Extracellular high K+ stimulates vesicular glutamate release from astrocytes by activating voltage-dependent calcium...

Extracellular high K(+) (75 mM) increased intracellular Ca(2+) concentrations in cultured rat hippocampal astrocytes, and the Ca(2+) rise was abolished by deleting extracellular Ca(2+) 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 alpha1B, alpha1C, alpha1D, and alpha1E. 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 Ca(2+), 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 Ca(2+), 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 Ca(2+) concentrations through VDCCs, which triggers vesicular glutamate release from astrocytes, independently of reverse transport through glutamate transporters.