Ascorbate and glutathione levels in the developing normal and dystrophic rat retina: effect of intense light exposure

Is There an Optimal Combination of AREDS2 Antioxidants Zeaxanthin, Vitamin E and Vitamin C on Light-Induced Toxicity of Vitamin A Aldehyde to the Retina?

Vitamins C and E and zeaxanthin are components of a supplement tested in a large clinical trial-Age-Related Eye Disease Study 2 (AREDS2)-and it has been demonstrated that they can inhibit the progression of age-related macular degeneration. The aim of this study was to determine the optimal combinations of these antioxidants to prevent the phototoxicity mediated by vitamin A aldehyde (ATR), which can accumulate in photoreceptor outer segments (POS) upon exposure to light. We used cultured retinal pigment epithelial cells ARPE-19 and liposomes containing unsaturated lipids and ATR as a model of POS. Cells and/or liposomes were enriched with lipophilic antioxidants, whereas ascorbate was added just before the exposure to light. Supplementing the cells and/or liposomes with single lipophilic antioxidants had only a minor effect on phototoxicity, but the protection substantially increased in the presence of both ways of supplementation. Combinations of zeaxanthin with α-tocopherol in liposomes and cells provided substantial protection, enhancing cell viability from ~26% in the absence of antioxidants to ~63% in the presence of 4 µM zeaxanthin and 80 µM α-tocopherol, and this protective effect was further increased to ~69% in the presence of 0.5 mM ascorbate. The protective effect of ascorbate disappeared at a concentration of 1 mM, whereas 2 mM of ascorbate exacerbated the phototoxicity. Zeaxanthin or α-tocopherol partly ameliorated the cytotoxic effects. Altogether, our results suggest that the optimal combination includes upper levels of zeaxanthin and α-tocopherol achievable by diet and/or supplementations, whereas ascorbate needs to be at a four-fold smaller concentration than that in the vitreous. The physiological relevance of the results is discussed.

Iatrogenic Damage of Eye Tissues: Current Problems and Possible Solutions

Visual system is at high risk of iatrogenic damage. Laser ocular surgery, the use of powerful illumination devices in diagnostics and surgical treatment of eye diseases, as well as long surgeries under general anesthesia provoke the development of chronic degenerative changes in eye tissues, primarily in the cornea and the retina. Despite the existence of approaches for prevention and treatment of these complications, the efficacy of these approaches is often limited. Here, we review the mechanisms of iatrogenic damage to eye tissues at the cellular and biochemical levels. It is well recognized that oxidative stress is one of the main factors hindering regeneration of eye tissues after injuries and, thereby, aggravating iatrogenic eye disorders. It is accompanied by the downregulation of low-molecular-weight antioxidants and antioxidant enzymes, as well as changes in the expression and redox status of proteins in the damaged tissue. In this regard, antioxidant therapy, in particular, the use of highly effective mitochondria-targeted antioxidants such as SkQ1, is considered as a promising approach to the prevention of iatrogenesis. Recent findings indicate that the most efficient protection of eye tissues from the iatrogenic injury is achieved by preventive use of these antioxidants. In addition to preventing corneal and retinal cell death induced by oxidative stress, SkQ1 contributes to the restoration of innate antioxidant defense of these tissues and suppresses local inflammatory response. Since the timing of routine medical manipulations is usually known in advance, iatrogenic damage to the ocular tissues can be successfully prevented using mitochondria-targeted therapy.

Redox modulation of homomeric rho1 GABA receptors

The activity of many receptors and ion channels in the nervous system can be regulated by redox-dependent mechanisms. Native and recombinant GABA(A) receptors are modulated by endogenous and pharmacological redox agents. However, the sensitivity of GABA(C) receptors to redox modulation has not been demonstrated. We studied the actions of different reducing and oxidizing agents on human homomeric GABArho(1) receptors expressed in Xenopus laevis oocytes. The reducing agents dithiothreitol (2 mM) and N-acetyl-L-cysteine (1 mM) potentiated GABA-evoked Cl(-) currents recorded by two-electrode voltage-clamp, while the oxidants 5-5'-dithiobis-2-nitrobenzoic acid (500 microM) and oxidized dithiothreitol (2 mM) caused inhibition. The endogenous antioxidant glutathione (5 mM) also enhanced GABArho(1) receptor-mediated currents while its oxidized form GSSG (3 mM) had inhibitory effects. All the effects were rapid and easily reversible. Redox modulation of GABArho(1) receptors was strongly dependent on the GABA concentration; dose-response curves for GABA were shifted to the left in the presence of reducing agents, whereas oxidizing agents produced the opposite effect, without changes in the maximal response to GABA and in the Hill coefficient. Our results demonstrate that, similarly to GABA(A) receptors and other members of the cys-loop receptor superfamily, GABA(C) receptors are subjected to redox modulation.

