Mechanisms of H+ and Na+ changes induced by glutamate, kainate, and D-aspartate in rat hippocampal astrocytes.
The excitatory transmitter glutamate (Glu), and its analogs kainate (KA), and D-aspartate (D-Asp) produce significant pH changes in glial cells. Transmitter-induced pH changes in glial cells, generating changes in extracellular pH, may represent a special form of neuronal-glial interaction. 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 intracellular Na+ concentration ([Na+]i), using fluorescence ratio imaging with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) or sodium-binding benzofuran isophthalate (SBFI). Glu, KA, or D-Asp evoked increases in [Na+]i; Glu or D-Asp produced parallel acidifications. KA, in contrast, evoked biphasic changes in [H+]i, alkaline followed by acid shifts, which were unaltered after Ca2+ removal and persisted in 0 CI(-)-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-nitroquinoxaline-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 induced by Glu. D-Asp, which is transported but does not act at Glu receptors, induced [H+]i and [Na+]i changes that were virtually unaltered by CNQX. Our study indicates that [Na+]i increases are not primarily responsible for Glu- or KA-induced acidifications in astrocytes. Instead, intracellular acidifications evoked by Glu or D-Asp are mainly caused by transmembrane movement of acid equivalents associated with 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- cotransport, reflecting KA-induced biphasic membrane potential changes.
Rose
C R
CR
Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Ransom
B R
BR
D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D001224:Aspartic Acid / Q000494:pharmacology; D001253:Astrocytes / Q000187:drug effects* / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism; D002021:Buffers; D002245:Carbon Dioxide / Q000502:physiology; D018698:Glutamic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology*; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects*; D006859:Hydrogen / Q000378:metabolism*; D007608:Kainic Acid / Q000494:pharmacology; D009994:Osmolar Concentration; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism*; D012965:Sodium Chloride / Q000737:chemistry
D001639:Bicarbonates; D002021:Buffers; D002245:Carbon Dioxide; D001224:Aspartic Acid; D018698:Glutamic Acid; D012965:Sodium Chloride; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D006859:Hydrogen; D012964:Sodium; D007608:Kainic Acid