Second Messenger Systems and Signal Transduction Mechanisms

Abstract

Hormones are secreted, reach their target, and bind to a receptor. The interaction of the hormone with the receptor produces an initial signal that through a series of steps, results in the final hormone action. How does the binding of a hormone to a receptor result in a cellular action? For example, in times of stress, epinephrine is secreted by the adrenal glands, is bound by receptors in skeletal muscle, and results in the hydrolysis of glycogen and the secretion of glucose. Signal transduction is the series of steps and signals that links the receptor binding of epinephrine to the hydrolysis of glycogen. Signal transduction can be simple or complex. There can be only one or two steps between receptor and effect, or intestinal tract and the massive diarrhea that characterizes cholera. Pertussis toxin causes the ADP ribosylation of the α subunit of G_j. This results in uncoupling of thr G-protein from the receptor, and leads to constitutive activation of adenylyl cyclase and increased leves of cAMP.