Background: The physiologic significance of milk-borne hormones and growth factors for internal organs of suckling animals is poorly understood. In this study the significance of milk-borne insulin was evaluated, as well as its combination with trypsin inhibitor, and its role in the development of pancreatic digestive capacity at the time of weaning was investigated.
Methods: Experiments were performed using insulin-deficient milk formula (standard formula), insulin (20 ng/ml) formula, or insulin with trypsin inhibitor (1 U/ml) formula by a rat artificial-rearing technique.
Results: In 17-day-old rats administered standard formula, the plasma insulin level was as low as that in 14-day-old rats. When insulin-trypsin inhibitor formula was administered to rat pups, the plasma insulin level was significantly higher than those in rats given standard or insulin formula. In rats artificially reared on standard formula, the usual developmental increases in pancreatic amylase activity and plasma insulin concentration at the beginning of weaning did not occur. Insulin formula elevated the pups' plasma insulin concentration and amylase activity at the onset of weaning but not to the levels observed in mother-reared rats. In rats reared on insulin-trypsin inhibitor formula, the developmental increases in the plasma insulin concentration and amylase activity observed in mother-reared rats were induced.
Conclusions: The present study demonstrates the necessity of milk-borne insulin for the development of pancreatic amylase during the weaning period.