Role of neuronal nitric oxide synthase in regulating retinal blood flow during flicker-induced hyperemia in cats.

PURPOSE To investigate how neuronal nitric oxide synthase (nNOS) contributes to regulation of the retinal circulation during rest and flicker stimulation in cats. METHODS Using laser Doppler velocimetry, we measured the vessel diameter and blood velocity simultaneously and calculated the retinal blood flow (RBF) in feline first-order retinal arterioles. After intravitreal injections of Nω-Nitro-L-arginine methyl ester (L-NAME), a nonselective NOS inhibitor, and Nω-propyl-L-arginine (L-NPA), a selective nNOS inhibitor, we continuously monitored the retinal circulation without any perturbations for 2 hours. We then examined the changes in the RBF in response to 16-Hz flicker stimuli for 3 minutes at 2 hours after intravitreal injection of phosphate-buffered saline (PBS) as a control, L-NAME, L-NPA, and thromboxane A2 (TXA2) analogue U46619 as a basal tone-adjusted control. RESULTS After intravitreal injection of L-NAME and L-NPA, the baseline RBF decreased gradually in a dose-dependent manner. In the PBS group, the RBF increased gradually and reached a maximal level after 2 to 3 minutes of flicker stimuli. After 3 minutes of 16-Hz flicker stimuli, the RBF increased by 53.5% ± 3.4% compared with baseline. In the L-NAME and L-NPA groups, the increases in RBF during flicker stimulation were attenuated significantly compared with the PBS group. In the TXA2 group, the reduction in the flicker-induced increase in RBF was comparable to that in the PBS group. CONCLUSIONS The current results suggested that increased RBF in response to flicker stimulation may be mediated by nitric oxide (NO) production via nNOS activation.