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

Abstract

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 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. RPE-choroid levels and ascorbate in the sclera were similar in the light exposed and dark maintained animals.

As measured by the level of rhodopsin present 2 weeks after intense light treatment, interperitoneal L- or D-ascorbate provided equivalent and significant protection for experiments lasting up to 7 hrs. For rats given ascorbate by the intraocular route, the two stereoisomers were also equally effective for exposures lasting 2.5 hrs. In these rats, rhodopsin levels were significantly higher in the L- or D- injected eyes than in the eyes of rats injected with saline, or in the uninjected fellow eyes. In experiments lasting 4 hrs, rhodopsin levels in the injected eyes were higher than in the uninjected eyes, but the differences were not significant. Our findings support an antioxidative role for ascorbate in retinal light damage and suggest that it exerts a protective effect when tissue levels are elevated.