[Mechanism of ascorbic acid transport in the aqueous humor]

The concentration of ascorbic acid in the aqueous humor in many species (including humans and oxen) is several times higher than the plasma concentration. The role of ascorbic acid as a protective mechanism in the intraocular tissues against light-induced damage is discussed. We have performed uptake studies using 14C-labeled ascorbic acid in cultured bovine pigmented ciliary epithelial cells in order to study the mechanisms for ascorbic acid transport from the ciliary body to the aqueous humor. We found that intracellular accumulation of ascorbic acid was Na(+)-dependent. Data from kinetic experiments suggest the presence of a co-transport system for 2 Na+ and 1 ascorbate. The oxidized form of the vitamin (dehydro-ascorbic acid, DHA) was taken up in the cell by a Na(+)-independent mechanism. Using thin-layer chromatographic assay, we found intracellularly only ascorbic acid after incubation with extracellular DHA, indicating the presence of DHA reductase activity in ciliary body epithelial cells. Thus, these cells possess two mechanisms for intracellular accumulation of ascorbic acid: (1) co-transport for 2 Na+ and 1 ascorbate and (2) a Na(+)-independent DHA transport in cooperation with intracellular DHA reductase. These mechanisms could be involved in the transport of ascorbic acid from the stroma to the aqueous humor.