Acute retinal ischemia inhibits endothelium-dependent nitric oxide-mediated dilation of retinal arterioles via enhanced superoxide production.

PURPOSE: Because retinal vascular disease is associated with ischemia and increased oxidative stress, the vasodilator function of retinal arterioles was examined after retinal ischemia induced by elevated intraocular pressure (IOP). The role of superoxide anions in the development of vascular dysfunction was assessed. METHODS: IOP was increased and maintained at 80 to 90 mm Hg for 30, 60, or 90 minutes by infusing saline into the anterior chamber of a porcine eye. The fellow eye with normal IOP (10-20 mm Hg) served as control. In some pigs, superoxide dismutase mimetic TEMPOL (1 mM) or vehicle (saline) was injected intravitreally before IOP elevation. After enucleation, retinal arterioles were isolated and pressurized without flow for functional analysis by recording diameter changes using videomicroscopic techniques. Dihydroethidium (DHE) was used to detect superoxide production in isolated retinal arterioles. RESULTS: Isolated retinal arterioles developed stable basal tone and the vasodilations to endothelium-dependent nitric oxide (NO)-mediated agonists bradykinin and L-lactate were significantly reduced only by 90 minutes of ischemia. However, vasodilation to endothelium-independent NO donor sodium nitroprusside was unaffected after all time periods of ischemia. DHE staining showed that 90 minutes of ischemia significantly increased superoxide levels in retinal arterioles. Intravitreal injection of membrane-permeable radical scavenger but not vehicle before ischemia prevented elevation of vascular superoxide and preserved bradykinin-induced dilation. CONCLUSIONS: Endothelium-dependent NO-mediated dilation of retinal arterioles is impaired by 90 minutes of ischemia induced by elevated IOP. The inhibitory effect appears to be mediated by the alteration of NO signaling via vascular superoxide.

Divergent roles of nitric oxide and rho kinase in vasomotor regulation of human retinal arterioles.

PURPOSE: Although the arteriolar segment contributes to flow regulation, there is sparse information at the single microvessel level on how vasomotor function is regulated in the human retina. The authors have previously reported vasoreactivity and its underlying mechanisms in isolated porcine retinal arterioles. Herein, they studied human retinal arterioles for comparison. METHODS: Retinal tissues were obtained from seven patients undergoing enucleation. Human and porcine retinal arterioles were isolated and pressurized to 55 cm H(2)O luminal pressure for vasoreactivity study using videomicroscopic techniques. RESULTS: Isolated human and porcine retinal arterioles developed myogenic tone and dilated dose dependently to bradykinin, adenosine, and sodium nitroprusside. Stepwise increases in luminal flow produced graded dilation with approximately 60% dilation at the highest flow tested. Nitric oxide (NO) synthase inhibitor L-NAME nearly abolished dilations to bradykinin and flow and attenuated the adenosine-induced dilation without altering the response to nitroprusside. Endothelin-1 caused dose-dependent constriction. Rho kinase (ROCK) inhibitor H-1152 blocked both myogenic tone and endothelin-1-induced constriction. Responses of retinal arterioles to all agonists and increased flow were similar between pigs and humans. CONCLUSIONS: Isolated human retinal arterioles dilate to bradykinin and increased flow in an NO-dependent manner. NO contributes, in part, to adenosine-induced vasodilation. Conversely, ROCK activation mediates myogenic tone and endothelin-1-induced vasoconstriction. Similarities in these vasoactive responses and the underlying mechanisms between human and porcine retinal arterioles support the latter as a viable experimental model of the human retinal microcirculation.

Functional and molecular characterization of the endothelin system in retinal arterioles.

PURPOSE: Activation of the endothelin (ET) system has been implicated in the pathogenesis of retinal ischemic disease. Although ET-1, the predominant endogenous isoform of ET, has been shown to cause constriction of retinal vessels, the expression and functional significance of its synthesis and the involved specific ET receptors in retinal arterioles remain unknown. The authors examined the roles of ET(A) and ET(B) receptors and of endothelin-converting enzyme (ECE)-1 in ET-1-induced vasomotor responses of single retinal arterioles. METHODS: To exclude systemic confounding effects, porcine retinal arterioles were isolated for vasoreactivity and molecular studies. RESULTS: Isolated and pressurized retinal arterioles developed basal tone and constricted in a manner dependent on concentration to ET-1. ET-1 precursor big ET-1 elicited time-dependent vasoconstriction over 20 minutes, which was blocked by the ECE-1 inhibitor phosphoramidon. ET(A) receptor antagonist BQ123 inhibited most (approximately 90%) of vasoconstrictions to ET-1 and big ET-1. ET(B) receptor agonist sarafotoxin also elicited concentration-dependent constriction of retinal arterioles but with significantly less potency than ET-1. ET(B) receptor antagonist BQ788 abolished vasoconstriction to sarafotoxin but only slightly reduced responses to ET-1 and big ET-1. Protein and mRNA expressions of ET(A), ET(B), and ECE-1 were detected in retinal arterioles. Immunohistochemistry revealed ET(A) and ET(B) receptors predominantly in smooth muscle and ECE-1 predominantly in endothelium and smooth muscle. CONCLUSIONS: ET-1 elicits constriction of retinal arterioles predominantly through the activation of smooth muscle ET(A) receptors. Endogenous production of ET-1 from vascular ECE-1 is sufficient to evoke ET(A) receptor-dependent constriction in retinal arterioles.

