MECHANISMS OF H-ASPARTATE IN RAT HIPPOCAMPAL ASTROCYTES( AND NA+ CHANGES INDUCED BY GLUTAMATE, KAINATE, AND D)

The excitatory transmitter glutamate (Glu), and its analogs kainate (K A), and D-aspartate (D-Asp) produce significant pH changes in glial ce lls. Transmitter-induced pH changes in glial cells, generating changes in extracellular pH, may represent a special form of neuronal-glial i nteraction. We investigated the mechanisms underlying these changes in intracellular H+ concentration ([H+](i)) in cultured rat hippocampal astrocytes and studied their correlation with increases in intracellul ar Na+ concentration ([Na+](i)), using fluorescence ratio imaging with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) or sodium-bind ing benzofuran isophthalate (SBFI). Glu, KA, or D-Asp evoked increases in [Na+](i); Glu or D-Asp produced parallel acidifications. KA, in co ntrast, evoked biphasic changes in [H+](i), alkaline followed by acid shifts, which were unaltered after Ca2+ removal and persisted in O Cl- -saline, but were greatly reduced in CO2/HCO3--free or Na+-free saline , or during 4,4'-diisothiocyanato-stilbene-2,2-'disulphonic acid (DIDS ) application. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxa line-2,3-dione (CNQX) blocked KA-evoked changes in [H+](i) and [Na+](i ), indicating that they were receptor-ionophore mediated. In contrast, CNQX increased the [H+](i) change and decreased the [Na+](i) change i nduced by Glu. D-Asp, which is transported but does not act at Glu rec eptors, induced [H+](i) and [Na+](i) changes that were virtually unalt ered by CNQX. Our study indicates that [Na+](i) increases are not prim arily responsible for Glu- or KA-induced acidifications in astrocytes. Instead, intracellular acidifications evoked by Glu or D-Asp are main ly caused by transmembrane movement of acid equivalents associated wit h Glu/Asp-uptake into astrocytes. KA-evoked biphasic [H+](i) changes, in contrast, are probably attributable to transmembrane ion movements mediated by inward, followed by outward, electrogenic Na+/HCO3- cotran sport, reflecting KA-induced biphasic membrane potential changes.