Bovine GH is a potent stimulant of lactation, and the insulin-like growth factors I and II (IGF-I and -II) are believed to mediate GH’s growth-promoting actions. Since all of IGF’s known actions are mediated through its receptor subtypes, we analyzed the distribution of IGF receptor subtypes in lactating and nonlactating bovine mammary tissue. Analysis of competition curves showed that IGF-I had greater potency than IGF-II in competing with [125I]IGF-I for binding to membranes prepared from both lactating and nonlactating animals. An insulin concentration of 4 μg/ml displaced less than 40% of the [125I]IGF-I bound to membranes prepared from both lactating and nonlactating animals, indicating that a high percentage of [125I]IGF-I was bound to the type II receptor. Lactation was associated with an increase in the total amount of [125I]IGF-I bound, and this change was due to an increase in binding to both receptor subtypes. Specifically, membranes prepared from lactating animals had a 3-fold increase in binding competed for by insulin and a 2-fold increase in binding not competed for by insulin. Affinity cross-linking of [125I]IGF-I to membranes prepared from both lactating and nonlactating animals, followed by polyacrylamide gel electrophoresis (PAGE) and autoradiography, showed that 260K and 135K bands were present. Competition experiments indicated that unlabeled IGF-I effectively competed for binding to the 260K band, whereas insulin did not. Binding to the 135K band could be inhibited by both IGF-I and insulin. The intensity of the labeled bands showed that type II receptors were relatively more abundant than type I receptors in membranes from both lactating and nonlactating animals. Membranes prepared from lactating animals showed both 135K and 127K species of the type I receptor, whereas nonlactating animals showed only the 135K band. We conclude that type I and II receptors are present in bovine mammary tissue, and type II predominate. Lactation is associated with increases in the concentration of both receptor subtypes, especially type I receptors. Lactation may be associated with structural changes in the type I receptor. These changes in receptor distribution could play a role in modulating the physiological effects of the IGFs on mammary tissue.
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