Profile of ligand binding to the porcine β2-adrenergic receptor

Chinese hamster ovary cells expressing the porcine beta (2)-adrenergic receptor (beta AR) were used to determine the binding kinetics of agonists and antagonists by competitive displacement of the radioligand [I-125]iodocyanopindolol. Several purported agonists, including isoproterenol, epinephrine, norepinephrine, dobutamine, salbutamol, and terbutaline, exhibited dual-affinity displacement curves, which is characteristic of agonist binding to beta AR. In each case, the addition of guanosine triphosphate (GTP) eliminated the high-affinity state and resulted in a one-site displacement curve. All of the antagonists modeled to only one site in the presence or absence of GTP. Several ligands, including ones used to promote animal growth (clenbuterol, L-644,969, and ractopamine) and the beta (3)AR-selective agonist BRL 37344 modeled to only one site, suggesting that these ligands would not be full agonists at the porcine beta (2)AR (p beta (2)AR). Most of the tested ligands exhibited binding affinities that were similar to published values for the beta (2)AR from other species. However, several exceptions were observed. The BRL 37344 ligand bound the p beta (2)AR with a 10-fold higher affinity than the human beta (2)AR, and the K-d Of this was similar to K-d values reported for the human and rat beta (3)AR. The K-d of the p beta (2)AR for ICI 118,551 was 50-fold higher than that for the beta (2)AR from rats and humans. For both BRL 37344 and ICI 118,551 the subtype-selective character of these ligands was different in the pig compared with the human and rat. These data demonstrate the value of using species-specific beta AR for selection of agonists and antagonists. Further, these data support the growing evidence that few ligands are full agonists for p beta AR and that binding data may be useful for identifying ligands With full agonist potential.