Catecholamine‐induced apoptosis and necrosis in cardiac and skeletal myocytes of the rat in vivo : the same or separate death pathways?

High levels of catecholamines are myotoxic but the relative amounts of apoptosis and necrosis have not been established in vivo in cardiac and skeletal muscles. Immunohistochemistry was used to detect and quantify myocyte‐specific necrosis (myosin antibody in vivo ) and apoptosis (caspase‐3 antibody in vitro ) in the heart and soleus muscles of male Wistar rats that had received single subcutaneous injections of isoprenaline over the range 1 μg to 5 mg [kg body weight (BW)] −1 . Peak myocyte apoptosis occurred 3–6 h after, and necrosis 18 h after, a single injection of 5 mg (kg BW) −1 isoprenaline in vivo . In the heart myocyte death was mediated through the β 1 ‐adrenergic receptor whereas myocyte death in the soleus muscle was mediated through the β 2 ‐adrenergic receptor. Cardiomyocyte death was heterogeneously distributed throughout the heart, being greatest in the left ventricle (LV) subendocardium and peaking close to the apex, but with significantly more necrosis than apoptosis. Extensive co‐localization of caspase‐3 and myosin labelling was found in the myocytes of both the heart and the slow‐twitch soleus muscle. This, together with similar spatial distributions and responses to catecholamine doses, suggests that either caspase‐3 activation occurs in necrotic as well as apoptotic myocytes or that a large proportion of apoptotic myocytes progress to secondary necrosis in vivo.