Effect of dietary carbohydrate on monosaccharide uptake by mouse small intestine in vitro.

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Abstract
Using intestinal sleeves in vitro, we studied the effect of dietary carbohydrate on active monosaccharide uptake in mice. Dietary carbohydrate did not affect numerous parameters of intestinal structure, such as length, circumference, weight, protein content, villus dimensions and density, and area at the villus level. Mice on a carbohydrate‐free diet had active D‐glucose uptake relatively independent of position along the small intestine. A carbohydrate‐containing diet reversibly and within 1 day stimulated uptake except in the ileum, restoring the proximal‐to‐distal gradient in glucose uptake normally observed. This stimulation involved a 81‐116% increase in the Michaelis‐ Menton constant Vmax, and also an apparent increase in the Michaelis‐ Menton constant Km, that may however be an artifact arising from unstirred‐layer effects. Active uptake of 3‐O‐methyl‐D‐glucose also increased, permeability to glucose remained unchanged, and proline uptake reversibly decreased (probably due to the lower protein content of the carbohydrate‐containing diets). The effect of fasting on active monosaccharide uptake seemed largely due to withdrawal of dietary carbohydrate, rather than of calories per se. It is concluded that dietary carbohydrate causes induction of monosaccharide carriers in the intestine, along with its more familiar induction of pancreatic amylase and intestinal disaccharidases. Substrate‐dependent carrier induction may be physiologically significant in maintaining the proximal‐to‐distal gradient of glucose transport. An appendix presents measurements of villus area as a function of position along the intestine.