Characteristics of ascorbate transport in the rabbit iris-ciliary body

The accumulation and efflux of [14C]ascorbic acid by the rabbit iris-ciliary body were examined in vitro. Accumulation of labelled ascorbate was observed to be reduced significantly by ouabain(10(-4) M), low sodium solutions, 0 degree C and dinitrophenol (10(-3) M). These findings suggest that the ascorbate uptake mechanism is linked to metabolism via the sodium gradient. In addition, labelled ascorbate uptake was diminished by both phloretin (10(-4) M) and phlorhizin-(10(-4) M), two glucose transport inhibitors. Clear differences were observed between the characteristics of [14C]ascorbic acid uptake and efflux. In contrast to the uptake, 14C-labelled, ascorbate efflux from the iris-ciliary body was not significantly altered by dinitrophenol, phloretin, or phlorhizin. The observation that the efflux of labelled ascorbate was not diminished by either dinitrophenol or ouabain supports the concept that the process of ascorbate efflux from the iris-ciliary body is passive and not sodium dependent. It was demonstrated that a vigorous rate of [14C]ascorbate efflux from the iris-ciliary body persisted even when the external ascorbate level was elevated to 1 mM, the concentration found in aqueous humor.