Voluntary resistance wheel exercise during post-natal growth in rats enhances skeletal muscle satellite cell and myonuclear content at adulthood

Aim: To determine whether voluntary free wheel (FW) or resistance wheel (RW) exercise or reduced muscle activity would influence maturational increases in muscle mass and the number of satellite cells (SCs) and myonuclei (MN) accrued by adulthood. Methods: Hind limb muscles of male rats housed with, or without, FWs from 4 to 5, 7 or 10 weeks of age, and rats housed with RWs from 4 to 10 week of age, were evaluated. To assess the effect of reduced muscle activity, gastrocnemius muscles of 4-week-old rats were injected with botulinum toxin (Btx) and collected at 7 weeks of age. Muscle fibre size and the frequency of Pax7-positive SCs and MN were determined in 7-and 10-weekold muscles via immunohistochemical methods. Results: Free wheel exercise enhanced muscle growth and the frequency of SCs in the medial gastrocnemius (MG) (threefold) and vastus lateralis (VL) (twofold) of rats at 10 week of age. Resistance wheel exercise increased the number of SCs and MN (22-30%), with more muscle fibre nuclei being associated with larger fibre size, in the soleus, MG and VL muscles. Btx impaired the normal increases in muscle fibre size and the accrual of MN but not SCs. Conclusion: A greater volume of exercise during maturational growth was important for enhancing SC numbers, whereas their conversion to MN required higher-intensity exercise. The enhanced muscle fibre nuclear populations may influence the capacity of the muscle to adapt to exercise, injury or disuse in later adulthood.