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 of a series of trimetoquinol [1-(3',4',5'-trimethoxybenzyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoqui nol ine; TMQ] analogs with beta adrenergic receptors (beta-AR). The results indicated that TMQ analogs bound with similar affinities to guinea pig (heart, lung and skeletal muscle) and human (beta-AR in Escherichia 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-isomers having affinities at least 112- and 8-fold greater, respectively, than their corresponding R-isomers. In general, a direct relationship existed 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-difluoro-TMQ and 5-iodo-TMQ) which had higher beta-AR affinities than TMQ, but were less potent beta-AR agonists than TMQ, this relationship was not seen. To explain this, the function of the TMQ analogs was analyzed at the level of the beta-AR-associated effector mechanism (i.e., G-protein and adenylyl cyclase). In Chinese hamster ovary cells expressing human beta-2-AR, TMQ and halogenated analogs bound to the receptor with high affinity (nanomolar range); however, they failed to effectively couple with beta-AR-associated G-protein and only partially activated 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 beta-2-AR Chinese hamster ovary cells. Thus, TMQ and halogenated TMQ derivatives bind stereoselectively to beta-AR with high affinity, and may be classified as partial beta-AR agonists.
Fraundorfer
P F
PF
Division of Pharmacology, College of Pharmacy, Ohio State University, Columbus.
Fertel
R H
RH
Miller
D D
DD
Feller
D R
DR
D000318:Adrenergic beta-Agonists / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive; D016466:CHO Cells; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D006168:Guinea Pigs; D006801:Humans; D007700:Kinetics; D011869:Radioligand Assay; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000235:genetics; D014213:Tretoquinol / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D011943:Receptors, Adrenergic, beta; D000242:Cyclic AMP; D014213:Tretoquinol