Beta-adrenergic receptors: structure, function and regulation.

The diverse effects of the catecholamines (CA) epinephine and norepinephrine are mediated by a family of specific receptors (adrenergic receptors, AR). The beta-AR is a glycoprotein present in the membrane of a number of cell types. This receptor is closely associated with at least two other proteins (Gs and adenylate cyclase enzyme, AC) to form the beta-AR complex. The beta-AR recognizes the CA and is coupled to Gs which stimulates the effector enzyme AC. This enzyme converts ATP to cAMP and is the effector of the beta-AR complex. Thus the beta-AR is a G-coupled receptor which acts by raising intracellular levels of cAMP. The beta-AR is an important site of regulatory modifications through a variety of mechanisms. The best characterized is known as homologous desensitization: when the receptor is exposed to repeated stimulus by the agonist (CA), its responsiveness wanes, probably to compensate this potentially dangerous overstimulation. The gene for mammalian beta2-AR has been recently cloned and the predicted amino-acid sequence now opens the field to identification of the protein structures involved in receptor functions. The beta2-AR protein is characterized by the presence of seven membrane spanning regions. Study of the structure, function and regulation of the beta-AR will extend our knowledge of the role of beta-AR in pathological conditions and suggest new therapeutic approaches.