Insulin receptors in the gastrointestinal tract of the rat fetus: quantitative autoradiographic studies.

Purified carrier-free 125I-insulin was injected into the vitelline vein of rat fetuses in utero after 17, 19 or 21 days of a 22-day gestation. Three minutes later, the weight and radioactivity of various organs and the remaining carcass were measured. A radioactivity concentration index was calculated by dividing the specific activity of each organ by that of the whole feto-placental unit. In each of the three age groups studied, the gastrointestinal tract radioactivity concentration indices were 1.7, 2 and 1.9 respectively, indicating that the gastrointestinal tract concentrated the labelled hormone. Three, 9 and 15 min after 125I-insulin injection, the gastrointestinal tract was removed, homogenized and chromatographed on a G-50 fine Sephadex column. At 3 min, 91.4% of gastrointestinal tract radioactivity co-eluted with a standard of 125I-insulin. At the later time intervals studied, the percentage of 125I-insulin decreased while that of low molecular weight degradation products increased. Quantitative autoradiographic study of the fetal gastrointestinal tract indicated that epithelial cells bound 125I-insulin and that this binding was inhibited by co-injection of large amounts of unlabelled insulin. 125I-insulin binding was highest in the proximal small bowel and lowest in the colon. Insulin binding did not appear to depend upon degree of cell maturation or cell type. These results indicate that the epithelium of the gastrointestinal tract is characterized by the presence of numerous insulin receptors and is a potentially important insulin target.