α- and β-adrenergic pathways differentially regulate cell type - Specific apoptosis in rat cardiac myocytes

Abstract

Background - The apoptosis of cardiac myocytes may play a role in the development of heart failure. Norepinephrine is one of the factors activated in heart failure and can induce myocardial cell apoptosis in culture. However, it is unknown if α- and β-adrenergic pathways coordinately or differentially regulate apoptosis and if this apoptotic pathway uses common or cell type-specific apoptotic signals. Methods and Results - We stimulated cultured neonatal rat cardiac myocytes with an α1-adrenergic agonist (PE, phenylephrine), a β-adrenergic agonist (isoproterenol [Iso]) or a membrane- permeable cAMP analogue (8-Br-cAMP) in serum-free conditions for 48 hours. Iso and 8-Br-cAMP markedly increased the number of TUNEL-positive cells (%TUNEL-positive nuclei >40%) compared with saline stimulation (< 10%). DNA fragmentation was also confirmed by ladder formation in agarose gels. Apoptotic myocytes were characterized by cell shrinkage and nuclear condensation, consistent with morphological features of apoptosis. The Iso- induced apoptosis was almost completely inhibited by the protein kinase A- specific inhibitor KT5720. In contrast, PE inhibited 8-Br-cAMP-induced myocardial cell apoptosis. The apoptosis-inhibitory effect by PE was negated by the α1-adrenergic receptor antagonist prazosin and the MEK-1-specific inhibitor PD098059. Interestingly, although 8-Br-cAMP markedly induced apoptosis in cardiac myocytes, it completely blocked serum depletion-induced apoptosis in PC12 cells, a rat pheochromocytoma cell line. Conclusions - These findings indicate that α- and β-adrenergic pathways differentially regulate myocardial cell apoptosis. The results also suggest that a cAMP- protein kinase A pathway is necessary and sufficient for β-adrenergic agonist-induced apoptosis and that this apoptotic pathway is not functional in other cell types, for example, PC12 cells.