Polyphosphoinositide Breakdown As the Initiating Reaction in Receptor-stimulated Inositol Phospholipid Metabolism

All cell-surface receptors that bring about a rise in cytosol Ca2+ concentration upon stimulation appear also to provoke enhanced metabolism of inositol phospholipids. For many years, it has been thought that the initiating reaction in this response is phosphodiesterase-catalysed breakdown of phosphatidylinositol (PtdIns). However, recent experiments with hepatocytes, parotid gland and blowfly salivary gland have demonstrated very rapid breakdown of phosphatidylinositol-4, 5-bisphosphate (PtdIns4,5P2), and maybe also of PtdIns4P, in cells stimulated by Ca2+-mobilizing stimuli (V1-vasopressin, angiotensin, alpha 1-adrenergic, muscarinic cholinergic, substance P and 5-hydroxytryptamine). As with the disappearance of PtdIns that had been studied previously, this response is not Ca2+-mediated and shows a receptor occupation dose-response curve. The PtdIns 'breakdown' studied previously was probably utilization of PtdIns for resynthesis of polyphosphoinositides to replace the degraded PtdIns4,5P2. We suggest that the primary event in receptor-stimulated inositol phospholipid metabolism is phosphodiesterase attack upon PtdIns4,5P2 to yield 1,2-diacylglycerol and inositol-1,4, 5-trisphosphate, and that this is an essential coupling event in a general mechanism by which receptors mobilize Ca2+ in the cytosol of stimulated cells.