Overexpression of β 1-adrenoceptors in adult rat ventricular myocytes enhances CGP 12177A cardiostimulation: Implications for 'putative' β 4-adrenoceptor pharmacology

Abstract

1. CGP 12177A mediates cardiostimulation by activation of the 'putative' β 4-adrenoceptor; however, it has recently been reported that disruption of the β 1-adrenoceptor gene abolishes this effect. We have adenovirally overexpressed β 1-adrenoceptors in isolated, cultured adult rat ventricular cardiomyocytes and observed the inotropic potency of isoprenaline and CGP 12177A (in the presence of 1 μM propranolol). 2. Isoprenaline was a full inotropic agonist at rat ventricular myocytes (pD 2 7.69±0.12). CGP 12177A was a nonconventional partial agonist (pD 2 6.34±0.09), increasing inotropy and lusitropy, with an intrinsic activity of 0.34 and antagonised by bupranolol. 3. β 1-adrenoceptor overexpression enhanced the inotropic potency of isoprenaline by 11.7-fold (pD 2 8.76±0.14) and CGP 12177A by 5.9-fold (7.11±0.10), respectively. Green fluorescent protein (GFP) overexpression did not alter the potency of isoprenaline or CGP 12177A (pD 2 7.41±0.24 and pD 2 6.60 + 0.50, respectively). 4. The cardiostimulant effects of CGP 12177A were enhanced by IBMX (phosphodiesterase inhibitor) and decreased by Rp-cAMPS (cAMP antagonist). CGP 12177A also increased cAMP levels. CGP 12177A but not isoprenaline initiated arrhythmias at lower concentrations following β 1-adrenoceptor overexpression. 5. 125I-Cyanopindolol saturation binding in Adv.β 1 myocytes demonstrated ∼18-fold increase in β 1-adrenoceptors. 3H-CGP 12177A saturation binding, in the presence of propranolol, increased ∼5-fold following overexpression of β 1-adrenoceptors. 6 This study demonstrates enhanced cardiostimulation by CGP 12177A (in the presence of propranolol) in rat ventricular myocytes overexpressing β 1-adrenoceptors, mediated by a Gs/cAMP signalling pathway. 'Putative' β 4-adrenoceptor pharmacology appears to be mediated by activation of a novel affinity state of the β 1-adrenoceptor.