CHARACTERIZATION OF IGF-I RECEPTORS IN THE PORCINE SMALL-INTESTINE DURING POSTNATAL-DEVELOPMENT

Receptors for insulin-like growth factor I were characterized on intes tinal membranes of jejunum from newborn, suckling, and weaned pigs. I- 125-labelled insulin-like growth factor I (IGF-I) binding was time-dep endent, saturable, linearly related to membrane protein, and reversibl e. Analysis of saturation curve data from membrane homogenates reveale d a single class of receptors, with receptor number declining during d evelopment. In vitro autoradiography localized receptors to the small intestinal submucosa and mucosa and further immunohistochemical locali zation identified receptors on the apical (microvillar membrane) and b asolateral surfaces of enterocytes. After affinity cross-linking of I- 125-labelled IGF-I to microvillar membranes, 135 and 260 kilodalton ba nds were visualized on polyacrylamide gels under reducing conditions. Parallel incubations with excess unlabeled IGF-I, -II, or insulin demo nstrated that these bands correspond to the a subunit and an incomplet ely reduced dimeric a complex of the IGF-I receptor. Receptor number o n microvillar membranes increased dramatically during the suckling per iod and was negatively correlated with maltase and sucrase activities. The functional activity of receptors on microvillar membranes was dem onstrated using an in vitro phosphorylation assay. The expression of f unctional IGF-I receptors during development on both apical and basola teral membranes suggests a regulatory role for this receptor and its l igands.