Norepinephrine is implicated in a wide range of physiological processes through activation of nine different G-protein-coupled receptors (alpha1a, alpha1b, alpha1d, alpha2a, alpha2b, alpha2c, beta1, beta2, beta3). The human beta2-adrenergic receptor was the first 7-transmembrane receptor for a hormone or neurotransmitter to have its crystal structure solved. It has been suggested that the beta 2-adrenoceptor may form homodimers as well as oligomers with other receptors. The beta 2-adrenoceptor mediates the actions of catecholamines in multiple tissues. They are responsible for relaxation of vascular, uterine, and airway smooth muscle, and are involved in metabolic and endocrine functions.