Signal Transduction and Second Messengers

Publisher Summary This chapter describes signal transduction as the mechanism by which cell surface receptors receive information from extracellular signals such as hormones and neurotransmitters, and amplify this information through the actions of second messengers. Receptors are the primary sensors of the cell environment, but ligand binding alone is useless without signal transduction. G-protein–coupled receptors are the most abundant class of receptors and are responsible for regulating a multitude of physiologic processes including vision, muscle contraction, secretion, and neurotransmission. The use of G-proteins as a relay between receptor ligand binding and effector activation has important implications toward effectiveness of signaling. Intracellular receptors require ligands that are membrane permeable and include receptors for steroid hormones, lipophilic vitamins, and small molecules such as nitric oxide and hydrogen peroxide. Despite the diversity of receptor subtypes, there is a relatively small subset of G-protein–coupled second messengers that are the convergence point for multiple receptors. Changes in gene transcription mediated by steroid receptors are relatively slow in onset yet result in long-term changes in gene expression.