ADMINISTRATION OF 3,5,3'-TRIIODOTHYRONINE INDUCES A RAPID INCREASE INENTEROCYTE LACTASE-PHLORHIZIN HYDROLASE ACTIVITY OF YOUNG-PIGS ON A LOW-ENERGY INTAKE

The rapid increase in plasma concentration of 3,5,3'-triiodothyronine (T3) which occurs after feeding may invoke changes in lactase-phlorizi n hydrolase (LPH) activity of the small intestine. This hypothesis has been tested in 6-week-old pigs living at thermal neutrality (26-degre es-C) on a low level of energy intake. Littermate pairs were infused w ith either saline or T3 at 30 min intervals over a 6 h period, 18-24 h after the last meal. The activity of LPH in mucosal homogenates incre ased significantly in test compared with control animals (P < 0.05; T3 37 % > saline). This was a specific effect on LPH since there was no effect of T3 on the activity of sucrase isomaltase. Further, it could not be attributed to changes in intestinal morphology since there were no differences in crypt depth, villus height or villus area between t he two groups. Enzyme-cytochemical analysis indicated that administrat ion of T3 increases LPH activity at all points along the villus axis, whereas there is no effect on alpha-glucosidase (combined sucrase--iso maltase and maltase) activities. These results indicate that there is unlikely to be a simple causal relation between the immediate increase in plasma T3 after feeding and the initial decline in LPH activity ob served previously in young pigs living in a cold environment. By contr ast, the subsequent increase in LPH activity could be under the direct control of the food-induced increase in plasma T3 concentration, and the present results suggest a potential role for T3 as an important sh ort-term homeostatic regulator of LPH in the small intestine.