<P> Calcium ions are important for cellular signalling, as once they enter the cytosol of the cytoplasm they exert allosteric regulatory effects on many enzymes and proteins . Calcium can act in signal transduction resulting from activation of ion channels or as a second messenger caused by indirect signal transduction pathways such as G protein - coupled receptors . </P> <P> The resting concentration of Ca in the cytoplasm is normally maintained around 100 nM, variously reported as 20,000 - to 100,000-fold lower than typical extracellular concentration . To maintain this low concentration, Ca is actively pumped from the cytosol to the extracellular space, the endoplasmic reticulum (ER), and sometimes into the mitochondria . Certain proteins of the cytoplasm and organelles act as buffers by binding Ca . Signaling occurs when the cell is stimulated to release calcium ions (Ca) from intracellular stores, and / or when calcium enters the cell through plasma membrane ion channels . </P> <P> Specific signals can trigger a sudden increase in the cytoplasmic Ca level up to 500--1,000 nM by opening channels in the endoplasmic reticulum or the plasma membrane . The most common signaling pathway that increases cytoplasmic calcium concentration is the phospholipase C pathway . </P> <Ul> <Li> Many cell surface receptors, including G protein - coupled receptors and receptor tyrosine kinases activate the phospholipase C (PLC) enzyme . </Li> <Li> PLC hydrolyses the membrane phospholipid PIP2 to form IP3 and diacylglycerol (DAG), two classical second messengers . </Li> <Li> DAG recruits protein kinase C (PKC), attaching it to the plasma membrane </Li> <Li> IP diffuses to the endoplasmic reticulum, and binds to an IP3 receptor, </Li> <Li> The IP3 receptor serves as a Ca channel, and releases Ca from the endoplasmic reticulum . </Li> <Li> The Ca ions bind to PKC, activating it . </Li> </Ul>

Where is the calcium that triggers muscle contraction stored