Enhanced G(i) Signaling Selectively Negates Beta2-adrenergic Receptor (AR)--but Not Beta1-AR-mediated Positive Inotropic Effect in Myocytes from Failing Rat Hearts

Background: Myocardial contractile response to beta1- and beta2-adrenergic receptor (AR) stimulation is severely impaired in chronic heart failure, in which G(i) signaling and the ratio of beta2/beta1 are often increased. Because beta2-AR but not beta1-AR couples to G(s) and G(i) with the G(i) coupling negating the G(s)-mediated contractile response, we determined whether the heart failure-associated augmentation of G(i) signaling contributes differentially to the defects of these beta-AR subtypes and, if so, whether inhibition of G(i) or selective activation of beta2-AR/G(s) by ligands restores beta2-AR contractile response in the failing heart.

Methods And Results: Cardiomyocytes were isolated from 18- to 24-month-old failing spontaneously hypertensive (SHR) or age-matched Wistar-Kyoto (WKY) rat hearts. In SHR cardiomyocytes, either beta-AR subtype-mediated inotropic effect was markedly diminished, whereas G(i) proteins and the beta2/beta1 ratio were increased. Disruption of G(i) signaling by pertussis toxin (PTX) enabled beta2- but not beta1-AR to induce a full positive inotropic response in SHR myocytes. Furthermore, screening of a panel of beta2-AR ligands revealed that the contractile response mediated by most beta2-AR agonists, including zinterol, salbutamol, and procaterol, was potentiated by PTX, indicating concurrent G(s) and G(i) activation. In contrast, fenoterol, another beta2-AR agonist, induced a full positive inotropic effect in SHR myocytes even in the absence of PTX.

Conclusions: We conclude that enhanced G(i) signaling is selectively involved in the dysfunction of beta2- but not beta1-AR in failing SHR hearts and that disruption of G(i) signaling by PTX or selective activation of beta2-AR/G(s) signaling by fenoterol restores the blunted beta2-AR contractile response in the failing heart.