Sildenafil (Viagra) evokes retinal arteriolar dilation: dual pathways via NOS activation and phosphodiesterase inhibition.

PURPOSE: Sildenafil (Viagra; Pfizer, New York, NY), a selective phosphodiesterase type-5 (PDE5) inhibitor, is widely used to treat impotence by improving penile blood flow via elevation of cGMP. However, its effect on ocular circulation is controversial and whether retinal arterioles are responsive to this drug remains unclear. In this study, the direct reaction of retinal arterioles to sildenafil was examined and the signaling pathway underlying this vasomotor activity was probed. METHODS: Retinal arterioles from porcine eyes were isolated, cannulated, and pressurized without flow. Diameter changes in response to sildenafil were recorded using videomicroscopic techniques. RESULTS: Retinal arterioles (67 +/- 2 microm) dilated dose dependently to sildenafil (1 ng/mL to 1 microg/mL). This dilation was inhibited by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), the guanylyl cyclase inhibitor 1H- [1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), the extracellular signal-regulated kinase (ERK) pathway inhibitor PD98059, the nonselective potassium channel blocker tetraethylammonium (TEA), and the selective adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel blocker glibenclamide. The vasodilation elicited by the NO donor S-nitroso-N-acetylpenicillamine (SNAP) was inhibited by ODQ and TEA but was insensitive to PD98059. In the presence of L-NAME, the addition of SNAP (1 microM) produced modest vasodilation and the inhibited sildenafil response was subsequently restored. The restored dilation was insensitive to PD98059 but was blocked by TEA. CONCLUSIONS: Activation of NO synthase, through ERK signaling, leading to NO production and subsequent guanylyl cyclase activation and K(ATP) channel opening is the major vasodilatory pathway for sildenafil in retinal arterioles. Moreover, the elevated cGMP, from endogenous or exogenous NO, plays a permissive role for sildenafil to exert vasodilation through inhibition of the PDE5 pathway independent of ERK signaling.

Brimonidine evokes heterogeneous vasomotor response of retinal arterioles: diminished nitric oxide-mediated vasodilation when size goes small.

Brimonidine, an alpha2-adrenergic receptor (AR) agonist, has been employed in the treatment of glaucoma due to its beneficial effects on intraocular pressure reduction and neuroprotection. In addition, some studies have implicated that brimonidine might influence ocular blood flow; however, its effect on the retinal microcirculation has not been documented. Herein, we examined the vasomotor action of brimonidine on different branching orders of retinal arterioles in vitro and determined the contribution of the alpha2-AR subtype and the role of endothelium-derived nitric oxide (NO) in this vasomotor response. First- and second-order retinal arterioles of pigs were isolated, cannulated, and pressurized for functional studies. Videomicroscopic techniques were employed to record diameter changes in response to brimonidine. RT-PCR was performed for detection of alpha-AR and endothelial NO synthase (eNOS) mRNA in retinal arterioles. All first-order arterioles (82 +/- 2 microm ID) dilated dose dependently to brimonidine (0.1 nM to 10 microM) with 10% dilation at the highest concentration. Second-order arterioles (50 +/- 1 microm ID) responded heterogeneously with either dilation or constriction. The incidence and magnitude of vasoconstriction were increased with increasing brimonidine concentration. Administration of the NO synthase inhibitor NG-nitro-L-arginine methyl ester abolished the brimonidine-induced vasodilation in first- and second-order arterioles. Regardless of vessel size, vasomotor responses (i.e., vasodilation and vasoconstriction) of retinal arterioles were sensitive to the alpha2-AR antagonist rauwolscine. Consistent with the functional data, alpha2A-AR and eNOS mRNAs were detected in retinal arterioles. Collectively, our data demonstrate that brimonidine at clinical doses evokes a consistent NO-dependent vasodilation in first-order retinal arterioles but a heterogeneous response in second-order arterioles. These vasomotor responses are mediated by the activation of alpha2-AR. It appears that brimonidine, depending on the concentration and vessel size, may alter local retinal blood flow.

Requisite roles of A2A receptors, nitric oxide, and KATP channels in retinal arteriolar dilation in response to adenosine.

