EVect of chronic inhibition of nitric oxide synthase on ocular blood flow and glucose metabolism in the rat

Aims—To investigate the eVects of chronic administration of nitric oxide synthase inhibition on ocular blood flow and metabolic demand in the rat and to compare these eVects with changes in the cerebral and peripheral circulation. Methods—Male Sprague-Dawley rats were injected with the nitric oxide synthase inhibitor L-NAME (75 mg/kg ip), either on a single occasion only or once daily for 10 consecutive days. Controls were injected with saline. Regional blood flow and glucose metabolism were measured from tissue samples, using [C]iodoantipyrine and [C]-2-deoxyglucose respectively, 1 hour after either acute L-NAME injection or 1 hour after the last injection of the chronic treatment protocol. Results—Mean arterial pressure was significantly increased (+31%) following the acute injection (indicating peripheral vasoconstriction) and this eVect was enhanced (+50%) following chronic treatment. In both the ocular and cerebral circulation, blood flow was decreased following acute treatment (−48% and −43% respectively). However, while this response was totally attenuated in the cerebral circulation following chronic L-NAME treatment (−4%), the ocular circulation remained responsive (−57%). Metabolic demand in brain and eye tissue, as reflected in the accumulation of 2-deoxyglucose, was unaVected by either acute or chronic treatment with L-NAME. Conclusion—Homeostatic mechanisms appear to be activated in the cerebral circulation which re-establish flow metabolism homeostasis, and the eVect of L-NAME on cerebral blood flow is attenuated following repeated exposure. This process does not seem to happen in the ocular circulation and, thus, the ocular vasculature appears to behave more like those blood vessels which determine total peripheral resistance than the cerebral circulation. It remains to be seen whether the sustained decrease in blood flow in the eye is suYcient to compromise ocular function and render the eye susceptible to damage from chronic L-NAME induced oligaemia. (Br J Ophthalmol 1997;81:68–71) Experiments using isolated intraand extraocular arterial vessels indicate that endothelium derived nitric oxide (NO) plays a prominent role in the regulation of vascular tone in the ocular circulation. Inhibition of nitric oxide synthase (NOS) by the arginine analogue N-nitro-L-arginine methyl ester (LNAME) has been shown to significantly reduce choroidal blood flow following a single intravenous dose. These previous studies have examined short term, acute eVects of NOS inhibition, and little is known of the longer term changes in the ophthalmic circulation following chronic repetitive injection of L-NAME. This is of potential interest as vascular endothelial dysfunction may contribute to ocular disease processes. In the present study, therefore, we evaluated the in vivo eVects of acute and chronic administration of L-NAME on the ocular circulation in the rat, and contrasted these eVects with those in the cerebral and peripheral circulations. Given the coupling that exists between blood flow and metabolic demand in the optic nerve head, a semiquantitative assessment of glucose metabolism was also made in ocular and cerebral tissues from parallel groups of rats similarly treated. Methods Male Sprague-Dawley rats (250–275 g at the outset) were injected intraperitoneally (ip) with the nitric oxide synthase inhibitor L-NAME (75 mg/kg; n=10) or saline (n=10) once daily for 10 consecutive days (chronic treatment group). A further group of rats (n=10) was injected with L-NAME only once (acute treatment group). A single ip injection of L-NAME at this dose has been shown to produce a near complete inhibition (93%) of cerebral NOS activity, and to produce vascular eVects which last for up to 18 hours after a single injection. Chronic L-NAME treatment, following the protocol described here, produces a level of NOS inhibition (96%) which is only slightly greater than the acute eVects. On the day of the experiment the rats were anaesthetised with halothane in a mixture of oxygen (30%) and nitrous oxide (70%) and cannulae were inserted into both femoral arteries (for the measurement of arterial pressure and sampling of arterial blood), and both femoral veins (for the injection of drugs and radiolabelled tracers). All surgery was British Journal of Ophthalmology 1997;81:68–71 68 Department of Ophthalmology, University of Edinburgh, Scotland