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

Abstract

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 μmol/L), NE+propranolol (2 μmol/L), NE+prazosin (0.1 μmol/L), or isoproterenol (ISO, 10 μmol/L) for 24 hours. NE and ISO decreased the number of viable myocytes by ≈35%. This effect was completely blocked by the β-adrenergic antagonist propranolol but was not affected by the α 1 -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 μmol/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 μmol/L) or the voltage-dependent calcium channel blockers diltiazem and nifedipine. Conclusions —NE, acting via the β-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.