PURPOSE: Adenosine is a potent vasodilator of retinal microvessels and is implicated to be a major regulator of retinal blood flow during metabolic stress. However, the receptor subtypes and the underlying signaling mechanism responsible for the dilation of retinal microvessels in response to adenosine remain unclear. In the present study, the roles of specific adenosine receptor subtypes, nitric oxide (NO), and adenosine triphosphate (ATP)-sensitive K(+) (K(ATP)) channels in adenosine-induced dilation of retinal arterioles in vitro were examined. METHODS: Porcine second-order retinal arterioles (40-70 mum in internal diameter) were isolated, cannulated, and pressurized to 55 cmH(2)O luminal pressure without flow. Diameter changes in response to agonists were recorded by using videomicroscopic techniques. RESULTS: All vessels exhibited basal tone and dilated dose dependently in reaction to adenosine, N(6)-cyclopentyladenosine (an adenosine A(1) receptor agonist), and 2-[p-(2-carboxyethyl)]phenylethyl-amino-5'-N-ethylcarboxamidoadenosine(CGS21680; an adenosine A(2A) receptor agonist). These responses were not altered by the selective adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, but were significantly attenuated by the selective adenosine A(2A) receptor antagonist 4-(2-{7-amino-2-(2-furyl)[1,2,4]-triazolo[2,3-a] [1,3,5]triazin-5-ylamino}ethyl)phenol. Blockade of NO synthase, but not of cyclooxygenase or cytochrome P-450 epoxygenase, significantly attenuated the vasodilations in response to adenosine and CGS21680. The residual vasodilative reactions to both agonists was nearly abolished by the K(ATP) channel inhibitor glibenclamide. CONCLUSIONS: These data suggest that adenosine evokes retinal arteriolar dilation via activation of A(2A) receptors and subsequent production of NO and opening of K(ATP) channels. A better understanding of the fundamental signaling pathways responsible for adenosine-induced dilation of retinal arterioles may help shed light on the possible mechanisms contributing to impaired retinal blood flow regulation in patients after retinal ischemia.

A 1-year study on carbon, titanium surface-modified intraocular lens in rabbit eyes.

PURPOSE: To evaluate biocompatibility of Carbon (C) and Titanum (T) surface modified Polymethyl-methacrylate (PMMA) Intraocular Lens (IOLs) in pseudophakic rabbit model. METHODS: Extracapsule Cataract Extraction (ECCE) and IOLs implantation were performed in Japanese albino rabbit eyes. The white cell concentration from the aqueous humor in the eyes was counted within 3 months post operation. The inflammatory cells in the eyes were noted and graded by slit lamp using a semiquantitive scale up to 1-year post operation. The rabbit eyes were inspected under light microscopy, where pathology of the eyes was caracterized. RESULTS: The white cell concentration in the aqueous humor was significantly attenuated in C and T IOL-implanted eyes compared with PMMA IOL-implanted eyes 1 week post operation. The exudate levels in the anterior ocular chamber and the posterior synechias were significant lower in T IOL-implanted eyes than in PMMA IOL-implanted eyes 1 week and 2 weeks after surgery. The exudate levels in the anterior chambers and the posterior synechias were not significantly different in C IOL-implanted versus PMMA IOL-implanted eyes. CONCLUSIONS: This in vivo study provides evidence of effectiveness of Carbon and Titanium IOLs in improving the biocompatibility of PMMA IOLs.

Contrast sensitivity and color vision with a yellow intraocular len.

OBJECTIVE: To evaluate contrast sensitivity and color vision of yellow ultraviolet (UV) intraocular len (IOL) in cataract patients. DESIGN: Randomized clinical trial. METHODS: Extracapsular cataract extraction was performed in 60 senile cataract patients. The patients were randomly assigned to receive 30 yellow UV IOLs and 30 ordinary UV IOLs. Visual acuity, contrast sensitivity, and color vision were examined up to 6 months postoperatively. RESULTS: The yellow UV IOLs showed statistically significantly higher spatial contrast sensitivity than ordinary UV IOLs in the low and middle frequencies. There was no significant difference between yellow and ordinary UV IOL in color vision. Incidences of photophobia and cyanopsia were less in patients who received the yellow UV IOLs. CONCLUSIONS: Yellow UV IOLs are preferable to ordinary UV IOLs in preserving spatial contrast sensitivity and cause less photophobia and cyanopsia in the early postoperative period.

Physical and cytological characters of carbon, titanium surface modified intraocular lens in rabbit eyes.

PURPOSE: To evaluate the physical and cytological characters of carbon (C), titanium (T) surface-modified intraocular lenses (IOLs) in rabbit eyes. METHODS: Water contact angle in air of C, T-IOLs was measured by goniometer using the sessile drop method. Extracapsular cataract extraction (ECCE) and IOL implantation were performed in rabbit eyes. Cells adhering to IOLs were measured using the MIAS-2000 Computer Analyzer System at 12 months after operation. The diopters and resolution of C, T-IOLs were measured after 1 year of implantation. RESULTS: Water contact angle in air of C, T-IOLs (80, 84.5 deg) were significant lower than that of PMMA-IOL (74.8 deg). C, T-IOLs became hydrophobic compared with PMMA-IOLs. C, T-IOLs did not change significantly in diopters and resolution after 1 year of implantation in rabbit eyes. There were fibroblasts, epitheloid cells, giant cells, lymphocyte cells and fibrous membrane attached to the surface of IOLs. CONCLUSIONS: The optical characteristics of C, T-IOL) were stable in rabbit eyes after 1 year of implantation. The C, T-IOLs became hydrophobic compared with PMMA-IOLs. They also induced foreign body reactions similar to those seen with PMMA-IOLs in rabbit eyes.