β4-adrenoceptors are more effective than β1-adrenoceptors in mediating arrhythmic Ca2+ transients in mouse ventricular myocytes

Putative beta(4)-adrenoceptors mediate cardiostimulation and arrhythmias in mammalian heart. Both beta(1)- and putative beta(4)-adrenoceptors mediate arrhythmias but through different mechanisms. To elucidate further the mechanisms of cardiostimulation and arrhythmias we measured Ca2+ transients and L-type Ca2+ currents in mouse ventricular myocytes, We used (-)-CGP 12177, an antagonist of beta(1)- and beta(2)-adrenoceptors with agonist properties at the putative beta(4)-adrenoceptor, and (-)-isoprenaline as an agonist for beta(1)- and beta(2)-adrenoceptors, (-)-CGP 12177 increased Ca2+ transients in electrically stimulated cells loaded with Indo-1. The maximum increase of Ca2+ transients caused by (-)-CGP 12177 amounted to approximately one-third of that caused by maximally effective (-)-isoprenaline concentrations. Both (-)-CGP 12177 and (-)-isoprenaline caused concentration-dependent arrhythmic Ca2+ transients. The arrhythmias appeared at paced Ca2+ transients and between paced Ca2+ transients. The arrhythmic potency of (-)-CGP 12177 (-logEC(50) = 9.4) was approximately 40 times greater than that of (-)-isoprenaline (-logEC(50) = 7.8). L-type Ca2+ current was measured in the whole cell configuration of the patch clamp technique. In the presence of both 3-isobutyl 1-methylxanthine (6 mu mol/l) and (-)-propranolol (500 nmol/l), (-)-CGP 12177 (100 nmol/l) increased significantly L-type Ca2+ current by 19% of the effect of (-)-isoprenaline. The (-)-CGP 12177-evoked increase of Ca2+ transients contrasts with the smaller effects on L-type Ca2+ current, suggesting that activation of the putative beta(4)-adrenoceptor causes a more efficient Ca2+-induced Ca2+ release than activation of the beta(1)-adrenoceptor. beta(4)-Adrenoceptors mediate arrhythmias with smaller Ca2+ transients and smaller increases of L-type Ca2+ current than beta(1)-adrenoceptors, in line with different but still unknown mechanisms as previously suggested for the intact heart.