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-[14C]ascorbate for 1 min at 37 degrees C. We observed that the apparent maximal rate of uptake (Vmax) increased rapidly (less than 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-[14C]ascorbate. The increase in Vmax 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.
Wilson
J X
JX
Department of Physiology, University of Western Ontario, London, Canada.
Jaworski
E M
EM
Kulaga
A
A
Dixon
S J
SJ
D000818:Animals; D000831:Animals, Newborn; D001205:Ascorbic Acid / Q000378:metabolism* / Q000494:pharmacology; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D001692:Biological Transport; D002250:Carbon Radioisotopes; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000378:metabolism*; D003847:Deoxyglucose / Q000378:metabolism; D007700:Kinetics; D051381:Rats; D011919:Rats, Inbred Strains
D002250:Carbon Radioisotopes; D003847:Deoxyglucose; D001205:Ascorbic Acid