Adrenoceptors can be subdivided into three major types, the α1-, α2-, and β-adrenoceptors. Each of these types can be further subdivided into three subtypes, based on pharmacological characteristics. Molecular cloning techniques have supported this subclassification. Recent data now suggest that α-adrenoceptor subtypes identified by pharmacological and molecular techniques correspond well, although species orthologs of several adrenoceptor subtypes have been identified. The secondary structure of the adrenoceptors has been elucidated and correlated with their interaction with second messenger molecules. α1-Adrenoceptors, β-adrenoceptors, and α2-adrenoceptors mediate their actions through stimulation of inositol phosphate release, stimulation of adenylate cyclase, and inhibition of adenylate cyclase, respectively. Site-directed mutagenesis and the preparation of chimeric receptors have located the site of receptor-second messenger interaction to the third intracellular loop for each of these adrenoceptors. While subtypes of each of these classes all interact with the same second messenger, studies with recombinant α2-adrenoceptors show subtype-related differences in receptor-second messenger interaction. Multiple α-adrenoceptor subtypes are expressed in vascular smooth muscle and are involved in various aspects of blood vessel function, including contraction, cellular growth, and proliferation. Various physiological factors can selectively influence responses to a particular subtype, and the relative roles of each subtype can vary between vascular beds and along an individual blood vessel as its caliber changes. Functional studies in blood vessels suggest the presence of additional α-adrenoceptor subtypes not yet identified via molecular techniques. Optimization of the therapeutic profile of an α-adrenoceptor antagonist may be possible via enhancement of selectivity for a particular subtype or by design of a specific profile of affinity for the individual subtypes.
- adrenoceptor subclassification
- second messenger
- smooth muscle proliferation
ASJC Scopus subject areas
- Physiology (medical)