Oxidative damage of retinal rod outer segment membranes and the role of vitamin E

Effects of Melatonin, Vitamin E and Octreotide on Lipid Peroxidation during Ischemia-Reperfusion in the Guinea Pig Retina

Purpose

The purpose of this study is to provide evidence that free radical damage is a component of retinal ischemia-reperfusion (I/R) injury, and to determine whether melatonin, vitamin E and octreotide can protect the retina from this injury.

Methods

The right eyes of 50 male guinea pigs weighing 500–600 g were used. The animals were randomly assigned to group 1 (control), group 2 (I/R), group 3 (melatonin + I/R), group 4 (vitamin E + I/R) and group 5 (octreotide + I/R). Groups 3, 4 and 5 received four subcutaneous injections with a 6-h interval for a total daily dose of 10 mg/kg melatonin, 150 mg/kg vitamin E and 22 μg/kg octreotide. The first dose of each substance was administered 5 minutes before retinal ischemia, which was induced for 1.5 hours, followed by reperfusion for 24 hours. All three substances were repeated for 6, 12 and 18 hours during reperfusion. The animals were killed at 24 hours of reperfusion. Retinas were isolated and processed for the quantification of malondialdehyde (MDA).

Results

The compounds had the following relationships: melatonin more than vitamin E more than octreotide in preventing retinal damage by ischemia-reperfusion. All three gave significant protection against the formation of MDA (10.4±2.3, 12.4±2.4, 13.9±1.5 nmol/100 mg tissue wet weight, respectively) compared to the control (3.7±1.3 nmol/100 mg tissue wet weight) and I/R groups (22.7±6.2 nmol/100 mg tissue wet weight).

Conclusions

This study demonstrates the inhibitory effect of melatonin, vitamin E and octreotide on MDA levels during retinal ischemia-reperfusion injury.

Light exposure induces antioxidant enzyme activities in eye tissues of frogs

Retina and pigment epithelium of eyes are prone to increased production of reactive oxygen species and are susceptible to oxidative damage. We studied antioxidant enzyme activities in eye tissues exposed to prolonged low intensity (2000 lx) and short-term high intensity light (50000 lx) of visible range. Both prolonged low intensity and short-term intensive light increased superoxide dismutase, glutathione peroxidase and catalase activities in the pigment epithelium of frogs' eyes, while the activity of these enzymes in the retina remained unchanged. Our results suggest that visible light may increase oxidative stress and cause compensatory up-regulation of antioxidant enzyme activities in frogs' eyes.

Susceptibility of rat retina acyl-CoA:1-acyl-sn-glycero-3-phosphocholine O-acyltransferase and CTP:phosphocholine cytidylyltransferase activity to lipid peroxidation and hydroperoxide treatment

Two enzyme activities involved in phospholipid metabolism in the rat retina were determined after in vivo and in vitro peroxidation according to several model systems. The in vivo models were based on: (i) intravenous administration of a sonicated emulsion of phospholipid and linoleate photooxidized mixture to normal rat for a period of one week; (ii) acute injection of Fe2+ solution (20 mM) or (iii) 0.5 mg of hydroperoxylinoleate into the vitreous body, and collection of retinal tissue 4 h or 4 days later, respectively. Oleoyl CoA:lysophosphatidylcholine acyltransferase activity was unchanged or exhibited significant inhibition. On the contrary, CTP:phosphocholine cytidylyltransferase activity was stimulated. By incubating in vitro the retina with: (i) Fe(2+)-ascorbate; (ii) photooxidized phospholipid mixture (0.1-5 mM) or individual phospholipid classes; (iii) hydroperoxylinoleate (0.25-2 mM), with or without Fe2+, a significant inactivation of acyltransferase (six-fold maximum loss of initial activity) and a slight stimulation of cytidylyltransferase were seen. Altogether, the results suggest that in situ oxygen radical generation by a variety of agents irreversibly perturbs enzymes and/or membrane structures in which the enzymes are inserted; these events may bea causal factor in retinal degeneration accompanying some ocular diseases.

Lipid peroxidation and antioxidant enzymatic systems in rat retina as a function of age

In the present study, we have assayed the enzymatic activity of Cu,Zn-SOD, Mn-SOD, GSH-Px, GSH-Red, Cat, and G6PD in rat retina as a function of age. Conjugated diene levels and MDA formation were also determined. The conjugated diene levels in rat retina were found to increase significantly with age, accompanied by a marked decrease in GSH-Px and Cat activities. No age-related change in MDA levels and in GSH-Red and G6PD activity was found, whereas a significant increase in SOD activity was observed between 1 and 4 months. Decreased GSH-Px and Cat activity is related to increased lipid peroxidation with age.

