The role of calcium in stimulus-secretion coupling in excitable and non-excitable cells.
Secretion of vesicular contents by exocytosis is a common feature of excitable (neurones, chromaffin cells, beta cells) and non-excitable cells (platelets, neutrophils, mast cells). The simplistic view that the universal mechanism controlling secretion is elevation of [Ca2+]i--whatever the source of this second messenger may be--is no longer tenable in view of recent reports demonstrating secretion at basal or even reduced [Ca2+]i. It is nevertheless clear that in excitable cells an increase in [Ca2+]i is the triggering event that induces secretion. In non-excitable cells, secretion is presumably triggered by other second messengers, although [Ca2+]i appears to act as an important modulator of the rate of secretion. Conversely, these second messenger systems may serve a regulatory function in excitable cells. Given the relative importance of [Ca2+]i in the regulation of cellular functions in excitable and non-excitable cells, it is not surprising that several mechanisms are expressed in these cells to regulate intracellular calcium concentration. The major pathway for Ca2+ in excitable cells is by voltage-activated Ca2+ channels, but release of Ca2+ from intracellular stores, via second messengers, predominates in non-excitable cells, and may also be important in excitable cells. In addition, receptor-operated channels and second messenger-gated conductances may prove to be important. All of these pathways are subject to regulation by a variety of interactive second messenger systems, which provide necessary tuning for an appropriate control of intracellular calcium level.
Penner
R
R
Max-Planck-Institut für Biophysikalische Chemie, Göttingen, FRG.
Neher
E
E
D000818:Animals; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology; D005089:Exocytosis; D019204:GTP-Binding Proteins / Q000502:physiology; D006801:Humans; D009489:Neurosecretion; D011956:Receptors, Cell Surface / Q000502:physiology; D015290:Second Messenger Systems
D015220:Calcium Channels; D011956:Receptors, Cell Surface; D019204:GTP-Binding Proteins; D002118:Calcium