Effect of bovine GH and ovariectomy on mammary tissue sensitivity to IGF-I in prepubertal heifers.

Sixteen prepubertal Holstein Friesian heifers were used to study the effect of long-term administration of bovine GH (bGH; 15 mg/day for 15 weeks) and ovariectomy (at 2.5 months of age) on GH and IGF-I receptors in mammary and liver parenchymal tissue. Heifers were slaughtered on the day after the last injection of bGH or excipient. Mammary and liver parenchymal tissue was collected for preparation of crude membranes for ligand-binding assays. Specific binding of [125I]IGF-I to mammary membranes increased linearly with increasing membrane protein concentration and was displaced by IGF-I, less effectively by IGF-II and least effectively by insulin. Linear Scatchard plots were obtained showing the presence of a single class of high-affinity binding sites for IGF-I in prepubertal mammary tissue. Specific binding of IGF-I was unaffected by bGH treatment but tended to be higher (P < 0.10) in membranes from ovariectomized heifers. Hepatic binding of GH and IGF-I was unaffected by bGH treatment and ovariectomy. However, specific binding of GH tended to be lower (P < 0.06) in liver tissue from bGH-treated intact heifers compared with control heifers. The present study confirms the presence of IGF-I receptors in prepubertal mammary tissue in agreement with the suggested involvement of IGF-I in the mechanism of action of GH on mammary glands in this period. The results, however, indicate that the stimulatory effect of GH on mammary growth is mediated via increased serum concentration of IGF-I and not by increased IGF-I binding to its mammary receptors.(ABSTRACT TRUNCATED AT 250 WORDS)