IP3 is the cytosolic messenger formed after activation of many types of receptors and then responsible for generating highly organized increases in intracelular Ca2+ concentration. A family of IP3 receptors has been identified and from the structures of these receptors we are beginning to understand how IP3 binding leads to opening of an intrinsic Ca2+ channel and how that process is modulated by other intracellular signals. Receptors that stimulate IP3 formation often trigger transient elevations in cytoplasmic Ca2+ concentration (Ca2+ spikes), the frequency of which may depend on the concentration of the extracellular stimulus. This frequency-coded Ca2+ signaling system may allow cells to maintain sustained responses to extracellular stimuli without the potentially damaging consequences of a sustained increase in cytoplasmic Ca2+ concentration. Our growing understanding of IP3 receptor structure, recognition of the close relationship between IP3 receptors and another family of intracellular Ca2+ channels, ryanodine receptors, and evidence that IP3 receptors are the site to which many intracellular signals converge, are together beginning to provide an understanding of the complex organization of intracellular Ca2+ signals. © 1997 Elsevier B.V. All rights reserved.
