Nitric oxide synthase activity and L-arginine metabolism in the retinas from streptozotocin-induced diabetic rats

Neuroinflammation and oxidative stress act in concert to promote neurodegeneration in the diabetic retina and optic nerve: galectin-3 participation

Diabetes is a lifelong disease characterized by glucose metabolic imbalance, in which low insulin levels or impaired insulin signaling lead to hyperglycemic state. Within 20 years of diabetes progression, 95% of patients will have diabetic retinopathy, the leading cause of visual defects in working-age people worldwide. Although diabetes is considered a microvascular disease, recent studies have shown that neurodegeneration precedes vascular changes within the diabetic visual system, albeit its mechanisms are still under investigation. Neuroinflammation and oxidative stress are intrinsically related phenomena, since macrophage/microglia and astrocytes are the main sources of reactive oxygen species during central nervous system chronic degenerative diseases, and both pathological processes are increased in the visual system during diabetes. The present review will focus on recent findings of the contribution of oxidative stress derived from neuroinflammation in the early neurodegenerative aspects of the diabetic visual system and their relationship with galectin-3.

Insulinotropic and antidiabetic effects of β-caryophyllene with l-arginine in type 2 diabetic rats

Beta-caryophyllene (BCP) is a flavoring agent, whereas l-arginine (LA) is used as a food supplement. They possess insulinotropic and β cell regeneration activities, respectively. We assessed the antidiabetic potential of BCP, LA, and its combination in RIN-5F cell lines and diabetic rats. Ex vivo studies were carried out for glucose uptake and absorption of the combination of BCP with LA. The results indicated that the combination of BCP with LA showed a significant decrease in glucose absorption and an increase in its uptake in tissues and also an increase in insulin secretion in RIN-5F cells. The combination treatment of BCP with LA showed a significant reduction in glucose, lipid levels, and oxidative stress in pancreatic tissue when compared with the diabetic group. Furthermore, the combination of BCP with LA normalized glucose tolerance and pancreatic cell damage in diabetic rats. In conclusion, the combinational treatment showed significant potentials in the treatment of type 2 diabetes mellitus. PRACTICAL APPLICATIONS: Type 2 diabetes mellitus is the most prevalent chronic metabolic disorder affecting a large population. Beta-caryophyllene is a CB₂ receptor agonist shown to have insulinotropic activity. l-Arginine is a food supplement that possesses beta-cell regeneration property. The combination of BCP with LA could work as a potential therapeutic intervention, considering the individual pharmacological activities of each. We evaluated the antidiabetic activity of the combination of BCP with LA in diabetic rats using ex vivo and in vitro experimentations. Results from the study revealed that the combination of BCP with LA showed a significant (p < .001) reduction in glucose and lipid levels as compared to individual treatment. In vitro study also supports the diabetic potential of the combination of BCP with LA in the glucose-induced insulin secretion in RIN-5F cell lines. The study indicates a therapeutic approach to treat T2DM by BCP and LA combination as food and dietary supplement.

Metabolism and Functions of Amino Acids in Sense Organs

Sense organs (eyes, ears, nose, tongue, and skin) provide senses of sight, hearing, smell, taste, and touch, respectively, to aid the survival, development, learning, and adaptation of humans and other animals (including fish). Amino acids (AAs) play an important role in the growth, development, and functions of the sense organs. Recent work has identified receptor-mediated mechanisms responsible for the chemosensory transduction of five basic taste qualities (sweet, sour, bitter, umami and salty tastes). Abnormal metabolism of AAs result in a structural deformity of tissues and their dysfunction. To date, there is a large database for AA metabolism in the eye and skin under normal (e.g., developmental changes and physiological responses) and pathological (e.g., nutritional and metabolic diseases, nutrient deficiency, infections, and cancer) conditions. Important metabolites of AAs include nitric oxide and polyamines (from arginine), melanin and dopamine (from phenylalanine and tyrosine), and serotonin and melatonin (from tryptophan) in both the eye and the skin; γ-aminobutyrate (from glutamate) in the retina; and urocanic acid and histamine (from histidine) in the skin. At present, relatively little is known about the synthesis or catabolism of AAs in the ears, nose, and tongue. Future research should be directed to: (1) address this issue with regard to healthy ageing, nasal and sinus cancer, the regulation of food intake, and oral cavity health; and (2) understand how prenatal and postnatal nutrition and environmental pollution affect the growth, development and health of the sense organs, as well as their expression of genes (including epigenetics) and proteins in humans and other animals.

Human vitreous in proliferative diabetic retinopathy: Characterization and translational implications

Diabetic retinopathy (DR) is one of the leading causes of visual impairment in the working-age population. DR is a progressive eye disease caused by long-term accumulation of hyperglycaemia-mediated pathological alterations in the retina of diabetic patients. DR begins with asymptomatic retinal abnormalities and may progress to advanced-stage proliferative diabetic retinopathy (PDR), characterized by neovascularization or preretinal/vitreous haemorrhages. The vitreous, a transparent gel that fills the posterior cavity of the eye, plays a vital role in maintaining ocular function. Structural and molecular alterations of the vitreous, observed during DR progression, are consequences of metabolic and functional modifications of the retinal tissue. Thus, vitreal alterations reflect the pathological events occurring at the vitreoretinal interface. These events are caused by hypoxic, oxidative, inflammatory, neurodegenerative, and leukostatic conditions that occur during diabetes. Conversely, PDR vitreous can exert pathological effects on the diabetic retina, resulting in activation of a vicious cycle that contributes to disease progression. In this review, we recapitulate the major pathological features of DR/PDR, and focus on the structural and molecular changes that characterize the vitreal structure and composition during DR and progression to PDR. In PDR, vitreous represents a reservoir of pathological signalling molecules. Therefore, in this review we discuss how studying the biological activity of the vitreous in different in vitro, ex vivo, and in vivo experimental models can provide insights into the pathogenesis of PDR. In addition, the vitreous from PDR patients can represent a novel tool to obtain preclinical experimental evidences for the development and characterization of new therapeutic drug candidates for PDR therapy.

Viewing the choroid: where we stand, challenges and contradictions in diabetic retinopathy and diabetic macular oedema

Diabetic macular oedema (DMO) is the leading cause of vision loss in the working-age population. Blood-retinal barrier (BRB) dysfunction in diabetic retinopathy (DR), mainly at the level of the retinal vessels, has long been related with leakage and fluid accumulation, leading to macular oedema. However, the nourishment of the macula is provided by the choroid and a diabetic choroidopathy has been described. Therefore, there has been a growing interest in studying the role of the choroid in the pathophysiology of DR and DMO, mainly by optical coherence tomography (OCT). Nevertheless, there are conflicting results in the different studies. We summarize the results from the available studies, describe the limitations and confounding factors and discuss future procedures to avoid bias.

Progression of Diabetic Capillary Occlusion: A Model

An explanatory computational model is developed of the contiguous areas of retinal capillary loss which play a large role in diabetic maculapathy and diabetic retinal neovascularization. Strictly random leukocyte mediated capillary occlusion cannot explain the occurrence of large contiguous areas of retinal ischemia. Therefore occlusion of an individual capillary must increase the probability of occlusion of surrounding capillaries. A retinal perifoveal vascular sector as well as a peripheral retinal capillary network and a deleted hexagonal capillary network are modelled using Compucell3D. The perifoveal modelling produces a pattern of spreading capillary loss with associated macular edema. In the peripheral network, spreading ischemia results from the progressive loss of the ladder capillaries which connect peripheral arterioles and venules. System blood flow was elevated in the macular model before a later reduction in flow in cases with progression of capillary occlusions. Simulations differing only in initial vascular network structures but with identical dynamics for oxygen, growth factors and vascular occlusions, replicate key clinical observations of ischemia and macular edema in the posterior pole and ischemia in the retinal periphery. The simulation results also seem consistent with quantitative data on macular blood flow and qualitative data on venous oxygenation. One computational model applied to distinct capillary networks in different retinal regions yielded results comparable to clinical observations in those regions.

