Absorption of Milk‐Borne Insulin‐Like Growth Factor‐I Into Portal Blood of Suckling Rats

ABSTRACT Background Insulin‐like growth factors (IGFs) are potent mitogens that have been implicated in control of growth and development during the perinatal period. These hormones are also present in biologically significant quantities in mammalian milks. Although one site of action of these IGFs may be at the intestinal level, current information about whether they pass intact into the circulation is conflicting. Methods To test the hypothesis that milk‐borne IGFs are absorbed into blood in receptor‐active form, suckling rats were given either recombinant human (rh)125I‐IGF‐I or ‐II (4 × 106 counts per minute [cpm]), and the activity present in portal and cardiac blood was examined at 5, 10, 20, and 30 minutes after ingestion for presence of appropriate molecular weight peptides in these samples. In selected samples, purified radioactive samples were tested for their ability to bind competitively to crude membranes bearing IGF receptors. Results The results of these studies indicate that rh125I‐IGF‐I is absorbed in receptor‐active form into the portal circulation and that maximal amounts are present 20 to 30 minutes after ingestion. Estimation of the presence of intact hormone was made on the basis of the elution profile of samples when run on gel chromatography as well as reversed‐phase high‐performance liquid chromatography. Isolated samples from portal blood also bound competitively to placental membranes bearing IGF receptors. In contrast, rh125I‐IGF‐II could not be demonstrated in receptor‐active form in portal blood. Chromatography showed appropriate sized peaks with greater activity in portal than cardiac samples, but competitive binding was not appreciated. Conclusions It is likely that at least milk‐borne IGF‐I is absorbed intact and may exert effects on liver and other peripheral tissues. In addition, this study lends further credence to the possibility of an enterohepatic circulation for IGF‐I.