INTRACELLULAR CA2- CALCIUM POOLS SENSITIVE TO INOSITOL 1,4,5-TRISPHOSPHATE AND THAPSIGARGIN( SIGNALS INDUCED BY ATP AND THAPSIGARGIN IN GLIOMA C6 CELLS )

In glioma C6 cells, extracellular ATP generates inositol 1,4,5-trispho sphate (InsP(3)), indicating the presence of purinergic receptors coup led to phosphoinositide turnover. To identify the effect of ATP (actin g via InsP(3)) and thapsigargin (acting without InsP(3) production as a specific inhibitor of the endoplasmic plasmic reticulum Ca2+-ATPase) on intracellular Ca2+ pools we used video imaging of Fura-2 loaded in to single, intact glioma C6 cells. It has been shown that ATP and thap sigargin initiate Ca2+ response consistent with the capacitative model of Ca2+ influx. When the cells were stimulated by increasing concentr ations of ATP (1, 10, 50 and 100 mu M) the graded, quantal Ca2+ respon se was observed. In the absence of extracellular Ca2+ thapsigargin and ionomycin-releasable Ca2+ pools are overlapping, demonstrating that C a2+ stores are located mainly in the endoplasmic reticulum. After maxi mal Ca2+ mobilization by ATP, thapsigargin causes further increase in cytosolic Ca2+: concentration, whereas emptying of thapsigargin-sensit ive intracellular stores prevents any further Ca2+ release by ATP. Thu s, the thapsigargin-sensitive intracellular pool of Ca2+ in glioma C6 cells seems to be larger than that sensitive to InsP(3). Two hypothesi s to explain this result are proposed. One postulates a presence of tw o different Ca2+ pools, sensitive and insensitive to InsP(3) and both discharged by thapsigargin, and the other,the same intracellular pool of Ca2+ completely emptying by thapsigargin and only partially by InsP (3). These results may contribute to understanding the mechanism of Ca 2+ signalling mediated by ATP, the most potent intracellular Ca2+ mobi lizing agonist in all types of glial cells. (C) 1997 Elsevier Science Ltd.