Vitamin C metabolomic mapping in experimental diabetes with 6-deoxy-6-fluoro-ascorbic acid and high resolution 19F-nuclear magnetic resonance spectroscopy

Metabolomic mapping is an emerging discipline geared at providing information on a large number of metabolites as a complement to genomics and proteomics. Here we have probed ascorbic acid homeostasis and degradation in diabetes using 6-deoxy-6-fluoro ascorbic acid (F-ASA) and 750 MHz (19)F-nuclear magnetic resonance (NMR) spectroscopy with proton decoupling In vitro, Cu(2+)-mediated degradation of F-ASA revealed the formation of 4 major stable degradation products at 24 hours. However, when normal or diabetics rats were injected with F-ASA intraperitoneally (IP) for 4 days, up to 20 fluorine-labeled compounds were observed in the urine. Their composition resembled, in part, metal catalyzed degradation of F-ASA and was not explained by spontaneous degradation in the urine. Diabetes led to a dramatic increase in urinary F-ASA loss and a relative decrease in most other urinary F-compounds. Diabetes tilted F-ASA homeostasis toward oxidation in liver (P <.01), kidney (P <.01), spleen (P <.01), and plasma (P <.01), but tended to decrease oxidation in brain, adrenal glands, and heart. Surprisingly, however, besides the major oxidation product fluoro-dehydroascorbic acid (F-DHA), no F-ASA advanced catabolites were detected in tissues at 5 micromol/L sensitivity. These findings not only confirm the key role of the kidney in diabetes-mediated loss of ascorbic acid, but demonstrate that only selected tissues are prone to increased oxidation in diabetes. While the structure of most degradation products needs to be established, the method illustrates the power of high resolution (19)F-NMR spectroscopy for the mapping of complex metabolomic pathways in disease states.

How does glucose generate oxidative stress in peripheral nerve?

Diabetes-associated oxidative stress is clearly manifest in peripheral nerve, dorsal root, and sympathetic ganglia of the peripheral nervous system and endothelial cells and is implicated in nerve blood flow and conduction deficits, impaired neurotrophic support, changes in signal transduction and metabolism, and morphological abnormalities characteristic of peripheral diabetic neuropathy (diabetic peripheral neuropathy). Hyperglycemia has a key role in oxidative stress in diabetic nerve, whereas the contribution of other factors, such as endoneurial hypoxia, transition metal imbalance, and hyperlipidemia, has not been rigorously proven. It has been suggested that oxidative stress, particularly mitochondrial superoxide production, is responsible for sorbitol pathway hyperactivity, nonenzymatic glycation/glycooxidation, and activation of protein kinase C. However, this concept is not supported by in vivo studies demonstrating the lack of any inhibition of the sorbitol pathway activity in peripheral nerve, retina, and lens by antioxidants, including potent superoxide scavengers. Its has been also hypothesized that aldose reductase (AR) detoxifies lipid peroxidation products, and therefore, the enzyme inhibition in diabetes is detrimental rather than benefical. However, the role for AR in lipid peroxdation product metabolism has never been demonstrated in vivo, and the effects of aldose reductase inhibitors and antioxidants on diabetic peripheral neuropathy are unidirectional, i.e., both classes of agents prevent and correct functional, metabolic, neurotrophic, and morphological changes in diabetic nerve. Growing evidence indicates that AR has a key role in oxidative stress in the peripheral nerve and contributes to superoxide production by the vascular endothelium. The potential mechanisms of this phenonmenon are discussed.

Isomerization of 11-cis-retinoids to all-trans-retinoids in vitro and in vivo

The regeneration of 11-cis-retinal, the universal chromophore of the vertebrate retina, is a complex process involving photoreceptors and adjacent retinal pigment epithelial cells (RPE). 11-cis-Retinal is coupled to opsins in both rod and cone photoreceptor cells and is photoisomerized to all-trans-retinal by light. Here, we show that RPE microsomes can catalyze the reverse isomerization of 11-cis-retinol to all-trans-retinol (and 13-cis-retinol), and membrane exposure to UV light further enhances the rate of this reaction. This conversion is inhibited when 11-cis-retinol is in a complex with cellular retinaldehyde-binding protein (CRALBP), providing a clear demonstration of the protective effect of retinoid-binding proteins in retinoid processes in the eye, a function that has been long suspected but never proven. The reverse isomerization is nonenzymatic and specific to alcohol forms of retinoids, and it displays stereospecific preference for 11-cis-retinol and 13-cis-retinol but is much less efficient for 9-cis-retinol. The mechanism of reverse isomerization was investigated using stable isotope-labeled retinoids and radioactive tracers to show that this reaction occurs with the retention of configuration of the C-15 carbon of retinol through a mechanism that does not eliminate the hydroxyl group, in contrast to the enzymatic all-trans-retinol to 11-cis-retinol reaction. The activation energy for the conversion of 11-cis-retinol to all-trans-retinol is 19.5 kcal/mol, and 20.1 kcal/mol for isomerization of 13-cis-retinol to all-trans-retinol. We also demonstrate that the reverse isomerization occurs in vivo using exogenous 11-cis-retinol injected into the intravitreal space of wild type and Rpe65-/- mice, which have defective forward isomerization. This study demonstrates an uncharacterized activity of RPE microsomes that could be important in the normal flow of retinoids in the eye in vivo during dark adaptation.

