The role of a Ca2+/calmodulin dependent plasma membrane Ca2+ channel during concanavalin A activation of MC9 mast cells.

The effects of Con A on free cytoplasmic calcium concentrations in the cloned murine mast cell, MC9, have been measured using the fluorescent calcium indicator quin 2. Con A causes a rapid, small yet sustained rise in free cytosolic calcium (up to 245 nM) followed closely by increased 45calcium uptake and more slowly by histamine release. The increases in 45calcium uptake and histamine release require extracellular calcium. However, the Ca2+ influx blockers, nifedipine and verapamil inhibit these responses only at concentrations significantly higher than those used in smooth muscle to oppose potential-dependent events, and diltiazem is inactive. These observations suggest that, in these mast cells, other types of channels control Ca2+ entry. In contrast, the intracellular Ca2+ blocker, TMB-8, inhibits both the Con A-induced histamine release and the Ca2+ changes. The calmodulin antagonists calmidazolium, trifluoperazine and W-7 are also highly effective inhibitors of both the Ca2+ changes and histamine release in direct proportion to their potency against calmodulin-dependent phosphodiesterase, implicating calmodulin in the regulation of stimulus-secretion in MC9 cells. These data imply that histamine release follows increases in intracellular Ca2+ concentration. Free intracellular Ca2+ results from rapid release from internal stores and is followed by a slower but more sustained influx of extracellular Ca2+.