Receptors, inositol polyphosphates and intracellular Ca2+.

1. A large number of cell-surface receptors catalyse the activation of phospholipase C via a guanine nucleotide exchange protein and generate at least two important intracellular second messengers. One of these, inositol(1,4,5) trisphosphate, binds to specific intracellular receptors and releases Ca2+ from intracellular stores. 2. Subsequent metabolism of this messenger is complex, proceeding either by dephosphorylation or phosphorylation routes with the latter generating inositol(1,3,4,5) tetrakisphosphate which may have additional functional significance in Ca2+ homeostasis. 3. Calculations of the relative rates of metabolism through these routes as well as the development of new analogues of inositol polyphosphates have helped our understanding of these important cell signalling systems.