Intestinal IGF binding protein profiles in piglets fed formula containing IGF-I

IGF binding proteins (IGFBPs) modulate IGF-I activity by affecting its availability to tissue receptors. Intestinal tissue is exposed to IGFBPs present in the circulation and in milk, as well as, IGFBPs produced within the intestine. We have shown that oral IGF-I stimulates intestinal villus growth and lactase activity in neonatal piglets, however, the effect of oral IGF-I on intestinal IGFBP populations has not been reported. Piglets were either fed milk replacer with or without IGF-I at 100 to 1000 μg/L, or were sow-reared for 14 d. Intestinal samples were homogenized and IGFBP profiles in homogenates and serum were characterized by Western ligand blotting. Jejunal IGFBP mRNA abundance was determined by Northern analysis. Serum and intestinal homogenates contained IGFBP-1,-2,-3 and -4, however, IGFBP-3 was lower in intestine than serum, which is consistent IGFBP-3's inability to cross vascular epithelium. The levels of all 4 IGFBPs were lower in IGF-I treated piglets than control, with a trend for the decrease to be dose-dependent. Oral IGF-I could decrease jejunal IGFBP either by affecting tissue permeability to serum IGFBPs or by modulating autocrine IGFBP production. Only IGFBP-2 mRNA was detected in jejunum, suggesting that other IGFBPs must be derived from the circulation. Jejunal IGFBP-2 levels were consistently higher in sow-reared than formula-fed piglets, however, IGFBP mRNA were comparable, suggesting that IGFBP-2 in the intestines of sow-reared piglets were derived either from the circulation or from sow milk. In conclusion, orally administered IGF-I down-regulates intestinal IGFBP content, which could enhance IGF-I bioactivity (Supported by HD-29264).