Transforming growth factor-β following skeletal muscle strain injury in rats

Transforming growth factor-β (TGF-β) is a multifunctional cytokine implicated in inflammatory processes, wound healing, and fibrosis. In muscle diseases (i.e., dystrophy and inflammatory myopathy) and in animal models of muscle injury (i.e., produced by cardiotoxin, laceration, and eccentric contractions), increased TGF-β was associated with muscle fibrosis and healing. Although TGF-β transcript abundance was increased following injury, many studies presume that TGF-β protein was also active as evident by increases in collagen transcript abundance. The purpose was to determine whether TGF-β protein is present and active 48 h following injury. Using female rats, muscle strains were produced by stretching (50 stretches) the plantar flexor muscles. Forty-eight hours following injury, the medial gastrocnemius was removed and compartmentalized into five equal segments. Damaged myofibers with intracellular concanavalin A staining were counted. The percentage of damaged myofibers was significantly greater in the distal-most segment. TGF-β was assessed by using immunohistochemistry, RT-PCR, and immunoblot analysis. Immunohistochemistry revealed the presence of TGF-β1in areas of myofiber injury, whereas TGF-β2was not detected. Increases in TGF-β1and TGF-β2transcript abundance following strain injury were documented by RT-PCR analysis. Increases in TGF-β1and TGF-β2precursor abundance were observed following strain injury by using immunoblot analysis but there was no change in active TGF-β abundance. Although there was no correlation between the amount of cellular injury and TGF-β transcript and protein abundance, elevated levels of TGF-β1and TGF-β2precursor proteins were present in strain-injured skeletal muscles 48 h after injury.