SECRETION OF INSULIN-LIKE GROWTH-FACTORS IN MILK AND THEIR EFFECT ON THE NEONATE

Many hormones and growth factors appear in milk and colostrum. Interes t and research concerning these factors has recently been revitalized to study their role in both mammary and neonatal development. Colostru m is rich in insulin-like growth factor-I (IGF-I), IGF-II and IGF bind ing proteins (IGFBPs). The appearance of IGFs in colostrum is specific and the pattern of appearance is similar to that of immunoglobulins. Blood concentrations of IGF-I are significantly altered by the cessati on and initiation of lactation. Increasing blood IGF-I concentrations by somatotropin treatment results in increased mass appearance of IGF- I into colostrum during the prepartum period with only modest increase s at parturition. Because local production of IGF-I is low and not epi thelial in origin, and because blood flux has been demonstrated, it is likely that most colostrum IGF-I is of blood origin. Conversely, mamm ary tissue epithelial cells synthesize and secrete IGFBPs. Peak secret ory IGF-I and IGFBP colostrum concentrations are 10-30-fold that of bl ood concentrations. Two possible mechanisms of appearance are describe d. Because of the occurrence of IGFs in colostrum, the dietary effects of IGF-I on the newborn were of interest. Both intestinal and systemi c changes were hypothesized. Calves were fed dietary treatments of. (1 ) milk replacer (MR), (2) pooled colostrum for four feedings, followed by MR, and (3) MR supplemented with 750 ng/ml of recombinant human (r h) IGF-I for one week. The dietary treatments had no effect on circula ting neonate IGFBP profiles, yet IGFBPs changed with neonate age. Bloo d insulin and prolactin concentrations, but not IGF-I were altered ini tially. A latent increase in blood IGF-I concentrations in IGF-I-fed a nimals appeared after 4 to 5 days of feeding. At slaughter, there was more growth in intestinal proximal regions in rhIGF-I-fedanimals than in the colostrum-fed controls. Thermodynamic analysis of the IGF type I and II receptors of intestinal mucosal microsomes indicated that IGF -I feeding up-regulated (increased binding capacity) the type I IGF re ceptor, whereas dietary colostrum down-regulated the type II IGF recep tor. These data support the concept of a physiological effect of colos trum-borne IGFs on the neonate.