Sodium-ascorbate cotransport controls intracellular ascorbate concentration in primary astrocyte cultures expressing the SVCT2 transporter

Expression of the Na+-ascorbate cotransporter, SVCT2, was detected in rat b rain and in primary cultures of cerebral astrocytes by Northern blot analys is. SVCT2 expression in cultured astrocytes increased in response to the cy clic AMP analog, dibutyryl cyclic AMP. A mathematical model of ascorbic aci d transport was developed to evaluate the hypothesis that Na+-ascorbate cot ransport across the plasma membrane regulates the steady state intracellula r concentration of ascorbic acid in these cells. The outcomes predicted by this model were compared to experimental observations obtained with primary cultures of rat cerebral astrocytes exposed to normal and pathologic condi tions. Both cotransport activity and intracellular ascorbic acid concentrat ion increased in astrocytes activated by dibutyryl cyclic AMP. Conversely t ransport activity and ascorbic acid concentration were decreased by hyposmo tic cell swelling, low extracellular Na concentration, and depolarizing lev els of extracellular K+. In cells incubated for up to 3 h in medium having an ascorbic acid concentration typical of brain extracellular fluid, the ch anges in intracellular ascorbic acid concentration actually measured were n ot significantly different from those predicted by modeling changes in Na+- ascorbate cotransport activity. Thus, it was not necessary to specify alter ations in vitamin C metabolism or efflux pathways in order to predict the s teady state intracellular ascorbic acid concentration. These results establ ish that SVCT2 regulates intracellular ascorbic acid concentration in prima ry astrocyte cultures. They further indicate that the intracellular-to-extr acellular ratio of ascorbic acid concentration at steady state depends on t he electrochemical gradients of Na+ and ascorbate across the plasma membran e. (C) 2000 Elsevier Science B.V. All rights reserved.