Calcium homeostasis and the activation of calcium channels in cells of the immune system.

In summary, calcium seems to play a central role in the activation of cells of the immune system. When the cells are stimulated, [Ca2+]i generally increases as a result of entry from the external medium, as well as mobilization of calcium from intracellular membrane-bound compartments. It is generally acknowledged that inositol 1,4,5-trisphosphate mediates the release of calcium from internal stores. By comparison, relatively little is known about the mechanism underlying entry of calcium across the plasma membrane. Currently available evidence seems to rule out the participation of classic voltage-gated channels. Similarly, it appears unlikely that the interaction of ligands with their surface receptors directly activates the channels. Instead, it is more likely that a second messenger produced by the formation of the receptor-ligand complex is responsible for gating the channels. Several mediators, including calcium itself and inositol phosphates, have been proposed to effect gating of the channels, but conclusive evidence is not yet available. Ongoing work is likely to reveal the nature of the putative second messenger(s) in the near future.