • intraocular pressure