<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>

Which second messenger molecule acts on the endoplasmic reticulum to release calcium ions