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.