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Second Messenger
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. |