Microdialysis measurement of ascorbic acid in rabbit vitreous after photodynamic reaction

A method for long-term intravitreous microdialysis was used to measure endogenous reduced ascorbic acid in the vitreous of rabbits by HPLC-ECD before and after exposure to intense visible light in the presence of fluorescein. Cellulose microdialysis probes were implanted into the vitreous humor of each eye and after stabilization ascorbic acid measurements were recorded over a 14 day period. Under this experimental condition, normal ascorbic acid concentrations in vitreous varied from 98.0 +/- 9.8 to 106.9 +/- 20.3 microM(mean +/- S.D.). The eyes received light irradiation (25 000 lux) for 2 hr and fluorescein was used as the photosensitizer once or twice. No immediate effects on ascorbic acid concentrations could be observed in the eyes irradiated twice without fluorescein i.v. injections and in the twice fluorescein injected without irradiation. However, in the eyes irradiated once with fluorescein (30 mg kg(-1)), ascorbic acid concentration after irradiation significantly decreased from day 2 and continued over a period of 10 days compared with that before irradiation and maximal reduction was 32.6% (P < 0.005) on day 6 after irradiation. By day 13, the ascorbic acid concentration returned to control levels (P > 0.01). In the eyes irradiated twice with fluorescein injections, ascorbic acid concentration after irradiation decreased even more over the experimental period and the maximal reduction was 65.5% (P < 0.005) on day 5 after irradiation and did not recover over the next 9 days. In the eyes irradiated twice with fluorescein injections plus administration of ascorbic acid (150 mg kg(-1)) 30 min before irradiation, a significant increase (52.5%) of ascorbic acid (P < 0.005) was found on day 1 and control levels of ascorbic acid were maintained from day 2 onward. The protective role of ascorbic acid in the vitreous humor against photodynamic reaction is suggested.

Antioxidant activity of retinol, glutathione, and taurine in bovine photoreceptor cell membranes

The antioxidant activities of compounds endogenous to mammalian rod outer segments (ROS) were investigated in vitro by measuring the oxidative loss of polyunsaturated fatty acids (PUFA's) from the membranes of intact ROS and from liposomes made from ROS phospholipids (PL) to which lipid soluble compounds had been added. The membranes were exposed to the water-soluble oxidant 2, 2'-azobis(2-amidinopropane) dihydrochloride (AAPH). Retinol protected PUFA's in ROS liposome PL's, whereas retinaldehyde promoted lipid peroxidation. When isolated ROS were stimulated to produce endogenous retinol, PUFA loss was inhibited by up to 17%. These findings suggest an antioxidant function for the enzymatic reduction of retinaldehyde to retinol during the visual cycle. Water-soluble antioxidants, taurine and reduced glutathione (GSH), were investigated individually and in combination with retinol in ROS PL liposomes. GSH protected PUFA's in ROS PL liposomes. Taurine alone showed little antioxidant activity, but in combination with retinol it protected lipids twice as much as retinol alone. These results support previous findings that taurine protects ROS lipids during exposure to cyclic light.

Changes in manganese superoxide dismutase expression after exposure of the retina to intense light

Manganese superoxide dismutase (Mn-SOD) is a naturally-occurring scavenger of superoxide, one of several reactive oxygen intermediates. To determine if Mn-SOD expression is enhanced as a defensive mechanism against oxidative challenges, such as intense light exposure, rats were exposed to cyclic light (80 lux) for 2 weeks, intense light (1,800 lux) for 24 h, and then again to cyclic light. Experimental and control (exposed to cyclic light only) eyes were enucleated 3 h, 1, 3, 7, and 14 days after light challenge. Protein expression was examined immunohistochemically using rabbit antisera against rat Mn-SOD. There was no significant difference between the light-exposed and the control groups in the thickness of the outer nuclear layers. Both retinal pigment epithelial cells and photoreceptor inner segments in the normal retina were labeled for Mn-SOD. Mn-SOD labeling was lost 3 h and day 1 after light challenge. It was re-expressed in the retinal pigment epithelial cells 3, 7, and 14 days after the light challenge, and in the photoreceptor inner segments after day 14. These results suggest that the retina might have a protective potential against light damage, in which Mn-SOD may play an important role.

