The binding of ligands (\u201cfirst messengers\u201d) to many cellsurface receptors leads to a short-lived increase (or decrease) in the concentration of certain low-molecular- weight intracellular signaling molecules termed second messengers. These molecules include 3',5'-cyclic AMP (cAMP), 3',5'- cyclic GMP (cGMP), 1,2-diacylglycerol (DAG), and inositol 1,4,5-trisphosphate (IP3). Other important second messengers are Ca2 and various inositol phospholipids, also called phosphoinositides, which are embedded in cellular membranes.\nThe elevated intracellular concentration of one or more second messengers following binding of an external signaling molecule triggers a rapid alteration in the activity of one or more enzymes or nonenzymatic proteins. In muscle, a signalinduced rise in cytosolic Ca2 triggers contraction. A similar increase in Ca2 induces exocytosis of secretory vesicles in endocrine cells and of neurotransmitter containing vesicles in nerve cells.