Cardiac &bgr;2-adrenergic receptor (&bgr;2AR) overexpression is a potential contractile therapy for heart failure. Cardiac contractility was elevated in mice overexpressing &bgr;2ARs (TG4s) with no adverse effects under normal conditions. To assess the consequences of &bgr;2AR overexpression during ischemia, perfused hearts from TG4 and wild-type mice were subjected to 20-minute ischemia and 40-minute reperfusion. During ischemia, ATP and pH fell lower in TG4 hearts than wild type. Ischemic injury was greater in TG4 hearts, as indicated by lower postischemic recoveries of contractile function, ATP, and phosphocreatine. Because &bgr;2ARs, unlike &bgr;1ARs, couple to Gias well as Gs, we pretreated mice with the Giinhibitor pertussis toxin (PTX). PTX treatment increased basal contractility in TG4 hearts and abolished the contractile resistance to isoproterenol. During ischemia, ATP fell lower in TG4+PTX than in TG4 hearts. Recoveries of contractile function and ATP were lower in TG4+PTX than in TG4 hearts. We also studied mice that overexpressed either &bgr;ARK1 (TG&bgr;ARK1) or a &bgr;ARK1 inhibitor (TG&bgr;ARKct). Recoveries of function, ATP, and phosphocreatine were higher in TG&bgr;ARK1 hearts than in wild-type hearts. Despite basal contractility being elevated in TG&bgr;ARKct hearts to the same level as that of TG4s, ischemic injury was not increased. In summary, &bgr;2AR overexpression increased ischemic injury, whereas &bgr;ARK1 overexpression was protective. Ischemic injury in the &bgr;2AR overexpressors was exacerbated by PTX treatment, implying that it was Gsnot Giactivity that enhanced injury. Unlike &bgr;2AR overexpression, basal contractility was increased by &bgr;ARK1 inhibitor expression without increasing ischemic injury, thus implicating a safer potential therapy for heart failure.