DL-Aspartic acid α-methyl ester

Diabetes alters retinal hemodynamics, but little is known about the impact of diabetes on the role of endothelium-derived hyperpolarizing factor (EDHF) in the regulation of retinal circulation. Therefore, we examined how diabetes affects the nitric oxide- and prostaglandin-independent vasodilation of retinal arterioles induced by acetylcholine. Male Wistar rats were treated with streptozotocin (80 mg/kg, i.p.) and experiments were performed 6-8 weeks later. Under artificial ventilation, rats were treated with tetrodotoxin (100 microg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye. Methoxamine was used to maintain adequate systemic circulation. Fundus images were captured by a digital camera that was equipped with a special objective lens. The vasodilator responses of retinal arterioles were assessed by measuring changes in diameters of the vessels. In streptozotocin-induced diabetic rats and the age-matched controls, acetylcholine increased diameters of retinal arterioles in a dose-dependent manner. The vasodilator responses to acetylcholine in diabetic rats were smaller than those in control rats. The nitric oxide- and prostaglandin-independent vasodilation of retinal arterioles observed under treatment with combination of N(G)-nitro-l-arginine methyl ester (30 mg/kg, i.v.) and indomethacin (5 mg/kg, i.v.) were also attenuated by diabetes. Diabetes did not alter the dilator responses of retinal arterioles to sodium nitroprusside and forskolin. These results suggest that diabetes impairs EDHF-mediated vasodilation of retinal arterioles induced by acetylcholine. The impaired EDHF-mediated vasodilation may contribute to alteration of retinal hemodynamics in diabetes.

Purpose: Results in a prior study have demonstrated that systemic administration of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor simvastatin to healthy subjects reduces intraocular pressure and increases retinal blood flow. However, it remains unclear whether simvastatin can directly elicit dilation of retinal microvessels. In the current study, the direct effect and the underlying mechanism of the vasomotor action of simvastatin in retinal arterioles was studied. Methods: Porcine retinal arterioles ( approximately 75 mum internal diameter) were isolated, cannulated, and pressurized (55 cmH(2)O) without flow for in vitro study. Diameter changes in response to simvastatin were recorded using videomicroscopic techniques. Results: Retinal arterioles dilated dose dependently to simvastatin (1 nM to 10 muM). This vasodilation was significantly reduced after removal of the endothelium. The nitric oxide (NO) synthase inhibitor l-NAME (N(G)-nitro-l-arginine methyl ester) markedly inhibited the vasodilation, and combined administration of l-NAME with cyclooxygenase inhibitor indomethacin mimicked the effect of denudation. Blockade of soluble guanylyl cyclase by ODQ (1H-[1,2,4] oxadiazolo[4,3,-a]quinoxalin-1-one) produced a similar inhibitory effect as that by l-NAME. In contrast, the dilation was unaffected by cytochrome-P450 epoxygenase inhibitor sulfaphenazole. Intraluminal incubation of vessels with mevalonate, an immediate metabolite of HMG-CoA reductase, partially inhibited vasodilation to simvastatin. The Rho kinase inhibitor Y-27632 abolished the antagonistic effect of mevalonate. Conclusions: Simvastatin elicits mainly an endothelium-dependent, NO-mediated dilation of retinal arterioles via activation of guanylyl cyclase; cyclooxygenase plays a relatively minor role. It appears that inhibition of the mevalonate-Rho kinase pathway in endothelial cells contributes in part to the simvastatin-induced vasodilation. A better understanding of the action of statins on retinal vasculature may help shed light on its therapeutic potential in retinal vascular disease.

Opioids contribute to the regulation of cerebral vascular tone. The purpose of this study was to examine the effects of herkinorin, a non-opioid μ-opioid receptor agonist derived from salvinorin A, on blood vessels in the rat retina and to investigate the mechanism underlying the herkinorin-induced retinal vasodilatory response. Ocular fundus images were captured using an original high-resolution digital fundus camera in vivo. The retinal vascular responses were evaluated by measuring the diameter of retinal arterioles in the fundus images. Both systemic blood pressure and heart rate were continuously recorded. Herkinorin increased the retinal arteriolar diameter without significantly changing mean blood pressure and heart rate. The retinal vasodilator response to herkinorin was almost completely prevented following treatment with naloxone, a nonselective opioid receptor antagonist and H-D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a selective μ-opioid receptor antagonist. Nω-nitro-L-arginine methyl ester, a nonselective nitric oxide (NO) synthase inhibitor, or indomethacin, a cyclooxygenase inhibitor, significantly attenuated the herkinorin-induced retinal vasodilator responses. In addition, Nω-propyl-L-arginine, an inhibitor of neuronal NO synthase, diminished the herkinorin-induced retinal vasodilator responses. Seven days after an intravitreal injection of N-methyl-D-aspartic acid, loss of inner retinal neurons and abolishment of the retinal vasodilator response to herkinorin were observed. These results suggest that herkinorin dilates rat retinal arterioles through stimulation of retinal μ-opioid receptors. The μ-opioid receptor-mediated retinal vasodilator response is likely mediated by NO generated by neuronal NO synthase. Retinal neurons play an important role in the retinal vasodilator mechanism involving μ-opioid receptors in rats.