The effect of Gi-protein inactivation on basal, and β1- and β2AR-stimulated contraction of myocytes from transgenic mice overexpressing the β2-adrenoceptor

1 The atria and ventricles of transgenic mice (TG beta(2)) with cardiac overexpression of the human beta(2)-adrenoceptor (beta(2)AR) were Initially reported 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. We have investigated the roles of cyclic AMP and Gi in basal and beta AR function 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 contraction in TG beta 2 myocytes, indicating that there as no tonic influence of raised cyclic AMP. These findings cannot be explained by the proposed model for inverse agonism. where the activated receptor (R*) raises cyclic AMP levels and so increases contraction In the absence of agonist. 3 After pertussis toxin (PTX) pretreatment to produce inactivation of Gi, the 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 isoprenaline 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 original desensitization and by the resensitization with PTX. 4 PTX treatment has therefore restored the original phenotype, with high basal contractility and little further ther effect of isoprenaline. We suggest 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 increased activity of PTX-sensitive G-proteins.