Periparturient Changes in Secretion and Mammary Uptake of Insulin and in Concentrations of Insulin and Insulin-Like Growth Factors in Milk of Dairy Cows

Mammary uptake of insulin from blood was quantified near parturition in nine dairy cows, some of which were milked prepartum to induce prepartum lactogenesis. Milk samples were immunoassayed for concentrations of insulin, insulin-like growth factor I, and insulinlike growth factor II. Cows milked prepartum, but in which prepartum lactogenesis did not occur (denoted as unsuccessful group), had higher arterial concentrations and greater mammary uptake of insulin during several days immediately preceding parturition than did those with prepartum lactogenesis. Although mammary uptake of insulin was measurable before parturition, estimated uptake on postpartum d 4 through 6 was negligible. Colostrum and milk contained very substantial quantities of insulin, insulin-like growth factor I, and insulin-like growth factor II. Cows in which prepartum lactogenesis was induced (denoted as successful group) had lower concentrations of both forms of insulin-like growth factor in prepartum samples of milk. Cows of the successful group also had a decreasing prepartum profile of milk insulin, but the unsuccessful group had an increasing trend until parturition. Milk concentrations of insulin and both forms of insulin-like growth factor declined rapidly after parturition and completion of lactogenesis.