Antioxidants and age-related eye disease. Current and future perspectives

Oxidative mechanisms may play an important role in the pathogenesis of age-related eye disease, in particular cataract and macular degeneration, the two most important causes of visual impairment in older adults. For this reason, there is considerable interest in determining whether vitamins and trace minerals with antioxidant properties can be of benefit in preventing the onset or progression of disabling eye disease. Basic research studies have shown that antioxidants can protect against the cumulative effects of oxidative stress in animal models of cataract and macular degeneration. Data from observational epidemiological studies in humans, however, are inconclusive. While results from several studies, primarily cross-sectional and case-control, are compatible with a possible protective role for micronutrients in disease development, data for specific nutrients or specific disease types have often been inconsistent. Further, these observational studies are limited because of the inherent imprecision of dietary exposure data and the likely effects of uncontrolled confounding. Thus, reliable data regarding a potentially important benefit of vitamin supplementation in eye disease will emerge mainly from well-designed, large-scale, randomized trials.

Endogenous ascorbate regenerates vitamin E in the retina directly and in combination with exogenous dihydrolipoic acid

Vitamin E (alpha-tocopherol) is the major lipid-soluble antioxidant of retinal membranes whose deficiency causes retinal degeneration. Its antioxidant function is realized via scavenging peroxyl radicals as a result of which phenoxyl radicals of alpha-tocopherol are formed. Our hypothesis is that alpha-tocopherol phenoxyl radicals can be reduced by endogenous reductants in the retina, providing for alpha-tocopherol recycling. The results of this study demonstrate for the first time that: (i) endogenous ascorbate (vitamin C) in retinal homogenates and in rod outer segments is able to protect endogenous alpha-tocopherol against oxidation induced by UV-irradiation by reducing the phenoxyl radical of alpha-tocopherol, (ii) in the absence of ascorbate, neither endogenous nor exogenously added glutathione (GSH) is efficient in protecting alpha-tocopherol against oxidation; (iii) GSH does not substantially enhance the protective effect of ascorbate against alpha-tocopherol oxidation; (iv) exogenous dihydrolipoic acid (DHLA), although inefficient in direct reduction of the alpha-tocopherol phenoxyl radical, is able to enhance the protective effect of ascorbate by regenerating it from dehydroascorbate. Thus, regeneration of alpha-tocopherol from its phenoxyl radical can enhance its antioxidant effectiveness in the retina. The recycling of alpha-tocopherol opens new avenues for pharmacological approaches to enhance antioxidants of the retina.

Induction of heme oxygenase 1 in the retina by intense visible light: suppression by the antioxidant dimethylthiourea

The effect of intense visible light (light damage) on the expression of heme oxygenase 1 (HO-1), a protein induced by oxidative stress, was investigated in the rat retina. A sensitive reverse transcription-PCR assay demonstrated the expression of mRNA for HO-1 as well as HO-2, the noninducible HO form, in the normal retina. As analyzed by Northern blotting, however, HO-1 mRNA was barely detectable under normal circumstances. After exposure to intense visible light, retinas had markedly higher HO-1 mRNA levels than unexposed controls, with increases up to 52- and 98-fold at 12 and 24 hr of exposure, respectively. Intense light exposure also resulted in an increase in HO-1 protein. In contrast, no appreciable change in HO-2 mRNA or protein was observed. The increase in HO-1 message was more pronounced in rats previously reared in the dark than in those reared in a weak cyclic-light environment. A marked decrease from the high level of HO-1 mRNA induced by light insult was observed when the animals were allowed to recover in the dark for 24 hr after light exposure. Most important, treatment of animals with 1,3-dimethylthiourea, a synthetic antioxidant, prior to light exposure effectively blocked the increase in HO-1 mRNA. Thus, HO-1 is a sensitive marker for assessing light-induced insult in the retina. Since increased expression of HO-1 is thought to be a cellular defense against oxidative damage, its expression may play an important role in protecting the retina against light damage.

Ultrastructural findings in solar retinopathy

This study documents the ultrastructural findings in a case of solar retinopathy, 6 days after sungazing. A malignant melanoma of the choroid was diagnosed in a 65-year-old man. On fundoscopy, the macula was normal. The patient agreed to stare at the sun prior to enucleation. A typical solar retinopathy developed, characterised by a small, reddish, sharply circumscribed depression in the foveal area. Structural examination of the fovea and parafovea revealed a spectrum of cone and rod outer segment changes including vesiculation and fragmentation of the photoreceptor lamellae and the presence of discrete 100-120 nm whorls within the disc membranes. Many photoreceptor cells, particularly the parafoveal rods, also demonstrated mitochondrial swelling and nuclear pyknosis. Scattered retinal pigment epithelial cells in the fovea and parafovea showed a degeneration characterised by loss of plasma membrane specialisations, swelling of the smooth endoplasmic reticulum and changes in the fine structure of the lipofuscin granules. The good visual prognosis in solar retinopathy was attributed to the resistance of the foveal cone cells to photochemical damage.

