Sucrase-isomaltase ontogeny: synergism between glucocorticoids and thyroxine reflects increased mRNA and no change in cell migration.

During postnatal maturation of the rat small intestine, glucocorticoid hormones (GC) and thyroxine (T4) act synergistically to elicit a precocious increase of sucrase activity. The current work shows that the synergistic effect on sucrase activity is paralleled by increased steady-state levels of sucrase-isomaltase mRNA. The enhancing effects of T4 on dexamethasone (DEX)-induced sucrase activity was seen even after prolonged treatment (9 days). Moreover, when the location of sucrase-bearing cells was examined after 2 days of hormone treatment, there was distinctly stronger immunostaining of sucrase in the presence of T4, and the sucrase-bearing cells were located on the lower quarter of the intestinal villi regardless of whether the animals received DEX or T4 plus DEX. Thus, despite predictions from the literature, there was no evidence for increased migration in the presence of T4. Instead, we conclude that the synergism between the two hormones is due to greater accumulation of sucrase-isomaltase per epithelial cell.