Insulin-like growth factor-I (IGF-I) is a mito- genic polypeptide postulated to mediate the effect of GH on adipose tissue development. To determine if the effect of GH could be mediated by the local production of IGF-I, we have characterized IGF-I RNA expression, polypeptide secretion, and binding protein activity in primary preadipocyte cultures derived from porcine adipose tissues. GH acutely regulated the abundance of multiple IGF-I RNA transcripts and resulted in a 2fold increase in secreted immunoreactive IGF-I (iIGF-I) poly-peptide in medium conditioned for 48 h by preadipocyte cultures relative to those not receiving GH. Immunocytochemical data indicated that IGF-I is synthesized by presumptive and mature adipocytes. The effect of GH on iIGF-I secretion was observed in cultures derived from both fetal and postnatal animals, while secreted IGF-binding protein activity was increased due to GH only in cultures from fetal animals. The increase in local IGF-I production in response to GH was associated with a decrease in adipocyte development, suggesting that local IGF-I may contribute to suppression of differentiated phenotype.
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