Overexpression of beta 1-adrenoceptors in adult rat ventricular myocytes enhances CGP 12177A cardiostimulation: implications for 'putative' beta 4-adrenoceptor pharmacology.
1. CGP 12177A mediates cardiostimulation by activation of the 'putative' beta(4)-adrenoceptor; however, it has recently been reported that disruption of the beta(1)-adrenoceptor gene abolishes this effect. We have adenovirally overexpressed beta(1)-adrenoceptors in isolated, cultured adult rat ventricular cardiomyocytes and observed the inotropic potency of isoprenaline and CGP 12177A (in the presence of 1 microm 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. beta(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 beta(1)-adrenoceptor overexpression. 5. (125)I-Cyanopindolol saturation binding in Adv.beta(1) myocytes demonstrated approximately 18-fold increase in beta(1)-adrenoceptors. (3)H-CGP 12177A saturation binding, in the presence of propranolol, increased approximately 5-fold following overexpression of beta(1)-adrenoceptors. 6. This study demonstrates enhanced cardiostimulation by CGP 12177A (in the presence of propranolol) in rat ventricular myocytes overexpressing beta(1)-adrenoceptors, mediated by a Gs/cAMP signalling pathway. 'Putative' beta(4)-adrenoceptor pharmacology appears to be mediated by activation of a novel affinity state of the beta(1)-adrenoceptor.
Lewis
Clive J
CJ
Clinical Pharmacology Unit, Department of Medicine, Cambridge University, Cambridge. clivel@cantab.net
Gong
Haibin
H
Brown
Morris J
MJ
Harding
Sian E
SE
D058665:Adrenergic beta-1 Receptor Agonists; D058671:Adrenergic beta-1 Receptor Antagonists; D000818:Animals; D002316:Cardiotonic Agents / Q000494:pharmacology*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D008297:Male; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D011412:Propanolamines / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011943:Receptors, Adrenergic, beta / Q000096:biosynthesis*; D018342:Receptors, Adrenergic, beta-1 / Q000096:biosynthesis*
D058665:Adrenergic beta-1 Receptor Agonists; D058671:Adrenergic beta-1 Receptor Antagonists; D002316:Cardiotonic Agents; D011412:Propanolamines; D011943:Receptors, Adrenergic, beta; D018342:Receptors, Adrenergic, beta-1; C120358:adrenergic beta-4 receptor; C033394:CGP 12177