Inositol Triphosphate (IP3)

Inositol trisphosphate is a secondary messenger molecule used in signal transduction in biological cells. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that is located in the plasma membrane, by phospholipase C. IP3 binds to and activates the InsP3 receptor on the membrane of the endoplasmic reticulum (ER), and sarcoplasmic reticulum (SR) opens a calcium channel, resulting in the release of Ca2 into the cytoplasm, and sarcoplasm respectively. This increase in Ca2 activates the ryanodine receptor-operated channel on the SR, leading to a further increase in the Ca2 .

Its main functions are to mobilize Ca²⁺ from storage organelles and to regulate cell proliferation and other cellular reactions.In smooth muscle cells, for example, the increase in concentration of cytoplasmic calcium results in the contraction of the muscle cell.