Glutamate receptor‐linked changes in membrane potential and intracellular Ca 2+ in primary rat astrocytes

Kainate‐, quisqualate‐ and glutamate‐induced depolarization and mobilization of intracellular Ca 2+ was determined in primary cultured astrocytes using the fluorescent probes DiBa‐C 4 ‐(3) and fura‐2, respectively. All three receptor agonists depolarized the cells in a Na + ‐dependent manner and increased the intracellular Ca 2+ concentration. The glutamate‐ and quisqualate‐induced increase in cytosolic Ca 2+ was only partially inhibited by removal of extracellular Ca 2+ , whereas the response to kainate was totally dependent on extracellular Ca 2+ . The mechanisms for depolarization and increases in cytosolic Ca 2+ appeared to be independent of each other, as extracellular Ca 2+ removal or intracellular Ca 2+ buffering with entrapped BAPTA did not affect the depolarization. Removal of extracellular Na + did not affect the agonist‐induced increase in Ca 2+ . If quisqualate was added after kainate, the cells were hyperpolarized in a Ca 2+ ‐ and K + ‐dependent manner. This could be due to effects on a Ca 2+ ‐dependent K + channel, the effects of which are normally hidden by the greater effect on Na + channels as a response to quisqualate.