Inositol trisphosphate (${\mathrm{IP}}_3$)

In signal transduction, inositol trisphosphate usually denotes inositol 1,4,5-trisphosphate, a small second messenger that can release stored calcium into the cytoplasm. It is distinct from the receptor sensing the extracellular ligand and from the ion channel carrying the eventual membrane electric current. The intracellular-injection experiment under metabotropic receptor illustrates that distinction.