Calcium: an important second messenger in mast cells.

Recently there has been considerable controversy over the mechanism(s) by which intracellular Ca2+ is elevated when receptors for IgE on the surface of mast cells are aggregated by antigen. The central role played by calcium in the initiation of secretion from these cells has also been called into question. In a mast cell line which has been widely used to study stimulus-secretion coupling in non-excitable cells it is now clear that calcium is indeed important in the physiological response of the cells but that other intracellular messengers are also involved. In addition it has been shown that while the increase in intracellular Ca2+ probably originates from intracellular stores it can only be sustained by the influx of calcium across the plasma membrane. The nature of the Ca2+ permeability pathway has yet to be elucidated although a number of candidates for the calcium channel in mast cells have been proposed. Significant oscillations and spatial gradients of Ca2+ are often seen when the responses of individual antigen-stimulated cells are measured using digital imaging microscopy. The complexity of these responses highlights the importance of single-cell measurements in elucidating the relationship between IgE receptor activation, Ca2+ movements and exocytosis.