Mechanism Involved in Initiation and Propagation of Receptor-Induced Intercellular Calcium Signaling in Cultured Rat Astrocytes

The mechanisms involved in the initiation and the propagation of intercellular calcium signaling (calcium waves) were studied in cultured rat astrocytes. The analysis of calcium waves, induced either by mechanical stimulation or by focal application of ionomycin, indicated that initiation was dependent on the presence of external calcium. In addition, pharmacological experiments indicate that intercellular propagation required PLC activation, integrity of IP3-sensitive internal calcium stores, and functional gap junctions. An extracellular action of ATP or glutamate and participation of voltage-dependent Ca2+ channels were tested by using enzymatic degradation, receptor antagonists, and channel blockers, respectively. Because neither the speed of propagation nor the extent of the calcium waves was affected by these treatments, these alternate mechanisms were excluded from playing a role in intercellular calcium signaling. Biochemical assays and focal applications of several agonists (methoxamine, carbachol, glutamate) of membrane receptors to neurotransmitters and peptides (endothelin 1) demonstrated that their ability to trigger regenerative calcium waves depended on phospholipase C activity and inositol phosphate production. Thus, in rat astrocytes, initiation and propagation of calcium waves involve a sequence of intra- and intercellular steps in which phospholipase C, inositol trisphosphate, internal calcium stores, and gap junction channels play a critical role. The identification of these different events allows us to determine several targets at which the level of long-range signaling in astrocytes may be controlled.