Detection of β-Adrenergic Receptors by Radioligand Binding

The catecholamines epinephrine and norepinephrine produce a wide variety of physiological responses in target cells. Some of these are mediated by β-adrenergic receptors (β-ARs) through the stimulation of adenylyl cyclase and the production of cyclic AMP as first demonstrated by Sutherland and his colleagues (1 ). Although β-ARs were initially divided into β₁ -AR (norepinephrine ≥epinephrine) and β₂ -AR (epinephrine > norepinephrine) subtypes by Lands et al. (2 ), a third subtype, the β₃ -AR, has now been recognized. The latter was first identified in adipocytes based on atypical responses to antagonists and later to novel agonists (reviewed in ref. 3 ). The three subtypes, from both human and rat, have been cloned, and although they share limited homology in amino acid sequences, they all are members of the seven membrane-spanning rhodopsin superfamily, and couple to the stimulatory G-protein and activate adenylyl cyclase.