Low cytoplasmic [Ca(2+)] activates I(CRAC) independently of global Ca(2+) store depletion in RBL-1 cells

Release of Ca(2+) from inositol (1,4,5)-trisphosphate-sensitive Ca(2+) stores causes "capacitative calcium entry," which is mediated by the so-called "Ca(2+) release-activated Ca(2+) current" (I(CRAC)) in RBL-1 cells. Refilling of the Ca(2+) stores or high cytoplasmic Ca(2+)] (Ca(2+)](cyt)) inactivate I(CRAC). Here we address the question if also Ca(2+)](cyt) lower than the resting Ca(2+)](cyt) influences store-operated channels. We therefore combined patch clamp and mag fura-2 fluorescence methods to determine simultaneously both I(CRAC) and Ca(2+)] within Ca(2+) stores of RBL-1 cells (Ca(2+)](store)). We found that low Ca(2+)](cyt) in the range of 30-50 nM activates I(CRAC) and Ca(2+) influx spontaneously and independently of global Ca(2+) store depletion, while elevation of Ca(2+)](cyt) to the resting Ca(2+)](cyt) (100 nM) resulted in store dependence of I(CRAC) activation. We conclude that spontaneous activation of I(CRAC) by low Ca(2+)](cyt) could serve as a feedback mechanism keeping the resting Ca(2+)](cyt) constant.