Structural and functional maturation of rat gastrointestinal barrier with thyroxine.

It has been noted that the closure of the intestinal barrier to immunoglobulins is a normal maturational process in the rat. It has also been noted that the microvillus membrane (MVM) of newborn animals differs from adult MVM. The purpose of this study is to document whether thyroid hormone can induce closure in vivo in the rat and to relate this effect of thyroxine to the structural and functional maturation of the intestinal MVM. To assess closure, 2-wk-old rats were fed a rat immunoglobulin G (IgG), and serum antibody binding activity was measured 4 h later. The antibody binding activity of treated animals (T) was 1.5-2 times less than that of controls (C) (P less than 0.001), indicating that thyroxine stimulates closure. The MVM similarly showed signs of maturation. Structural maturation was demonstrated by the lower fluidity of the thyroid-treated animals' membranes. Under the influence of thyroxine, the number of receptors on the MVM for IgG had decreased [2.8 X 10(-7) M (C) vs. 1.7 X 10(-7) M (T)], while the Ka remained the same, demonstrating the functional maturation of the MVM. In conclusion, thyroid hormone can induce both structural and functional maturation of the intestinal MVM and can enhance the intestinal mucosal barrier by decreasing the penetration of antibodies.