Characterization of the Change in Type I and II Insulin-Like Growth Factor Receptors of Bovine Mammary Tissue during the Pre- and Postpartum Periods*

Abstract

Type I and II receptors for the insulin-like growth factors (IGF) were characterized in microsomes from bovine mammary tissue. Specific binding of [125I]IGF-I increased linearly with increasing microsomal protein concentrations. In contrast, IGF-II binding showed a curvilinear relationship over the concentration range tested, with a maximum at 120 micrograms/ml. Kinetic studies conducted at 4 C showed the binding reactions to be reversible. Competition studies showed recombinant human IGF-I (rh-IGF-I) to be 8-, 18-, and 1000-fold more potent at inhibiting [125I]rh-IGF-I binding than recombinant bovine IGF-II (rb-IGF-II), multiplication-stimulating activity, and insulin, respectively. rbIGF-II was 1.8- and 6-fold more potent at inhibiting [125I]rbIGF-II binding than rhIGF-II and multiplication-stimulating activity. Specific binding of [125I]IGF-I and -II was measured in microsomes prepared from cows (n = 47) ranging from 138 days prepartum to 411 days postpartum. IGF-I binding declined during the prepartum period, increased 75% with the onset of lactation, and then declined during the postpartum period. Scatchard analysis showed the presence of a single class of high affinity binding sites for both ligands, with type II receptors being about 10-fold more prevalent than type I receptors. The survey and Scatchard data support the conclusion that the onset of lactation coincides with an increase in the number of type I receptors present in mammary tissue. This increase supports the contention that IGF-I may play an important role in modulating the metabolic activity of the bovine mammary gland.