Toward a Consensus on the Operation of Receptor-Induced Calcium Entry Signals

Receptor-induced Ca 2+ signals involve both Ca 2+ release from intracellular stores and extracellular Ca 2+ entry across the plasma membrane. The channels mediating Ca 2+ entry and the mechanisms controlling their function remain largely a mystery. Here we critically assess current views on the Ca 2+ entry process and consider certain modifications to the widely held hypothesis that Ca 2+ store emptying is the fundamental trigger for receptor-induced Ca 2+ entry channels. Under physiological conditions, receptor-induced store depletion may be quite limited. A number of distinct channel activities appear to mediate receptor-induced Ca 2+ entry, and their activation is observed to occur through quite diverse coupling processes.