Ischemic conditioning protects the rat retina in an experimental model of early type 2 diabetes

Diabetic retinopathy is a leading cause of acquired blindness in adults, mostly affected by type 2 diabetes mellitus (T2DM). We have developed an experimental model of early T2DM in adult rats which mimics some features of human T2DM at its initial stages, and provokes significant retinal alterations. We investigated the effect of ischemic conditioning on retinal changes induced by the moderate metabolic derangement. For this purpose, adult male Wistar rats received a control diet or 30% sucrose in the drinking water, and 3 weeks after this treatment, animals were injected with vehicle or streptozotocin (STZ, 25mg/kg). Retinal ischemia was induced by increasing intraocular pressure to 120 mm Hg for 5 min; this maneuver started 3 weeks after vehicle or STZ injection and was weekly repeated in one eye, while control eyes were submitted to a sham procedure. Fasting and postprandial glycemia, and glucose, and insulin tolerance tests were analyzed. At 12 weeks of treatment, animals which received a sucrose-enriched diet and STZ showed significant differences in metabolic tests, as compared with control groups. Brief ischemia pulses in one eye and a sham procedure in the contralateral eye did not affect glucose metabolism in control or diabetic rats. Ischemic pulses reduced the decrease in the electroretinogram a-wave, b-wave, and oscillatory potential amplitude, and the increase in retinal lipid peroxidation, NOS activity, TNFα, Müller cells glial fibrillary acidic protein, and vascular endothelial growth factor levels observed in diabetic animals. In addition, ischemic conditioning prevented the decrease in retinal catalase activity induced by T2DM. These results indicate that induction of ischemic tolerance could constitute a fertile avenue for the development of new therapeutic strategies to treat diabetic retinopathy associated with T2DM.

High glucose decreases expression and activity of p-glycoprotein in cultured human retinal pigment epithelium possibly through iNOS induction

Inhibition of p-glycoprotein under hyperglycemic conditions has been reported in various barrier tissues including blood-brain barrier, intestine, and kidney, and has been linked to significant clinical complications. However, whether this is also true for the outer blood-retinal barrier constituted by retinal pigment epithelium, or has a role in pathogenesis of diabetic retinopathy is not yet clear. In this study, using cultured human retinal pigment epithelium cell line D407, we found that high glucose exposure induced a significant decrease in p-glycoprotein expression both at mRNA and at protein levels, accompanied by an attenuated p-glycoprotein activity determined by intracellular rhodamine 123 retention. In marked contrast, the expressions of both mRNA and protein levels of inducible nitrate oxide synthase (iNOS) increased, and were accompanied by increased extracellular nitrate/nitrite production by Griess reaction. In addition, mRNA levels of nuclear receptors revealed a decreased expression of pregnane X receptor after the exposure of high glucose. However, the subsequent alterations in production of nitrate/nitrite, functional expression of p-glycoprotein, and mRNA levels of pregnane X receptor were partially blocked when pretreated with S,S′-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea•2HBr (PBITU), a selective iNOS inhibitor. Moreover, the effects of PBITU were antagonized with the addition of L-arginine, a substrate for NO synthesis. Our in vitro results suggest for the first time that iNOS induction plays a novel role in decreased p-glycoprotein expression and transport function at the human outer blood-retinal barrier under hyperglycemic conditions and further support the concept of inhibiting iNOS pathway as a therapeutic strategy for diabetic retinopathy.

Melatonin as a therapeutic tool in ophthalmology: implications for glaucoma and uveitis

Several lines of evidence support the view that increased free radical generation and altered nitric oxide (NO) metabolism play a role in the pathogenesis of highly prevalent ocular diseases, such as glaucoma and uveitis. Data are discussed indicating that melatonin, being an efficient antioxidant that displays antinitridergic properties, has a promising role in the treatment of these ocular dysfunctions. Melatonin synthesis occurs in the eye of most species, and melatonin receptors are localized in different ocular structures. In view of the fact that melatonin lacks significant adverse collateral effects even at high doses, the application of melatonin could potentially protect ocular tissues by effectively scavenging free radicals and excessive amounts of NO generated in the glaucomatous or uveitic eye.

Diabetic eNOS-knockout mice develop accelerated retinopathy

PURPOSE

Dysfunction of endothelial nitric oxide synthase (eNOS) has been implicated in the pathogenesis of diabetic vascular complications. This study was undertaken to determine the role of eNOS in the development of diabetic retinopathy (DR), by investigating the functional consequences of its deficiency in the diabetic state.

METHODS

Diabetes was induced in eNOS-knockout (eNOS(-/-)) and C57B/6 mice by streptozotocin (STZ) injection. Retinal vasculature was evaluated by albumin extravasation, to quantitatively measure vascular permeability, and by trypsin-digested retinal vascular preparations, to quantify acellular capillaries. Gliosis was evaluated by immunofluorescent techniques. Retinal capillary basement membrane thickness was assessed by transmission electron microscopy. Total retinal nitric oxide level was assessed by measuring nitrate/nitrite using a fluorometric-based assay, iNOS expression was examined by real-time PCR.

RESULTS

Diabetic eNOS(-/-) mice exhibit more severe retinal vascular permeability than age-matched diabetic C57BL/6 mice, detectable as early as 3 weeks after diabetes induction. Diabetic eNOS(-/-) mice also show earlier onset and an increased number of acellular capillaries, sustained gliosis, and increased capillary basement membrane thickness. Total nitric oxide (NO) level was also increased, concomitant with elevated iNOS expression in diabetic eNOS(-/-) retina.

CONCLUSIONS

Diabetic eNOS(-/-) mice exhibit A significantly wider range of advanced retinal vascular complications than the age-matched diabetic C57BL/6 mice, supporting the notion that eNOS-derived NO plays an essential role in retinal vascular function. This mouse model also faithfully replicates many of the hallmarks of vascular changes associated with human retinopathy, thus providing a unique model to aid in understanding the pathologic mechanisms of and to develop effective therapeutic strategies for diabetic retinopathy.

Increased nitric oxide synthase activity is essential for electromagnetic-pulse-induced blood-retinal barrier breakdown in vivo

PURPOSE

To examine whether electromagnetic pulses (EMPs) affected the permeability of the blood-retinal barrier (BRB), gene expression of occludin and activity of nitric oxide synthase (NOS).

METHODS

Sprague-Dawley (SD) rats were used and randomized into EMP and control groups. Retinas were removed immediately, and 2 h or 24 h after EMP radiation. BRB permeability was analyzed by transmission electron microscopy and Evans Blue staining. Retinal NOS activity and concentrations of nitrite and nitrate were measured. Occludin mRNA and protein levels were detected by RT-PCR and Western blotting.

RESULTS

Exposure of SD rats to EMP resulted in increased BRB permeability, with the greatest decrease in occludin at 24 h. Moreover, this permeability defect was also correlated with significant increases in the formation of NO and induction of NOS activity in SD rats. Furthermore, we found that treatment with NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME) blocked BRB breakdown and prevented the increase in NO formation and induction of NOS activity, as well as the decrease in occluding expression.

CONCLUSION

Taken together, these results support the view that NOS-dependent NO production is an important factor that contributes to EMP-induced BRB dysfunction, and suggests that NOS induction may play an important role in BRB breakdown.

