Site of thyroxine-evoked decrease of jejunal lactase in the rat.

Localization of thyroid-mediated decrease of lactase activity along the villus-crypt unit in adult rat jejunum was studied 24 and 48 h after first injection of L-thyroxine (200 micrograms/100 g body wt) every 24 h. [3H]thymidine was also given at time of first thyroxine injection. Serum thyroid-stimulating hormone, food intake, and body weight were significantly decreased within 24 h. Total jejunal protein and villus-crypt height were unchanged during the time period studied. Lactase activity (expressed both as per tissue protein and per intestinal segment) was significantly decreased in jejunum and midjejunum within 24 h. Serial sectioning of the jejunal villus-crypt unit in a cryostat showed that the site of decrease in lactase activity at 24 h was in the apical villus and by 48 h extended along the entire height of the villus. Epithelial cell migration measured both by histoautoradiography and scintillation counting of [3H]thymidine in cryostat sections revealed no difference between control and thyroid-treated animals at both 24 and 48 h. The decrease in lactase activity at 24 h was in advance of the leading edge of radioactivity, indicating that the thyroid-evoked response occurred in mature enterocytes already on the villus.