ONTOGENIC DIFFERENCES IN THE NUTRITIONAL REGULATION OF CIRCULATING IGF BINDING-PROTEINS IN SHEEP PLASMA

Well-fed castrated male sheep (N = 3) and 125 days gestation pregnant ewes (N = 6) with chronically catherized fetuses were fasted for 72 h. Insulin-like growth factor-binding protein (IGFBP) levels in fed and starved fetal, maternal and castrated male sheep plasma were measured using ligand blot analysis. IGFBPs in adult and fetal sheep differed in distribution both before and after 72 h starvation. IGFBP-3 was the major postnatal binding protein, while in the fetus IGFBP-2, IGFBP-3 and the circulating IGF type 2 receptor fragment each contributed 25-30% of total IGF binding capacity. After starvation, total IGF binding capacity and IGFBP-3 fell in plasma of maternal and castrated male sheep (p < 0.05). Total IGF binding capacity rose with starvation in fetal plasma (p < 0.05) as a result of an increase in IGFBP-1 (p < 0.01) and IGFBP-2 (p < 0.05). The different nutritional control of the IGFBPs in the fetus and the adult may reflect ontogenic differences in the regulation and function of circulating IGFs and their binding proteins.