β1-Adrenergic receptor activation induces mouse cardiac myocyte death through both L-type calcium channel-dependent and -independent pathways

Abstract

Cardiac diseases persistently increase the contractility demands of cardiac myocytes, which require activation of the sympathetic nervous system and subsequent increases in myocyte Ca2+ transients. Persistent exposure to sympathetic and/or Ca2+ stress is associated with myocyte death. This study examined the respective roles of persistent β-adrenergic receptor (β-AR) agonist exposure and high Ca2+ concentration in myocyte death. Ventricular myocytes (VMs) were isolated from transgenic (TG) mice with cardiac-specific and inducible expression of the β2a-subunit of the L-type Ca2+ channel (LTCC). VMs were cultured, and the rate of myocyte death was measured in the presence of isoproterenol (ISO), other modulators of Ca2+ handling and the β-adrenergic system, and inhibitors of caspases and reactive oxygen species generation. The rate of myocyte death was greater in TG vs. wild-type myocytes and accelerated by ISO in both groups, although ISO did not increase LTCC current ( ICa-L) in TG-VMs. Nifedipine, an LTCC antagonist, only partially prevented myocyte death. These results suggest both LTCC-dependent and -independent mechanisms in ISO induced myocyte death. ISO increased the contractility of wild type and TG-VMs by enhancing sarcoplasmic reticulum function and inhibiting sarco(endo)plasmic reticulum Ca2+-ATPase, Na+/Ca2+ exchanger, and CaMKII partially protected myocyte from death induced by both Ca2+ and ISO. Caspase and reactive oxygen species inhibitors did not, but β2-AR activation did, reduce myocyte death induced by enhanced ICa-L and ISO stimulation. Our results suggest that catecholamines induce myocyte necrosis primarily through β1-AR-mediated increases in ICa-L, but other mechanisms are also involved in rodents.