Inositol trisphosphate

Inositol trisphosphate or inositol 1,4,5-trisphosphate (also commonly known as triphosphoinositol; abbreviated InsP3 or IP3), together with diacylglycerol (DAG), is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that is located in the plasma membrane, by phospholipase C.

Mechanism
IP3 binds to and activates the InsP3 receptor on the membrane of the endoplasmic reticulum (ER) and sarcoplasmic reticulum (SR), opening 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+ through a process known as Calcium-induced calcium release.

Human
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.

In the nervous system, IP3 serves as a second messenger, with the cerebellum containing the highest concentration of IP3 receptors. There is evidence that IP3 receptors plays an important role in the induction of plasticity in cerebellar Purkinje cells.

Sea urchin eggs
The slow block to polyspermy in the sea urchin is mediated by the phosphatidyl inositol diphosphate (PIP2) secondary messenger system. Activation of the binding receptors activates phospholipase C, which cleaves PIP2 in the egg plasma membrane, releasing inositol triphosphate (IP3) into the egg cell cytoplasm. IP3 diffuses to the endoplasmic reticulum, where it opens calcium channels.