Role of nitric oxide in mediating retinal blood flow regulation in cats

The role of nitric oxide (NO) was evaluated in mediating control of retinal blood flow in the anesthetized cat. This was done under resting conditions and as a function of decreases in perfusion pressure. The retinal blood flow was calculated by measuring blood velocity with laser Doppler velocimetry (LDV) and retinal blood vessel diameter with image analysis. Graded decreases in perfusion pressure were obtained by increasing intraocular pressure (IOP). Following intravenous administration of 30 mg/kg NG-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, the arterial blood pressure increased under resting conditions by 34+/-3% (n = 7), the blood vessel diameter decreased by 18+/-2%, and retinal blood flow significantly declined by 27+/-6% (n = 6). Irrespective of the presence or absence of L-NAME (30 mg/kg), stepwise reductions in perfusion pressure that reached a level that was 35% of the baseline value had no significant effect on retinal blood flow. These results suggest that, under resting conditions, retinal blood flow decreases in response to a putative fall in NO levels. However, NO does not appear to be involved in mediating the autoregulatory response to a decrease in perfusion pressure.