Mechanism Of Oxidative Damage And The Antioxidative Defence In The Rat Lens After In Vivo Exposure To Ultraviolet Radiation

Clinical significanceThe major cause of blindness in the world is cataract. Cataract is believed to develop secondary to oxidative stress. Ultraviolet radiation (UVR) is considered the most important changeable risk factor for cortical cataract. The increasing age of the world population will increase the world prevalence of cataract to a level that makes it necessary to further elucidate the machinsm of oxidative damage and the antioxidative defence system in the lens after in vivo UVR exposure, and to find a non-surgical solution for prevention or delay of onset of cataract. Alpha-tocopherol has strong vitamin E activity and is believed to be an effective antioxidant for protection against UVR induced cataract. A good knowledge of the dose response relationship and the protective factor for the protective effect of a-tocopherol are necessary to further elucidate the possibility to prevent cataract with a-tocopherol.In this project we aimed to investigate the evolution of cataract development and glutathione redox balance in the rat lens after in vivo exposure to close-to-threshold dose of UVR. Furthermore, we decided to establish the dose response relationship forα-tocopherol protection of UVR-induced cataract in rat, and to elucidate the effects ofα-tocopherol on the glutathione redox balance in rat lens and the possible mechanisms underlying.Experiment 1:Evolution of cataract development and redox balance in the rat lens after in vivo exposure to ultraviolet radiation Purpose:To investigate the evolution of cataract development and glutathione redox balance in the rat lens after in vivo close-to-threshold dose exposure to ultraviolet radiation (UVR) around 300 nm.Methods:Three groups of 10 Sprague-Dawley rats were unilaterally exposed to 8 kJ/m2 at UVR-300 nm wavelength region for 15 minutes, and a fourth group of 10 rats was kept without UVR exposure as nonexposed control animals. The exposed animals were sacrificed at 1,3 and 7 days after exposure. Both lenses from all animals were extracted and photographed and the intensity of forward light scattering was measured quantitatively. Thereafter, the lenses were homogenized. The concentration of glutathione, reduced (GSH) and oxidized (GSSG), and the activity of glutathione reductase (GR) and peroxidase (GPx) respectively, were determined spectrophotometrically. The mean paired differences between exposed and nonexposed lenses were used as primary data in the statistical analyses.Results:All exposed lenses developed cataract. Lens light scattering increased throughout the 7 days after UVR exposure. GSH concentration and GPx rate transiently increased at 1 day after exposure and then decreased throughout follow-up, with GSH concentration having a negative balance at the end. GSSG concentration and GR activity did not change after UVR exposure.Conclusion:In vivo close-to-threshold UVR exposure induces a gradual increase in rat lens opacification/cataract development and time dependently alters the redox balance in the lens.Experiment 2:Dose-response relationship for a-tocopherol prevention on in vivo UVR induced cataract in ratPurpose:To establish the dose response relationship forα-tocopherol protection of UVR-induced cataract in rat. Further, the effect of the concentration of glutathione in reduced/oxidized form (GSH/GSSG), and the glutathione reductase/peroxidase (GR/GPx) were studies. Methods:Five groups of twenty six-week-old albino Sprague-Dawley rats were used in this study. One group without anyα-tocopherol feeding was the control group, whilst the other 4 groups were received 5,25,50, and 100 IU/day ofα-tocopherol. After 4 weeks of feeding, each rat was unilaterally exposed to 8 kJ/m2 at UVR-300 nm wavelength region for 15 min. At 1 week after exposure to UVR, the rats was sacrificed, the lenses were extracted, and forward light scattering was measured. Lens total reduced (GSH) and oxidized (GSSG) glutathione; glutathione-reductase (GR) and peroxidase (GPx) were determined spectrophotometrically. The mean paired differences between exposed and nonexposed lenses were used as primary data in the statistical analyses.Results: The UVR-exposed lenses in the a-tocopherol fed groups developed superficial and slight equatorial cataract, whereas lenses in the control group developed cortical and dense equatorial opacities. Forward light scattering in lenses from theα-tocopherol-supplemented rats was lower than in lenses from the control group. The difference of light scattering between the exposed lens and its contralateral nonexposed lens decreased with increasing doses of a-tocopherol supplementation with a rate constant of 4.8 IU/day to an asymptote level. The exposure to UVR caused a significant depletionof lens GSH in rats without or at low a-tocopherol supplementation that recovered towards higher a-tocopherol supplementation with a rate constant of 52 IU/day. There was no detectable difference in lens GSSG, GR or GPx activity at any level of a-tocopherol supplementation.Conclusion:Orally administered a-tocopherol dose dependently protects against UVR-induced cataract. But the oral supplementation of a-tocopherol can not completely retard cataract development. The protection of UVR-induced light scattering is associated with an a-tocopherol dose-dependent GSH depletion secondary to UVR exposure without any effects on total lens GSSG, GR or GPx activity. UVR-induced light scattering only occurs if the GSH depletion exceeds a threshold.