Contributions of inflammatory processes to the development of the early stages of diabetic retinopathy

Diabetes causes metabolic and physiologic abnormalities in the retina, and these changes suggest a role for inflammation in the development of diabetic retinopathy. These changes include upregulation of iNOS, COX-2, ICAM-1, caspase 1, VEGF, and NF-κB, increased production of nitric oxide, prostaglandin E2, IL-1β, and cytokines, as well as increased permeability and leukostasis. Using selective pharmacologic inhibitors or genetically modified animals, an increasing number of therapeutic approaches have been identified that significantly inhibit development of at least the early stages of diabetic retinopathy, especially occlusion and degeneration of retinal capillaries. A common feature of a number of these therapies is that they inhibit production of inflammatory mediators. The concept that localized inflammatory processes play a role in the development of diabetic retinopathy is relatively new, but evidence that supports the hypothesis is accumulating rapidly. This new hypothesis offers new insight into the pathogenesis of diabetic retinopathy, and offers novel targets to inhibit the ocular disease.

A review of the role of glial cells in understanding retinal disease

Retinal vascular diseases such as diabetic retinopathy and retinopathy of prematurity are major causes of visual loss. Although the focus of a great deal of research has been on the aetiology of vascular growth, it is now emerging that anomalies in other retinal cell types, especially glial cells, occur very early in the course of the disease. Glial cells have major roles in every stage of disease, from the earliest subtle variations in neural function, to the development of epi-retinal membranes and tractional detachment. Therefore, having a firm understanding of the function of retinal glia is important in our understanding of retinal disease and is crucial for the development of new treatment strategies.

Simultaneous determination of L-arginine and 12 molecules participating in its metabolic cycle by gradient RP-HPLC method: application to human urine samples

We have developed and described a highly sensitive, accurate and precise reversed-phase high-performance liquid chromatography (RP-HPLC) method for the simultaneous determination of L-arginine and 12 molecules participating in its metabolic cycle in human urine samples. After pre-column derivatization with ortho-phthaldialdehyde (OPA) reagent containing 3-mercaptopropionic acid (3MPA), the fluorescent derivatives were separated by a gradient elution and detected by fluorescence measurement at 338 nm (excitation) and 455 nm (emission). L-Arginine (ARG) and its metabolites: L-glutamine (GLN), N(G)-hydroxy-L-arginine (NOHA), L-citrulline (CIT), N(G)-monomethyl-L-arginine (NMMA), L-homoarginine (HARG), asymmetric N(G),N(G)-dimethyl-L-arginine (ADMA), symmetric N(G),N(G')-dimethyl-L-arginine (SDMA), L-ornithine (ORN), putrescine (PUT), agmatine (AGM), spermidine (SPERMD) and spermine (SPERM) were extracted in a cation-exchange solid-phase extraction (SPE) column and after derivatization separated in a Purospher STAR RP-18e analytical column. The calibration curves of analysed compounds are linear within the range of concentration: 45-825, 0.2-15, 16-225, 12-285, 0.1-32, 15-235, 0.1-12, 0.1-12, 10-205, 0.02-12, 0.1-24, 0.01-10 and 0.01-8 nmol mL(-1) for GLN, NOHA, CIT, ARG, NMMA, HARG, ADMA, SDMA, ORN, PUT, AGM, SPERMD and SPERM, respectively. The correlation coefficients are greater than 0.9980. Coefficients of variation are not higher than 6.0% for inter-day precision. The method has been determined or tested for limits of detection and quantification, linearity, precision, accuracy and recovery. All detection parameters of the method demonstrate that it is a reliable and efficient means of the comprehensive determination of ARG and its 12 main metabolites, making this approach suitable for routine clinical applications. The levels of analysed compounds in human urine can be successfully determined using this developed method with no matrix effect.

Effect of cyclic guanosine-monophosphate on porcine retinal vasomotion

PURPOSE

Vasomotion refers to periodic oscillations in vascular tone that ensure the intermittent supply of blood to adjacent microvascular units. Previous evidence from vessels outside the eye suggests that cyclic guanosine-monophosphate (cGMP) is involved in the regulation of vasomotion, but it is unknown whether this compound has an effect on vasomotion in retinal vessels.

METHODS

Retinal arterioles from porcine eyes were studied in a wire myograph. After initiation of vasomotion, the vessels were stimulated with increasing concentrations of the cGMP agonist 8-Br-cGMP (n = 6), the phosphodiesterase inhibitor zaprinast (n = 6) and the cGMP synthesis inhibitor L-NAME (n = 6). High concentrations of L-NAME blocked vasomotion, and control experiments (n = 20) using 8-Br-cGMP, S-nitroso-N-acetylpenicillamine (SNAP), adenosine and pinacidil were carried out to elucidate whether this effect was related to changes in the general tone of the vessel. Additionally, the relationship between oscillations in vascular tone and intracellular calcium concentration was studied.

RESULTS

Induction of cGMP agonistic activity with either 8-Br-cGMP or zaprinast lowered the vasomotion frequency significantly, whereas L-NAME-induced inhibition of cGMP increased this frequency. Neither of the agents affected the amplitude of the oscillations. The control experiments indicated that the effect of cGMP on vasomotion frequency was independent of the accompanying increase in tone. The oscillations in tone during vasomotion were accompanied by similar oscillations in intracellular calcium concentration.

CONCLUSION

Cyclic GMP lowers the frequency without affecting the amplitude of vasomotion in isolated porcine retinal arterioles.

Contribution of Nitric Oxide-Producing Cells in Normal and Diabetic Rat Retina

PURPOSE

To examine the immunohistochemical localization of L-arginine and L-citrulline and determine where and how nitric oxide (NO) is produced in the normal and streptozotocin (STZ)-induced diabetic rat retinas.

METHODS

NO is produced when L-arginine is changed to L-citrulline by NO synthase (NOS). In normal and STZ-induced diabetic rats, using an immunohistochemical method, we examined the retinal distribution of L-arginine and L-citrulline after intracardiac perfusion. We studied the distribution of NOS after immersed fixation and analyzed the number of neuronal NOS (nNOS)-positive neurons.

RESULTS

We observed L-arginine localization in the internal limiting membrane (ILM), the ganglion cell layer (GCL), and the inner nuclear layer (INL). L-Arginine immunoreactivity in the diabetic rat retinas was found in the inner plexiform layer (IPL), as well as in the normal retina. L-Citrulline immunoreactivity in the normal and diabetic retinas was observed in the ILM, the GCL, the IPL, and the INL. nNOS staining in the normal and diabetic rat retinas was observed in the GCL, the IPL and the INL. The number of nNOS-positive amacrine cells was less in the diabetic rat retinas.

CONCLUSION

NO might be produced in the GCL and amacrine cells, which show immunoreactivity to L-arginine, L-citrulline, and nNOS. In the early stage of diabetic retinopathy in STZ rat retinas, diabetes disturbed the function of the nNOS-positive amacrine cells and reduced NO production via nNOS.

Peroxynitrite decomposition catalyst, FP15, and poly(ADP-ribose) polymerase inhibitor, PJ34, inhibit leukocyte entrapment in the retinal microcirculation of diabetic rats

PURPOSE

Oxidative and nitrosative stress and activation of poly(ADP ribose) polymerase (PARP) play a role in the pathogenesis of diabetic complications. We evaluated the effectiveness of the peroxynitrite decomposition catalyst, FP15, and the PARP inhibitor, PJ34, in the treatment of leukocyte entrapment in the retinal microcirculation of diabetic rats.

METHODS

Diabetes was induced in rats by intraperitoneal injection of 60 mg/kg of streptozotocin. Rats were divided into four groups: controls; untreated diabetes; diabetes treated with FP15 (10 mg/kg oral gavage twice daily) and diabetes treated with PJ34 (10 mg/kg oral gavage twice daily). All experiments were performed 4 weeks after initiation of treatment. Leukocyte entrapment in the retinal microcirculation was quantitatively evaluated in vivo with acridine orange digital fluorography.

