Primary cultures of skeletal muscle satellite cells were induced to proliferate by exposure to physiologic levels of somatomedins and pharmacologic levels of insulin. Dexamethasone inclusion in serum containing medium facilitated the ovine somatomedin (oSm) (P < 0.05), but that both were different than the proliferation induced by MSA/rIGF-II (P < 0.05). In the presence of insulin concentrations that promote maximum proliferation, addition of oSm did not produce an additive effect, whereas the addition of MSA/rIGF-II did produce a significant increase in satellite cell proliferation above that induced by insulin. A more, in depth, analysis of the interaction of MSA/rIGF-II with its satellite cell receptor under a variety of experimental conditions revealed that binding of ¹²⁵I-MSA/rIGF-II was inhibited by oSm and MSA/rIGF-II, but not by insulin. Migration, and localization of ¹²⁵I-MSA/rIGF-II-receptor complexes in 7% sodium dodecyl sulfate polyacrylamide gels suggest that these complexes are Type II IGF receptors. In addition, this receptor system of satellite cells was shown to be modulated by other hormones; notably, pre-exposure of cells with insulin increased ¹²⁵I-MSA/rIGF-II binding, while oSm, or MSA/rIGF-II preincubation decreased the binding of ¹²⁵I-MSA/rIGF-II. Therefore, the proliferative effects of MSA/rIGF-II appeared not as a consequence of MSA/rIGF-II induction of other receptor types such as the insulin, or Type I IGF receptor systems. Concommitant to the previous experimentation, oSm was further examined in an initial attempt to elucidate its biologic binding mechanism in myogenic satellite cells. Binding of ¹²⁵I-oSm was inhibited by MSA/rIGF-II, insulin and IGF-I; thus these data suggest that oSm may be the ovine analog to human IGF-I. In addition, pre-exposure of cells to MSA/rIGF-II and oSm down-regulated the ability of satellite cells to bind oSm, while only concentrations of insulin greater than 550 ng insulin had this ability. Collectively, these data support the hypothesis that somatomedins play an important role in the control of postnatal muscle growth by providing a link between these hormones and satellite cells, one of the significant target cells involved in the growth process.

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