Calcium is a universal intracellular messenger controlling many cellular
processes from fertilisation to cell death. Changes in intracellular
calcium levels are strictly governed within defined parameters. In order to
regulate levels of cytosolic calcium within safe limits the cell has
developed a toolkit of release channels, pumps and storage/buffering
organelles. The main location of calcium storage within cells is the
endoplasmic reticulum (ER) or its muscle equivalent the sarcoplasmic
reticulum (SR). Calcium
is primarily released from stores by the dual action calcium and
Inositol, 1,4,5-trisphosphate (InsP3) on InsP3 receptors (R) or by the
action of calcium alone on receptors (RyR) a process known as calcium
induced calcium release (CICR). In addition to these direct modulators of
channel activity, both channels are regulated by numerous accessory
proteins and by secondary protein modifications such as
phosphorylation. The role and mechanism of action of some of these
accessory proteins will be
discussed.
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