Abstract
Cardiac-specific overexpression of the human beta(2)-adrenergic receptor (AR) in transgenic mice (TG4) enhances basal cardiac function due to ligand-independent spontaneous beta(2)-AR activation. However, agonist-mediated stimulation of either beta(1)-AR or beta(2)-AR fails to further enhance contractility in TG4 ventricular myocytes. Although the lack of beta(2)-AR response has been ascribed to an efficient coupling of the receptor to pertussis toxin-sensitive G(i) proteins in addition to G(s), the contractile response to beta(1)-AR stimulation by norepinephrine and an alpha(1)-adrenergic antagonist prazosin is not restored by pertussis toxin treatment despite a G(i) protein elevation of 1.7-fold in TG4 hearts. Since beta-adrenergic receptor kinase, betaARK1, activity remains unaltered, the unresponsiveness of beta(1)-AR is not caused by betaARK1-mediated receptor desensitization. In contrast, pre-incubation of cells with anti-adrenergic reagents such as muscarinic receptor agonist, carbachol (10(-5)m), or a beta(2)-AR inverse agonist, ICI 118,551 (5 x 10(-7)m), to abolish spontaneous beta(2)-AR signaling, both reduce the base-line cAMP and contractility and, surprisingly, restore the beta(1)-AR contractile response. The "rescued" contractile response is completely reversed by a beta(1)-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 beta(2)-AR results in elevated base-line cAMP and contraction associated with a marked attenuation of beta(1)-AR response; carbachol pretreatment fully revives the diminished beta(1)-AR contractile response. Thus, we conclude that constitutive beta(2)-AR activation induces a heterologous desensitization of beta(1)-ARs independent of betaARK1 and G(i) proteins; suppression of the constitutive beta(2)-AR signaling by either a beta(2)-AR inverse agonist or stimulation of the muscarinic receptor rescues the beta(1)-ARs from desensitization, permitting agonist-induced contractile response.
Highlights
Vestigations of transmembrane signal transduction, those signals mediated by G protein-coupled receptors
Our results indicate that the defect of 1-AR stimulation in TG4 cardiomyocytes is conferred to the enhanced constitutive 2-AR activation and is likely caused by PKA-dependent heterologous receptor desensitization, independent of ARK1 and Gi activation
Loss of Contractile Response to 1-AR Stimulation in TG4 Ventricular Myocytes—Fig. 1A shows that a non-selective -AR agonist, isoproterenol (ISO, 10Ϫ7 M) robustly increases contraction amplitude in a representative ventricular myocyte from a wild type (WT) littermate
Summary
Vol 275, No 28, Issue of July 14, pp. 21773–21779, 2000 Printed in U.S.A. Inhibition of Spontaneous 2-Adrenergic Activation Rescues 1-Adrenergic Contractile Response in Cardiomyocytes Overexpressing 2-Adrenoceptor*. The chronic spontaneous 2-AR activation in TG4 cardiomyocytes could result in an increased G protein-coupled receptor kinase activity, as is the case in transgenic mice overexpressing Gs␣ [18] In both humans [19] and animals [20], chronic heart failure is often associated with elevated plasma catecholamine levels [21,22,23] and an increase in ARK1 abundance and enzymatic activity, which at least in part contribute to the reduced efficacy of -AR stimulation under these pathophysiological conditions. We have determined 1) whether 1-AR in TG4 hearts is desensitized via a PKA- or ARK1-dependent mechanism, 2) whether inhibitory mechanisms, Gi proteins, cross-talk with 1-AR-coupled Gs signaling and counteract its contractile response, and 3) whether inhibition of the spontaneous 2-AR activation by either a 2-AR inverse agonist or stimulation of Gi-coupled muscarinic receptor restores the desensitized 1-AR signaling. Our results indicate that the defect of 1-AR stimulation in TG4 cardiomyocytes is conferred to the enhanced constitutive 2-AR activation and is likely caused by PKA-dependent heterologous receptor desensitization, independent of ARK1 and Gi activation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
