Molecular cloning has accelerated discovery of novel receptor subtypes, and their expression as recombinant proteins has facilitated discovery of subtype-selective drugs. Distinct but related receptors may, but may not, display distinctive patterns of selectivity among agonist or antagonist ligands. When selective ligands are not known, the receptors are more commonly referred to as isoforms rather than as subtypes. The distinction between classes and subtypes of receptors, however, often is arbitrary or historical. The a, aT and j receptors differ from each other both in ligand selectivity among drugs and in coupling to G proteins (Gq, G,, and Gs, respectively), yet a and j are considered receptor classes and a, and a2 are considered subtypes. The a^, a,B, and a1C receptor isoforms differ little in their biochemical properties, although their tissue distributions are distinct. The j,, P2, and fi3 adrenergic receptor subtypes exhibit both differences in tissue distribution and phosphorylation by either GRKs or PKA.
Pharmacological differences among receptor subtypes are exploited therapeutically through the development and use of receptor-selective drugs. Such drugs may be used to elicit different responses from a single tissue when receptor subtypes initiate different intracellular signals, or they may serve to differentially modulate different cells or tissues that express one or another receptor subtype. Increasing the selectivity of a drug among tissues or among responses elicited from a single tissue may determine whether the drug's therapeutic benefits outweigh its unwanted effects.
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