Elevation of nitric oxide production in human trabecular meshwork by increased pressure

The presence of the nitric oxide (NO)-producing enzyme nitric oxide synthase (NOS) in the trabecular meshwork and the effect of various drugs acting through the NO pathway on the outflow facility and trabecular contractility suggest a role for NO in the regulation of outflow and intraocular pressure (IOP).

To model the effect of elevated IOP on the NO production in the trabecular meshwork, we perfused anterior segments of human donor eyes in vitro and studied the effect of raised perfusion pressure on NO levels in the perfusate. Furthermore, we evaluated using quantitative PCR whether enhanced perfusion pressure had an effect on the NOS gene expression levels.

Elevating the pressure from 10 mmHg to 25 mmHg caused a significant increase in NO production from 3.6+/-0.9 pmol/min to 5.9+/-1.6 pmol/min (mean +/- SEM), corresponding to an average increase of 66%. These high NO levels were reduced by application of L-NAME, an NOS inhibitor, to 2.6+/-0.6 pmol/min. Addition of L-NAME before raising the pressure decreased the basal NO production but was not able to block the increase in NO production after raising the pressure. The transcript level of iNOS was significantly increased in the trabecular meshwork after raising the perfusion pressure.

These results show that NO production increased after elevation of the pressure gradient over the trabecular meshwork, accompanied by an upregulation of iNOS gene expression. Previous studies have demonstrated that enhanced NO levels facilitate outflow. Taken together, the data indicate the existence of a regulatory feedback mechanism in the trabecular meshwork, which may contribute to the regulation of the IOP. In this system an increase in IOP will enhance NO production, which, in turn, increases the outflow facility, leading to a normalization of the IOP.