Substrate regulation of ascorbate transport activity in astrocytes |
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Authors: | John X. Wilson Ewa M. Jaworski Andrew Kulaga S. Jeffrey Dixon |
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Affiliation: | (1) Department of Physiology, University of Western Ontario, Medical Sciences Building, N6A 5C1 London, Ontario, Canada;(2) Division of Oral Biology, University of Western Ontario, N6A 5C1 London, Ontario, Canada |
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Abstract: | Astrocytes possess a concentrativel-ascorbate (vitamin C) uptake mechanism involving a Na+-dependentl-ascorbate transporter located in the plasma membrane. The present experiments examined the effects of deprivation and supplementation of extracellularl-ascorbate on the activity of this transport system. Initial rates ofl-ascorbate uptake were measured by incubating primary cultures of rat astrocytes withl-[14C]ascorbate for 1 min at 37°C. We observed that the apparent maximal rate of uptake (Vmax) increased rapidly (<1 h) when cultured cells were deprived ofl-ascorbate. In contrast, there was no change in the apparent affinity of the transport system forl-[14C]ascorbate. The increase inVmax was reversed by addition ofl-ascorbate, but notD-isoascorbate, to the medium. The effects of external ascorbate on ascorbate transport activity were specific in that preincubation of cultures withl-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. |
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Keywords: | Ascorbate astrocytes autoregulation rat brain transport mechanism vitamin C |
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