Beta-Adrenergic receptor interactions. Direct comparison of receptor interaction and biological activity

Iodohydroxybenzylpindolol (I-HYP) is a chemically defined, high affinity, high specific activity beta-adrenergic antagonist that interacts with a single site on the turkey erythrocyte membrane. Study of the interaction of agonists, antagonists, and congeners with this site and concomitant alterations in adenylate cyclase activity have been carried out in the presence of high or low concentrations of guanine nucleotide. The results help clarify the relationship between binding and activation or inhibition of adenylate cyclase and the role of guanine nucleotides in modulating this interaction. There is a close correlation between binding constants (KD) for inhibitors determined by analysis of competitive displacement of 125I-HYP from receptor, and apparent affinities (Ki) for inhibition of adenylate cyclase. For activators, however, there is up to a 10-fold difference between KD and apparent affinity (KDapp) for adenylate cyclase activation at low guanine nucleotide concentration (10(-6) M guanylylimidodiphosphate). This difference is virtually abolished by employing higher nucleotide concentrations (10(-5) M guanylylimidodiphosphate) without significantly altering receptor affinity. This suggests that guanine nucleotides act by modulating receptor-enzyme interactions rather than hormone-receptor interactions. Moreover, several beta-adrenergic analogs previously shown to have no effect on adenylate cyclase in the absence of nucleotide, are partial agonists in the presence of 10(-5) M guanylylimidodiphosphate. Parallel analyses for a series of agonists and antagonists for adenylate cyclase activation and receptor interaction show affinities for levorotatory isomers generally 100-fold greater than for dextrorotatory isomers. Thus stereoconfiguration at the beta carbon clearly influences affinity of agonists or antagonists. Affinity is also importantly influenced by the nature of the aromatic ring as well as the N-alkyl group. The complexity of structure-function relationships for these compounds requires a redefinition of structural requirements for beta-adrenergic activity.