The putative β4-adrenergic receptor is a novel state of the β1-adrenergic receptor

The atypical beta (3)-adrenergic receptor (AR) agonist CGP-12177 has been used to define a novel atypical beta -AR subtype, the putative beta (4)-AR. Recent evaluation of recombinant beta -AR subtypes and beta -AR-deficient mice, however, has established the identity of the pharmacological beta (4)-AR as a novel state of the beta (1)-AR protein. The ability of aryloxypropanolamine ligands like CGP-12177 to independently interact with agonist and antagonist states of the beta (1)-AR has important implications regarding receptor classification and the potential development of tissue-specific beta -AR agonists.