Aryloxypropanolamine and catecholamine ligand interactions with the beta(1)-adrenergic receptor: evidence for interaction with distinct conformations of beta(1)-adrenergic receptors

Pharmacological responses to aryloxypropanolamines were examined in cells expressing rat or human beta(1)-adrenergic receptors (ARs) using adenylyl cyclase assays. The aryloxypropanolamines CGP 12177 and LY 362884, originally developed as beta(3)-AR agonists, were found to stimulate the beta(1)-AR. Interestingly, both CGP 12177 and LY 362884 exhibited an anomalous biphasic effect on beta(1)-AR. Low concentrations of either CGP 12177 or LY 362884 potently blocked isoproterenol-induced stimulation of beta(1)-AR, whereas higher concentrations of these compounds stimulated the beta(1)-AR. The unusual interaction of these aryloxypropanolamine ligands with the beta(1)-AR was further characterized using beta-AR antagonists. Activation of beta(1)-AR by CGP 12177 or LY 362884 was observed to be significantly more resistant to blockade by beta-AR antagonists compared with activation by catecholamines. These results suggest that catecholamines and aryloxypropanolamines interact with distinct active conformations of the beta(1)-AR: a state that is responsive to catecholamines and is blocked with high affinity by CGP 12177 and LY 362884, and a novel state that is activated by aryloxypropanolamines but is resistant to blockade by standard beta-AR antagonists. Moreover, dependence of antagonist affinity on agonist structure is unprecedented, and its implications on the use of beta-AR agonists such as CGP 12177 in receptor classification are discussed.