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Simvastatin Elicits Dilation of Isolated Porcine Retinal Arterioles: Role of Nitric Oxide and Mevalonate-Rho Kinase Pathways

¹From the Department of Ophthalmology, Scott & White Eye Institute, Temple, Texas; the ²Department of Ophthalmology, Asahikawa Medical College, Asahikawa, Japan; and the ³Department of Systems Biology and Translational Medicine, College of Medicine, Texas A&M Health Science Center, Temple, Texas.

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 (75 µm internal diameter) were isolated, cannulated, and pressurized (55 cmH₂O) 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 µM). 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.

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