Enhanced G_i signaling selectively negates β ₂-adrenergic receptor (AR)- but not β₁-AR-mediated positive inotropic effect in myocytes from failing rat hearts

Background - Myocardial contractile response to β₁- and β₂-adrenergic receptor (AR) stimulation is severely impaired in chronic heart failure, in which G_i signaling and the ratio of β₂/β₁ are often increased. Because β ₂-AR but not β₁-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 β-AR subtypes and, if so, whether inhibition of G_i or selective activation of β ₂-AR/G_s by ligands restores β₂-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 β-AR subtype-mediated inotropic effect was markedly diminished, whereas G_i proteins and the β₂/β₁ ratio were increased. Disruption of G_i signaling by pertussis toxin (PTX) enabled β₂- but not β₁-AR to induce a full positive inotropic response in SHR myocytes. Furthermore, screening of a panel of β₂-AR ligands revealed that the contractile response mediated by most β₂-AR agonists, including zinterol, salbutamol, and procaterol, was potentiated by PTX, indicating concurrent G_s and G_i activation. In contrast, fenoterol, another β ₂-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 β₂- but not β₁-AR in failing SHR hearts and that disruption of G_i signaling by PTX or selective activation of β₂-AR/G_s signaling by fenoterol restores the blunted β₂-AR contractile response in the failing heart.