Calcium ions are the most versatile and universal intracellular messengers discovered so far. They are involved in the regulation of almost all known cellular functions and reactions. The exceptions are few, the most notable probably being
Long-term adaptive changes
Figure 5.9 Versatility and ubiquity of calcium signalling. Calcium signals occur within different spatial and temporal domains controlling a wide variety of physiological reactions, ranging from immediate (exocytosis, modulation of ion channels, muscle contraction), to delayed (memory and long-term adaptive processes; see the text for further explanation). (From Toescu EC, Verkhratsky A (1998) Principles of calcium signalling. In: Integrative Aspects of Calcium Signalling, Verkhratsky A Toescu EC, Eds, Plenum Press, p. 2-22)
the propagation of nerve action potentials, which depends on Na+ and K+ channels that are not acutely Ca2+-regulated. The most important properties of Ca2+ signalling are the promiscuity with respect to its effector systems and its autoregulation. Indeed, Ca2+ regulates a truly remarkable variety of intracellular processes, within extremely different temporal domains, from microseconds (e.g. exocytosis) to months or even years (e.g. memory processes - see Figure 5.9). Physiological effects of Ca2+ ions are produced by intracellular Ca2+ sensors, represented by enzymes, which, upon Ca2+ binding, change their activity. These enzymes have different affinity and therefore sensitivity to Ca2+ ions (e.g. Ca2+ sensors in the cytosol are regulated by Ca2+ in concentrations of hundreds of nM, whereas Ca2+-dependent enzymes in the endoplasmic reticulum are sensitive to Ca2+ concentrations in the range of 100-1000 ^M). Furthermore the intracellular Ca2+ sensors are localized in different parts of the cell, and therefore local Ca2+ gradients may specifically regulate particular sets of Ca2+ -dependent processes. These peculiarities of intracellular Ca2+ effector systems allow for amplitude and space encoding of Ca2+ signals and solve the problem of specificity of such an intrinsically promiscuous system.
The actual molecular systems responsible for controlling intracellular Ca2+ homeostasis and producing Ca2+ signalling events are limited to several protein families represented by Ca2+ channels and Ca2+ transporters. These systems are very much conserved and ubiquitously expressed within the cellular kingdom. Most importantly, all these systems are regulated by Ca2+ itself, thus making a very robust, albeit versatile and adaptable piece of molecular machinery.
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