Abstract 1995: Upregulation of Cardiac SERCA 2a by Chronic β3-Adrenergic Receptor (AR) Deficiency: A Key Molecular Mechanism of the Protective Effect of β3-AR Deficiency on Myocardial Aging
Background. We have shown previously that aging-induced cardiomyocyte dysfunction and impaired β-adrenergic regulation were prevented in β3-adrenergic receptor (AR) knockout (β3KO) aged mice. However, the molecular mechanism is unclear. We hypothesize that reversal of aging-induced down-regulation of SR Ca2+-ATPase (SERCA 2a) and β1-ARs by β3-AR deficiency may play a key role for the protective effect.
Methods. We compared cardiac SERCA 2a, phospholamban (PLB), β1- and β3-AR protein expression, myocyte contractile, and [Ca2+]i transient ([Ca2+]iT) responses to isoproterenol (ISO, 10−8 M) in left ventricle (LV) myocytes obtained from 2 young (Y)(~4 – 6 mo) and 2 aged (A) (~18 –24 mo) groups (7/group) of wild-type (WT) and β3KO mice, respectively.
Results. Compared with YWT, AWT myocytes had significantly decreased SERCA 2a (66%, AWT: 0.21 vs YWT: 0.61), SERCA 2a/PLB ratio (0.76 vs 1.77) and β1-AR (33%, 0.39 vs 0.58), but increased β3-AR (93%, 0.27 vs 0.14). These changes were associated with reduced basal cell contraction (dL/dtmax) (AWT: 84.3 vs YWT: 124.8 μm/s), relaxation (dR/dtmax) (−60.1 vs −89.8 μm/s), and [Ca2+]iT (0.17 vs 0.22). This was accompanied by diminished ISO-stimulated positive inotropic effect. In AWT myocytes, ISO (10−8 M) caused significantly less increases in dL/dtmax (AWT: 33% vs YWT: 81%), dR/dtmax (24% vs 58%), and [Ca2+]iT (14% vs 34%). Compared with YWT, Yβ3KO did not alter basal contraction and ISO response of myocytes, but SERCA 2a (Yβ3KO: 1.2 vs YWT: 0.61) and SERCA 2a/PLB ratio were doubled with relatively unchanged β1-AR (0.62 vs 0.58). Aβ3KO mice had similar alterations. Importantly, in contrast to AWT, in Aβ3KO myocytes, the increased SERCA 2a (1.1) and β1-AR (0.61) correlated with normal basal cell contraction and relaxation with preserved ISO-stimulated positive inotropic response. ISO caused similar increases in dL/dtmax (84% vs 82%) and [Ca2+]iT (32% vs 34%) compared to Yβ3KO mice.
Conclusions. Chronic β3-AR deficiency upregulates cardiac SERCA 2a, prevents aging-induced downregulation of β1-ARs, and leads to the preservation of myocyte function, [Ca2+]iT, and β-adrenergic responsiveness in aged hearts. Thus, antagonizing β3-AR could be a new therapeutic strategy for age-related decline in myocardial function.