| Summary |
Skeletal muscle remodeling in response to an overload hypertrophy stimulus is highly dependent upon satellite cell proliferation, a phenomenon which is mediated in part by growth factor and extracellular matrix secretion from neighboring fibroblasts. Acute loading (in vitro) and acute traumatic skeletal muscle damage (in vivo) both stimulate fibroblast proliferation, and fibroblast number is elevated in chronically overloaded skeletal muscle. However, the fibroblast response to an acute overload hypertrophy stimulus in skeletal muscle has yet to be investigated. Therefore the aim of this investigation was to determine the effect of an acute bout of resistance exercise on skeletal muscle fibroblast proliferation. It was hypothesized that one acute bout of high intensity resistance exercise would increase skeletal muscle fibroblast content in the vastus lateralis muscles of young men. Seven healthy, untrained men (mean ± SE: age. 24.3 ± 0.9 yrs; height, 179.80 ± 2.98 cm; weight, 78.79 ± 4.29 kg; body fat, 14.29 ± 2.02 %) performed an acute bout of resistance exercise (leg extensions; 3 warm-up sets and 3 sets to exhaustion at a 10-repetition maximum resistance, each separated by l.Sminutes rest). Vastus lateralis biopsies were obtained before, 24 hours post-, 48 hours post-, and 72 hours post-exercise. Fibroblast content in biopsy cross-sections was assessed by immunohistochemical staining for vimentin (a marker for fibroblasts). Contrary to the hypothesis, skeletal muscle fibroblast content as measured by vimentin-positive area remained unchanged at all post-exercise timepoints compared to pre-exercise. Likewise, total nuclear number in muscle biopsy cross-sections (as assessed by hematoxylin staining) was also unaffected, indicating a lack of general cellular proliferation in the muscles in response to the exercise bout. Venous serum creatine kinase, an indicator of skeletal muscle damage, was only mildly, but significantly (p<0.05), elevated by 72 hours post-exercise in these subjects. Lastly, a small group of non-exercising control subjects (n=3) displayed no changes in any of these variables. These data indicate that fibroblasts do not proliferate in response to one acute bout of high-intensity resistance exercise in the skeletal muscles of young, untrained men. It is possible that a more chronic stimulus (such as multiple resistance exercise bouts or chronic overload) is necessary to stimulate measurable fibroblast proliferation in an overload-induced skeletal muscle hypertrophy model when muscle damage is not extensive. |
