The Role of Second Messengers in Receptormediated Responses

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The adrenomimetic drugs, including the naturally occurring catecholamines, initiate their responses by combining with a-, (3-, or dopamine adrenoceptors. This interaction triggers a series of biochemical events starting within the effector cell membrane that eventually culminates in the production of a physiological response, for example contraction, secretion, relaxation, or altered metabolism. The total process of converting the action of an external signal (e.g., norepinephrine interacting with its receptor) to a physiological response (e.g., vascular smooth muscle contraction) is called signal transduction.

Following the binding of the agonist (the first messenger) to its appropriate receptor on the external surface of the effector cell, a second messenger is generated (or synthesized) and participates in a particular series of biochemical reactions that ultimately result in the generation of a specific physiological response by that cell (Figs. 10.2 and 10.3). For both a- and (-adrenoceptors, the signal transduction process seems to involve the participation of G proteins (see Chapter 2).

The specific second-messenger pathways constitute a highly versatile signaling system that can modify (stimulate or inhibit) numerous cellular processes including secretion, contraction and relaxation, metabolism, neuronal excitability, cell growth, and apoptosis. The second messengers that participate in signal transduction include cyclic adenosine monophosphate (cAMP), diacylglycerol, and inositol triphosphate. Once liberated within the cell, second messengers will activate specific



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