RESULTS

The density of leukocytes trapped in the retinal microcirculation 30 minutes after dye injection was significantly greater in untreated diabetes (32.1 +/- 4.7 cells/mm2) than in controls (11.3 +/- 4.5 cells/mm2) (p < 0.05). Compared with untreated diabetes, the density of trapped leukocytes significantly decreased in diabetes treated with FP15 (14.5 +/- 5.1 cells/mm2) (p < 0.0001) and diabetes treated with PJ34 (24.1 +/- 4.2 cells/mm2) (p < 0.05).

CONCLUSIONS

Treatment with FP15 and PJ34 decreased enhanced leukocyte entrapment in the retinal microcirculation during the early diabetic period. The current study suggests a role for peroxynitrite production and for PARP activation in the pathogenesis of retinal microvascular leukostasis in early diabetes.

Simvastatin inhibits leukocyte accumulation and vascular permeability in the retinas of rats with streptozotocin-induced diabetes

Leukocytes play important roles in the pathogenesis of diabetic retinopathy. Recently, 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors have been reported to exert various effects in addition to their lipid-lowering ability. We investigated the effects of simvastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, on leukocyte-induced diabetic changes in retinas. Diabetes was induced in Long-Evans rats with streptozotocin, and simvastatin administration was begun immediately after the induction of diabetes. Two weeks of treatment with simvastatin suppressed significantly the number of leukocytes adhering to retinal vessel endothelium and the number of leukocytes accumulated in the retinal tissue by 72.9% and 41.0%, respectively (P < 0.01). The expression of intercellular adhesion molecule-1 (ICAM-1) and the CD18 (the common beta-chain of ICAM-1 ligands) were both suppressed with simvastatin. The amount of vascular endothelial growth factor in the retina was attenuated in the simvastatin-treated group. To evaluate the effects of simvastatin on leukocyte-induced endothelial cell damage, vascular permeability in the retina was measured with fluorescein-labeled dextran. Treatment with simvastatin markedly reduced retinal permeability (P = 0.014). This suggests that simvastatin attenuates leukocyte-endothelial cell interactions and subsequent blood-retinal barrier breakdown via suppression of vascular endothelial growth factor-induced ICAM-1 expression in the diabetic retina. Simvastatin may thus be useful in the prevention of diabetic retinopathy.

Regulation of the early subnormal retinal oxygenation response in experimental diabetes by inducible nitric oxide synthase

We aimed to test the hypothesis that the inducible form of nitric oxide synthase (iNOS) contributes to the development of an early subnormal retinal oxygenation response in preclinical models of diabetic retinopathy. In urethane anesthetized Sprague Dawley rats or C57BL/6 mice, functional magnetic resonance imaging was used to noninvasively measure the change in retinal oxygen tension (Delta PO(2)) during a carbogen-inhalation challenge. In the rat experiments, the retinal Delta PO(2) of the following groups were compared: control rats (n = 9), 3-month diabetic rats (n = 5), and 3-month diabetic rats treated orally with L-N(6)-(1-iminoethyl)lysine 5-tetrazole amide, a prodrug of an inhibitor of iNOS (n = 6). In addition, the retinal Delta PO(2) of the following mouse groups were compared: C57BL/6 mice (n = 20), C57BL/6-Nos2(tm1 Lau) mice (n = 10), 4-month diabetic mice (n = 13), and 4-month diabetic knockout mice (n = 6). Only the Delta PO(2) of the superior hemiretina of the diabetic rat and mice groups were significantly subnormal (P < 0.05). The superior Delta PO(2) of the diabetic rats treated with the prodrug was not significantly (P > 0.05) different from their respective normal controls. In the mice experiments, the superior retinal Delta PO(2) of the iNOS null mice was not statistically different (P > 0.05) from that of normal control mice. iNOS is required for the development of an early subnormal Delta PO(2) in experimental diabetic retinopathy.

Hyperglycemia increases mitochondrial superoxide in retina and retinal cells

Oxidative stress is believed to play a significant role in the development of diabetic retinopathy. In this study, we have investigated the effects of elevated glucose concentration on the production of superoxide anion by retina and retinal cells, the cellular source of the superoxide, the effect of therapies that are known to inhibit diabetic retinopathy on the superoxide production, and the role of the superoxide in cell death in elevated glucose concentration. Superoxide release was measured from retinas collected from streptozotocin-diabetic rats (2 months) treated with or without aminoguanidine, aspirin, or vitamin E, and from transformed retinal Müller cells (rMC-1) and bovine retinal endothelial cells (BREC) incubated in normal (5 mM) and high (25 mM) glucose. Diabetes (retina) or incubation in elevated glucose concentration (rMC-1 and BREC cells) significantly increased superoxide production, primarily from mitochondria, because an inhibitor of mitochondrial electron transport chain complex II normalized superoxide production. Inhibition of reduced nicotinamine adenine dinucleotide phosphate (NADPH) oxidase or nitric oxide synthase had little or no effect on the glucose-induced increase in superoxide. Treatment of diabetic animals with aminoguanidine, aspirin, or vitamin E for 2 months significantly inhibited the diabetes-induced increase in production of superoxide in the retinas. Despite the increased production of superoxide, no increase in protein carbonyls was detected in retinal proteins from animals diabetic for 2-6 months or rMC-1 cells incubated in 25 mM glucose for 5 d unless the activities of calpain or the proteosome were inhibited. Addition of copper/zinc-containing superoxide dismutase to the media of rMC-1 and BREC cells inhibited the apoptotic death caused by elevated glucose. Diabetes-like glucose concentration increases superoxide production in retinal cells, and the superoxide contributes to impaired viability and increased cell death under those circumstances. Three therapies that inhibit the development of diabetic retinopathy all inhibit superoxide production, raising a possibility that these therapies inhibit retinopathy in part by inhibiting a hyperglycemia-induced increase in superoxide production.

Possible involvement of nitric oxide in the progression of diabetic retinopathy

Abnormal nitric oxide (NO) synthesis has been implicated in the pathogenesis of diabetes mellitus. The aim of our study was to elucidate the relationship between the stages of diabetic retinopathy (DR) and the NO levels in aqueous humor and plasma. Using the chemiluminescence assay, we measured the concentrations of NO in aqueous humor and plasma samples obtained during intraocular surgery from 45 diabetic patients and 19 nondiabetic cataract patients. The patients with diabetes were classified into 4 groups: proliferative DR (PDR) with active neovascularization (active PDR; 9 cases), PDR with quiescent neovascularization (regressed PDR; 6 cases), background DR (BDR; 16 cases) and no DR (14 cases). We found that the aqueous NO levels (mean +/- SE) of the active PDR group (83.2 +/- 13.9 microM) were significantly higher than those of the BDR group (45.8 +/- 6.0 microM, p = 0.049) and the diabetics without DR (33.3 +/- 5.2 microM, p = 0.011), and, although not statistically significantly, they were also higher than those of the regressed PDR group (52.1 +/- 10.3 microM, p = 0.224). However, no significant differences were observed between any of the diabetic subgroups in the plasma NO levels (p = 0.345). We therefore concluded that NO present in the ocular tissues may play important roles in the progression of DR.

