= IP3-sensitive calcium store
{c}
An <IP3-sensitive calcium store> contains releasable <calcium> behind a membrane bearing functional <IP3 receptors>. The <endoplasmic reticulum> is an important example. Pumping <calcium> into it or exporting <calcium> across the <cell membrane> lowers free cytosolic <calcium>; intracellular redistribution and loss from the whole <biological cell> are different balances.<The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper/Pathways of Intracellular Signal Transduction>[Phospholipids and Ca2+, paragraph accompanying Figure 13.27: calcium pumps across plasma and ER membranes]{ref}
Finite availability matters: a release pathway cannot deliver more <calcium> than its accessible store contains unless replenishment occurs. A falling cytosolic <calcium> response can reflect reduced release, increased removal, or both. Store content, release rate and free cytosolic <concentration> therefore answer different questions. Binding to <proteins> can also change the free fraction without changing the total number of <calcium> <ions>.
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