β-Adrenergic Receptor Subtypes Differentially Affect Apoptosis in Adult Rat Ventricular Myocytes

Background —Catecholamine-induced apoptosis is mediated by activation of the β-adrenergic signaling pathway. We tested the hypothesis that β1 - and β2 -adrenergic receptor (AR) subtypes differentially affect apoptosis in adult rat ventricular myocytes in vitro.Methods and Results —Myocytes were first exposed to norepinephrine (NE) alone (10 μmol/L) or NE+atenolol (AT) (10 μmol/L) for 12 hours. AT, a β1 -selective AR antagonist, abolished the NE-induced increase in nick end-labeling (TUNEL)–positive cells compared with control (NE, 33±3% versus control, 3±1%,P <0.0001; NE+AT, 4±2% versus control, 3±1%,P =0.98). Annexin V staining, DNA laddering, and caspase activity determinations corroborated these results. Subsequent experiments under prazosin treatment established the apoptosis dose-response curves for the increasingly β2 -selective AR agonists isoproterenol (ISO) (β1 ≈β2 ) and albuterol (ALB) (β2 >β1 ). ISO and ALB induced significantly less apoptosis than NE (β1 >β2 ) at equimolar concentrations as assessed by TUNEL staining [1 μmol/L: NE (8±2%)≈ISO (7±1%)>ALB (2±1%); 10 μmol/L: NE (35±2%)>ISO (23±1%)>ALB (3±1%); 100 μmol/L: NE (50±2%)>ISO (29±2%)>ALB (14±1%),P <0.0001 except for NE versus ISO at 1 μmol/L withP =0.62]. ALB-induced apoptosis at 100 μmol/L was abolished by AT (10 μmol/L), indicating a β1 AR-mediated effect. Importantly, ICI 118551 (0.1 μmol/L), a highly selective β2 AR antagonist, did not decrease the percentage of NE-, ISO-, and ALB-induced apoptosis. Reverse transcription–polymerase chain reaction studies revealed that AT completely reversed the β-adrenergic signaling–induced changes in the Bcl-2–to-Bax ratio.Conclusions —These observations provide evidence that βAR-mediated apoptotic death signaling is largely dissociated from β2 ARs and selectively mediated by β1 ARs in adult rat ventricular myocytes.