The effects of L-and D-ascorbic acid administration on retinal tissue levels and light damage in rats

To assess the protective effect of ascorbic acid in retinal light damage of rats, we have determined the uptake and retinal tissue distributions of its L- and D- stereoisomers following interperitoneal or intraocular injections. The effects of intense-intermittent light exposure and darkness on tissue ascorbate were compared by measuring its levels in retina and retinal pigment epithelial tissues at various times after administration. The protective effects of the two forms of ascorbate against retinal light damage were also compared by measuring rhodopsin levels 2 weeks after intense light exposure. After interperitoneal injection, both forms of ascorbic acid were higher in the retinal pigment epithelial-choroid-scleral complex (eye cup) than in the retina. Over a 2 hr post-injection period, L-ascorbate in the eye cup was 2 to 4 fold higher than normal (10-11 nmol); D-ascorbate levels were between 15 and 30 nmol. During the same period retinal L-ascorbate was just above normal (12-14 nmol), whereas less than 5 nmol of D-ascorbate was present. When ascorbate was given by the intraocular route the opposite effect was found. During the 2 hr post-injection period retinal L-ascorbate levels were 2 to 5 fold higher than normal; D-ascorbate was between 25 and 50 nmol/retina. Within 1 hr post-injection, L-ascorbate in the eye cup was near normal and D-ascorbate levels were 10 nmol or less. In uninjected rats perfused with normal saline, the endogenous L-ascorbate was distributed 55% in the retina with 9% and 36%, respectively, in the RPE-choroid and sclera. Ten-thirty min after interperitoneal peritoneal injection about 40% of the L-ascorbate was present in the retina with 17% and 44% in the RPE-choroid and sclera. Total ascorbate (L + D) levels in the same tissues of D- injected rats were similar to those found for rats given L-ascorbate. Following 7 hrs of darkness, tissue ascorbate levels in the injected rats decreased to approximately the same levels present in uninjected animals. For rats exposed to intense light average retinal ascorbate levels decreased further, while RPE-choroid and scleral levels were largely unchanged from the dark control levels. About 50% of the tissue ascorbate was present in the retina 10-30 min after intraocular injection. The RPE-choroid contained between 10 and 14% of the ascorbate, with 35-40% present in the sclera. Retinal ascorbate levels remained high in the injected eyes following 2.5 hrs of darkness, but decreased as a result of intense light treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties and function of the ocular melanin--a photobiophysical view

This paper reviews the biosynthesis and physicochemical properties of the ocular melanin. Age-related changes of melanin granules and the corresponding formation of lipofuscin pigments in the retinal pigment epithelium (RPE) are also described. Adverse photoreactions of the eye and, in particular, light-induced damage to the RPE-retina are reviewed in relation to the ocular pigmentation. A hypothesis on the photoprotective role of the RPE melanin is presented that is based on the ability of the cellular melanin to bind redoxactive metal ions. Since bound-to-melanin metal ions are expected to be less damaging to the pigment cells, it is proposed that sequestration of heavy metal ions by the RPE melanin is an efficient detoxifying mechanism. It is postulated that oxidative degradation of RPE melanin may lower its metal-binding capability and decrease its anti-oxidant efficiency. Cellular and environmental factors that may contribute to possible oxidative damage of the RPE melanin are discussed in connection with the etiology of age-related macular degeneration.

Effect of species differences and dietary vitamin C on the concentration of ascorbate- and acid-soluble thiol in fish eye

Data presented confirm the essentiality of modification of the dinitrophenylhydrazine (DNPH) method to analyze the total ascorbic acid and dehydroascorbic acid in ocular tissues and stress the need of corrections for the interfering substances. Variations in ascorbate and thiol concentrations in the lens, retina and aqueous humour of freshwater fish belonging to the Cyprinidae family were examined. The interspecific variability of ascorbate concentration was highest in the aqueous humour and lowest in the retina. The high ascorbate concentration in the retina seems to reflect the importance of the sense of vision in fish life-style as compared to chemo- and acoustico lateralis senses. The regional distribution of the total ascorbate is in the order of decreasing concentrations: retina, lens and aqueous humour. However, the retinal ascorbate is almost exclusively in the oxidized form, and the lenticular ascorbate is almost exclusively in the reduced form. Thiol concentration in the lens is five- to tenfold that in the retina and aqueous humour. This explains the oxidation status of ascorbate in different eye compartments of the eye. After 30 days on diets containing various levels of ascorbic acid or ascorbic acid sulphate, the ascorbate concentration in the eye compartments of common carp (Cyprinus carpio L.) was determined. Ocular tissue can be used to monitor the development of the ascorbate status in fish, and the retina is the most responsive tissue to the enhanced or depleted ascorbate levels.

