The binding of ligands (“first messengers”) 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.
The 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.