IP3
IP3 is a key that unlocks the cell's calcium pantry. Cells keep large amounts of calcium stockpiled in an internal compartment, sealed away from the rest of the cell. When IP3 appears, it opens that store and lets calcium flood out to act as a signal.
Chemically, IP3 stands for inositol trisphosphate. It is generated when a hormone activates a Gq-coupled receptor, which switches on the enzyme phospholipase C; that enzyme splits a membrane lipid into two second messengers — IP3 and diacylglycerol. IP3 diffuses into the cytoplasm and binds channels on the endoplasmic reticulum, releasing stored calcium into the cell.
This makes IP3 the trigger for the calcium half of one of the body's two great signaling systems (the other being cyclic AMP). Many hormones that need rapid calcium-driven effects — such as some actions of angiotensin II, vasopressin, and GnRH — work through the IP3 pathway.
When angiotensin II binds cells of the adrenal zona glomerulosa, IP3 releases internal calcium, helping trigger aldosterone production.
IP3 unlocking internal calcium stores in response to a hormone.
IP3 and diacylglycerol are produced together from the same membrane lipid, so the same hormone often launches both the calcium arm (via IP3) and the protein kinase C arm (via diacylglycerol) at once.