Glutamate effects on calcium homeostasis in cerebellar granule cells in primary cultures grown under depolarizing and nondepolarizing conditions

Abstract

Cerebellar granule cell cultures are normally grown under partly depolarizing conditions (in a medium with approximately 25 mM K+), but these cultures can also be grown at a normal potassium concentration (5.4 mM K+), although some of their characteristics are altered. In this study, intracellular free calcium concentration and 45Ca uptake were measured in cerebellar granule cell cultures grown at either 25 or 5.4 mM extracellular potassium in the presence of glutamate, and/or some of its subtype-specific agonists and antagonists. Granule cells in cultures grown at 25 mM K+ responded to glutamate, but not to quisqualate, with an increase in free cytosolic calcium concentration and in 45Ca uptake. This increase in free cytosolic calcium concentration was dependent on extracellular calcium and it was antagonized by AP5 and ketamine, NMDA receptor antagonists. In contrast, granule cells in cultures grown at 5.4 mM K+ responded to both glutamate and quisqualate, and these responses were independent of extracellular calcium and not sensitive to AP5 and ketamine. In agreement with this, 45Ca uptake was not affected by glutamate. Neither of the two culture types responded to kainate with an increase in calcium concentration or uptake. These observations indicate that calcium uptake in granule cells in cultures grown at 25 mM K+ reflect NMDA activation of calcium influx, whereas the cells in cultures grown at 5 mM K+ increase cytosolic calcium concentration on account of intracellular release of bound calcium, caused by activation of the metabotropic receptor.(ABSTRACT TRUNCATED AT 250 WORDS)