Dietary Insulin Affects Leucine Aminopeptidase, Growth Hormone, Insulin-Like Growth Factor I and Insulin Receptors in the Intestinal Mucosa of Neonatal Pigs

Abstract

Background: Recent studies suggest that milk-borne insulin may regulate the development of the gastrointestinal tract in neonatal mammals. Objectives: To explore the mechanism by which milk-borne insulin affects gastrointestinal tract development, we examined the effect of dietary insulin on the expression levels of leucine aminopeptidase (LAP) and insulin-like growth factor I (IGF-I), as well as its effect on growth hormone (GH), IGF-I and insulin receptors in the small intestinal mucosa of neonatal pigs. Methods: Five piglets were anesthetized and sampled within 2–4 h after birth. They were not allowed to suckle and were used as newborn controls (group N). Ten other piglets from 5 litters were randomly divided into group M (n = 5), which was fed cow’s milk, and group MI (n = 5), which was fed cow’s milk and insulin (2.5 mg/l). Piglets in groups M and MI were artificially fed for 3 days and then sampled. Total RNA in their intestinal mucosa was extracted with Tripure reagents (Roche, USA). Reverse transcription PCR (RT-PCR) was used to semi-quantify mRNA levels of target genes and 18S rRNA was used in an RT-PCR system as an internal control. PCR products were loaded onto a 9% nondenaturing polyacrylamide gel. The gel was stained by silver staining agents. Digital photos were taken and the strength of the band areas was quantified using software. Results: The results showed that the DNA contents and LAP activity in the small intestines of the piglets in group MI were higher (p < 0.05) than in the piglets in group N. Compared with group M, piglets in group MI exhibited significantly increased expression levels of both insulin and GH receptor in the ileum, and LAP in the jejunum (p < 0.05); IGF-I receptor expression levels in both the jejunum and ileum were significantly decreased (p < 0.01 and p < 0.05, respectively), while IGF-I expression was unchanged (p > 0.05). Conclusion: Collectively, dietary insulin increased mRNA levels of insulin and GH receptor, which could help explain the effect of dietary insulin on receptor-mediated postnatal development of the small intestine. Dietary insulin suppressed IGF-I receptor expression, which may be the result of negative feedback caused when insulin binds to IGF-I receptors.