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

Abstract.

Putative β₄-adrenoceptors mediate cardiostimulation and arrhythmias in mammalian heart. Both β₁- and putative β₄-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 β₁- and β₂-adrenoceptors with agonist properties at the putative β₄-adrenoceptor, and (-)-isoprenaline as an agonist for β₁- and β₂-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 (–logEC₅₀=9.4) was approximately 40 times greater than that of (-)-isoprenaline (–logEC₅₀=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 µmol/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 β₄-adrenoceptor causes a more efficient Ca²⁺-induced Ca²⁺ release than activation of the β₁-adrenoceptor. β₄-Adrenoceptors mediate arrhythmias with smaller Ca²⁺ transients and smaller increases of L-type Ca²⁺ current than β₁-adrenoceptors, in line with different but still unknown mechanisms as previously suggested for the intact heart.