Extracellular High K+ Stimulates Vesicular Glutamate Release From Astrocytes by Activating Voltage-Dependent Calcium Channels

Extracellular high K+ (75 mM) increased intracellular Ca2+ concentrations in cultured rat hippocampal astrocytes, and the Ca2+ rise was abolished by deleting extracellular Ca2+ or cadmium, a non-selective inhibitor of voltage-dependent calcium channels (VDCCs). In the reverse transcription-polymerase chain reaction analysis, cultured astrocytes expressed mRNAs for L type-VDCC subunits such as alpha 1B, alpha 1C, alpha 1D, and alpha 1E. Extracellular high K+ (75 mM) stimulated glutamate release from astrocytes. The glutamate release was not prevented by the glutamate transporter inhibitor, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), or deleting extracellular Na+, but otherwise it was clearly inhibited by deleting extracellular Ca2+, cadmium, vesicular transport inhibitors such as brefeldin A, bafilomycin A1, and latrunculin B, or botulinum toxin-A, an exocytosis inhibitor. Extracellular high K+ (75 mM) bleached fluorescent signals of FM1-43, taken up into the vesicular membrane in astrocytes, that was also inhibited by deleting extracellular Ca2+, cadmium, brefeldin A, bafilomycin A1, latrunculin B, or botulinum toxin-A, but not by PDC. Taken together, the results of the present study indicate that extracellular high K+-evoked depolarization activates VDCCs expressed in astrocytes, causing an increase in intracellular Ca2+ concentrations through VDCCs, which triggers vesicular glutamate release from astrocytes, independently of reverse transport through glutamate transporters. J. Cell. Physiol. 225: 512-518, 2010. (C) 2010 Wiley-Liss, Inc.