Dynamic Gradient of Glutamate Across the Membrane: Glutamate/Aspartate-Induced Changes in the Ambient Level of l-[14C]glutamate and d-[3H]aspartate in Rat Brain Nerve Terminals

Extracellular/intracellular l-[C-14]glutamate exchange and conservativeness of the extracellular level of l-[C-14]glutamate was analyzed in isolated rat brain nerve terminals. l-Glutamate-, dl-threo-beta-hydroxyaspartate (dl-THA)-, and d-aspartate-induced increase in the ambient level of l-[C-14]glutamate or d-[H-3]aspartate was evaluated comparatively. 100 mu M cold nonradiolabeled l-glutamate, dl-THA, d-aspartate extruded a quarter of radioactivity from l-[C-14]glutamate-preloaded synaptosomes for 6 min. The similar results were obtained with l-glutamate-evoked extracellular/intracellular redistribution of d-[H-3]aspartate. Contribution of presynaptic glutamate receptors to an increase in the extracellular l-[C-14]glutamate level was evaluated using receptor agonists NMDA, AMPA, and kainate (100 mu M), and it consisted of less than 5 % of total accumulated label. The existence of the efficient extracellular/intracellular glutamate exchange, and so dynamic glutamate gradient across the plasma membrane of nerve terminals was demonstrated. A two-substrate kinetic algorithm that included transporter reversal was considered. The extracellular level of l-[C-14]glutamate and d-[H-3]aspartate in nerve terminals depended on the amount of exogenous substrates of glutamate transporter available. Taking into account that l-glutamate, dl-THA, and d-aspartate are the substrates of glutamate transporters, and also the similarity in their effectiveness in the establishment of new extracellular level of the neurotransmitters, the central role of glutamate transporters in permanent glutamate turnover in nerve terminals was demonstrated.