The influence of growth factors on turkey embryonic myoblasts and satellite cells in vitro.

The influence of growth factors on the proliferation and differentiation of clonal-derived embryonic myoblasts and satellite cells was examined in hormonally controlled serum-free media. Although insulin-like growth factor-I (IGF-I) and IGF-II each stimulated proliferation of both cell types, IGF-II was the more potent mitogen for embryonic myoblasts (P < or = 0.05). Exogenously added IGFs also stimulated differentiation of embryonic myoblasts (P < or = 0.05) but did not influence the differentiation of satellite cells (P > or = 0.05). Fibroblast growth factor (FGF) was required for proliferation of either cell type to occur. Platelet-derived growth factor-BB acted synergistically with IGF-I and FGF to stimulate proliferation. Unlike human muscle cells, neither avian satellite cells nor embryonic myoblasts responded to epidermal growth factor. The influence of IGFs and insulin on IGF-binding protein (IGFBP) release from myogenic cells was also examined. Both cell types secreted a major binding protein of 34 kDa and a second protein of 36 kDa. Satellite cells secreted a prominent IGFBP at 30.8 kDa, while embryonic myoblasts secreted higher molecular weight forms of 37 and 41 kDa. Highest IGFBP release was seen when cells from either type were administered IGF-II. Equimolar levels of IGF-I or insulin stimulated IGFBP release, however, at levels lower than that induced by IGF-II. Release of IGFBP was nearly undetectable in the presence of cycloheximide (50 micrograms/ml), suggesting that protein synthesis was necessary for IGFBP release.