The rapid increase in plasma concentration of 3,5,3'-triiodothyronine (T3) which occurs after feeding may invoke changes in lactase-phlorizin hydrolase (LPH) activity of the small intestine. This hypothesis has been tested in 6-week-old pigs living at thermal neutrality (26 degrees C) on a low level of energy intake. Littermate pairs were infused with 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 increased 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 the two groups. Enzyme-cytochemical analysis indicated that administration of T3 increases LPH activity at all points along the villus axis, whereas there is no effect on alpha-glucosidase (combined sucrase-isomaltase 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 observed previously in young pigs living in a cold environment. By contrast, 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 short-term homeostatic regulator of LPH in the small intestine.

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