Downhill running in rats: influence on neutrophils, macrophages, and MyoD+ cells in skeletal muscle.

The accumulation of neutrophils and macrophages, as well as the activation of satellite cells, are early events following skeletal muscle injury. We examined the temporal relationship between changes in neutrophils, macrophages, and MyoD protein, a marker of satellite cell activation, after injurious exercise. Male rats ( n=47) performed an intermittent downhill (-16% grade) running (17 m/min) protocol and the solei were obtained at 0, 2, 6, 24, 48, or 72 h post-exercise. Neutrophils, macrophages (ED1 and ED2), and MyoD+ cells were determined in muscle cross sections using immunohistochemistry. Downhill running increased ( P<or=0.05) the percentage of injured fibers and elevated blood creatine kinase activity. Neutrophils were elevated 18-fold relative to controls at 24 h post-exercise. ED1 macrophages were elevated four- and twofold at 24 and 48 h post-exercise, respectively. Neither ED2 macrophages nor MyoD+ cells were elevated post-exercise. These observations may indicate that elevations in neutrophils and ED1 macrophages after injurious exercise are not temporally associated with an increase in satellite cell activation.