Glucose transport by rat small intestine after extensive small-bowel resection

Uptake of lipids into rabbit jejunum and colon following ileal resection. Effect of variations in dietary content of carbohydrate

This study was undertaken to determine the effect of variations in the dietary content of carbohydrate (sucrose) and intestinal resection on the passive jejunal and colonic uptake of short-, medium-, and long-chain length fatty acids, cholesterol, and decanol. A previously validated in vitro technique was used, and studies were performed in sham-operated control animals and in rabbits submitted to the surgical removal of the distal half of the small intestine. After six weeks feeding of a high- or low-carbohydrate diet, the uptake of lipids was altered, but the direction and extent of changes was different among jejunum, ileum, and colon in control animals, and between the jejunum or colon of control vs resected animals. The intestinal membrane is likely heterogeneous with respect to passive permeability pathways since dietary manipulation of sucrose had a different effect on the uptake of each lipid probe. The finding of lower jejunal and colonic cholesterol uptake in animals fed a high- as compared with a low-carbohydrate diet reflects the importance of dietary effects on intestinal permeation. Studies must now be performed to establish the mechanisms responsible for these diet-related changes in intestinal permeability.

Absorption studies after massive small bowel resection and antiperistaltic colon interposition in rhesus monkeys

An experimental model of massive (80%) small bowel resection and antiperistaltic colon interposition in between the remnant jejunum and ileum was created in rhesus monkeys. The gastrointestinal functions were assessed preoperatively and at different periods after the operation. Progressive increase in the transit time and enhanced absorption of D-xylose and vitamin B12 was observed in these animals. The transit time marginally increased till three months postoperatively, and a significant increase was observed later (4-12 months) in the colon autografted monkeys. After three months, the D-xylose absorption in the colon interpositioned animals was nearly equal to that of normal monkeys. There was no significant difference in the vitamin B12 absorption in the normal and operated animals up to three months, but a significant rise was observed in the latter group after this period. The fecal fat excretion was 33-44% initially, and it gradually decreased to near normal values after nine months. The antiperistaltic colon interposition was effective in improving the early postoperative survival in animals after massive small bowel resection and enhanced the bowel absorption considerably.

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

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.

Regulation of proline and glucose transport in mouse intestine by dietary substrate levels

Active uptake of D-glucose and L-proline at 50 mM was measured in everted intestinal sleeves of mice whose dietary carbohydrate and protein levels were being varied experimentally. Compared to a nearly carbohydrate-free meat diet, a 50% carbohydrate laboratory chow diet stimulated active glucose uptake in the proximal intestine without affecting proline uptake, passive glucose permeability, or several measures of mucosal mass. Switching from a low-protein high-carbohydrate to a high-protein no-carbohydrate diet reversibly stimulated proline uptake while inhibiting glucose uptake. For each solute and diet switch, the stimulation of transport was complete within 1 day, while the inhibition required several days. The results imply induction and repression of intestinal glucose and proline transport by dietary substrate levels. This mechanism, in conjunction with the normal gradient of nutrient concentrations along the intestine, is probably largely responsible for the gradient in nutrient transport along the intestine.

Morphologic and functional adaptations of large bowel after small-bowel resection in the rat

Seventy percent small bowel was resected in rats. Two and four weeks later transport of sodium, chloride, and water was examined in cecum and more distal large bowel (colon) using a well-established in vivo luminal perfusion technique. Sham-operated and unoperated rats served as controls. In cecum mucosa grew 29% by two weeks after resection but transport remained unchanged. There were no further adaptive changes by four weeks after resection. Unexpectedly, cecum of all rats secreted water and electrolytes. The mechanism remains unclear. In colon there were no adaptive changes by two weeks after resection but by four weeks colon mucosa increased 14% and luminal absorption increased proportionately. Separate studies showed hexose transport could not be induced in cecum or colon, although we have previously demonstrated its induction in contiguous remnant ileum.

Nutritional aspects of malabsorption. Short gut adaptation

Adaptation of remaining small bowel after extensive intestinal resection plays an important part in maintaining adequate nutrition. The metabolic consequences of short bowel depend upon the anatomic extent of bowel removed, the functional condition of remaining digestive organs and the degree to which adaptive changes occur. The pathophysiology of short bowel results primarily from the abnormal absorption of nutrients and selective metabolites leading to malnutrition and other organ complications. Nutrients in the lumen of the small intestine are required to stimulate the process of intestinal adaptation (mucosal hyperplasia). This stimulation may be mediated via direct mucosal absorption or metabolism of nutrients, the presence of pancreaticobiliary secretions in the intestine, trophic effects of circulating enteric hormones, or neurovascular influences. Understanding the mechanisms by which mucosal growth may be stimulated could lead to important clinical applications.