THE EFFECTS OF GROWTH-HORMONE ON AVIAN SKELETAL-MUSCLE SATELLITE CELL-PROLIFERATION AND DIFFERENTIATION

Growth hormone receptor (GH-R) mRNA was expressed in avian skeletal mu scle tissue and satellite cells in culture, and was capable of binding chicken growth hormone (cGH). In the satellite cells, GH-R gene expre ssion was regulated by cGH in a biphasic manner which correlated with the GH effect on cell proliferation: 2-10 ng/ml of the hormone increas ed GH-R mRNA and DNA synthesis, whereas higher concentrations attenuat ed these effects. GH induced insulin-like growth factor I (IGF-I) mRNA , a potential factor for satellite cell proliferation and differentiat ion. However, GH inhibited the gene expression of myogenin and the exp ression of muscle-specific proteins in a dose-dependent manner. These results suggest a role of GH for inhibiting satellite cell differentia tion in an IGF-I-independent manner. During satellite cell differentia tion, both GH-R mRNA expression and cGH binding peaked when cells were still proliferating and beginning to fuse, and then declined as cells fully differentiated. GH-R mRNA expression in muscle tissue and the s atellite cell fraction was evaluated during chicken growth. In both fr actions, GH-R mRNA peaked at 4 days of age and then declined in correl ation with the reduction of muscle regulatory gene expression. Our res ults are in contrast with previous studies on rat muscle satellite cel ls, suggesting a difference between mammalian and avian species in the mode of action of GH in these cells. Our notion is that GH, via its o wn receptor, promotes more satellite cells to proliferate by inhibitin g their differentiation, leading to the addition of more nuclei to the growing muscle. (C) 1996 Academic Press, Inc.