 Inositol trisphosphate

In signal transduction, inositol trisphosphate usually means inositol 1,4,5-trisphosphate: a soluble intracellular second messenger. An extracellular ligand, its cell-surface receptor, IP3 and the released calcium ions are distinct components. IP3 binds an IP3 receptor on the endoplasmic reticulum, promoting stored ${\mathrm{Ca}}^{2+}$ release into the cytosol.^[ref]

The useful separation is spatial: an extracellular message can control an internal store without entering it. The existing intracellular-injection experiment under metabotropic receptor tests bypassing reception; the Pancreatic IP3 store fractionation experiment below tests which internal compartment supplies the released calcium. Neither question is answered merely by observing a downstream membrane electric current.