Inositol trisphosphate Created 2026-10-13 Updated 2026-10-14
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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 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.
Figure 1.
From extracellular reception to IP3-dependent release of stored calcium
. 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. Details.
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IP3 production by phospholipase C 2026-10-14
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Phospholipase C catalyzes hydrolysis of membrane PIP2, producing soluble IP3 and membrane-associated diacylglycerol. This consumes a phospholipid substrate, rather than manufacturing calcium. G protein-regulated and receptor tyrosine kinase-regulated forms of PLC provide different routes to this reaction.[ref]
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