EFFECT OF ADENOPHOSTIN-A ON CA2-ACTIVATED CALCIUM CURRENT (I-CRAC) INRAT BASOPHILIC LEUKEMIA-CELLS( ENTRY AND CALCIUM RELEASE)

In most non-excitable cells, calcium influx is signaled by depletion o f intracellular calcium stores, a process known as capacitative calciu m entry. Adenophostin A, a potent activator of the inositol 1,4,5-tris phosphate receptor, has been reported to activate Ca2+ entry in Xenopu s oocytes to a greater extent than expected on the basis of its abilit y to release calcium stores. In this study, we compared the abilities of adenophostin A and inositol 2,4,5-trisphosphate ((2,4,5)IP3) to rel ease Ca2+ from intracellular stores, to activate Ca2+ entry, and to ac tivate calcium release-activated calcium current (I-crac) in rat basop hilic leukemia cells. Under conditions of low intracellular Ca2+ buffe ring (0.1 mM BAPTA), adenophostin A-induced Ca2+ release and activatio n of I-crac could be monitored simultaneously. However, other reagents that would be expected to deplete Ca2+ stores ((2,4,5)IP3, 3-fluoro-i nositol 1,4,5-trisphosphate, thapsigargin, and ionomycin) were unable to activate I-crac under this low Ca2+ buffering condition. Adenophost in A activated I-crac after a significant delay, longer than the delay for Ca2+ release. Thus, adenophostin A activates I-crac as a conseque nce of release of intracellular Ca2+, rather than directly acting on s tore-operated channels. The unique ability of adenophostin A to activa te I-crac under conditions of low intracellular Ca2+ buffering suggest s an additional site of action, perhaps in preventing or reducing rapi d Ca2+-dependent inactivation of store-operated Ca2+ channels.