Figure 16-14 Molecular structure of the β-adrenergic receptor. Notice the three domains. The transmembrane domains act as a ligand-binding pocket. Cytoplasmic domains can interact with G proteins and kinases, such as β-adrenergic receptor kinase (β-ARK). The latter can phosphorylate and desensitize the receptor. (From Opie L: Receptors and signal transduction. In Opie LH [ed]: The Heart: Physiology and Metabolism. New York, Raven Press, 1991.)