Sarcopenia-related apoptosis is regulated differently in fast- and slow-twitch muscles of the aging F344/N × BN rat model

Age-related decreases in muscle mass have been associated with the loss of myonuclei, possibly through a mechanism involving mitochondria. It is unclear if age-related apoptotic mechanisms vary by fiber type. Here we investigate indices of apoptosis along with the regulation of apoptotic mediators in the extensor digitorum longus (EDL) and soleus of adult (6 month), old (30 month), and very old (36 month) Fischer 344/NNiaHSD × Brown Norway/BiNia (F344/N × BN) rats. Compared to 6-month muscles, aged muscles exhibited decreases in muscle mass along with increases in the number of nuclei staining positively for DNA fragmentation. The expression of Bax, Bcl-2, caspase-3 and caspase-9 was regulated differently with aging between muscle types and in a manner not consistent with mitochondria-mediated apoptosis. To investigate the potential of calpain involvement in age-related myonuclear loss, the calpain-dependent cleavage of α-fodrin was examined. The proteolytic cleavage of α-fodrin by calpains was increased in both muscles with only the 36-month soleus exhibiting increased caspase-dependent α-fodrin cleavage. Taken together, these data suggest that apoptotic regulatory events differ between fiber types in the aging F344/N × BN and that mitochondrial-dependent apoptosis pathways may not play a primary role in the loss of muscle nuclei with aging.