Catherine Communal

Norepinephrine Stimulates Apoptosis in Adult Rat Ventricular Myocytes by Activation of the β-Adrenergic Pathway

BACKGROUND Myocardial sympathetic activity is increased in heart failure. We tested the hypothesis that norepinephrine (NE) stimulates apoptosis in adult rat ventricular myocytes in vitro. METHODS AND RESULTS Myocytes were exposed to NE alone (10 micromol/L), NE+propranolol (2 micromol/L), NE+prazosin (0.1 micromol/L), or isoproterenol (ISO, 10 micromol/L) for 24 hours. NE and ISO decreased the number of viable myocytes by approximately 35%. This effect was completely blocked by the beta-adrenergic antagonist propranolol but was not affected by the alpha1-adrenergic antagonist prazosin. NE increased DNA laddering on agarose gel electrophoresis and increased the percentage of cells that were stained by terminal deoxynucleotidyl transferase-mediated nick end-labeling from 5.8+/-1. 0% to 21.0+/-2.3% (P<0.01; n=4). NE likewise increased the percentage of apoptotic cells with hypodiploid DNA content as assessed by flow cytometry from 7.8+/-0.7% to 16.7+/-2.2% (P<0.01; n=6), and this effect was abolished by propranolol but not prazosin. ISO and forskolin (10 micromol/L) mimicked the effect of NE, increasing the percentage of apoptotic cells to 14.7+/-1.9% and 14. 4+/-2.2%, respectively. NE-stimulated apoptosis was abolished by the protein kinase A inhibitor H-89 (20 micromol/L) or the voltage-dependent calcium channel blockers diltiazem and nifedipine. CONCLUSIONS NE, acting via the ss-adrenergic pathway, stimulates apoptosis in adult rat cardiac myocytes in vitro. This effect is mediated by protein kinase A and requires calcium entry via voltage-dependent calcium channels. NE-stimulated apoptosis of cardiac myocytes may contribute to the progression of myocardial failure.

Opposing Effects of β1- and β2-Adrenergic Receptors on Cardiac Myocyte Apoptosis Role of a Pertussis Toxin–Sensitive G Protein

Background—β-Adrenergic receptor (β-AR) stimulation increases apoptosis in adult rat cardiac (ventricular) myocytes (ARVMs) via activation of adenylyl cyclase. β2-ARs may couple to a Gi-mediated signaling pathway that can oppose the actions of adenylyl cyclase. Methods and Results—In ARVMs, β-AR stimulation for 24 hours increased the number of apoptotic cells as measured by flow cytometry. β-AR–stimulated apoptosis was abolished by the β1-AR–selective antagonist CGP 20712A (P<0.05 versus β-AR stimulation alone) but was potentiated by the β2-AR–selective antagonist ICI 118,551 (P<0.05 versus β-AR stimulation alone). The muscarinic agonist carbachol also prevented β-AR–stimulated apoptosis (P<0.05 versus β-AR stimulation alone), whereas pertussis toxin potentiated the apoptotic action of β-AR stimulation (P<0.05 versus β-AR stimulation alone) and prevented the antiapoptotic action of carbachol. Conclusions—In ARVMs, stimulation of β1-ARs increases apoptosis via a cAMP-dependent mechanism, whereas stimulatio...

Adrenergic regulation of myocardial apoptosis

Increased sympathetic nerve activity to the myocardium is a central feature in patients with heart failure. Norepinephrine, the primary transmitter of the sympathetic nervous system, signals via binding to alpha- and beta-adrenergic receptors (AR) that are coupled to G-proteins. Pharmacologic studies of cardiac myocytes in vitro demonstrate that beta-AR can stimulate apoptosis. Likewise, in transgenic mice overexpression of beta 1-AR or G alpha s is associated with myocyte apoptosis and the development of dilated cardiomyopathy. Whereas beta 1-AR stimulate apoptosis in vitro and in vivo, beta 2-AR may either stimulate or inhibit apoptosis and myocardial failure depending on the level of expression. Receptors coupling to Gi and Gq may also be able to mediate or modulate apoptosis and the development of myocardial failure, suggesting the potential for interactions between the beta-AR system and numerous remodeling stimuli that act through Gi or Gq signaling pathways. It appears likely that the mitogen-activated protein kinase superfamily plays a key role in mediating the actions of adrenergic pathways on myocyte apoptosis. These observations suggest that the adrenergic nervous system plays an important role in the regulation of myocyte apoptosis, and may thus contribute to the development of myocardial failure.

Inhibition of protein phosphatase 1 induces apoptosis in neonatal rat cardiac myocytes: role of adrenergic receptor stimulation.

Abstract The mechanisms that regulate cardiac myocyte apoptosis are not well understood. To study the role of protein phosphatase 1 (PP1) and 2A (PP2A) in apoptosis, we exposed cultured neonatal rat cardiac myocytes to the phosphatase inhibitor okadaic acid (OA). Exposure (18 h) to 100 nM OA (a concentration which inhibits both PP1 and PP2A) decreased the number of adherent cells, caused genomic DNA fragmentation, and increased the percentage of apoptotic cells. These effects did not occur at a lower concentration of OA (1 nM) which is relatively specific for PP2A. Stimulation of α1- or β-adrenergic receptors with norepinephrine (NE) in the presence of propranolol or prazosin partially blocked OA-induced apoptosis as measured by flow cytometry. Likewise, stimulation of adenylyl cyclase with forskolin reduced OA-induced apoptosis. Conversely, inhibition of protein kinase A with H89 or protein kinase C with chelerethrine potentiated OA-induced apoptosis. OA increased caspase-3 activity, and this effect was reduced by NE. Thus, inhibition of PP1 stimulates apoptosis in NRVM and stimulation of adrenergic receptors protects against OA-induced apoptosis.