Measurement of inositol(1,4,5)trisphosphate using a stereospecific radioreceptor mass assay.

Inositol(1,4,5)trisphosphate [Ins(1,4,5)P(3)] is an intracellular second messenger that plays an important role in calcium homeostasis and, thus, many diverse cellular processes including neuronal signaling, smooth muscle contraction, fertilization, and sensory perception. Ins(1,4,5)P(3) formation is triggered by the activation of a wide variety of seven-transmembrane, G protein-linked receptors, e.g., muscarinic, glutamate, dopamine, and opioid receptors (1-3), as well as by the activation of the tyrosine kinase-linked growth factor receptors. Ins(1,4,5)P(3) is produced by the phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate, and is metabolized by 3-kinase and 5-phosphatase, with the actual intracellular concentration of Ins(1,4,5)P(3) being dependent on the balance between formation and metabolism. Ins(1,4,5)P(3) in turn binds to the Ins(1,4,5)P(3) receptor on the smooth endoplasmic reticulum, causing a conformational change that opens the intrinsic calcium channel in the receptor, thus allowing the efflux of calcium ions from the intracellular stores. For further details, see the reviews by Berridge and Furuichi and Mikoshiba.