= Inositol trisphosphate
{title2=$\mathrm{IP}_3$}
{wiki}
= IP3
{c}
{synonym}
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>.<The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper/Pathways of Intracellular Signal Transduction>[Phospholipids and Ca2+, Figure 13.27: soluble IP3 and intracellular calcium release]{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>.
\Image[/ip3-membrane-to-calcium-store-pathway.png]
{title=From extracellular reception to <IP3>-dependent release of stored <calcium>}
{description=Original <G protein-coupled receptor> pathway schematic. Dashed arrows relay activation; solid arrows show products or <calcium> movement. Membrane-associated <PIP2> yields soluble <IP3> and membrane-associated <diacylglycerol>. Release, pumping into the <ER> and export are separate flows; shapes encode no measured geometry or timing.}
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