Calcium sequestration in the liver.
Hepatic parenchymal cells maintain intracellular total and cytosolic free Ca2+ levels by: entry of Ca2+ through channels, extrusion of Ca2+ by an outwardly directed Ca2+ pump, and controlled sequestration into intracellular pools. The mechanism of Ca2+ inflow is poorly characterized. The plasma membrane Ca2+ channels seem to share some of the characteristics of Ca2+ channels in excitable cells, but also differ from them. The outwardly directed plasma membrane Ca2(+)-ATPase is a calmodulin independent, P-type enzyme. Ca2+ uptake into the endoplasmic reticulum is due to the activity of a different Ca2(+)-ATPase, which is similar in molecular weight and shares antigenic determinants with the sarcoplasmic reticulum enzyme. In addition, mitochondria and nuclei also take up calcium. The exact mechanism by which Ca2+ is released from intracellular organelles is not well known. Several mechanisms for Ca2+ release from the endoplasmic reticulum were reported, including IP3 and GTP-induced. The most effective identified way of eliciting Ca2+ release from microsomal fraction is by the oxidation of critical -SH groups. This mechanism is likely to be involved in the rise of cytosolic Ca2+ observed in many situations of hepatocellular injury. In addition to being sequestered into subcellular organelles, some of the intracellular Ca2+ is bound to specific Ca2+ binding proteins. Both calmodulin and members of the annexin family were identified in the liver. Stimulation of the liver with gluconeogenic hormones results in increased Ca2+ entry into the cell, the release of Ca2+ from intracellular pools, and an oscillatory increase in free cytosolic Ca2+ levels. Extensive research is still needed for the elucidation of the exact mechanisms by which these events occur.
Kraus-Friedmann
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Department of Physiology and Cell Biology, University of Texas Medical School, Houston.
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D015388:Organelles / Q000378:metabolism; D015290:Second Messenger Systems / Q000502:physiology
D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D002118:Calcium