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 inter cellular calcium signaling (calcium waves) were studied in cultured ra t astrocytes. The analysis of calcium waves, induced either by mechani cal stimulation or by focal application of ionomycin, indicated that i nitiation was dependent on the presence of external calcium. In additi on, pharmacological experiments indicate that intercellular propagatio n 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 chan nel blockers, respectively. Because neither the speed of propagation n or the extent of the calcium waves was affected by these treatments, t hese alternate mechanisms were excluded from playing a role in interce llular calcium signaling. Biochemical assays and focal applications of several agonists (methoxamine, carbachol, glutamate) of membrane rece ptors to neurotransmitters and peptides (endothelin 1) demonstrated th at their ability to trigger regenerative calcium waves depended on pho spholipase C activity and inositol phosphate production. Thus, in rat astrocytes, initiation and propagation of calcium waves involve a sequ ence of intra- and intercellular steps in which phospholipase C, inosi tol trisphosphate, internal calcium stores, and gap junction channels play a critical role. The identification of these different events all ows us to determine several targets at which the level of long-range s ignaling in astrocytes may be controlled.