Effects of dietary n-3 polyunsaturated fatty acids on phospholipid composition and calcium transport in mouse cardiac sarcoplasmic reticulum

The effects of dietary n-3 and n-6 polyunsaturated fatty acids on the fatty acid composition of phospholipid, Ca++. Mg++ ATPase and Ca++ transport activities of mouse sarcoplasmic reticulum were investigated. Mice were fed a 2 weight percent fat diet containing either 0.5 weight percent ethyl esters of 18:3n-3, 20:5n-3 or 22:6n-3 as a source of n-3 polyunsaturated fatty acid or 0.5 weight percent safflower oil as a source of n-6 polyunsaturated fatty acid for 10 days. Olive oil (2 weight percent) was used as a control diet. Although feeding n-6 polyunsaturated fatty acid induced very little modifications of the phospholipid sarcoplasmic reticulum fatty acid composition, feeding n-3 polyunsaturated fatty acid altered it markedly. Inclusion of 18:3n-3, 20:5n-3 or 22:6n-3 in the diet caused an accumulation of 22:6n-3, which replaced 20:4n-6 and 18:2n-6 in phospholipid sarcoplasmic reticulum. The saturated fatty acids were significantly increased with a concurrent reduction of 18:1n-9. These changes in the fatty acid composition resulted in a decrease in the values of the n-6/n-3 polyunsaturated fatty acid ratio and a decrease in the ratio of 20 carbon to 22 carbon fatty acids esterified in the phospholipid sarcoplasmic reticulum. This was associated with a decrease in Ca++ uptake by n-3 polyunsaturated fatty acid enriched sarcoplasmic reticulum vesicles as compared with n-6 fatty acid and control diet sarcoplasmic reticulum vesicles. However, neither the affinity for Ca++ nor the maximal velocity of ATP hydrolysis activity of Ca++.MG++ATPase were altered by the different diets.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of iodinated density gradient media for the isolation of rod outer segments

Sucrose, nycodenz, metrizamide and a mixture of equal volumes of sucrose and metrizamide were used as density gradient media for the isolation of retinal rod outer segments. The high osmolarity of sucrose had a strongly negative effect on the nature of the rod outer segments, whereas they were much better preserved using iodinated density gradient media such as nycodenz and metrizamide for their isolation.

Alteration of lipid peroxide and endogenous antioxidant contents in retina of streptozotocin-induced diabetic rats: effect of vitamin A administration

Possible involvement of lipid peroxide (LPO) in the occurrence of diabetic retinal lesion was investigated using streptozotocin-induced diabetic rats. Young male Wistar rats weighing 100-150 g were made diabetic by daily intraperitoneal injection of 30 mg/kg streptozotocin (STZ) for 5 days. Five weeks after the termination of STZ-treatment, when animals maintained typical hyperglycemia, the tissue level of LPO, estimated by the thiobarbituric acid method in the presence of 0.5 mM EDTA, was found to be augmented in the kidney. At 7 to 9 weeks after the STZ-treatment, the content of LPO in the retina also exhibited a significant increase, while those in the serum, brain and peripheral nerves showed no alteration. This increment of LPO in the kidney and retina was accompanied by the concomitant reduction of fat-soluble antioxidants determined by the ferric chloride-bipyridyl reaction, and insulin treatment (10 u/rat/day, s.c.) completely eliminated the increased formation of LPO in these organs. When diabetic rats were treated with retinol acetate, which had an inhibitory effect on LPO formation in retinal homogenate, the increase in LPO content was found to be significantly suppressed, especially in the retina. These results suggest that the STZ-induced diabetic state may elicit an increased formation of LPO in the retina and kidney, both of which are known to be main organs having typical diabetic lesions.

Photoreceptor outer segment and retinal pigment epithelium in vitamin E deficient rats. An electron microscopic and electron histochemical study

The photoreceptor outer segments and retinal pigment epithelium of vitamin E deficient rats were examined by electron microscope using electron histochemical techniques. In rats fed a vitamin E deficient diet for more than 6 months starting 20 days after birth, the photoreceptor outer segment showed vesiculation of the discs near the apical portion of the retinal pigment epithelium. In the retinal pigment epithelium, secondary lysosomes were increased in number. Reaction products of acid phosphatase activity were located in secondary lysosomes in the retinal pigment epithelium and on the disc membranes of the photoreceptor outer segment. These alterations became more prominent with the duration of vitamin E deficiency. It was assumed that the disc degeneration of the photoreceptor outer segment might be due to enhanced activity of lysosomal enzymes in the retinal pigment epithelium and the effect of released lysomal enzymes on the photoreceptor outer segment.

Effects of vitamin E and selenium deficiency on the fatty acid composition of rat retinal tissues

The fatty acid composition of retinal tissues was measured in rats maintained for 26--32 weeks on each of the following diets: a purified basal diet deficient in alpha-tocopherol and selenium, an identical control diet supplemented with alpha-tocopherol and selenium, and a commerical laboratory rat chow. Dietary deficiencies of antioxidant nutrients were found to cause a large decrease in total polyunsaturated fatty acids in the retinal pigment epithelium, a small decrease in the retinal rod outer segments, but no change in the whole retina or liver when compared to tissues from animals fed the vitamin E- and selenium-supplemented control diet. The polyunsaturated fatty acid content which we have observed for the retinal pigment epithelium from rats fed commerical lab chow is similar to that which we observed for bovine retinal pigment epithelium. Our results indicate that changes in fatty acid composition are not generalized to all tissues in severely antioxidant-deficient animals, but that changes do occur in some tissues, such as the retinal pigment epithelium, which appears to be particularly sensitive to in vivo lipid peroxidation.

Retinal damage by light: possible implication of singlet oxygen

A new hypothesis is proposed in an attempt to explain the mechanism of the irreversible damage which can be induced in the retina by visible light. Upon illumination, retinal generates singlet oxygen and this reactive species can produce lipid peroxidation which in turn may induce membrane instability.