Experimental diabetes causes breakdown of the blood-retina barrier by a mechanism involving tyrosine nitration and increases in expression of vascular endothelial growth factor and urokinase plasminogen activator receptor

The purpose of these experiments was to determine the specific role of reactive oxygen species (ROS) in the blood-retinal barrier (BRB) breakdown that characterizes the early stages of vascular dysfunction in diabetes. Based on our data showing that high glucose increases nitric oxide, superoxide, and nitrotyrosine formation in retinal endothelial cells, we hypothesized that excess formation of ROS causes BRB breakdown in diabetes. Because ROS are known to induce increases in expression of the well-known endothelial mitogen and permeability factor vascular endothelial growth factor (VEGF) we also examined their influence on the expression of VEGF and its downstream target urokinase plasminogen activator receptor (uPAR). After 2 weeks of streptozotocin-induced diabetes, analysis of albumin leakage confirmed a prominent breakdown of the BRB. This permeability defect was correlated with significant increases in the formation of nitric oxide, lipid peroxides, and the peroxynitrite biomarker nitrotyrosine as well as with increases in the expression of VEGF and uPAR. Treatment with a nitric oxide synthase inhibitor (N-omega-nitro-L-arginine methyl ester, 50 mg/kg/day) or peroxynitrite scavenger (uric acid, 160 mg/kg/day) blocked the breakdown in the BRB and prevented the increases in formation of lipid peroxides and tyrosine nitration as well as the increases in expression of VEGF and uPAR. Taken together, these data indicate that early diabetes causes breakdown of the BRB by a mechanism involving the action of reactive nitrogen species in promoting expression of VEGF and uPAR.

Experimental diabetes causes breakdown of the blood-retina barrier by a mechanism involving tyrosine nitration and increases in expression of vascular endothelial growth factor and urokinase plasminogen activator receptor

The purpose of these experiments was to determine the specific role of reactive oxygen species (ROS) in the blood-retinal barrier (BRB) breakdown that characterizes the early stages of vascular dysfunction in diabetes. Based on our data showing that high glucose increases nitric oxide, superoxide, and nitrotyrosine formation in retinal endothelial cells, we hypothesized that excess formation of ROS causes BRB breakdown in diabetes. Because ROS are known to induce increases in expression of the well-known endothelial mitogen and permeability factor vascular endothelial growth factor (VEGF) we also examined their influence on the expression of VEGF and its downstream target urokinase plasminogen activator receptor (uPAR). After 2 weeks of streptozotocin-induced diabetes, analysis of albumin leakage confirmed a prominent breakdown of the BRB. This permeability defect was correlated with significant increases in the formation of nitric oxide, lipid peroxides, and the peroxynitrite biomarker nitrotyrosine as well as with increases in the expression of VEGF and uPAR. Treatment with a nitric oxide synthase inhibitor (N-omega-nitro-L-arginine methyl ester, 50 mg/kg/day) or peroxynitrite scavenger (uric acid, 160 mg/kg/day) blocked the breakdown in the BRB and prevented the increases in formation of lipid peroxides and tyrosine nitration as well as the increases in expression of VEGF and uPAR. Taken together, these data indicate that early diabetes causes breakdown of the BRB by a mechanism involving the action of reactive nitrogen species in promoting expression of VEGF and uPAR.

Inducible nitric oxide synthase gene and diabetic retinopathy in Asian Indian patients

Nitric oxide, a signal transduction molecule, when modulated causes various diseases including diabetic retinopathy. In diabetes, allelic polymorphism of the inducible nitric oxide synthase (iNOS) gene is associated with retinopathy in the Northern Irish population. In the present study we investigated the Asian Indian population. One hundred and ninety-nine unrelated Asian Indian patients with 15 or more years of type 2 diabetes were divided into two groups: (a) diabetic retinopathy (DR) and (b) diabetic nonretinopathy (DNR) subjects. In these groups the pentanucleotide microsatellite repeat located 2.5 kb upstream of the transcription start site of the iNOS gene was amplified by polymerase chain reaction and analyzed. Eleven alleles, 175-225 bp, were identified. Allele 210 bp was significantly associated with retinopathy (p = 0.044). Individuals carrying this allele had twice the risk of developing retinopathy compared with those who did not carry this allele [odds ratio (OR) - 2.03; 95% CI 0.96-4.35]. Alleles 200 and 220 bp were also significantly associated with no retinopathy and no serious retinopathy complications, respectively. In the Asian Indian population, allele 210 bp of the iNOS gene is a high-risk allele for developing retinopathy and alleles 200 and 220 bp protect an individual from developing retinopathy or its complications.

Effect of age receptor blocker and/or anti-inflammatory coadministration in relation to glycation, oxidative stress and cytokine production in stz diabetic rats

Until now the relation between advanced glycation end products (AGEs) and vascular lesion is still controversial. However, the interaction of the former with a receptor triggers the synthesis of cytokines particularly interleukin 1- beta(IL-1 beta) and tumour necrosis factor- alpha(TNFalpha ). Subsequent release of nitric oxide (NO) may in turn induce certain damage to beta cell islets. Several arguments indicated that AGEs and reactive oxygen intermediates (ROIs) could alter the function of the vessel wall. Therefore, this study was undertaken to investigate the effectiveness of aminoguanidine, AG (inhibitor of AGE formation) joined with omega -3-fatty acids, omega 3FAs (anti-inflammatory immunosuppressive drug) in STZ diabetic rats. Diabetes was induced in 48 female albino Wistar rats by a single intraperitoneal (i.p.) injection of streptozotocin (STZ, 50 mg kg (-1)). Diabetic animals were treated with AG (50 mg kg(-1) ) and/or omega 3FAs (12 mg kg (-1)) daily and orally for 4 weeks. Groups of age matched diabetic rats ( n= 10) and healthy animals ( n= 10) served as positive and negative controls. At the end of the study, plasma glucose, fructosamine, total cholesterol (TC), high density lipoprotein cholesterol (HDLC), low density lipoprotein cholesterol (LDLC), the susceptibility of LDL to copper-catalysed oxidation, catalase activity, NO, IL-1 beta, TNF alpha were measured. Histopathological assessment of pancreatic slices were also determined. Diabetes remarkably increased plasma glucose, fructosamine and dyslipidaemia (increased TC, LDLC and decreased HDLC). Oxidative markers like oxidative susceptibility of LDL, catalase activity and NO levels were greatly enhanced. Finally, it increased the synthesis and release of cytokine (IL-1beta and TNF alpha). Treatment of diabetic rats with AG and omega 3FAs markedly reduced the above mentioned parameters. Combined form therapy has a better effect regarding oxidative cell markers, specifically NO level. Finally, omega 3FAs coadministration with AG nearly restored the atrophy of islets of Langerhan's and the peripheral lymphocytic infiltration compared to diabetic and AG treated groups. In conclusion, there is a direct correlation between glycation, oxidative stress and cytokine production with increased propensity of microvascular disorder in STZ diabetic rats. omega 3FA administration with AGE receptor blocker may represent a possible avenue of research for therapeutics directed for alleviating the complication associated with diabetes.

Regulation of taurine transporter expression by NO in cultured human retinal pigment epithelial cells

Taurine is actively transported at the retinal pigment epithelial (RPE) apical membrane in an Na(+)- and Cl(-)-dependent manner. Diabetes may alter the function of the taurine transporter. Because nitric oxide (NO) is a molecule implicated in the pathogenesis of diabetes, we asked whether NO would alter the activity of the taurine transporter in cultured ARPE-19 cells. The activity of the transporter was stimulated in the presence of the NO donor 3-morpholinosydnonimine. The stimulatory effects of 3-morpholinosydnonimine were not observed during the initial 16-h treatment; however, stimulation of taurine uptake was elevated dramatically above control values with 20- and 24-h treatments. Kinetic analysis revealed that the stimulation was associated with an increase in the maximal velocity of the transporter with no significant change in the substrate affinity. The NO-induced increase in taurine uptake was inhibited by actinomycin D and cycloheximide. RT-PCR analysis and nuclear run-on assays provided evidence for upregulation of the transporter gene. This study provides the first evidence of an increase in taurine transporter gene expression in human RPE cells cultured under conditions of elevated levels of NO.

