Fate of insulin-like growth factors I and II administered orogastrically to suckling rats

Milk-borne insulin-like growth factors I and II (IGF-I and -II) may be of importance in the differentiation of the gastrointestinal tract of the suckling. To test this hypothesis, 10- to 11-d-old suckling rats were given via an orogastric tube ¹²⁵I-IGF-I (n = 6) or ¹²⁵I-IGF-II (n = 6) in rat milk and killed 30 min later. The results of this study demonstrated that approximately 40% of the radioactivity administered was detected in the gastrointestinal tract for both ¹²⁵I-IGF-I and ¹²⁵I-IGF-II experiments. Gel chromatography of acid extracts of homogenates of gastrointestinal tissues and luminal contents demonstrated that a significant fraction of recovered radioactivity eluted in a position identical to 'native' IGF. These findings were confirmed by subjecting similarly treated samples to high performance liquid chromatography. In addition, radioactive material recovered from M(r) 7,500 fractions bound specifically to crude membrane IGF- I and -II receptor preparations, further suggesting the preservation of biologic activity of the recovered peptides. Although skin homogenates contained large peptide fragments of ¹²⁵I-IGF-I, no 'intact' IGF was found in the blood or other tissues. These findings suggest that milk-borne IGFs are stable in the neonatal gastrointestinal tract and remain biologically active for as long as 30 min postingestion.