Transport of Ascorbic Acid into the Eye

It seems clear that ascorbic acid enters the eye as a constituent of aqueous humor. From there it can be shifted into the cells of the lens and also into the endothelial monolayer which lines the inner surface of the cornea. It is likely that some ascorbic acid diffuses from the aqueous humor into the vitreous cavity. However, McGahan (1985) suggested that ascorbic acid may also enter the vitreous humor from other sources perhaps some passes out of the retina. In most species, the concentration of ascorbic acid in the vitreous humor is the same as that in aqueous humor or slightly lower (DiMattio, 1989a McGahan, 1985 Varma and Richards, 1988). 

The suggestion that there may be a sodium-dependent ascorbic acid transport mechanism in the pigmented layer of the ciliary epithelium (the layer which faces the blood side) fits well with a study conducted by Chu and Candia (1988), who isolated the rabbit iris-ciliary body and measured trans-tissue fluxes of labeled ascorbic acid. A net flux of ascorbic acid was observed in what would have been the blood-to-aqueous humor direction. Importantly, this net flux could be inhibited by phloridzin added to the blood side but not when added to the aqueous side. This finding is consistent with a model where active ascorbic acid is accumulated by the pigmented cells on the blood side of the ciliary epithelium bilayer, then passes via gap junctions into the nonpigmented cells, and finally diffuses into the aqueous humor. However, the situation may be more complex since cultured cells derived from the nonpigmented ciliary epithelium have also been shown to be capable of sodium-dependent ascorbic acid accumulation (Delamere et al., 1993). 

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