A quantitative analysis of beta-adrenergic receptor interactions: resolution of high and low affinity states of the receptor by computer modeling of ligand binding data.

The properties of ligand binding to the beta-adrenergic receptor have been studied using a computer modeling technique to analyze data obtained by indirect binding methods. Antagonists are shown to bind to the receptor with one homogeneous state of uniform affinity, while agonists manifest heterogeneous binding. For agonists, two distinct binding states are apparent; one of high and one of low affinity. Affinity states of the receptor are characterized by specific macroscopic dissociation constants, and the proportion of total receptors in each state can be determined. The ability of an agonist to activate adenylate cyclase (intrinsic activity) correlates closely with the amount of high affinity state formed in the presence of the agonist (p < 0.001). A similar correlation exists between adenylate cyclase activation by an agonist and the ratio of dissociation constants of the agonist for the high and low affinity states of the receptor (p < 0.001). There is an apparent impairment of high affinity state formation in membranes derived from desensitized cells. Along with a decrease in the total number of receptors, this impairment of high affinity state formation may be responsible for the decreased activation of adenylate cyclase observed after desensitization. Guanyl nucleotides mediate the transition of high affinity receptors to the low affinity state. The extent of this transition is dose dependent and is evident even at low nucleotide concentrations. Our data indicate that high and low affinity states of the beta-adrenergic receptor are interconvertible, a finding incompatible with models of ligand-receptor interaction involving independent binding sites. Computer modeling of receptor binding data significantly enhances the ability to describe the interactions for adenylate cyclase-coupled receptor systems and validates the concept and biological importance of receptor affinity states for the beta-adrenergic receptor.