Abstract 3220: β1-Adrenergic Receptor Activation Induces Cardiac Myocyte Death Through Ca2+ Influx Dependent and Independent Pathways
Persistent stimulation of β1-adrenergic receptor (β1-AR) in congestive heart failure increases the L-type Ca2+ current (ICa) to maintain contractility, but this is proarrhythmic and promotes myocyte loss to enhance heart failure progression. The effects of β2-AR in heart failure are less well known but are thought to promote myocyte survival signaling. We determined if β1- and β2-AR signaling induce myocyte death through ICa dependent pathways.
Methods: Double transgenic (DTG) mouse line with inducible (Tet-off system; tTA-TG) and cardiac myocyte specific (αMHC promoter) overexpression of the β2a subunit of the L-type Ca2+ channel were used to increase ICa independent of β-AR. Myocytes isolated from wild type (WT) and DTG mice were exposed for 12 h to: 1 μM isoproterenol (ISO, mixed β-AR agonist) with or without 300 nM CGP 20712A (β1-AR antagonist); 1 μM Zinterol (β2-AR agonist) + 300 nM CGP 20712A. Myocyte viability, contraction, Ca2+ transients and ICa were measured.
Results: As shown in table 1⇓, DTG mice with increased ICa had greater death rates than WT. ISO treatment aggravated myocyte death in both WT and DTG to a similar extent (~50%), which were eliminated by β1-AR antagonists. β2-AR stimulation significantly prevented cell death in WT but not DTG myocytes. In both WT and DTG mice, ISO increased cell shortening (WT: 7.84±0.97% vs. 10.66±0.70%, n=19, P<0.05; DTG: 12.31±0.74% vs. 14.47±0.86%, n=9, P<0.05) and [Ca2+]i (F/F0: WT: 2.92±0.33% vs. 3.30±0.43%, n=8, P<0.05; DTG: 3.82±0.43% vs. 4.72±0.69%, n=5, P<0.05). However, ISO increased ICa significantly in WT (−13.3±1.9 pA/pF vs. −17.9±3.9 pA/pF, n=4, P<0.05) but not in DTG myocytes (−27.9±2.7 pA/pF vs. −20.2±1.90 pA/pF, n=7), implying that ISO induced inotropic effect and aggravated cell death in DTG mice are ICa independent.
Conclusion: β1-AR signaling mediates myocyte death through ICa dependent and independent pathways. â2-AR signaling protects myocytes from ICa-mediated cell death.