Microdomains of high calcium are not required for exocytosis in RBL-2H3 mucosal mast cells

We have previously shown that store-associated microdomains of high Ca2+ are not essential for exocytosis in RBL-2H3 mucosal mast cells. We have now examined whether Ca2+ microdomains near the plasma membrane are required, by comparing the secretory responses seen when Ca2+ influx was elicited by two very different mechanisms. In the first, antigen was used to activate the Ca2+ release-activated Ca2+ (CRAC) current (I-CRAC) through CRAC channels. In the second, a Ca2+ ionophore was used to transport Ca2+ randomly across the plasma membrane. Since store depletion by Ca2+ ionophore will also activate I-CRAC, different means of inhibiting I-CRAC before ionophore addition were used. Ca2+ responses and secretion in individual cells were compared using simultaneous indo-1 microfluorometry and constant potential amperometry. Secretion still takes place when the increase in intracellular Ca2+ occurs diffusely via the Ca2+ ionophore, and at an average intracellular Ca2+ concentration that is no greater than that observed when Ca2+ entry via CRAC channels triggers secretion. Our results suggest that microdomains of high Ca2+ near the plasma membrane, or associated with mitochondria or Ca2+ stores, are not required for secretion. Therefore, we conclude that modest global increases in intracellular Ca2+ are sufficient for exocytosis in these nonexcitable cells.