To establish an assay system for evaluation of the uptake and reversed transport of glutamate, we examined the effects of Na
+-concentration and pharmacological agents on the extracellular glutamate concentration ([Glu]
o) in rat cortical synaptosomes
in vitro. There was a decrease and increase of the [Glu]
o at high and low Na
+ concentrations, respectively, in a Ca
2+-free medium. The changes in [Glu]
o in both directions were temperature-sensitive, and reversed at around 30 m
M of Na
+. Dihydrokainate (DHK), a non-transportable inhibitor selective for glial glutamate transporter GLT-1, suppressed the decrease in [Glu]
o, and the reversal of [Glu]
o change was shifted to about 60 m
M Na
+. There was no change in the maximum [Glu]
o at total Na
+ substitution. Further pharmacological analysis revealed that
D-aspartate and
DL-
threo-β-hydroxy-aspartate (THA), transportable substrates of glutamate transporters, increased the [Glu]
o in standard media. In contrast, β-phenylglutamic acid, a structural analogue of glutamate, suppressed both the decrease in [Glu]
o in standard medium and the increase in [Glu]
o in low Na
+ medium. It is, thus, concluded that both the direction and the amount of [Glu]
o changes are determined by a balance of the uptake and reversed transport of glutamate, and that this assay system is suitable for evaluation of the effect of this on glutamate transporters.
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