Abstract 087: Reversal of Aging-Induced Contrast Changes of Cardiac SERCA 2a and Inducible Nitric Oxide Synthase in Mice Deficient in Beta3-Adrenoreceptor: Insights into the Molecular Mechanism of Cardiac Aging

Abstract

Background: We have shown previously that aging-induced cardiac dysfunction and β-adrenergic desensitization were prevented in β 3 -adrenergic receptor (AR) knockout (β 3 KO) aged mice. However, the molecular mechanism is unclear. We hypothesize that reversal of aging-induced alterations of cardiomyocyte SR Ca 2+ -ATPase (SERCA 2a) and inducible nitric oxide (NO) synthase by β 3 -AR deficiency may play a key role for the protective effect. Methods: We compared SERCA 2a, iNOS, β 1 - and β 3 -AR protein expression, myocyte contractile, and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses to isoproterenol (ISO, 10 -8 M) in cardiomyocytes obtained from 2 young (Y) (~6 mo) and 2 aged (A) (~26-30 mo) groups (5/group) of wild-type (WT) and β 3 KO mice, respectively. Results: Compared with YWT, AWT myocytes had significantly decreased protein levels of SERCA 2a (AWT: 0.22 vs YWT: 0.61) and β 1 -AR (0.34 vs 0.56), but increased iNOS (0.49 vs 0.24) and β 3 -AR (0.29 vs 0.14). These changes were associated with reduced basal cell contraction (dL/dt max ) (84.3 vs 124.8 μm/s), relaxation (dR/dt max ) (-66.1 vs -98.8 μm/s), and [Ca 2+ ] iT (0.19 vs 0.23). This was accompanied by diminished ISO-stimulated inotropic response. In AWT myocytes, ISO caused significantly less increases in dL/dt max (34% vs 82%), dR/dt max (22% vs 60%), and [Ca 2+ ] iT (15% vs 35%). Compared with YWT, Yβ 3 KO did not alter basal myocyte contraction and relaxation and response to ISO stimulation, but had significantly increased protein levels of SERCA 2a (Yβ 3 KO: 1.3 vs YWT: 0.61) and reduced iNOS (0.17 vs 0.24) with relatively unchanged β 1 -AR (0.63 vs 0.60). Aβ 3 KO mice had similar alterations. Importantly, in contrast to AWT, in Aβ 3 KO myocytes, the increased SERCA 2a (1.1) and reduced iNOS (0.19) correlated with normal basal cell contraction and relaxation with preserved ISO-stimulated inotropic response. ISO caused similar increases in dL/dt max (82% vs 84%) and [Ca 2+ ] iT (31% vs 33%) compared to Yβ 3 KO mice. Conclusions: β 3 -AR deficiency prevents aging-caused downregulation of cardiac β 1 -ARs and reverses increased iNOS and decreased SERCA 2a, leading to the preservation of myocyte function, [Ca 2+ ] iT , and β-adrenergic reserve in aged hearts. Thus, blocking β 3 -AR may provide a new strategy for myocardial aging.