Overexpression of wild-type Galpha(i)-2 suppresses beta-adrenergic signaling in cardiac myocytes.

The role of Galpha(i)-2 overexpression in desensitization of beta-adrenergic signaling in heart failure is controversial. An adenovirus-based approach was used to investigate whether overexpression of Galpha(i)-2 impairs beta-adrenergic stimulation of adenylyl cyclase (AC) activity and cAMP levels in neonatal rat cardiac myocytes (NRCM) and cell shortening of adult rat ventricular myocytes (ARVM). Infection of NRCM with Ad5Galpha(i)-2 increased Galpha(i)-2 by 50-600% in a virus dose-dependent manner. Overexpression was paralleled by suppression of GTP- and isoprenaline-stimulated AC by 10-72% (P<0.001) in a PTX-sensitive manner. Isoprenaline-stimulated shortening of Ad5Galpha(i)-2-infected ARVM was attenuated by 34% (P<0.01). Ad5Galpha(i)-2/GFP (Galpha(i)-2, green fluorescent protein; bicistronic) was constructed to monitor transfection homogeneity and target Galpha(i)-2 overexpression to levels found in heart failure. At Galpha(i)-2 levels of 93% above control, isoprenaline-stimulated AC activity and cAMP levels were reduced by 17% and 40% (P<0.02), respectively. Beta1- and beta2-adrenergic stimulation was reduced similarly. Our results suggest that (a) the Galpha(i)-2 system exhibits tonic inhibition of stimulated AC in cardiac myocytes, (b) Galpha(i)-2-mediated inhibition is concentration-dependent and occurs at Galpha(i)-2 levels seen in heart failure, and (c) Galpha(i)-2-mediated inhibition affects both beta1- and beta2-adrenergic stimulation of AC. The data argue for an important, independent role of the Galpha(i)-2 increase in heart failure.