BIOCHEMICAL AND PHARMACOLOGICAL CHARACTERIZATION OF HIGH-AFFINITY TRIMETOQUINOL ANALOGS ON GUINEA-PIG AND HUMAN BETA-ADRENERGIC-RECEPTOR SUBTYPES - EVIDENCE FOR PARTIAL AGONISM

Radioligand binding assays were used to characterize the interaction o f a series of trimetoquinol zyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoqu inoline; TMQ} analogs with beta adrenergic receptors (beta-AR). The re sults indicated that TMQ analogs bound with similar affinities to guin ea pig (heart, lung and skeletal muscle) and human (beta-AR in Escheri chia coli) beta-1- and beta-2-AR subtypes. However, the isomers of TMQ and 8-fluoro-TMQ bound stereoselectively to beta-AR with the S-isomer s having affinities at least 112- and 8-fold greater, respectively, th an their corresponding R-isomers. In general, a direct relationship ex isted between TMQ analog binding to guinea pig beta-AR and functional activity on guinea pig right atria (beta-1) and trachea (beta-2). For selected halogenated TMQ analogs (3',5'-diiodo-TMQ, 3'-iodo-TMQ, 5,8-d ifluoro-TMQ and 5-iodo-TMQ) which had higher beta-AR affinities than T MQ, but were less potent beta-AR agonists than TMQ, this relationship was not seen. To explain this, the function of the TMQ analogs was ana lyzed at the level of the beta-AR-associated effector mechanism (i.e., G-protein and adenylyl cyclase). In Chinese hamster ovary cells expre ssing human beta-2-AR, TMQ and halogenated analogs bound to the recept or with high affinity (nanomolar range); however, they failed to effec tively couple with beta-AR-associated G-protein and only partially act ivated receptor-associated adenylyl cyclase. Receptor occupancies of 0 .14, 2 and 23% were required for (-)-isoproterenol, S-(-)-TMQ and 3'5' -diiodo-TMQ to produce equivalent cyclic AMP accumulations in human be ta-2-AR Chinese hamster ovary cells. Thus, TMQ and halogenated TMQ der ivatives bind stereoselectively to beta-AR with high affinity, and may be classified as partial beta-AR agonists.