Satellite cell proliferation in response to pituitary hormones

Proliferation of rat skeletal muscle satellite cells was studied in vitro, and their ability to respond to a variety of protein hormones was examined, including: growth hormone, prolactin, luteinizing hormone, thyrotropin and fibroblast growth factor. All experiments were conducted in serum-free medium to avoid complicating interactions with serum constituents such as other hormones or binding proteins. Dose-response curves were constructed for each protein and encompassed the physiological range plus concentrations two to three orders of magnitude greater than the physiological range. Of the proteins examined, the only one shown to have the ability to directly stimulate satellite cell proliferation was fibroblast growth factor. None of the anterior pituitary protein hormones had the ability to interact directly with satellite cells to stimulate proliferation in vitro. Therefore, satellite cells seem to be selective in their response to protein hormones, and the classes identified thus far are hormones in the insulin-like growth factor family and fibroblast growth factor. These two classes of protein hormones are quite different and would not be expected to act through a common pathway. Consequently, we have proposed a dual regulatory system that may allow for local as well as systemic stimulation of satellite cells.