Operating microscope-induced retinal phototoxicity: pathophysiology, clinical manifestations and prevention

Retinal light damage is a poorly understood phenomenon, due to its multifactorial etiology and relatively infrequent recognition. It has been increasingly identified following ocular surgery involving the intense light of the operating microscope. The authors describe the clinical entity, review salient features of its pathophysiology and provide guidelines for prevention of retinal phototoxicity.

Induction of stress proteins in cultured human RPE-derived cells

The expression and induction of stress protein families were examined in cultured human fetal retinal pigment epithelial (RPE)-derived cells. These stress proteins (SPs) include the heat-shock proteins (HSPs) that have been shown to be highly inducible following treatment by heat, amino acid analogues, and various chemical oxidants. Three sets of proteins with molecular weights of 70, 84, and 110 kilodaltons were elevated simultaneously from constitutive levels after treatment with azetidine-2-carboxylic acid (AzC), an amino acid analogue of proline. Further experiments demonstrated that incubation of cultured human fetal RPE-derived cells with hydrogen peroxide (H2O2) at concentrations ranging from 10(-5) M to 10(-3) M for 30 minutes to 60 minutes did not elevate the levels of the common families of HSPs as with AzC. These results indicate that cultured human fetal RPE-derived cells are capable of elevated HSP biosynthesis after AzC exposure but appear resistant to H2O2 treatment.

Solar radiation and age-related macular degeneration

Age-related macular degeneration (AMD) involves a progressive impairment of the outer layers in the center of the retina. Experimental studies have demonstrated that bright light preferentially damages precisely the region that degenerates in AMD. The evidence that solar radiation is responsible for some of the deteriorative changes that lead to AMD is examined in this review. In the primate eye, the high-energy portion of the solar spectrum is most hazardous to retinal molecules, with damaging effects increasing as photon energy rises. This action spectrum is explicable by the quantum laws which describe the interaction of radiation with matter. High-energy visible and ultraviolet photons can produce molecular damage by a photochemical mechanism. The lesion is exacerbated by oxygen, which initiates free-radical chain reactions (photodynamic effects). Melanin exerts a protective effect against damage from sunlight. In the human retina, documented lesions from solar radiation range from the acute effects of sun-gazing to injuries resulting from prolonged periods of exposure in brightly illuminated environments. The damage occurs in the same region that degenerates in AMD. A cataractous lens and ocular melanin both protect the retina against AMD, as predicted by the radiation hypothesis. Identification of an environmental factor that evidently plays a role in the etiology of AMD provides the basis for a program of preventive medicine.

Effects of the antioxidant butylated hydroxytoluene (BHT) on retinal degeneration induced transplacentally by a single low dosage of N-methyl-N-nitrosourea (MNU)

A 1 mg/kg dose of the DNA alkylating agent, N-methyl-N-nitrosourea (MNU), when administered on day 16 of gestation provokes a progressive retinal degeneration in CD-1 albino mice reared under standard fluorescent lighting conditions (12 hr light: 12 hr dark); this degeneration begins at about 4 weeks post-natally and worsens with age. It is accelerated by constant fluorescent light exposure but is retarded greatly by constant darkness, suggesting the importance of secondary insults in the post-natal period for development of the degenerative disease. To determine whether the secondary photochemical damage might be specifically blocked, MNU-exposed and control animals in the present study were fed an antioxidant-enriched diet of Purina mouse chow supplemented with 0.75% butylated hydroxytoluene (BHT). A second group of MNU-exposed and control animals were fed a non-BHT supplemented standard Purina mouse chow diet. Systematic measurements of the number of rows of photoreceptor cell nuclei, the thickness of the inner/outer segment layer, and the thickness of the whole retina were made, to quantify and degenerative changes in animals 2, 4, 6, and 8 weeks of age. By 8 weeks, retinas of BHT-fed, MNU-exposed animals were significantly thicker and had more rows of photoreceptor cell nuclei than regular-diet, MNU-exposed animals. Moreover, the retinas of BHT-fed animals, both for MNU-exposed and controls, demonstrated sporadic morphologic changes in the form of circular configurations composed of ganglion cells, arcades of nuclear and plexiform layers, and, in one control animal, a hyperplastic nodule. These experiments suggested that MNU-induced retinal degeneration may be retarded by a BHT-enriched diet; however, continuous high doses of this compound pre- and postnatally may induce other retinal abnormalities.

