Age-related apoptotic responses to stretch-induced hypertrophy in quail slow-tonic skeletal muscle

In the present study, we examined the responses of apoptosis and apoptotic regulatory factors to muscle hypertrophy induced by stretch overload in quail slow-tonic muscles. The wings from one side of young and aged Japanese quails were loaded by attaching a tube weight corresponding to 12% of the bird's body weight for 7 or 21 days. Muscle from the contralateral side served as the intraanimal control. Relative to the intraanimal contralateral control side, the muscle wet weight increased by 96% in young birds, whereas the muscle weight gain in aged birds was not significant after 7 days of loading. After 21 days of loading, muscle weight significantly increased by 179% and 102% in young and aged birds, respectively. Heat shock protein (HSP) 72 and HSP27 protein contents in the loaded sides were higher than on the control sides exclusively in young birds after 7 days of loading. Compared with the contralateral control muscle, the extent of apoptotic DNA fragmentation and the total cytosolic apoptosis-inducing factor protein content were reduced in all loaded muscles except for the 7-day-loaded muscles from the aged birds. Bax protein content was diminished in the loaded muscle relative to the control side from all groups, whereas Bcl-2 protein content was reduced in the young and aged muscles after 21 days of loading. The total cytosolic cytochrome c protein content was decreased and the X chromosome-linked inhibitor of apoptosis protein content was elevated in 7- and 21-day-loaded muscles relative to the intraanimal control muscle from young birds. Furthermore, after 7 days of loading the muscles of aged birds, H2O2 content and the total cytosolic protein content of second mitochondrial activator of caspases/direct inhibitor of apopto-sis-binding protein with low isoelectric point were elevated compared with the intraanimal control side. These data suggest that stretch overload-induced muscle hypertrophy is associated with changes in apoptosis in slow-tonic skeletal muscle. Moreover, discrepant apoptotic responses to muscle overload in young and aged muscles may account in part for the age-related decline in the capability for muscle hypertrophy.