Intracellular Signal Transduction

The various intracellular pathways that transduce signals downstream from activated cell-surface receptors differ in their complexity and in the way they transduce signals. We describe the components and operation of many individual pathways later in this chapter and in other chapters. Some general principles of signal transduction, applicable to different pathways, are covered in this section.

Second Messengers Carry Signals from Many Receptors

The binding of ligands ("first messengers") to many cell-surface 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), whose structures are shown in Figure 13-7. Other important second messengers are Ca2+ and various inositol phospholipids, also called phosphoinosi-tides, 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 signal-induced rise in cytosolic Ca2+ triggers contraction (see Figure 19-28); a similar increase in Ca2+ induces exocytosis of secretory vesicles in endocrine cells and of neurotransmit-ter-containing vesicles in nerve cells (see Figure 7-43). Similarly, a rise in cAMP induces various changes in cell metabolism that differ in different types of human cells. The

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