Primary structure of the human kidney a2-adrenoceptor. The amino acid sequence is represented by the one-letter code. (Reprinted with permission from Regan JW et al. Cloning and expression of a human kidney cDNA. Proc Natl Acad Sci USA 85:6301, 1988.)

The variety of possible responses is further increased by the fact that receptor-coupled G proteins can either activate enzymes other than adenylate cyclase or can directly influence ion channel functions.

Many different receptor types are coupled to G proteins, including receptors for norepinephrine and epinephrine (a- and (3-adrenoceptors), 5-hydroxytrypta-mine (serotonin or 5-HT receptors), and muscarinic acetylcholine receptors. Figure 2.1 presents the structure of one of these, the a2-adrenoceptor from the human kidney. All members of this family of G protein-coupled receptors are characterized by having seven membrane-enclosed domains plus extracellular and intracellular loops. The specific binding sites for agonists occur at the extracellular surface, while the interaction with G proteins occurs with the intracellular portions of the receptor. The general term for any chain of events initiated by receptor activation is signal trans-duction.

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