Figure 10. Possible relations between calcium stores and plasmalemmal store-operated calcium channels in glioblastoma cells.

Activation of metabotropic receptors triggers Ca²⁺ release from the intracellular stores. On the basis of our results it is conceivable to speculate that glioma cells are endowed with two functionally distinct sets of InsP₃-sensitive stores. The bulk of the Ca²⁺ comes from the ‘Ca²⁺ release compartment’ of the ER. Emptying of this compartment does not release an appropriate signal for activation of store-operated Ca²⁺ entry. The latter is triggered only while the other ER compartment (with presumably lower sensitivity to InsP₃) is depleted; this part of the ER store (the ‘Ca²⁺ entry regulating compartment’) controls the store-operated Ca²⁺ influx. Some of the Ca²⁺ ions entering the cell through store-operated channels are accumulated by mitochondria, which might be located in the vicinity of Ca²⁺ entry sites.