Stable Activation of Single Ca2+ Release-activated Ca2+ Channels in Divalent Cation-free Solutions

The regulation of store-operated, calcium-selective channels in the plasma membrane of rat basophilic leukemia cells (RBL-2H3 m1), an immortalized mucosal mast cell line, was studied at the single-channel level with the patch clamp technique by removing divalent cations from both sides of the membrane. The activity of the single channels in excised patches could be modulated by Ca2+, Mg2+, and pH. The maximal activation of these channels by divalent cation-free conditions occurred independently of depletion of intracellular Ca2+stores, whether in excised patches or in whole cell mode. Yet, a number of points of evidence establish these single-channel openings as amplified store-operated channel events. Specifically, (i) the single channels are exquisitely sensitive to inhibition by intracellular Ca2+, and (ii) both the store-operated current and the single-channel openings are completely blocked by the capacitative calcium entry blocker, 2-aminoethoxydiphenyl borane. In addition, in Jurkat T cells single-channel openings with lower open probability have been observed in the whole cell mode with intracellular Mg2+ present (Kerschbaum, H. H., and Cahalan, M. D. (1999) Science 283, 836–839), and in RBL-2H3 m1 cells a current with similar properties is activated by store depletion.