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