AN EFFICACY-DEPENDENT EFFECT OF CARDIAC OVEREXPRESSION OF BETA(2)-ADRENOCEPTOR ON LIGAND AFFINITY IN TRANSGENIC MICE

In previous studies, it was shown that the overexpression of beta(2)-a drenoceptor (beta(2)AR) in the hearts of transgenic mice (Tg) leads to agonist-independent activation of adenylate cyclase and enhanced myoc ardia[ function. Here, we measured the physical coupling of beta(2)AR and G(s) by evaluating the coimmunoprecipitation of beta(2)AR and G(s) and the ligand binding properties of beta(2)AR in the hearts of Tg mi ce to investigate the details of the interaction among ligand, recepto r, and G protein. The following results were obtained: (i) coimmunopre cipitation of beta(2)AR and G(s) was increased in the absence of agoni st in Tg mice compared with the control animals. This demonstrates dir ectly the increased interaction between unliganded beta(2)AR and G(s), which is consistent with increased background cAMP production and car diac function in the hearts of Tg mice. (ii) Guanosine-5'-(beta,gamma- imido)triphosphate abolished the association of beta(2)AR/G(s) in the immunoprecipitate. (iii) The affinities for ligands that show agonist (isoproterenol, clenbuterol, and dobutamine), neutral antagonist (alpr enolol and timolol), and negative antagonist (propranolol and ICI 1185 51) activities in this experimental system were increased, not changed and decreased, respectively, in Tg mice compared with the controls. ( iv) This efficacy-dependent alteration in ligand affinities was still observed in the presence of a guanosine-5'-(beta,gamma-imido)triphosph ate concentration that abolishes beta(2)AR/G(s) coupling. This suggest s that the altered beta(2)AR binding affinities in Tg mice are not due to the increased interaction between beta(2)AR and G(s). These data c annot be explained by using ternary, quinternary, two-state extended t ernary, or cubic ternary complex models. We therefore discuss the resu lts using a ''two-state polymerization model'' that includes an isomer ization step for the conversion of receptor between an inactive and an active form (denoted as R and R, respectively) and a polymerization of the active state (R(n)). The simplest form of this model (i.e., no ncooperative dimerization of the receptor) is found to be consistent w ith the experimental data.