Electrogenic uptake of glutamate and aspartate into glial cells isolated from the salamander (Ambystoma) retina

Abstract

The effects of excitatory amino acids on the membrane current of isolated retinal glial cells (Müller cells) were investigated using whole-cell patch clamping. 2. L-Glutamate evoked an inward current at membrane potentials between -140 and +50 mV. The current was larger at more negative potentials. 3. The glutamate-evoked current was activated by external cations with relative efficacies: Na+ much greater than Li+ greater than K+ greater than Cs+, choline. It was activated by internal cations with relative efficacies K+ greater than Rb+ greater than Cs+ much greater than choline. Chloride and divalent cations did not affect the glutamate-evoked current. 4. Raising the intracellular sodium or glutamate concentrations, or raising the extracellular potassium concentration, reduced the current evoked by external glutamate. The suppressive effect of internal glutamate was larger when the internal sodium concentration was high. 5. Some analogues of glutamate also evoked an inward current. Responses to L-aspartate resembled those to glutamate, but for aspartate the apparent affinity was higher and the voltage dependence of the current was steeper. In the physiological potential range the current evoked by a saturating dose of aspartate was less than that evoked by a saturating dose of glutamate. 6. The uptake blocker threo-3-hydroxy-DL-aspartate (30 microM) reduced the glutamate-evoked current, but also generated a current itself. Dihydrokainate (510 microMs) weakly inhibited the glutamate-evoked current without generating a current itself. 7. The commonly used blockers of glutamate-gated ion channels, 2-amino-5-phosphonovalerate (APV; 100 microMs), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microMs), and kynurenate (1mM) had no effect on the glutamate-evoked current. 8. The voltage dependence, cation dependence and pharmacological profile of the current evoked by excitatory amino acids indicate that it is caused by activation of the high-affinity glutamate uptake carrier. This carrier appears to transport one glutamate anion into the cell, one K+ ion out of the cell, and two or more Na+ ions into the cell, on each carrier cycle. At the inner membrane surface some or all of the transported Na+ dissociates from the carrier after the transported glutamate has dissociated. 9. In addition to glutamate, the uptake carrier can also transport aspartate and threo-3-hydroxy-DL-aspartate, but not dihydrokainate.