Eccentric contraction induces inflammatory responses in rat skeletal muscle: Role of tumor necrosis factor-α

Eccentric contraction (EC) is known to elicit inflammation and damage in skeletal muscle. Proinflammatory cytokine TNF-α plays an important role in this pathogenesis, but the time course of its response to EC and the regulatory mechanisms involved are not clear. The purpose of the study is twofold: 1) to investigate the gene expression of TNF-α in rat muscle during and after an acute bout of downhill running and the associated oxidoreductive (redox) changes; and 2) to examine whether EC activates muscle ubiquitin-proteolytic pathway resulting in necrosis and oxidative damage. Female Sprague-Dawley rats (age 3 mo) were randomly divided into five groups (n = 6) that ran on treadmill at 25 m/min at - 10% grade for 1 h (group 1) or 2 h (group 2) and were killed immediately; ran for 2 h and killed at 6 h after exercise (group 3), ran for 2 h and killed at 24 h after exercise (group 4); and killed at rest as controls (group 5). TNF-α mRNA and protein content showed progressive increases in the deep portion of vastus lateralis (DVL) and gastrocnemius muscles during and after EC. These changes were accompanied by a progressive decrease of mitochondrial aconitase activity and NF-κB activation. After 2 h of exercise, elevated levels of serum TNF-α, endotoxin, creatine kinase, and lipid peroxidation marker were evident and persisted through 24 h postexercise. At 24 h, there were marked increases in H₂O₂ concentration, myleoperoxidase activity, and endotoxin level, along with nuclear accumulation of p65, in both muscles. mRNA level of ubiquitin-conjugating enzymes (E₂)-14k was progressively upregulated during exercise and recovery, whereas the expression of the Toll-like receptor 4 (TLR4) in DVL was downregulated in both muscles. We conclude that prolonged EC induces TNF-α expression possibly due to NF-κB activation stimulated by increased reactive oxygen species generation and endotoxin release. These inflammatory and prooxidative responses may underlie the processes of muscle proteolysis and oxidative damage.