Evaluation of nitric oxide synthesis in the optic nerve head in vivo using microdialysis and high-performance liquid chromatography and its interaction with endothelin-1

The influence of retinal blood flow on open-angle glaucoma in patients with and without diabetes

PURPOSE

To evaluate the impact of retinal blood flow on optic nerve head (ONH) morphology in patients with open-angle glaucoma (OAG) with and without diabetes mellitus (DM).

METHODS

A total of 66 patients with OAG (14 with DM, 52 without DM) were assessed at baseline and 3-year follow-up for retinal capillary blood flow using confocal scanning laser Doppler and ocular structure using Heidelberg retinal tomography and optical coherence tomography.

RESULTS

Change in retinal tissue with zero blood flow in the superior and inferior retina was found to have a strong correlation with ONH changes in diabetic patients (r≥0.90, p≤0.03); however, no relation was found in the nondiabetic cohort. There were also significant changes in inferior mean flow that strongly correlated with changes in cup area (r = 0.97, p = 0.0029), cup/disc area ratio (r = 0.96, p = 0.0070), linear cup/disc ratio (r = 0.93, p = 0.0172), rim area (r = -0.97, p = 0.0036), and rim volume (r = -0.95, p = 0.0084) in diabetic patients only, while changes in the superior mean flow were only significantly associated with cup area (r = -0.30, p = 0.0498), cup volume (r = -0.36, p = 0.0178), and rim volume (r = 0.35, p = 0.0193) in nondiabetic patients.

CONCLUSIONS

In this cohort of patients with OAG, changes in retinal capillary blood flow correlated more strongly with changes in ONH morphology in patients with DM than in those without DM. These data suggest that changes in retinal blood flow may play a larger role in glaucomatous ONH progression in patients with OAG with DM.

The role of endothelin in the pathophysiology of glaucoma

IMPORTANCE OF THE FIELD

Glaucoma is the second leading cause of blindness worldwide. To date, treatment of glaucoma has focused on lowering intraocular pressure (IOP) though there are other mechanisms that might damage the optic nerve, leading to characteristic visual field loss. Endothelin, a potent vasoconstrictor, is believed to play a role in the pathogenesis of glaucomatous optic neuropathy.

AREAS COVERED IN THIS REVIEW

We review the evidence from the last 20 years exploring the action of endothelin in the eye, its association with the pathophysiology of glaucoma, as well as the potential therapeutic role of targeting the endothelin pathway to affect disease progression in glaucomatous eyes.

WHAT THE READER WILL GAIN

The goal of this paper is to inform readers about endothelin structure, actions, and role in ocular pathology, pharmacology, and potential areas of future research.

TAKE HOME MESSAGE

Overall, we believe that the body of evidence supports the following conclusions; i) endothelin is a potent vasoconstrictor that plays a role in ocular physiology, ii) endothelin may play a role in the pathophysiology of glaucoma and iii) modulation of the endothelin system with newly discovered potent antagonists holds promise in treating glaucoma through both pressure-dependent and pressure-independent pathways.

Measurement of region-specific nitrate levels of the posterior chamber of the rat eye using low-flow push-pull perfusion

The determination of the presence of nitric oxide metabolites in the rat vitreous cavity in a regioselective manner is complicated by the size and shape of the eye as well as the diffusivity of the molecule and its metabolites. In this work, in vivo low-flow push-pull perfusion sampling was utilized with a rapid capillary electrophoretic assay to monitor levels of the major NO metabolite, nitrate, at the vitreoretinal interface (VRI) of normal and aged rat models. The sampling probe tips were placed in three different positions in the posterior chamber through a 29-gauge guide needle. Sampling was performed along the VRI over the optic nerve head and regions peripheral to the optic nerve head. Additionally, samples were collected from the middle vitreous region to compare to VRI sampling. A significant (P < 0.05) difference in the perfusate nitrate concentration was observed in each location, which may be due to the source of NO production or the clearance mechanism of the molecule from the vitreous cavity. Infusion of L-NAME with physiological saline led to a significant decrease (35%) in the observed nitrate level. LFPPP was then utilized to observe nitrate levels after an average of 4.5 months of aging. A 3-fold increase in the mean level of nitrate over the optic nerve head was observed in mature animals compared to younger control animals. Precise measurement of NO metabolites along the VRI may provide insights into the function of NO in maintaining homeostatic conditions and the molecular changes at the diseased retina.