Pharmacological inhibition of diabetic retinopathy: aminoguanidine and aspirin

Effects of aminoguanidine and aspirin on the development of retinopathy have been examined in 5-year studies of diabetic dogs. Either agent was administered daily in doses of 20-25 mg. kg(-1). day(-1). Because severity of hyperglycemia greatly influences development of the retinopathy, special effort was devoted to maintaining comparable glycemia in experimental and control groups. The retinal vasculature was isolated by the trypsin digest method, and retinopathy was assessed by light microscopy. Diabetes for 5 years resulted, as expected, in saccular capillary aneurysms, pericyte ghosts, acellular capillaries, retinal hemorrhages, and other lesions. Administration of aminoguanidine essentially prevented the retinopathy, significantly inhibiting the development of retinal microaneurysms, acellular capillaries, and pericyte ghosts compared with diabetic controls. Aspirin significantly inhibited the development of retinal hemorrhages and acellular capillaries over the 5 years of study, but had less effect on other lesions. Although diabetes resulted in significantly increased levels of advanced glycation end products (AGEs) (namely, pentosidine in tail collagen and aorta, and Hb-AGE), aminoguanidine had no significant influence on these parameters of glycation. Nitration of a retinal protein was significantly increased in diabetes and inhibited by aminoguanidine. The biochemical mechanism by which aminoguanidine has inhibited retinopathy thus is not clear. Aminoguanidine (but not aspirin) inhibited a diabetes-induced defect in ulnar nerve conduction velocity, but neither agent was found to influence kidney structure or albumen excretion.

Retinal glutamate in diabetes and effect of antioxidants

Diabetes results in various biochemical abnormalities in the retina, but which of these abnormalities are critical in the development of retinopathy is not known. The aim of this study is to examine the effect of antioxidant supplementation on diabetes-induced alterations of retinal glutamate, and to explore the inter-relationship between alterations of retinal glutamate, oxidative stress, and nitric oxide (NO) in diabetes. Glutamate was measured in the retina at 2 months of diabetes in rats receiving diets supplemented with or without a mixture of antioxidants containing ascorbic acid, Trolox, DL alpha-tocopherol acetate, N-acetyl cysteine, beta-carotene and selenium. The relationship between glutamate, oxidative stress and NO was evaluated using both bovine retinal endothelial cells and normal rat retina. In diabetes, retinal glutamate was elevated by 40, thiobarbituric acid-reactive substances (TBARS) by 100, and NO by 70%, respectively. Administration of antioxidants inhibited the diabetes-induced increases in glutamate, TBARS and NO. Incubation of bovine retinal endothelial cells or normal rat retina with glutamate significantly increased TBARS and NO, and addition of either antioxidant (N-acetyl cysteine) or a NO synthase inhibitor prevented the glutamate-induced elevation in oxidative stress and NO. Incubation of retina with a glutamate agonist, likewise elevated oxidative stress and NO, and memantine inhibited such elevations. Thus, the alterations of retinal glutamate, oxidative stress and NO appear to be inter-related in diabetes, and antioxidant therapy may be a suitable approach to determine the roles of these abnormalities in the development of diabetic retinopathy.

Tetrahydrobiopterin reverses the impairment of acetylcholine-induced vasodilatation in diabetic ocular microvasculature

The purpose of this study is to test whether tetrahydrobiopterin, an essential cofactor in nitric oxide synthesis, can reverse endothelium dysfunction in diabetic ocular circulation. Using the streptozotocin-induced diabetic rat model and the isolated perfusion eye technique, the response to the acetylcholine (an endothelium-dependent vasodilator mediated by stimulated nitric oxide release) induced vasodilatation of the diabetic ocular vasculature before and after tetrahydrobiopterin administration was compared. Age matched normal rats were used for reference response. Six streptozotocin-induced diabetic rats and eleven control rats at 21.5 +/- 0.2 weeks and 21.2 +/- 2.1 weeks postinduction, respectively, were used. The dose response curve from the diabetic eyes was found to be significantly different from that of the control eyes (p < 0.001) with significantly reduced responses to 10(-4)M acetylcholine. After 30 min of administration of tetrahydrobiopterin to the diabetic eyes, however, the acetylcholine-induced vasodilatation response was significantly (p < 0.001) increased compared with the response prior to tetrahydrobiopterin administration. The vasodilatory response in the diabetic eyes after tetrahydrobiopterin administration was at a level that was comparable with the control response (p = 0.742). We have shown that acute administration of tetrahydrobiopterin is effective in reversing to control level the impaired acetylcholine-induced vasodilatory response at 21.5 +/- 0.2 weeks postinduction. Our result suggests that a decreased level of tetrahydrobiopterin in the eyes of the streptozotocin-induced diabetic rats may be responsible for the ocular vascular endothelium dysfunction.

Effects of aminoguanidine and desferrioxamine on some vascular and biochemical changes associated with streptozotocin-induced hyperglycaemia in rats

The effects of aminoguanidine (AG; 100 mg x kg(-1)) and desferrioxamine (DFO; 50 mg x kg(-1)) on some vascular and biochemical changes associated with streptozotocin (STZ; 65 mg x kg(-1); i.p.)-induced hyperglycaemia were investigated in rats. Both AG and DFO were administered i.p., once daily, for 14 consecutive days to normal and hyperglycaemic animals. The responsiveness of the isolated aortic rings to phenylephrine (PE) was tested. In addition, biochemical markers for oxidative stress such as plasma levels of lipid peroxides and total thiols, as well as the activities of erythrocytic superoxide dismutase (SOD) and whole blood glutathione peroxidase (GSH-Px) were assessed. Results of the present study indicated that induction of hyperglycaemia was associated with increased aortic ring responsiveness to PE, loss in body weight, increase in urine volume, elevation of plasma total thiols and lipid peroxide levels and elevated SOD and GSH-Px enzymatic activities. Treatment of normal rats with AG reduced the response of their aortae to PE. Furthermore, a profound increase in body weight without any significant change in the measured biochemical parameters was observed. In hyperglycaemic animals, AG tended to normalize the enhanced aortic response to PE and modulated STZ-induced biochemical changes without affecting the elevated plasma glucose level. Treatment of normal rats with DFO reduced the response of their aortae to PE and decreased their body weight without altering any of the chosen biochemical parameters. In hyperglycaemic animals, DFO attenuated the responsiveness of their aortae to PE and at the same time, did not affect the loss in body weight and the elevation of plasma glucose level observed in the hyperglycaemic group. Additionally, DFO normalized the elevated plasma level of total thiols and exerted a modulatory influence on the enhanced activities of SOD and GSH-Px as well as on the increased levels of lipid peroxides. Our data lend further credence for the contribution of oxidative stress in the vascular and biochemical changes associated with STZ-induced hyperglycaemia. It is also apparent that advanced glycosylation end products and nitric oxide might be involved. Until clinical studies prove the efficacy and safety of these drugs, specific agents which could scavenge free radicals and block protein glycosylation seem beneficial as a helpful adjunct to the therapy of diabetes.

Nitric oxide synthase activity in retinas from non-insulin-dependent diabetic Goto-Kakizaki rats: correlation with blood-retinal barrier permeability

The aim of this work was to examine whether the non-insulin-dependent diabetic Goto-Kakizaki (GK) rats develop retinal changes with similar characteristics to those observed in insulin-dependent diabetic rats in what concerns blood-retinal barrier (BRB) permeability, nitric oxide (NO) production, and retinal IL-1beta level. BRB permeability was evaluated by vitreous fluorophotometry. NO synthase (NOS) activity was assessed by the production of l-[(3)H]-citrulline and retinal IL-1beta level was determined by ELISA. The expression of the inducible isoform of NOS (iNOS) protein was evaluated by Western blot analysis and immunohistochemistry. The in vivo studies indicated that in GK rats the BRB permeability to fluorescein was increased (787.81 +/- 68 min(-1)) in comparison to that in normal Wistar rats (646.6 +/- 55 min(-1)). The ex vivo studies showed that in retinas from GK rats the NOS activity was higher (207 +/- 28.9 pmol l-[(3)H]-citrulline/mg protein/30 min) than that in normal Wistar rats (125 +/- 32.3 pmol l-[(3)H]-citrulline/mg protein/30 min). These results were correlated with an increase in the protein level of iNOS in the retinas of GK rats, which was confirmed not only by the study of the iNOS protein expression but also by the use of NOS activity inhibitors. Indeed, the data about the effect of specific inhibitors on the NOS activity revealed that in retinas from GK rats the most effective inhibitor was aminoguanidine, which predominantly inhibits the iNOS isoform whereas in retinas from normal Wistar rats it was N(G) nitro l-arginine that predominantly inhibits the constitutive isoforms of NOS. In summary, in retinas from GK rats there is an increased production of NO which may contribute to the BRB breakdown.

