Elevated Intraocular Pressure Inhibits Endothelium–Dependent Nitric Oxide–Mediated Dilation of Retinal Arterioles: Role of Superoxide

Abstract

Abstract: : Purpose: Numerous studies have suggested that vascular dysfunction contributes to the development of glaucoma. Although an acute elevation of intraocular pressure (IOP) has been shown to reduce retinal blood flow, its direct effect on the retinal microvascular function remains elusive. Since glaucoma has been associated with increased oxidative stress and the potent vasodilator nitric oxide (NO) has been implicated in the regulation of retinal blood flow, we examined the effect of elevated IOP on NO–mediated dilation of retinal arterioles and assessed the possible role of superoxide anion in vascular dysfunction. Methods: IOP was increased and maintained at 60–70 mmHg for 60 minutes by infusing Balanced Saline Solution into the anterior chamber of a porcine eye. The fellow eye with normal IOP (10–20 mmHg) served as control. IOP was measured using a Schiotz tonometer. After enucleation, retinal arterioles (40–60 µm in diameter) were isolated from both eyes for functional and molecular analysis. For functional studies, arterioles were cannulated and pressurized to 55–cmH₂O lumenal pressure without flow. Concentration–dependent vasodilations to endothelium–dependent NO–mediated agonist bradykinin (10 pM to 10 nM) and endothelium–independent NO donor sodium nitroprusside (10 nM to 0.1 mM) were examined in the absence and presence of superoxide anion scavenger TEMPOL (1 mM). RT/PCR was performed for detection of endothelial NO synthase (eNOS) mRNA. Results: Retinal arterioles from both groups developed similar levels of basal tone. In control arterioles, bradykinin and nitroprusside evoked concentration–dependent dilations. L–NAME, a NOS inhibitor, significantly attenuated vasodilation to bradykinin but did not alter vasodilation to nitroprusside. In elevated IOP arterioles, the dilation to bradykinin but not to nitroprusside was significantly reduced. Treating these vessels with TEMPOL restored bradykinin–induced dilation. RT–PCR data revealed that elevated IOP did not alter eNOS mRNA expression in retinal arterioles. Conclusions: These data show that an acute elevation of IOP inhibits endothelium–dependent NO–mediated dilation of retinal arterioles. The inhibitory effect appears to be mediated by NO inactivation via increased levels of superoxide without an alteration of eNOS mRNA expression. A better understanding of the mechanisms associated with vascular dysfunction observed during acute elevation of IOP and glaucoma may be gleaned from these findings.

Keywords: oxidation/oxidative or free radical damage • nitric oxide • intraocular pressure