Endothelin-1 (ET-1) causes death of retinal neurons through activation of nitric oxide synthase (NOS) and production of superoxide anion

Endothelin-1 (ET-1) is the most potent and long-acting vasoconstricting peptide presently known. In addition to its vascular effects, endothelin signaling pathway exists in the central nervous system (CNS), which is deeply related to neuronal degeneration. In the present study, we evaluated the effect of ET-1 on death of retinal neurons consisting mainly of amacrine cells, and its interaction with nitric oxide synthase (NOS) and superoxide production. Cultured retinal neurons from fetal rats were exposed to various doses of ET-1 (0.1, 1.0, 10 and 100nM). Neuronal toxicity of ET-1 was assessed by trypan blue exclusion, Hoechst 33,258 staining and TUNEL assay at different times. Intracellular levels of nitric oxide (NO), superoxide and peroxynitrite were determined semiquantitatively by DAF2-DA, hydroethidine and dihydrorhodamine-123, respectively. The effects of adding SOD (100U/ml) and L-NAME with ET-1 on these changes were evaluated. In addition, the receptor mechanisms involved in these reactions were determined by BQ-123 and BQ-788, receptor antagonists for ET A and ET B receptors, respectively. Exposure of cultured retinal neurons to ET-1 reduced the percentage of living cells in a dose- and time-dependent way, and the percentage of living cells was significantly increased by addition of SOD and L-NAME. Fluorometric analyses revealed that ET-1 increased the intracellular NO level in a dose- and time-dependent manner. The intracellular superoxide and peroxynitrite levels were also significantly increased 24h after incubation with 100nM of ET-1, and this elevation was suppressed by SOD and L-NAME. These ET-1-induced alterations were significantly suppressed when both BQ-123 and BQ-788 were added simultaneously with ET-1 to the medium. These results indicate that the neuronal death caused by ET-1 is most likely mediated by the activation of NOS in association with the formation of superoxides and peroxynitrite.

Analysis of nitrite and nitrate in biological fluids by assays based on the Griess reaction: appraisal of the Griess reaction in the L-arginine/nitric oxide area of research

In the Griess reaction, first reported by Johann Peter Griess in 1879 as a method of analysis of nitrite (NO(2)(-)), nitrite reacts under acidic conditions with sulfanilic acid (HO(3)SC(6)H(4)NH(2)) to form a diazonium cation (HO(3)SC(6)H(4)-N[triple bond]N(+)) which subsequently couples to the aromatic amine 1-naphthylamine (C(10)H(7)NH(2)) to produce a red-violet coloured (lambda(max) approximately 540 nm), water-soluble azo dye (HO(3)SC(6)H(4)-NN-C(10)H(6)NH(2)). The identification of nitrite in saliva has been the first analytical application of this diazotization reaction in 1879. For a century, the Griess reaction has been exclusively used to identify analytically bacterial infection in the urogenital tract, i.e. to identify nitrite produced by bacterial reduction of nitrate (NO(3)(-)), the major nitrogen oxide anion in human urine. Since the discovery of the l-arginine/nitric oxide (l-Arg/NO) pathway in 1987, however, the Griess reaction is the most frequently used analytical approach to quantitate the major metabolites of NO, i.e. nitrite and nitrate, in a variety of biological fluids, notably blood and urine. The Griess reaction is specific for nitrite. Analysis of nitrate by this reaction requires chemical or enzymatic reduction of nitrate to nitrite prior to the diazotization reaction. The simplicity of the Griess reaction and its easy and inexpensive analytical feasibility has attracted the attention of scientists from wide a spectrum of disciplines dedicated to the complex and challenging L-Arg/NO pathway. Today, we know dozens of assays based on the Griess reaction. In principle, every laboratory in this area uses its own Griess assay. The simplest Griess assay is performed in batch commonly as originally reported by Griess. Because of the recognition of numerous interferences in the analysis of nitrite and nitrate in biological fluids and of the desire to analyze these anions simultaneously, the Griess reaction has been repeatedly modified and automated. In recent years, the Griess reaction has been coupled to HPLC, i.e. is used for post-column derivatization of chromatographically separated nitrite and nitrate. Such a HPLC-Griess system is even commercially available. The present article gives an overview of the currently available assays of nitrite and nitrate in biological fluids based on the Griess reaction. Special emphasis is given to human plasma and urine, to quantitative aspects, as well as to particular analytical and pre-analytical factors and problems that may be associated with and affect the quantitative analysis of nitrite and nitrate in these matrices by assays based on the Griess reaction. The significance of the Griess reaction in the L-Arg/NO pathway is appraised.

Glutamate level in optic nerve head is increased by artificial elevation of intraocular pressure in rabbits

Neurons can be damaged by the activation of glutamate receptors, but whether glutamate is related to the development of glaucomatous optic neuropathy is still controversial. The purpose of this study was to measure the acute changes in the glutamate levels in the optic nerve head (ONH) of rabbits induced by an artificial elevation of the intraocular pressure (IOP). A concentric microdialysis probe was inserted into the ONH of rabbits via the pars plana. The probe was perfused with Ringer's solution, and the levels of glutamate in 10-min dialysate samples were measured repeatedly using high-performance liquid chromatography. After the glutamate level was stabilized for at least 60 min, the IOP was adjusted to three levels; 120 mm Hg (n=11), 60 mm Hg (n=12), and 15 mm Hg (control group; n=11). The IOP was altered by changing the height of a bottle of Ringer's solution, which was connected to the anterior chamber by a 23-gauge needle. The IOP levels were maintained for 60 min, and the glutamate levels were determined every 10 min during the 60 min. The mean basal levels of glutamate in the 10-min dialysate were not significantly different among the three groups. The glutamate levels remained unchanged and stable in the controls, but elevation of the IOP significantly increased the level of the glutamate in the dialysate (IOP60, P=0.012; and IOP120, P=0.005: repeated measures ANOVA). Elevation of the IOP causes an increase in the glutamate levels in the ONH of rabbits. This suggests a possible interaction between glutamate metabolism and the IOP in the ONH.

