The phosphoinositide cycle is now established as a major pathway by which a variety of hormones, neurotransmitters, and other signaling molecules can generate second messengers and thus affect the intracellular milieu (1 ). Receptor-mediated activation of membrane-associated enzymic activities act on the minor membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2 ] to generate three second messenger molecules: inositol 1,4,5-trisphosphate [Ins(1,4,5)P3 ] and 1,2-diacylglycerol via inositol phospholipid-specific phospholipase C (PI-PLC) activity, and phosphatidylinositol 3,4,5-trisphosphate via phosphoinositide 3-kinase activity (2 ).
