ATP CAUSES RELEASE OF INTRACELLULAR CA2-BETA( VIA THE PHOSPHOLIPASE C)IP3 PATHWAY IN ASTROCYTES FROM THE DORSAL SPINAL-CORD/

Calcium signaling within astrocytes in the CNS may play a role compara ble to that of electrical signaling within neurons. ATP is a molecule known to produce Ca2+ responses in astrocytes, and has been implicated as a mediator of intercellular Ca2+ signaling in other types of nonex citable cells. We characterized the signal transduction pathway for AT P-evoked Ca2+ responses in cultured astrocytes from the dorsal spinal cord. Nearly 100% of these astrocytes respond to extracellularly appli ed ATP, which causes release of Ca2+ from an intracellular pool that i s sensitive to thapsigargin and insensitive to caffeine. We found that intracellular administration of IP3 also caused release of Ca2+ from a thapsigargin-sensitive intracellular pool, and that IP3 abolished th e response to ATP. The ATP-evoked Ca2+ response was blocked by the IP3 receptor antagonist heparin, applied intracellularly, but not by N-de sulfated heparin, which is not an antagonist at these receptors. The C a2+ response caused by ATP was also blocked by a phospholipase C inhib itor, U-73122, but not by its inactive analog, U-73343. Increases in [ Ca2+](i) were elicited by intracellular application of activators of h eterotrimeric G-proteins, GTP gamma S and AIF(4)(-). On the other hand , [Ca2+](i) was unaffected by a G-protein inhibitor, GDP beta S, but i t did abolish the Ca2+ response to ATP. Pretreating the cultures with pertussis toxin did not affect responses to ATP. Our results indicate that in astrocytes ATP-evoked release of intracellular Ca2+ is mediate d by IP3 produced as a result of activating phospholipase C coupled to ATP receptors via a G-protein that is insensitive to pertussis toxin. ATP is known to be released under physiological and pathological circ umstances, and therefore signaling via the PLC-IP3 pathway in astrocyt es is a potentially important mechanism by which ATP may play a role i n CNS function.