Strain differences in sensitivity to light-induced photoreceptor degeneration in albino mice

The sensitivity to light-induced photoreceptor degeneration was examined in 7 different inbred strains of albino mice. The mice were exposed to 3 weeks of constant fluorescent light at an illuminance level of 115-130 ft-c (ca. 1,265-1,430 lux), after which the eyes were examined histologically. The degree of light-induced photoreceptor degeneration was compared to that found in BALB/cByJ (BALB/c) albino mice, which have previously been described as sensitive to the damaging effects of light, and to C57BL/6J-c2J albino mice, which have been shown to be resistant to light-induced damage. Mice of the A/J, AKR/J and NZW/LacJ strains were indistinguishable from BALB/c mice in light sensitivity, as measured by mean outer nuclear layer thickness and the presence or absence of outer segment membranes. Mice of the Ma/MyJ and RF/J strains were somewhat more sensitive to light than BALB/c mice, and those of the RIIIs/J were far more sensitive than all of the other strains. The LG/J strain differed from other strains by individual mice displaying one of two degrees of light sensitivity, those similar to the light-sensitive Ma/MyJ and RF strains and those remarkably more resistant to light, with photoreceptor outer segment integrity even greater than that of the light-resistant C57BL/6J-c2J strain. These findings demonstrate that different inbred strains of a given species may exhibit a wide range of sensitivities to constant light exposure and that most albino mouse strains examined thus far are highly sensitive to the damaging effects of light.

Perinuclear lens retrodots: a role for ascorbate in cataractogenesis

Lens retrodots are round, oblong, or oval features in the perinuclear zone of the adult lens after the fifth decade of life and associated with cataract. Retrodots were found in 47 out of 121 eyes with cataract (39%) in the present series. They show birefringence in vivo and in vitro, and chemical studies suggest that they contain calcium oxalate. It is proposed that ascorbic acid, which is abundant in the normal human lens, is the most likely source for this oxalate. Ascorbic acid is thought to have a protective role against oxidative stress in the lens and other parts of the eye, and its level is known to be reduced in senile cataract. The presence of the retrodots may identify lenses which have been exposed to oxidative stress and are less capable of resisting oxidative damage.

Distribution of ascorbate in the retina, subretinal fluid and pigment epithelium

The posterior segment of the eye was divided into four compartments: retinal cytosol (R), subretinal fluid on the retinal surface (S/R), retinal pigment epithelial (RPE) cytosol, and subretinal fluid on the RPE surface (S/RPE). The volume of each compartment was estimated from the dilution of creatinine (in the extraction buffer) by the endogenous tissue fluid. The ascorbate concentrations in R, S/R, S/RPE, and RPE were 20.6, 12.3, 3.7, and 5.8 mg/dl respectively. Dehydroascorbate was observed only in the RPE and S/RPE. The decreasing ascorbate concentration from the retina to RPE, and the distribution of dehydroascorbate suggest a movement of ascorbate from the vitreous cavity into the subretinal space. The permeability of retinal cell layers to ascorbate was confirmed by the high radioactivity observed in the subretinal space after an intravitreal injection of C14-ascorbate. The occurrence of dehydroascorbate in the RPE and the S/RPE indicates the presence of oxidative reaction of ascorbate in these compartments, where light induced free radicals are located.

Ascorbate transport in cultured cat retinal pigment epithelial cells

Transport of ascorbate by primary cultures of cat retinal pigment epithelial cells (RPE) was studied. Confluent primary cultures were incubated with 10-500 microM L-[carboxyl-14C] ascorbic acid in balanced salt solution (BSS) at 37 degrees C for 1 to 40 min. The uptake of radioactive ascorbate followed saturation kinetics with a Km of 42 microM and Vmax of 117 pmol min-1 microgram-1 DNA. Cells incubated with 10 microM radioactive ascorbate for 40 min showed a ratio of intracellular to extracellular radioactive ascorbate of greater than 40. The transport of ascorbate was sodium- and energy-dependent. Replacement of 150 mM NaCl in BSS with 150 mM LiCl reduced ascorbate uptake significantly. Ouabain, 2,4-dinitrophenol, alpha-D-glucose, 3-O-methyl-D-glucose, and the ascorbate analogues, D-isoascorbate and dehydroascorbate, each inhibited ascorbate uptake into RPE cells. The efflux of radioactivity into the incubation media was slow when cells were preloaded with either 50- or 500 microM radioactive ascorbate, but increased when cells preloaded with 50 microM ascorbate were incubated in the presence of excess non-radioactive ascorbate. These studies demonstrated that a sodium-dependent carrier system is involved in transport of ascorbate in primary cultures of cat RPE.

