Inhibition of spontaneous β2-adrenergic activation rescues β1- adrenergic contractile response in cardiomyocytes overexpressing β2- adrenoceptor

Abstract Cardiac-specific overexpression of the human β2-adrenergic receptor (AR) in transgenic mice (TG4) enhances basal cardiac function due to ligand-independent spontaneous β2-AR activation. However, agonist-mediated stimulation of either β1-AR or β2-AR fails to further enhance contractility in TG4 ventricular myocytes. Although the lack of β2-AR response has been ascribed to an efficient coupling of the receptor to pertussis toxin-sensitive Gi proteins in addition to Gs, the contractile response to β1-AR stimulation by norepinephrine and an α1-adrenergic antagonist prazosin is not restored by pertussis toxin treatment despite a Gi protein elevation of 1.7-fold in TG4 hearts. Since β-adrenergic receptor kinase, βARK1, activity remains unaltered, the unresponsiveness of β1-AR is not caused by βARK1-mediated receptor desensitization. In contrast, pre-incubation of cells with anti-adrenergic reagents such as muscarinic receptor agonist, carbachol (10− 5 m), or a β2-AR inverse agonist, ICI 118,551 (5 × 10− 7 m), to abolish spontaneous β2-AR signaling, both reduce the base-line cAMP and contractility and, surprisingly, restore the β1-AR contractile response. The “rescued” contractile response is completely reversed by a β1-AR antagonist, CGP 20712A. Furthermore, these results from the transgenic animals are corroborated by in vitro acute gene manipulation in cultured wild type adult mouse ventricular myocytes. Adenovirus-directed overexpression of the human β2-AR results in elevated base-line cAMP and contraction associated with a marked attenuation of β1-AR response; carbachol pretreatment fully revives the diminished β1-AR contractile response. Thus, we conclude that constitutive β2-AR activation induces a heterologous desensitization of β1-ARs independent of βARK1 and Giproteins; suppression of the constitutive β2-AR signaling by either a β2-AR inverse agonist or stimulation of the muscarinic receptor rescues the β1-ARs from desensitization, permitting agonist-induced contractile response.