Glutamate efflux via the reversal of the sodium-dependent glutamate transporter caused by glycolytic inhibition in rat cultured astrocytes

Abstract

[3H]L-Glutamate uptake in cultured rat astrocytes was completely reduced by 30 min preincubation with 1 mM of iodoacetic acid, a glycolytic inhibitor. This treatment significantly reduced the energy charge potential, but did not cause membrane destruction in the cultured astrocytes. To examine the effect of iodoacetic acid on the glutamate release, [3H]L-glutamate was preloaded into astrocytes in the presence of methionine sulfoximine, a glutamine synthetase inhibitor, and the total intracellular radioactivity was measured after 30-min treatment with 1 mM iodoacetic acid for comparison with non-treated astrocytes. During the treatment, about 40% of the total intracellular glutamate content was effluxed. This efflux could be decreased by reducing the extracellular potassium ion concentration. The intracellular sodium concentration, measured with a sodium ion-sensitive fluorescent probe (sodium-binding benzofuran isophtalate), gradually increased to 30 mM on addition of 1 mM iodoacetic acid. These results indicate that the glutamate efflux via reversal of the Na(+)-dependent transporter occurred during glycolytic inhibition, and which may be caused by intracellular Na+ overload. Such an iodoacetic acid-induced Na+ overload could be completely diminished by pretreatment with 1 microM 5-[N- ethyl-N-isopropyl]amiloride, a selective Na(+)-H+ antiporter inhibitor, but even this did not stop the iodoacetic acid-induced glutamate efflux. The intracellular pH, measured by a pH-sensitive fluorescent probe [2',7'-bis(carboxyethy)-5,6-carboxy-fluorescein], was gradually decreased to 7.1 by the iodoacetic acid treatment. On the other hand, iodoacetic acid-induced intracellular acidosis was more rapid and severe in the presence of 5-[N-ethyl-N-isopropyl]amiloride. These results suggest that the reversal of the Na+-dependent glutamate transporter may be caused by not only intracellular Na+ overload but also intracellular acidosis.