ONTOGENIC AND NUTRITIONAL CHANGES IN CIRCULATING INSULIN-LIKE-GROWTH-FACTOR (IGF)-I, IGF-II AND IGF-BINDING PROTEINS IN GROWING EWE AND RAMLAMBS

The ontogeny of the IGF endocrine system was investigated in 15 young lambs before and after weaning at 62 days of age. Before weaning, plas ma IGF-I concentrations were higher in rams than eu es, and plasma con centrations of IGF-II and IGF-binding protein-3 (IGFBP-3) also tended to be higher in rams than in ewes. Feed intake of ewes and rams was re stricted after weaning to remove sex differences in feed intake. Plasm a concentrations of IGF-I and IGFBP-3 did not differ between rants and ewes at 100 days of age, but plasma IGF-II was higher in rams than in ewes at this time. Since circulating concentrations of GH were higher in rams than in ewes at 100 days of age, this implies that the restri cted feed intake blocked the IGF-I and IGFBP-3 responses to GH. We con clude that sex differences in circulating IGF-I and IGFBP-3 concentrat ions in the growing lamb alter with age, and are not present when nutr ition is restricted.