Myocardial overexpression of the C-terminus of b-adrenergic receptor kinase (bARKct) has been shown to result in a positive inotropic effect or an improvement of survival in heart failure. However, it is not clear whether this beneficial effect is mainly because of dominant-negative inhibition of bARK1, and a consecutive resensitization of b-adrenergic receptors (bAR), or rather due to inhibition of other Gbgmediated effects. In this study, we tested whether overexpression of N-terminally truncated phosducin (nt-delphosducin), another Gbg-binding protein that does not resensitize bARs owing to simultaneous inhibition of GDP release from Ga subunits, shows the same effects as bARKct. Adenoviral gene transfer was used to express ntdel-phosducin and bARKct in isolated ventricular cardiomyocytes and in myocardium of rabbits, which suffered from heart failure because of rapid ventricular pacing. bARstimulated cAMP formation was increased by bARKct, but not by nt-del-phosducin, whereas both proteins inhibited Gbg-mediated effects. Both transgenes also increased contractility of normal and failing isolated cardiomyocytes and improved contractility in rabbits with heart failure after gene transfer in vivo. In conclusion, overexpression of nt-delphosducin enhances the contractility of cardiomyocytes to the same extent as bARKct, suggesting that the effects of bARKct might be owing to inhibition of Gbg rather than to bAR resensitization.