Pharmacological activities of trimetoquinol and 1-benzyl halogen-substituted analogues on rat beta-adrenoceptor subtypes

Stereoselectivity for interactions of agonists and antagonists at mouse, rat and human β3-adrenoceptors

This study examines the stereoselectivity profile of recombinant mouse, rat and human beta(3)-adrenoceptors expressed in Chinese Hamster Ovary (CHO-K1) cells using radioligand binding, in comparison with endogenously expressed beta(3)-adrenoceptors mediating relaxation responses in mouse ileum. The enantiomeric ratios for several beta-adrenoceptor agonists and antagonists at the cloned mouse, rat and human beta(3)-adrenoceptor were less than those reported at the cloned beta(1)-/beta(2)-adrenoceptor but higher than those reported in previous studies. The degree of stereoselectivity was relatively low for the agonists isoprenaline and noradrenaline but higher for antagonists and, in particular, tertatolol and propranolol. In mouse ileum, stereoselectivity of propranolol and tertatolol was observed under beta(1)-/beta(2)-adrenoceptor blockade. The (-)-enantiomers of propranolol and tertatolol were more effective at antagonism of (-)-isoprenaline-mediated relaxation of mouse ileum than their (+)-enantiomers. The recombinant mouse, rat and human beta(3)-adrenoceptors display stereoselective interactions for agonists and antagonists similar to the stereoselective profile of beta(3)-adrenoceptors in mouse ileum. The degree of stereoselectivity varied between species and the human beta(3)-adrenoceptor displayed higher affinities and enantiomeric ratios than the mouse or rat beta(3)-adrenoceptors.

Biochemical and functional characterization of 1-benzyl substituted trimetoquinol affinity analogs on rat and human beta-adrenoceptors

The site of interaction for the 1-(3',4',5'-trimethoxybenzyl) group of trimetoquinol (TMQ) with beta-adrenoceptors (beta-ARs) is important for the rational design of highly potent and beta3-AR-selective analogs. 1-Benzyl ring-substituted TMQ analogs were evaluated for binding affinities and biochemical activities (cyclic AMP accumulations) in Chinese hamster ovary (CHO) cells expressing the rat and human beta3-AR, and for functional activities on isolated rat tissues. Binding affinities (K1 approximately 0.055 to 1.5 microM) for the rat beta3-AR and potencies for adenylyl cyclase activation (K(act) approximately 0.43 to 2;5 nM) of the 3'-monoiodo or 3',5'-diiodo derivatives with 4'-isothiocyanato-, 4'-amino, 4'-acetamido, or 4'-alpha-haloacetamido substitutions were higher than those of (-)-isoproterenol, and comparable to those of BRL 37344 [(+/-)-(R*,R*-[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]ph enoxy]-acetic acid sodium]. A similar rank order of binding affinities (K(i) approximately 0.11 to 2.5 microM) and potencies (K(act) approximately 0.45 to 9.5 nM) was obtained for TMQ analogs on the human beta3-AR. The 4'-acetamido and 4'-alpha-chloroacetamido analogs of 3',5'-diiodoTMQ were more potent than (-)-isoproterenol in rat atria (beta1-AR) and rat trachea (beta2-AR) and exhibited partial agonist activities, whereas full agonist activities were observed in rat esophageal smooth muscle (EC50 approximately 2-8 nM, beta3-AR). 4'-alpha-Chloroacetamido-3',5'-diiodoTMQ-mediated chronotropic responses in atria were sustained and resistant to washout. Further, the 4'-alpha-chloroacetamido and 4'-alpha-bromoacetamido analogs of 3',5'-diiodoTMQ demonstrated significant concentration-dependent irreversible binding to the rat beta3-AR. Reversible beta-AR agonists such as (-)-isoproterenol, BRL 37344, and 4'-acetamido-3',5'-diiodoTMQ or nucleophilic 1-amino acids (lysine, glutathione, cysteine) did not protect against this irreversible binding. Thus, the lipophilic 1-benzyl ring of TMQ analogs interacts with a hydrophobic region of the beta-AR that may represent an exo-site or an allosteric binding site.