Changes in insulin-like growth factor-binding proteins in bovine mammary secretions associated with pregnancy and parturition

Abstract

The bovine mammary gland accumulates large quantities of insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs) during late gestation which are secreted at parturition. The present study was conducted to determine the changes in the profiles of IGFBPs secreted by the mammary gland and in blood during late gestation and early lactation in dairy cows. Ligand blotting of serum and mammary secretions showed that IGFBPs of M_r 25 000, 30000, 34000, 42 000, 46 000 and > 200 000 were present in both fluids. The binding activity of the 42–46 000 M_r IGFBP predominated in prepartum mammary secretions and colostrum but was reduced postpartum. The binding activities of the 30 000 and 34 000 M_r IGFBPs, relative to other IGFBPs, were increased postpartum. Concentrations of IGF-I and IGF-II in mammary secretions declined from 347·1 and 181·1 nmol/litre 1 week prepartum to 0·7 and 0·3 nmol/litre 1·5 weeks postpartum. The volume of mammary secretions obtained was 0109 litre and 6·690 litres at 1 week prepartum and 1·5 weeks postpartum respectively. In prepartum serum, the greatest binding activity was at M_r 42–46 000. The activity at this M_r decreased at parturition but was restored postpartum. The binding activities of the 30 000 and 34 000 M_r IGFBPs were increased around parturition. The 25 000 M_r IGFBP had minor activity during all periods. IGF-I concentrations decreased from 10·6 nmol/litres 1 week prepartum to 4·7 nmol/litres 1·5 weeks postpartum but IGF-II concentrations remained constant. In conclusion, IGFBP activity secreted by the mammary gland shifts from primarily M_r 42–46 000 prepartum to M_r 30 000 postpartum. The 42–46 000 M_r IGFBP activity in serum was not abolished during late gestation as reported in several monogastric species. Journal of Endocrinology (1991) 131, 127–133