(-)-
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.