Abnormalities of retinal metabolism in diabetes or experimental galactosemia VIII. Prevention by aminoguanidine

PURPOSE

Aminoguanidine has been found to inhibit the development of some retinal lesions in diabetic rats and diabetic dogs, thereby raising a possibility that the formation of glycation end products (AGEs) may be an essential step in the pathogenesis of the retinopathy. The purpose of this study is to investigate the effect of aminoguanidine administration on other metabolic abnormalities which might be involved in the development of retinopathy in two models of the retinopathy, alloxan diabetes and experimental galactosemia.

METHODS

Oxidative stress, nitric oxide (NO) and the activity of protein kinase C (PKC, total activity) were measured in the retina of the rats experimentally diabetic or galactosemic for 2 months. Effect of aminoguanidine administration on the inhibition of hyperglycemia-induced retinal dysmetabolism was investigated.

RESULTS

Two months of diabetes or experimental galactosemia in rats resulted in elevation of retinal oxidative stress (increase in thiobarbituric acid reactive substances, TBARS, and decrease in glutathione, GSH), NO, and PKC activity. Aminoguanidine supplementation (2.5 g aminoguanidine/kg rat diet) significantly inhibited each of these abnormalities in retinas of diabetic rats and galactosemic rats, and did so without lowering the blood hexose levels of these animals.

CONCLUSIONS

The ability of aminoguanidine to normalize the hyperglycemia-induced increases in retinal oxidative stress, NO and PKC in diabetic rats and galactose-fed rats suggests that these abnormalities may be inter-related in the retina, and that the biochemical mechanism by which aminoguanidine inhibits retinal microvascular disease in diabetes may be complex.

Nitric oxide synthase activity and expression in experimental diabetic neuropathy

The changes of nitric oxide synthase (NOS) activity and expression in experimental diabetic neuropathy have not been examined. Increases in ganglia NOS might be similar to those that follow axotomy, whereas declines in endothelial NOS (eNOS) and immunological NOS (iNOS) might explain dysfunction of microvessels or macrophages. In this work, we studied NOS activity in lumbar dorsal root ganglia (DRG) of rats with both short- and long-term experimental streptozotocin-induced diabetes and correlated it with expression of each of the 3 NOS isoforms. NOS enzymatic activity in DRG increased after 12 months of diabetes. This increase, however, was not accompanied by an increase in neuronal NOS immunohistochemistry or mRNA. Immunohistochemical and RT-PCR studies did not identify changes of eNOS expression in 12-month sciatic nerves or DRG from diabetics. Two-month diabetic DRG had increased eNOS mRNA and there was novel eNOS labeling of capsular DRG and perineurial cells. iNOS mRNA levels were lower in diabetics at both time points in peripheral nerves but were unchanged in DRG. Diabetic ganglia showed an increase in NOS activity not explained by novel NOS isoform synthesis. The increases may compensate for NO "quenching" by endproducts of glycosylation. Declines in iNOS may indicate impaired macrophage function.

Elevated vitreous nitric oxide levels in patients with proliferative diabetic retinopathy

PURPOSE

The aim of this study was to determine nitric oxide levels in the vitreous of patients with proliferative diabetic retinopathy.

METHODS

Using the spectrophotometric method based on Griess reaction, we measured levels of nitrite, the stable product of nitric oxide, in the vitreous of 21 eyes of 21 patients who underwent vitrectomy for the treatment of proliferative diabetic retinopathy with tractional retinal detachment, prospectively. Three samples were excluded from the study because of blood contamination. The control group was made up of vitreous samples from 15 eyes of 15 normal cadavers and five eyes of five patients who were undergoing vitrectomy for macular hole surgery.

RESULTS

Nitrite levels were 0. 524 +/- 0.27 microM and 0.383 +/- 0.17 microM in the vitreous of patients with proliferative diabetic retinopathy of diabetes type I and type II, respectively. In 15 cadaver eyes and five vitreous samples from patients who underwent macular hole surgery, nitrite levels were below the detection limit (less than 0.08 microM). There was no significant difference between nitrite levels in patients with type I and type II diabetes (P =.56), whereas there was a significant difference between diabetes groups and controls (P <. 00001).

CONCLUSION

Vitreous nitric oxide levels are elevated in patients with proliferative diabetic retinopathy with tractional retinal detachment. Nitric oxide may play a role in the pathogenesis of proliferative diabetic retinopathy.

Diabetes reduces glutamate oxidation and glutamine synthesis in the retina. The Penn State Retina Research Group

Retinas of diabetic individuals develop early functional changes measurable by electrophysiological and psychometric testing. Using a rat model of diabetes, we previously identified diabetes-induced alterations in metabolism of the neurotransmitter glutamate which may ultimately lead to accumulation of glutamate in the retina (Diabetes, 47: 815, 1998). We therefore investigated the function of enzymes that mediate the synthesis and breakdown of glutamate in retinas from rats made diabetic by injection of streptozotocin. De novo synthesis of nitrogen-containing amino acids including glutamate, glutamine and aspartate was assessed by measuring the rate of carbon fixation in freshly dissected retinas, and was unchanged by diabetes. In contrast, the oxidation of glutamate was significantly reduced in retinas from diabetic rats (62%, P < 0.05). Furthermore, diabetic retinas were less susceptible to inhibition of glutamate oxidation by the transaminase inhibitor aminoxyacetate (80%, N.S.), compared to the significant decrease seen in control rats (61%, P < 0.001). The activity and content of glutamine synthetase were also significantly reduced in retinas from rats diabetic for 2-6 months [range of 48% (P < 0.005) to 83% (P < 0.05) compared to control]. The activity of glutamine synthetase was normalized by acute injections of insulin, but not by reducing blood sugar levels with injections of phlorizin. These results indicate two enzymatic abnormalities in the glutamate metabolism pathway in the retina during diabetes: transamination to alpha-ketoglutarate and amination to glutamine. The reduced flux through these pathways may be associated with the accumulation of glutamate. These results are also consistent with the possibility that some of the glial changes in the retina during diabetes may be caused by hypoinsulinemia rather than hyperglycemia.

Effect of cyclosporin-A on the blood--retinal barrier permeability in streptozotocin-induced diabetes

BACKGROUND

Our previous results showed that in retinas from streptozotocin (STZ)-induced diabetic rats there is an increased level of interleukin-1beta (IL-1beta). This cytokine may be involved in the expression of the inducible isoform of the nitric oxide synthase (iNOS), with consequent synthesis of large amounts of NO and blood-retinal barrier (BRB) breakdown.

AIMS

The aim of this work was to examine whether the administration of cyclosporin-A (Cs-A) to STZ-induced diabetic rats inhibits the synthesis of IL-1beta and the expression of the inducible proteins, iNOS and cyclo-oxygenase-2 (COX-2) in retinal cells, and whether the activity of these proteins contribute to BRB breakdown.

