Early changes in brush border disaccharidase kinetics in rat jejunum following subcutaneous administration of tetraiodothyronine: a quantitative histochemical study on villi revealing normal morphology.

In order to test the influence of thyroid hormones on small intestine function, adult female Wistar rats were injected daily with either 100 microgram/100 g body weight tetraiodothyronine or placebo. After 12 days, jejunal segments were removed and processed for morphometric analysis of mucosal architecture and quantitative histochemical determinations of the apparent Km- and Vmax-values of lactase/beta-glucosidase and neutral alpha-glucosidase at constant basal and apical measuring positions along the villi. The villus-crypt-architecture was the same in both experimental groups. At the cellular level, however, application of tetraiodothyronine resulted in a marked decrease in the apparent Vmax of lactase/beta-glucosidase at both villus positions, maintaining the normal activity gradient along the villi. In comparison with the controls, a less pronounced but significant reduction in activity was also demonstrated for the neutral alpha-glucosidase. Substrate affinity, however, was only increased for this enzyme, the apparent Km of lactase/beta-glucosidase not being affected by the hormone. The results indicate a direct effect of tetraiodothyronine on jejunal brush border disaccharidases of the rat. The alternative mechanism, an effect mediated by an altered enterocyte turnover is unlikely to occur.