Biological activity of bovine milk - Contribution of IGF-I and IGF bindingproteins

The mitogenic activity of milk from different stages of lactation was studi ed in primary cultures of undifferentiated bovine mammary epithelial cells. The mitogenic activity, measured as DNA synthesis, was 3-4-fold higher in colostrum than in basal medium. The mitogenic activity declined rapidly aft er calving, and in mid and late lactation the effect was inhibitory, not st imulatory. The content of IGF-I in milk varied with stage of lactation decl ining from more than 300 ng per mi in colostrum to 1-2 ng per ml in mid lac tation and increasing to 20 ng per mi in late lactation. The difference in mitogenic activities between colostrum and mature milk was closely related to the difference in IGF-I content. The changes in IGF-I, however, cannot a ccount for the inhibitory effect of mid and late lactation milk and the dif ference in the mitogenic effect of colostrum and BPMS (bovine prepartum mil k-like secretion). The amounts of IGF binding proteins (24, 28, 34 and 41-4 4 kDa) in milk followed the same pattern as IGF-I with high content in earl y lactation and low content in mid lactation. The discrepancies between mit ogenic activity and IGF-I content may in some, but not all, cases be relate d to the content of binding proteins. (C) 2001 Elsevier Science B.V. All ri ghts reserved.