Plasticity of aged skeletal muscle: chronic contractile activity‐induced adaptations in muscle and mitochondrial function

The intent of the present study was to investigate the adaptive potential of skeletal muscle, as well as subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria in young (6 mo) and senescent (36 mo) animals in response to a standard regimen of chronic contractile activity (CCA). The TA and EDL muscles of 6 and 36 mo F344XBN rats were chronically stimulated (10 Hz, 3 h/day, 7 days) to induce CCA. The contralateral limb served as a non‐stimulated control. Subsequent to CCA, acute stimulation (1 Hz, 5 min) of the TA muscle in situ revealed greater fatigue resistance in both 6 and 36 mo animals. However, the improvement in endurance was 50% greater in the young, compared to the old animals. The CCA‐mediated increase in Hsp70 was also greater in the young animals. The expression of the longevity protein SIRT1 was increased 40–50% with CCA in both age groups. CCA also induced similar 30% elevations in COX enzyme activity in young and old animals. This may be due to the CCA‐elicited increases in mitochondrial biogenesis regulatory proteins PGC‐1α and Tfam that were also comparable (40–70%) between groups. CCA reduced mitochondrial ROS production by 40–80% in both age groups, whereas mitochondrial VO 2 in the presence of succinate was increased with CCA only in the 6 mo group. Thus, muscle and mitochondria from senescent animals readily adapt to CCA, however the magnitude of the changes are less compared to young animals.