Ultraviolet radiation cataract development and ascorbate supplementation

Abstract

Background: Cataract is the major cause of blindness in the world, and long-term solar ultraviolet radiation (UVR) is a major risk factor. The pathogenesis of UVRinduced cataract is studied in various animal models and cell systems. The significance of oxidation processes in cataract and other eye diseases has made the study of protective antioxidants increasingly important. Ascorbate, or vitamin C, is an important dietary antioxidant and essential nutrient in the human and guinea pig, while the rat is capable of synthesizing ascorbate. Ascorbate has been implicated as a protectant in cataract development.

Purpose: The main purpose of this thesis was to establish two new animal models for the investigation of the effect of ascorbate supplementation in UVR cataract development.

Methods: Paper I. Albino rats were kept on chow (solid diet) supplemented with ascorbate for four weeks. The animals were then sacrificed, the lenses extracted and homogenized. Ascorbate and other low molecular weight compounds were isolated with ultrafiltration and ascorbate was quantified with subsequent high performance liquid chromatography (HPLC) with detection at 254 nm.

Paper II. Rats were exposed to five doses of UVR 300 nm and the degree of cataract was quantified after one week by measurement of lens forward light scattering. The safety measure Maximal Tolerable Dose (MTD2.3:16) for avoidance of UVR-induced cataract was calculated. Lens ascorbate concentration was measured using HPLC.

Paper III. Pigmented guinea pigs received ascorbate supplementation via the drinking water for four weeks. Since guinea pigs cannot synthesize ascorbate, a low concentration of essential ascorbate was distributed through the chow to avoid illness in the zero supplementation group. Lens ascorbate was analyzed with HPLC.

Paper IV. Ascorbate supplemented guinea pigs were exposed to 80 kJ/m2 UVR 300 nm. One day later, the animals were sacrificed and lenses were extracted, cataract degree quantified and lens ascorbate analyzed.

Results: The ascorbate supplementation did not induce cataract or signs of illness in the animals. Rat lenses contain ascorbate, even without ascorbate in the food, while guinea pig lenses had very low ascorbate level. For both animal models, the lens ascorbate increased significantly, 19% for the rats and 39% for the guinea pigs. The MTD2.3:16 in the albino rat was 3.01 kJ/m2. UVR exposure leads to a significant consumption of ascorbate in the exposed lenses, but also a small decrease in the the non-exposed lenses. UVR-exposed albino rat lenses exhibited mainly cortical cataract while UVR-exposed pigmented guinea pig lenses developed superficial anterior cataract.

Conclusions: The lens sample preparation and subsequent ascorbate analysis by HPLC is easy and feasible for animal lenses. Ascorbate supplementation via food or drinking water both increases lens ascorbate levels. The MTD2.3:16 for albino rat is similar to that in the pigmented mouse, rat and rabbit, while pigmented guinea pig has several fold higher tolerance to UVR. UVR exposure leads to a consumption of lens ascorbate in rats and guinea pigs. Ascorbate supplementation does not protect guinea pig lenses against UVR cataract development.