Ascorbate regeneration in bovine ocular tissues by NADH-dependent semidehydroascorbate reductase

Despite its fast autoxidation in vitro, ascorbate remains in its reduced form in vivo, indicating a special mechanism may be involved in its regeneration. The presence of an NADH-dependent reductase system, semidehydroascorbate reductase (SDR), for regeneration of ascorbate from its partially oxidized form, semidehydroascorbate (SDA), was demonstrated in bovine ocular tissues after extraction in Triton X-100. Highest SDR activity was detected in retinal extracts in the order of retina greater than pigment epithelium-choroid = ciliary body greater than iris. Minimal or no activity was observed in lens extracts or in aqueous fluid. Freezing and thawing, or boiling, destroyed the NADH-dependent SDR activity. NADH oxidation was significantly reduced (22% of total activity) when assays with retinal extracts were performed at 5 degrees C. Treatment with 4 mM, N-ethylmaleimide reduced the rate of NADH oxidation to 73 or 42% compared with control values with retinal or ciliary body extracts, respectively.

Rod outer segment lipids in vitamin A-adequate and -deficient rats

Weanling albino rats were fed a vitamin-A-adequate diet or vitamin-A-deficient diet and maintained in a cyclic light or dark environment for up to 14 weeks. One half of the rats were supplemented with additional dietary linolenic acid in the form of linseed oil. The lipid composition and rhodopsin-opsin contents of isolated rod outer segments were determined after 6-7 weeks or 12-14 weeks on diet. This study shows that feeding rats a standard vitamin A-adequate or -deficient diet results in an age-dependent loss of omega three docosahexaenoic acid and a concomitant increase in omega six docosapentanoic acid in the rod outer segments. The loss of docosahexaenoate appears to be caused by insufficient dietary omega three fatty acids. The increase in omega six docosapentanoic acid appears to arise from the high concentration of linoleic acid in standard diets containing either cottonseed, or peanut oil or supplemental corn oil. Feeding rats diets supplemented with linseed oil, however, results in a rod outer-segment lipid profile which is the same as for chow-fed animals. The same effects were seen in the fatty-acid profile of lipids from liver, although the content of polyunsaturates was much lower than in rod outer segments. Vitamin A deficiency, itself, does not lead to changes in the fatty-acid composition of either the rod outer segments or liver. After 6-7 weeks on A+ or A- diet, rhodopsin levels were, as expected, higher in dark-reared rats than in cyclic-light animals. Although the rhodopsin levels in dark-reared vitamin A-adequate rats were significantly higher than in vitamin A-deficient animals, measurements of the lipid to opsin ratio of rod outer segments indicate that the rods of vitamin A-deficient rats are not markedly different than those of vitamin A-adequate rats. It is concluded that these diets may be useful in providing a means for evaluating the role of docosahexaenoic acid in visual cell death from damaging light.

Pathogenetic factors of aging macular degeneration

In order to explore the pathogenetic factors of aging macular degeneration, risk factors from epidemiologic studies and causative factors from clinical and laboratory studies were reviewed. From the epidemiologic studies, the risk factors of senile macular degeneration included: personal characteristics such as age, sex, race, height, family history, and strength of hand grip; ocular characteristics such as hyperopia, color of iris and senile cataract; and systemic diseases and environmental exposure such as cardiovascular diseases, smoking, lung infections, and chemical exposures. The causative factors suggested from clinical and laboratory studies include: drusen, choroidal vascular diseases, photic injury, and vitamin C deficiency. It is postulated that aging macular degeneration is a multifactorial syndrome. Different etiologic factors may inflict damaging effect on the macula, resulting in a common set of clinical manifestations that are interpreted as aging macular degeneration.

alpha-Tocopherol in the developing rat retina: a high pressure liquid chromatographic analysis

High pressure liquid chromatography was used to measure alpha-tocopherol in the retinas of rats reared in a cyclic light or dark environment. These measurements were performed on extracts of whole retinas during the developmental period, 18-60 days, and on isolated ROS from adult animals. Similar alpha-tocopherol determinations were performed on retinas and isolated ROS following exposure of rats to intense visible light for 24 hr periods. The results show that alpha-tocopherol is chromatographically separated from the vitamin A derivatives found in the retina and is pure, as judged by mass spectrometry. In the retinas of cyclic light and dark reared rats, alpha-tocopherol accumulates in an age dependent fashion, so that at 60 days the level is nearly double that of animals at 18-20 days of age (P less than 0.001). Because the age dependent accumulation of rhodopsin is greater in dark reared rats, the average molar ratio of rhodopsin to alpha- tocopherol in the retina of dark reared animals is 25% higher than in cyclic light rats. Following exposure of rats to intense visible light for 24 hr periods, alpha-tocopherol concentrations in the retina were unchanged from the levels in control animals. In adult animals the concentration of alpha-tocopherol in ROS is 2.5-3.5 times higher than in whole retina. ROS from adult cyclic light reared rats also contain an average of 43% more alpha-tocopherol per mg protein than ROS from dark maintained animals (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)