METHODS

The level of IL-1beta was evaluated by ELISA and the NO production by L-[3H]-citrulline formation. Expression of iNOS and COX-2 proteins was determined by two methods, western blot and immunohistochemistry. The permeability of the BRB was assessed by quantification of the vitreous protein.

RESULTS AND DISCUSSION

Our results indicated that the levels of IL-1beta and NO in retinas from Cs-A-treated diabetic rats are significantly reduced, as compared to that in non-treated diabetic rats. The treatment of diabetic rats with Cs-A also significantly inhibited the expression of the inducible proteins, iNOS and COX-2. The evaluation of the vitreous protein content revealed that Cs-A also reduces the BRB permeability. Taken together, these results suggest that the increased production of the inflammatory mediators, IL-1beta and NO, in diabetes may affect the BRB permeability and therefore contribute to the development of diabetic retinopathy.

Increased susceptibility of diabetic rat gastric mucosa to food deprivation during cold stress

BACKGROUND

We investigated the mechanisms responsible for the increased susceptibility of diabetic rat gastric mucosa to damage inflicted by overnight food deprivation (18 h) and its worsening by the cold restraint stress (4 degrees C, 3 h).

METHODS

Gastric damage was measured in fasted animals, some of which were rendered diabetic by a single intraperitoneal injection of streptozotocin (STZ; 70 mg/kg) 5 weeks before experiments (STZ 5W).

RESULTS

STZ 5W rodents showed a number of hemorrhagic lesions in corpus mucosa (26. 8 +/- 5.2 mm(2)) which could be prevented by insulin or nitric oxide synthase (NOS) inhibitors: aminoguanidine or L-NAME (N(omega)-nitro-L-arginine methyl ester). Mucosal injury was further aggravated by low temperature (51.1 +/- 7.8 mm(2)), the damage ameliorated by insulin, aminoguanidine, or L-NAME. The salutary actions of L-NAME were L-arginine sensitive. Low temperature and L-NAME did not significantly influence the gastric secretory parameters in normal rats. On the other hand, L-NAME and aminoguanidine counteracted the attenuation of gastric juice acidity and acid output in STZ 5W rodents. Blood plasma nitrite and nitrate levels and outputs in gastric juice were augmented in STZ 5W animals in comparison to controls. The total activities of NOS including inducible NOS but not constitutive NOS were markedly enhanced by fasting and cold restraint in gastric mucosa of STZ 5W animals (2.2- and 3.7- or 2.4- and 17.9-fold respectively).

CONCLUSIONS

Stressful stimuli, such as food bereavement and cold challenge contribute to the elevated susceptibility of diabetic gastric mucosa to damage, even though the main aggressive factor, i.e., gastric acid secretion, is attenuated. The enhanced production of nitric oxide by inducible NOS during food deprivation and cold exposure seems to play an important role in gastric mucosal integrity disturbances during diabetes.

L-arginine transport in retinas from streptozotocin diabetic rats: correlation with the level of IL-1 beta and NO synthase activity

Several evidences suggest that the pro-inflammatory cytokines IL-1 beta and the radical NO are implicated as effectors molecules in the pancreatic beta-cells dysfunction; an event preceding the pathogenesis of diabetes. IL-1 beta induces the expression of the inducible isoform of NO synthase (iNOS), which use L-arginine as substrate to overproduce NO. However, it is not known whether these events may participate in the development of diabetic retinopathy, which is the main cause of blindness. In this work, we found an increased level of IL-1 beta in retinas from streptozotocin-induced (STZ) diabetic rats. We also observed that the activity of the NO synthase (NOS) and the L-arginine uptake are enhanced in retinas from STZ-induced diabetic rats as compared to retinas from control rats. We found that the uptake of L-arginine in retinas from control and diabetic rats occurs through a transporter resembling the Y + system, i.e. it is saturable, not affected over the pH range 6.5 to 7.4, and is independent of the extracellular Na+. Nevertheless, the L-arginine transport in retinas from diabetic rats occurs through a carrier with lower affinity (K(m) = 25 microM) and higher capacity (Vmax = 295 +/- 22.4 pmol L-arginine/mg protein) than in retinas from control rats (K(m) = 5 microM and Vmax = 158 +/- 12.8 pmol L-arginine/mg protein) which is correlated with the increased NOS activity and consequent depletion of the intracellular pool of L-arginine.

Effects of diabetes on non-adrenergic, non-cholinergic relaxation induced by GABA and electrical stimulation in the rat isolated duodenum

1. The effects of streptozotocin (STZ)-induced experimental diabetes on nitrergic-mediated responses to GABA and electrical field stimulation (EFS) have been evaluated in rat isolated duodenum. 2. In the presence of noradrenergic and cholinergic blockade, EFS (60 V, 1 ms, 0.1-32 Hz) induced frequency dependent relaxations of the preparation. GABA also caused submaximal relaxation of the rat duodenum. The relaxations induced by GABA and EFS were reduced in duodenal tissues from diabetic rats compared with control rats. 3. Neither ATP- nor sodium nitroprusside-induced relaxations were altered in diabetic duodenal tissues. GABA- and EFS-induced relaxations were inhibited by NG-nitro-L-arginine methyl ester (L-NAME; 300 mmol/L) in both diabetic and control rats. Although the inhibition caused by L-NAME of GABA- and EFS-induced relaxation was partially reversed by L-arginine (1 mmol/L), L-arginine alone had no effect on GABA- and EFS-induced relaxation in diabetic rats. 4. These results suggest that STZ-induced diabetes impairs non-adrenergic, non-cholinergic relaxation induced by EFS and GABA. Impairment of nitrergic innervation of the rat duodenum may contribute to the abnormalities of intestinal motility abnormalities associated with diabetes.

Functional role of nitric oxide in regulation of ocular blood flow

Nitric oxide generated by three distinct enzyme systems appears to play a critical role in many diverse physiological processes. Using both conventional and immunohistochemical techniques, nitric oxide synthases have been identified throughout the body, including all regions of the eye. A large number of in vitro and in vivo preparations have been utilized showing nitric oxide to have an important role in regulation of regional ocular blood flow. Nitric oxide-mediated control of basal ocular blood flow is demonstrated by vasoconstriction seen in experiments where vascular endothelial cells are removed, or when nitric oxide synthase is inhibited. The endogenous source of nitric oxide in the eye appears to be both endothelial and neural. In addition, administration of drugs that can 'donate' nitric oxide produces vasodilation of the eye vasculature. Local vasodilation in response to illumination of the retina is controlled by generation and release of nitric oxide, whereas most other physiological adjustments of ocular blood flow (i.e., autoregulation and responses to altered blood gas levels) seem to be relatively independent of nitric oxide mechanisms. Nitric oxide is implicated in a variety of ocular pathophysiological states including uveitis, retinal ischemic disease, diabetes and glaucoma.

A review of current research on the effect of diabetes mellitus on the eye

It is estimated that almost one million Australians will have diabetes by the year 2000. Of those with diabetes a significant proportion will have eye-related conditions, the most debilitating being diabetic retinopathy. Appropriate identification and treatment can result in prevention of visual loss and blindness. The importance of diabetes as a cause of blindness in our community is realised by the commencement of a national program by the National Health and Medical Research Council to develop clinical practice guidelines for the management of diabetic retinopathy. The development of these guidelines was based on available evidence following an extensive review of the literature up to May 1996. This review is a summary of our advances in research on the effect of diabetes on various aspects of the eye and vision over the past two years. This review is a compilation of articles of research on the effect of diabetes on various aspects of the eye and vision. As a result of the enormous amount of effort and work by scientists and clinicians around the world, as well as space restrictions, the review covers the past two years only. Although every effort has been made to include as many research articles as possible, not all articles of research are covered. It is intended that this review provide an overview of the latest trends in research, particularly relating to new techniques and methods in the study of diabetes in ocular tissue as well as the new theories in the development of ocular damage to each of the tissue.