SUBSTRATE REGULATION OF ASCORBATE TRANSPORT ACTIVITY IN ASTROCYTES

Astrocytes possess a concentrative L-ascorbate (vitamin C) uptake mechanism involving a Na+- dependent L-ascorbate transporter located in the plasma membrane. The present experiments examined the effects of deprivation and supplementation of extracellular L-ascorbate on the activity of this transport system. Initial rates of L-ascorbate uptake were measured by incubating primary cultures of rat astrocytes with L-[C-14]ascorbate for 1 min at 37-degrees-C. We observed that the apparent maximal rate of uptake (V(max)) increased rapidly (<1 h) when cultured cells were deprived of L-ascorbate. In contrast, there was no change in the apparent affinity of the transport system for L-[C-14]ascorbate. The increase in V(max) was reversed by addition of L-ascorbate, but not D-isoascorbate, to the medium. The effects of external ascorbate on ascorbate transport activity were specific in that preincubation of cultures with L-ascorbate did not affect uptake of 2-deoxy-D-[3H(G)]glucose. We conclude that the astroglial ascorbate transport system is modulated by changes in substrate availability. Regulation of transport activity may play a role in intracellular ascorbate homeostasis by compensating for regional differences and temporal fluctuations in external ascorbate levels.