Inositol triphosphate

Inositol trisphosphate or inositol 1,4,5-trisphosphate (also commonly known as triphosphoinositol; abbreviated InsP3 or IP3), together with diacylglycerol, is a second 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.

Its main functions are to mobilize Ca2+ from storage organelles and to regulate cell proliferation and other cellular reactions. For example, in the fruit fly Drosophila, InsP3 is used for intracellular transduction of light recognition in eye cells.

In smooth muscle cells, IP3 binds to and activates the InsP3 receptor on the membrane of the sarcoplasmic reticulum (SR) opens a calcium channel, resulting in the release of Ca2+ into the sarcoplasm. This increase in Ca2+ activates the ryanodine receptor-operated channel on the SR, leading to a further increase in the Ca2+ concentration in the muscle cell, resulting in the contraction of the muscle cell.

Inositol tetra-, penta-, and hexa-phosphates have been implicated in gene expression by Shen and Steger (both in Science Magazine).