Insulin-like growth factor-I and its N-terminal modified analogues induce marked gut growth in dexamethasone-treated rats.

The effects of insulin-like growth factor-I (IGF-I) on the gut of 150 g dexamethasone-treated rats were compared with those of two analogues with reduced affinity for IGF-binding proteins, des(1-3)IGF-I and LR3IGF-I, an N-terminal-extended variant. Administration of IGF-I for 7 days to rats made catabolic by co-treatment with dexamethasone induced a dose-dependent increase in total gut weight, with the highest dose of IGF-I (695 micrograms/day) increasing gut weight by up to 60%, and gut weight as a fraction of body weight by up to 32%. Effects were apparent in all regions of the gut examined, including the stomach, small intestine and colon. Histological and biochemical analyses of the intestine showed that cross-sectional mass, rather than gut length, was increased, and proportional increases in wet weight, protein and DNA content per unit length were measured in both the mucosa and muscularis layers. The rate of duodenal protein synthesis measured on day 7 of treatment was not increased by IGF-I treatment. The IGF-I analogues had qualitatively similar effects to IGF-I, but were consistently severalfold more potent, providing evidence that IGF-binding proteins reduce the biological activity of exogenous IGF-I in the gut. The results indicate that the gut is one of the most sensitive IGF-I target tissues, and that potency in vivo correlates with a reduced interaction with IGF-binding proteins.