Shallow agonist competition binding curves for beta-adrenergic receptors: the role of tight agonist binding

The beta 2-adrenergic receptors of bovine trapezius muscle membranes demonstrate tight agonist binding as a result of the formation of complexes between agonists, receptors, and Ns. Preincubation of the membranes with (-)-isoproterenol (followed by washing) causes a time- and concentration-dependent decrease in the number of radioligand-binding sites to a plateau value of 41.5 +/- 4%. The affinity of the remaining sites for the radioligand (-)-[3H]dihydroalprenolol is unchanged. This decrease is stable under radioligand binding conditions but is readily reversed in the presence of GTP. The isoproterenol/(-)-[3H]dihydroalprenolol competition binding curves are shallow. Such a phenomenon is usually interpreted in terms of two interconvertible affinity states of the receptor: the high affinity state reflecting the coupling of the agonist X receptor complex to Ns and the low affinity state not interacting with Ns. However, the competition curves undergo time-dependent shifts to the left. This apparent non-equilibrium can be explained by a model in which tight agonist binding to part of the receptor population is included. The usual computerized interpretation of the competition binding curves do not allow the correct evaluation of agonist binding parameters in the presence of tight agonist binding.