Abstract 644: Cardiac Beta3-adrenoceptors: A Putative Anti-arrhythmic Target
The stimulation of cardiac β3-adrenoceptors (β3-AR) produces a negative inotropic effect in dogs and humans through Gi/0 protein and NO pathway activation. Some studies associate this effect to a reduction of Ca2+ load during contraction. However, controversial data exist regarding the effects of β3-AR stimulation on Ca2+ and repolarizing currents. In addition, those data were not obtained in the same model and/or species leading to inconclusive results. Thus, the aim of the present work was to assess the effect of β3-AR stimulation on contractility, intracellular Ca2+ and ionic currents on isolated rabbit ventricular cardiomyocytes, in which we detected β3-AR transcripts and proteins using RT-PCR and Western blots. We measured cell shortening by video imaging, L-type Ca2+ current (ICa,L) and delayed rectifier K+ current (IKs) using whole-cell patch-clamp technique and the amplitude of Ca2+ transient (CaT) using fluo-4 die. The effects of 10 nM SR 58611A (SR), a preferential β3-AR agonist, were evaluated in the presence of 10 μM nadolol, a β1/β2-AR antagonist. SR decreased cell shortening by −29±2% (n=16, p<0.05). This effect was abolished by 1 μM L-748,337, a specific β3-AR antagonist, and significantly decreased after pre-treatment with 1.5 μg.mL−1 pertussis toxin, a Gi/o protein inhibitor, and in the presence of 10 μM NG-monomethyl-L-arginine (L-NMMA), a NO synthase inhibitor. This negative inotropic effect was associated to a 29±6% reduction of ICa,L amplitude (n=7, p<0.05) and a 43±3% reduction of CaT (n=14, p<0.001). In addition, SR increased IKs by 47±18% (n=6, p<0.05). SR effects on both currents and CaT were significantly antagonized by L-748,337 and L-NMMA. In conclusion, β3-AR stimulation
reduces cardiac contractility at least in part by reducing ICa,L and CaT through Gi/o-NOS pathway and
These results suggest that β3-AR could be a new anti-arrhythmic target by decreasing Ca2+ overload and preventing prolongation of ventricular repolarization in pathologies associated to an increased adrenergic tone, such as heart failure.