Sf. Mahmoud et C. Fewtrell, Microdomains of high calcium are not required for exocytosis in RBL-2H3 mucosal mast cells, J CELL BIOL, 153(2), 2001, pp. 339-349
We have previously shown that store-associated microdomains of high Ca2+ ar
e not essential for exocytosis in RBL-2H3 mucosal mast cells. We have now e
xamined whether Ca2+ microdomains near the plasma membrane are required, by
comparing the secretory responses seen when Ca2+ influx was elicited by tw
o 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 act
ivate I-CRAC, different means of inhibiting I-CRAC before ionophore additio
n were used. Ca2+ responses and secretion in individual cells were compared
using simultaneous indo-1 microfluorometry and constant potential amperome
try. Secretion still takes place when the increase in intracellular Ca2+ oc
curs diffusely via the Ca2+ ionophore, and at an average intracellular Ca2 concentration that is no greater than that observed when Ca2+ entry via CR
AC channels triggers secretion. Our results suggest that microdomains of hi
gh Ca2+ near the plasma membrane, or associated with mitochondria or Ca2+ s
tores, are not required for secretion. Therefore, we conclude that modest g
lobal increases in intracellular Ca2+ are sufficient for exocytosis in thes
e nonexcitable cells.