title: Cardiostimulant and cardiodepressant effects through overexpressed human beta2-adrenoceptors in murine heart: regional differences and functional role of beta1-adrenoceptors.
abstract: (-)-Isoprenaline enhances cardiac contractility through beta-adrenoceptors. However, in cardiac tissue from transgenic mice with a 200-400-fold cardiac overexpression of the human beta(2)-adrenoceptor (TG4) we observed a pronounced cardiodepression at high (-)-isoprenaline concentrations. Here, we investigated the functional role of the coexisting beta(1)-, beta(2)-, and beta(3)-adrenoceptor subtypes in several regions of the TG4 heart, and in particular their contribution to the negative inotropic effect. In paced TG4 left atria, (-)-isoprenaline produced bell-shaped concentration-effect curves increasing (-logEC(50)M=9.0) and decreasing (-logIC(50)M=6.4) contractile force. These effects were unaffected by the beta(1)-selective CGP 20712A (300 nM). The beta(2)-selective inverse agonist ICI 118,551 (30-1,000 nM) antagonised in surmountable manner both the positive and negative inotropic effects of (-)-isoprenaline with similar concentration-dependence, consistent with an exclusive mediation through beta(2)-adrenoceptors. The beta(3)-adrenoceptor-selective agonist BRL37344 (1 nM-10 microM) failed to produce significant inotropic effects in TG4 left atria. Subsequently, we measured left atrial action potentials accompanying the inotropic changes induced by (-)-isoprenaline. Action potentials tended to have shorter duration in left atria from TG4 mice than from non-transgenic littermate mice. However, (-)-isoprenaline prolonged the duration of 30% repolarisation in atria from non-transgenic littermate but not from TG4 mice, while 90% repolarisation was abbreviated in both groups of atria. Negative inotropic effects of (-)-isoprenaline were also observed in right ventricular preparations. Pertussis toxin-treatment of the mice abolished the negative inotropic effects in left atria and reduced cardiodepression in right ventricle, indicating an involvement of beta(2)-adrenoceptor coupling to PTX-sensitive G-proteins. In additional experiments, designed to study the native murine beta(1)-adrenoceptor function, we used the physiological beta(1)-adrenoceptor agonist (-)-noradrenaline. In the presence of 600 nM ICI 118,551 we failed to find a functional role of the beta(1)-adrenoceptors in left atria, and detected only a marginal contribution to the positive chronotropic effect in right atria. We also investigated the effects of the non-conventional partial agonist (-)-CGP 12177 (0.2 nM-6 microM), which in wild-type mice causes tachycardia through beta(1)-adrenoceptors. In TG4 right atria, however, (-)-CGP 12177-evoked tachycardia was resistant to blockade by CGP 20712A but antagonised by ICI 118,551, consistent with mediation through human beta(2)-adrenoceptors. The results from TG4 mice suggest that the positive and negative inotropic effects of (-)-isoprenaline are mediated through human overexpressed beta(2)-adrenoceptors coupled to G(s) protein and G(i) protein, respectively. The (-)-isoprenaline-evoked shortening of the atrial action potential combined with reduced responses of L-type Ca(2+) current may contribute to the negative inotropic effects. The function of murine cardiac beta(1)-adrenoceptors is suppressed by overexpressed human beta(2)-adrenoceptors.
authors.affiliation: Institut für Pharmakologie und Toxikologie, Medizinische Fakultät Carl Gustav Carus der TU Dresden, 01307 Dresden, Germany.
authors.forename: Jürgen F
authors.initials: JF
authors.lastname: Heubach
authors.forename: Marianne
authors.initials: M
authors.lastname: Blaschke
authors.forename: Sian E
authors.initials: SE
authors.lastname: Harding
authors.forename: Ursula
authors.initials: U
authors.lastname: Ravens
authors.forename: Alberto J
authors.initials: AJ
authors.lastname: Kaumann
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D005260:Female; D006321:Heart / Q000187:drug effects* / Q000502:physiology; D006325:Heart Atria / Q000187:drug effects; D007093:Imidazoles / Q000494:pharmacology; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D009200:Myocardial Contraction / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D037342:Pertussis Toxin / Q000494:pharmacology; D011412:Propanolamines / Q000494:pharmacology; D018342:Receptors, Adrenergic, beta-1 / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D018343:Receptors, Adrenergic, beta-2 / Q000235:genetics / Q000502:physiology*; D022702:Receptors, Adrenergic, beta-3 / Q000235:genetics / Q000502:physiology
chemical_list: D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D007093:Imidazoles; D011412:Propanolamines; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D022702:Receptors, Adrenergic, beta-3; C026777:ICI 118551; C049763:CGP 20712A; D037342:Pertussis Toxin; D007545:Isoproterenol; D009638:Norepinephrine