Receptor-Mediated Ca2+ Signaling
Over the past few years it has become apparent that Ca2+ signaling in many different cell types is initiated by an agonist-induced breakdown of inositol lipids in the plasma membrane (Berridge and Irvine, 1984; Williamsonet al., 1985; Hokin, 1985). This is mediated by a receptor-coupled GTP-binding protein, which is thought to dissociate upon binding of suitable agonists to their receptors to aßγ-subunit complex and an α subunit containing bound GTP (Fitzgeraldet al., 1986; Dickeyet al, 1987). The GTP-boundαsubunit then promotes the activity of an inositol lipid phosphodiesterase (phospholipase C) that splits phosphatidylinositol-4,5-bisphosphate to a lipophilic 1,2-diacylglycerol (the phys iological activator of protein kinase C) and a water-soluble product, inositol-1,4,5-trisphosphate (Ins-1,4,5-P3). This inositol polyphosphate isomer acts as a Ca2+ -mobilizing second messenger by opening a Ca2+ channel located in a specialized intracellular pool of calcium (Joseph and Williamson, 1986).
KeywordsInositol Phosphate Inositol Trisphosphate Inositol Polyphosphate Myoinositol Phosphate Inositol Lipid
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