Gastrointestinal absorption of insulin-like growth factor in the mouse in the absence of salivary insulin-like growth factor binding protein

Based on previous observations of the presence of both insulin-like growth factors I and II (IGF-I and IGF-II) in murine saliva (Kerr et al., Biochem Pharmacol 49: 1521–1531, 1995), the saliva from BALB/c and Non-obese diabetic (NOD) mice was examined for the presence of insulin-like growth factor binding proteins (IGFBPs). Using a western-blot type ligand binding assay with 125I-labeled IGF-I, a series of binding proteins with molecular masses ( M r ) between 25 and 45 kDa were detected in the sera, but not saliva, from both BALB/c and diabetic NOD mice. In the diabetic NOD mice, there were detectable changes in the concentrations of several of the IGFBPs relative to BALB/c mice. Using specific antibody to the binding proteins, one of these was identified as IGFBP-2. Gavage administration of [ 125I]IGF-I indicated substantial uptake from the gastrointestinal tract and significant tissue distribution. There was an increase in serum concentrations of radiolabeled IGF-I in diabetic NOD mice over that in BALB/c mice but less recovered from most of the tissues. Intact 125I-labeled IGF-I was extracted and purified from various tissues, following gavage, and shown to retain biological activity. Thus, the uptake of biologically active IGFs from saliva would appear to take place independently of specific binding proteins.