β-Adrenergic receptor subtypes differentially affect apoptosis in adult rat ventricular myocytes

Abstract

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 μmo]/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 with P=0.62]. ALB-induced apoptosis at 100 μmol/L was abolished by AT (10 μmol/L), indicating a β1AR-mediated effect. Importantly, ICI 118551 (0.1 μmol/L), a highly selectiveβ2AR 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 β2ARs and selectively mediated by β1ARs in adult rat ventricular myocytes.