The effect of Gi‐protein inactivation on basal, and β 1 ‐ and β 2 AR‐stimulated contraction of myocytes from transgenic mice overexpressing the β 2 ‐adrenoceptor

The atria and ventricles of transgenic mice (TGβ 2 ) with cardiac overexpression of the human β 2 ‐adrenoceptor (β 2 AR) were initially reported to show maximum contractility in the absence of β‐AR stimulation. However, we have previously observed a different phenotype in these mice, with myocytes showing normal contractility but reduced βAR responses. We have investigated the roles of cyclic AMP and Gi in basal and βAR function in these myocytes. ICI 118,551 at inverse agonist concentrations decreased contraction by 32%. However, the cyclic AMP antagonist Rp‐cAMPS had no effect on contraction in TGβ 2 myocytes, indicating that there was 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. After pertussis toxin (PTX) pretreatment to produce inactivation of Gi, the basal contraction in 1 m M Ca 2+ was increased in TGβ 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 TGβ 2 mice (9.40±1.22%, n =8) and was no longer reduced compared to LM mice (8.93±1.50%, n =11). Both β 1 ‐ and β 2 AR subtypes were affected both by the original desensitization and by the resensitization with PTX. PTX treatment has therefore restored the original phenotype, with high basal contractility and little further effect of isoprenaline. We suggest that both β‐AR desensitization and lack of increased basal contraction in ventricular myocytes from our colony of TGβ 2 mice were due to increased activity of PTX‐sensitive G‐proteins.British Journal of Pharmacology (2000) 131 , 594–600; doi: 10.1038/sj.bjp.0703591