β2‐adrenoceptor agonist clenbuterol reduces infarct size and myocardial apoptosis after myocardial ischaemia/reperfusion in anaesthetized rats

Background and purpose: Considerable evidence indicates that the β2‐adrenoceptor agonist clenbuterol decreases apoptosis in a rodent model of ischaemic cardiomyopathy. In this study, we investigated the effects of clenbuterol on infarct size caused by myocardial ischaemia/reperfusion (I/R) in anaesthetized rats. Experimental approach: Rats were randomly assigned to the following groups: (i) sham (ii) I/R (iii) clenbuterol + I/R (iv) ICI 118551 + clenbuterol + I/R (v) metoprolol + clenbuterol + I/R (vi) metoprolol + I/R (vii) pertussis toxin + clenbuterol + I/R. Under anaesthesia, left anterior descending coronary artery was occluded for 30 min followed by reperfusion for 2 h. Key results: Compared with the control I/R group,the clenbuterol (0.5 mg·kg−1, i.p.) group had reduced infarct size, improved diastolic function and sarcoplasmic/endoplasmic reticulum Ca2+‐ATPase (SERCA) activity, increased superoxide dismutase activity, and decreased malondialdehyde (MDA) level and LDH, CK release. Clenbuterol increased the phosphorylation of ERK1/2, which resulted in inhibition of myocardial apoptosis as indicated by the reduction of terminal deoxynucleotidyltransferase end labelling‐positive staining, Bax/Bcl‐2 mRNA and caspase‐3 protein expression. The Gi‐protein inhibitor pertussis toxin blocked the clenbuterol‐induced improvement in cardiac function and infarct size. Pretreatment with ICI 118551(a selective β2‐adrenoceptor antagonist) inhibited the effects of clenbuterol mentioned above. The β1‐adrenoceptor agonist metoprolol had similar effects to clenbuterol but failed to reduce MDA and improve SERCA activity. When administered together, metoprolol and clenbuterol did not induce synergistic effects. Conclusions and implications: Clenbuterol pretreatment provides significant cardioprotection against ischaemia/reperfusion injury and this is mediated by the β2‐adrenoceptor–Gi‐protein signalling. A combination of the β2‐adrenoceptor agonist clenbuterol and the β1‐antagonist metoprolol did not lead to a synergistic anti‐apoptotic effect.