Horse conceptuses secrete insulin-like growth factor-binding protein 3

Insulin-like growth factor-I (IGF-I) promotes early embryonic development i n several species. In the rabbit, ICF-I binds to the embryonic coats from D ay 3 of development onward by a 38-kDa protein that is probably insulin-lik e growth factor-binding protein 3 (IGFBP3). In the present study, ligand, W estern, and Northern blot analyses were used to demonstrate the presence of IGF-I-binding activity, several immunoreactive IGFBP3 proteins, and IGFBP3 mRNA in horse conceptuses with particularly large amounts of immunoreactiv e IGFBP3 in the conceptus capsule. In addition, immunoprecipitation of radi olabeled proteins showed that cultured horse conceptuses secreted ICFBP3 in to the culture medium. Endometrial samples from mares also contained ICFBP3 mRNA and protein; but there was no evidence of secretion of IGFBP3 into th e uterine lumen by ligand blot analysis, and there was evidence of only ver y small amounts by Western blot analysis. These results indicate that the horse conceptus secretes significant quanti ties of ICFBP3 toward the conceptus capsule from as early as Day 10 after o vulation. Thus, most of the IGFBP3 contained within the capsule, which bind s ICF-I to this special extracellular matrix of the preimplantation horse c onceptus, is likely to be embryonic in origin. ICFBP3 in the horse conceptu s capsule may enhance or modulate the action of IGFs on the developing conc eptus.