The effect of Gi-protein inactivation on basal, and beta(1)- and beta(2)AR-stimulated contraction of myocytes from transgenic mice overexpressing thebeta(2)-adrenoceptor

1 The atria and ventricles of transgenic mice (TG beta(2)) with cardiac ove rexpression of the human beta(2)-adrenoceptor (beta(2)AR) were Initially re ported to show maximum contractility in the absence of beta AR stimulation. However. we have previously observed a different phenotype in these mice, with myocytes showing normal contractility but reduced beta AR responses. W e have investigated the roles of cyclic AMP and Gi in basal and beta AR fun ction in these myocytes. 2 ICI 118,551 at inverse agonist concentrations decreased contraction by 32 %. However, the cyclic.Sh IP antagonist Rp-cAMPS had no effect on contracti on in TG beta 2 myocytes, indicating that there as no tonic influence of ra ised cyclic AMP. These findings cannot be explained by the proposed model f or inverse agonism. where the activated receptor (R*) raises cyclic AMP lev els and so increases contraction In the absence of agonist. 3 After pertussis toxin (PTX) pretreatment to produce inactivation of Gi, t he basal contraction in mM Ca2+ was increased in TG beta(2) mice (7.82+/-0. 47%, n = 23) compared to LM mice (3.60 +/- 0.59%, n = 11) (P < 0.001). The contraction amplitude of myocytes to the maximal concentration of isoprenal ine was also increased significantly by PTX in TG beta(2) mice (9.40 +/- 1. 22%, n = 8) and was no loner reduced compared to LM mice (8.93+/-1.50%, n = 11). Both beta(1)-, and beta(2)AR subtypes were affected both by the origi nal desensitization and by the resensitization with PTX. 4 PTX treatment has therefore restored the original phenotype, with high ba sal contractility and little further ther effect of isoprenaline. We sugges t that both beta-AR desensitization and lack of increased basal contraction in ventricular myocytes from our colony of TG beta(2) mice were due to inc reased activity of PTX-sensitive G-proteins.