Ascorbate transport and intracellular concentration in cerebral astrocytes

Regulation of the initial rate of uptake and steady-state concentration of ascorbate (reduced vitamin C) was investigated in rat cerebral astrocytes. Although these cells did not synthesize vitamin C, they accumulated millimolar concentrations of ascorbate when incubated with medium containing the vitamin at a level (200 μM) typical of brain extracellular fluid. Initial rate of [¹⁴C]-ascorbate uptake and intracellular ascorbate concentration were dependent on extracellular Na⁺ and sensitive to the anion transport inhibitor sulfinpyrazone. Comparison of the efflux profiles of ascorbate and 2',7'-bis (carboxyethyl)-5 (or -6)-carboxyfluorescein from astrocytes permeabilized with digitonin localized most intracellular ascorbate to the cytosol. Pretreatment of astrocytes with dibutyryl cyclic AMP (dBcAMP) doubled their initial rate of sulfinpyrazone-sensitive [¹⁴C]ascorbate uptake compared with cells treated with either n-butyric acid or vehicle. dBCAMP also increased steady-state intracellular ascorbate concentration by 39%. The relatively small size of the change in astrocytic ascorbate concentration was explained by the finding that dBCAMP increased the rate of efflux of the vitamin from ascorbate-loaded cells. These results indicate that uptake and efflux pathways are stimulated by cyclic AMP- dependent mechanisms and that they regulate the cytosolic concentration of ascorbate in astrocytes.