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FACILITATED MATURATION OF THE INTESTINAL MICROVILLUS MEMBRANE (MVM) BY THYROXINE: A CLUE TO THE DEVELOPMENT OF THE INTESTINAL BARRIER

It has been noted in the past that the MVM of newborn animals differs from adult MVM in IgG binding, enzyme activity, and membrane fluidity (MF). In this investigation we studied the influence of thyroid hormone (T4) on MVM maturation, particularly in relation to MF and IgG binding, two factors that control antigen uptake and transport in the intestine. Pregnant (17 day gestation) and 11 day old rats were given intraperitoneal T4 (50mcg/ 100g body weight) for three successive days. Control and T4-treated rats delivered normally and the MVM of the newborns (37 treated, 82 controls) and the 14 day olds (15 treated, 21 controls) were examined for enzyme activities. Results confirmed that sucrase activity can be induced and lactase activity is depressed by T4 induction. MF was performed using electron spin resonance spectroscopy. IgG binding to 0.2 mg/ml MVM protein was done using 125I-rat IgG. Results indicated that: 1) MF is decreased (MVM is more organized) in T4-treated newborns; 2) IgG binding is decreased in T4 treated animals as compared to controls (13% vs 20% in newborns and 19% vs 43% in 14 day olds at 2mcg/ml IgG). IgG binding to MVM is concentration dependent and saturable (range .17-100 mcg/ml IgG) in all groups, demonstrating specificity of binding. In conclusion, T4 induces MVM maturational changes, particularly decreased MF and decreased IgG binding, factors important in macromolecular uptake. This may play a role in mucosal host defense in the newborn period.