Sulfisoxazole, an endothelin receptor antagonist, protects retinal neurones from insults of ischemia/reperfusion or lipopolysaccharide

Endothelins exert pathological effects in the eye and much interest centres on their role in causing retinal neuronal death in ischemic diseases like glaucoma. In the present study the influence of the non-selective endothelin antagonist, sulfisoxazole on raised intraocular pressure-induced ischemia to the rat retina was investigated. Moreover, in vitro studies on primary rat retinal cultures were undertaken to see whether sulfisoxazole is able to blunt the toxic effect of lipopolysaccharide (LPS) to retinal neurones. In order to determine whether sulfisoxazole provides protection to the retina the a- and b-wave amplitudes of the electroretinogram (ERG), the localisation of retinal choline acetyltransferase (ChAT), nitric oxide synthase (nNOS) and Thy-1 and the retinal mRNA levels of Thy-1 and FGF-2 were deduced in retinas subjected to ischemia in the absence or presence of sulfisoxazole. The results showed that the ischemia-induced changes to the a- and b-wave amplitudes of the ERG and changes associated with the localisation of ChAT, nNOS and Thy-1 to be significantly blunted by sulfisoxazole. However, while the ischemia-induced changes to Thy-1 and FGF-2 mRNAs were reduced by sulfisoxazole, the reduction was non-significant. The in vitro studies provided support for the protective effect of sulfisoxazole. Here, it was clearly shown that sulfisoxazole attenuated the elevation of nitric oxide (deduced by measuring nitrite) and the reduction in numbers of GABA-containing neurones caused by LPS. The present study provides evidence for the first time that endothelin antagonist can protect the retina from ischemic-like insults as occurs in glaucoma.

Ocular blood flow changes after dynamic exercise in humans

PURPOSE

To investigate control mechanisms for ocular blood flow changes after dynamic exercise using two different methods.

METHODS

Changes over time in the tissue blood flow in the retina and choroid-retina of healthy volunteers were determined after dynamic exercise (Master's double two-step test), using scanning laser Doppler flowmetry (SLDF) and laser speckle flowgraphy (LSFG). Changes in intraocular pressure (IOP), blood pressure, plasma CO(2) gas concentration (pCO(2)), and levels of nitric oxide (NO) metabolites were examined.

RESULTS

Retinal blood flow measured by SLDF increased significantly only at 15 min after exercise. In contrast, normalized blur (NB) values in the choroid-retina, obtained by LSFG, increased significantly up to 60 min after exercise. Ocular perfusion pressure (OPP), calculated from IOP and blood pressure, increased significantly immediately and 15 min after exercise. The plasma NO metabolite levels increased significantly, although pCO(2) levels were unchanged.

CONCLUSIONS

Dynamic exercise changes OPP and produces increased tissue blood flow in the retina in the immediate postexercise period, while blood flow increases more persistently in the choroid-retina. Difference in control of blood flow in these two regions may be related to stronger autoregulatory mechanism of blood flow in the retina. Nitric oxide may play a role in the regulation of blood flow.

Effect of elevated intraocular pressure on endothelin-1 in a rat model of glaucoma

The role of endothelin-1 (ET-1) a potent vasoactive peptide, in glaucoma pathogenesis is receiving increasing attention, particularly in astroglial activation in optic nerve damage. Our laboratory has also shown that ET-1 treatment causes proliferation of cultured human optic nerve head astrocytes to possibly initiate astrogliosis. ET-1 is distributed in retina, optic nerve, and ciliary epithelium, however the effects of elevated intraocular pressure (IOP) (as seen in glaucoma) on ET-1 and ET(B) receptors are not clearly understood. In the present study, the levels of immunoreactive ET-1 (ir-ET-1) in aqueous humour (AH) and optic nerve head (ONH) were determined in the Morrison elevated IOP model of glaucoma. Additionally in the ONH of these rats, immunohistochemical analyses of ET(B) receptors and glial fibrillary acidic protein (GFAP; a marker for astroglial cells and for astrogliosis) were performed. There was 2- to 2.5-fold increase in AH ir-ET-1 levels for rats subjected to elevated IOP, compared to their respective controls. In the Morrison rat model of glaucoma, elevated IOP increased optic nerve ir-ET-1 with concomitant increases in ir-ET(B) and ir-GFAP labelling (possibly indicative of astrogliosis and hypertrophy). As seen in brain astrocytes subjected to neurotrauma, the present findings are suggestive of ET-1's role in astroglial activation, particularly in response to elevated IOP in glaucoma.