Release of l-aspartate by reversal of glutamate transporters

Aspartate is released in the brain during metabolic inhibition and can activate NMDA receptors. We compared the characteristics of aspartate and glutamate release mediated by reversed operation of GLAST glutamate transporters in salamander retinal glial cells, when high [K+]o solution was applied to mimic the ionic conditions of stroke or glaucoma. In the absence of Cl−, to isolate the transport-associated current of the transporters, reversed uptake of aspartate and glutamate had similar characteristics. Both were increased strongly by depolarisation, inhibited by the transport inhibitor TBOA (dl-threo-beta-benzyloxyaspartate), and activated in a first order manner by intracellular amino acid (in the presence of 20 mM [Na+]i) with an EC50 of 0.8 mM for aspartate and 2.3 mM for glutamate. In stroke the extracellular pH shifts acid by around a pH unit: this reduced the release of aspartate and glutamate by reversed uptake by a factor of 8-20. The external Cl− concentration had only a small effect on the current associated with reversed uptake of aspartate and glutamate. Tamoxifen, which reduces amino acid release through swelling-activated anion channels in glial cells, was found to inhibit reversed uptake with an IC50 which was >100 μM. Part of the activation of NMDA receptors which occurs in ischaemia is likely to reflect the release of aspartate by reversed uptake.