Insulin-like growth factor I in suckling rat gastric contents

The small intestinal mucosa of the neonatal rat expresses primarily lactase activity until just prior to weaning when lactase falls to low levels and a full complement of adult digestive enzymes appears. Insulin-like growth factor 1 (IGF-I) is a normal component of maternal milk of humans and experimental animals. Experiments were performed to examine the concentrations of IGF-I in dam milk and the gastric content of suckling pups. Lactase activity in 1-day-old neonates was 0.66 micromol glucose formed/mg protein/hr (unit) and fell progressively until day 25, whereas sucrase activity at day 1 postpartum was 0.07 units and rose progressively to 0.21 units at day 25. The IGF-I content of dam milk was measured at 1, 5, 10, 15, 18, and 20 days postpartum by radioreceptor assay (RRA). Milk contained 1.02 pmol IGF-I/ml milk at one day postpartum, peaked at day 18 with 5.08 pmol IGF-I/ml, and fell to 2.31 pmol/ml at day 20. By day 25, dams were dry. The IGF-I content of the neonate gastric lumen was also measured by RRA. At day 1 the gastric lumen contained 2.63 pmol IGF-I/ml of luminal contents, fell to 1.06 pmol IGF-I/ml at day 5, and then rose again to peak at 3.37 pmol/ml at day 15 just prior to weaning. Two days after weaning, the level of luminal IGF-I had fallen to 1.15 pmol/ml. These data demonstrate the concentration of IGF-I in maternal milk is reflected in the concentration of the peptide in gastric contents of suckling pups and that the concentration in the gastric lumen may be high enough to affect epithelial cell proliferation and differentiation.