The small intestine

Little information exists on the influence of bile acids on electrolyte and water movements in the small intestine. Forth et al. (107) noted that deoxy-cholic acid, but not conjugated bile acids, inhibited water and electrolyte absorption systematic examination of the effect of nucleus substitution and presence and type of conjugation was not carried out. No information is available at present on the influence of bile acids on water movements in the human jejunum and ileum. 

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As we have seen in this chapter steroids have a number of functions in human physiology Cholesterol is a component part of cell mem branes and is found in large amounts in the brain Derivatives of cholic acid assist the digestion of fats in the small intestine Cortisone and its derivatives are involved in maintaining the electrolyte balance in body fluids The sex hormones responsible for mascu line and feminine characteristics as well as numerous aspects of pregnancy from conception to birth are steroids... 

Lipids present in the diet may become rancid. When fed at high (>4-6%) levels, Hpids may decrease diet acceptabiUty, increase handling problems, result in poor pellet quaUty, cause diarrhea, reduce feed intake, and decrease fiber digestion in the mmen (5). To alleviate the fiber digestion problem, calcium soaps or prilled free fatty acids have been developed to escape mminal fermentation. These fatty acids then are available for absorption from the small intestine (5). Feeding whole oilseeds also has alleviated some of the problems caused by feeding Hpids. A detailed discussion of Hpid metaboHsm by mminants can be found (16). 

Vitamins A, D, and E are required by mminants and, therefore, their supplementation is sometimes necessary. Vitamin A [68-26-8] is important in maintaining proper vision, maintenance and growth of squamous epitheHal ceUs, and bone growth (23). Vitamin D [1406-16-2] is most important for maintaining proper calcium absorption from the small intestine. It also aids in mobilizing calcium from bones and in optimizing absorption of phosphoms from the small intestine (23). Supplementation of vitamins A and D at their minimum daily requirement is recommended because feedstuffs are highly variable in their content of these vitamins. 

Castor Oil. Castor oil [8001-79-4] (qv) is the fixed oil from the seeds of Picinus communis Linne. Pale yellowish or almost colorless, it is a transparent viscid Hquid with a faint, mild odor and a bland taste followed by a slightly acrid and usually nauseating taste. Its specific gravity is between 0.945 and 0.965. Castor oil is soluble in alcohol, and miscible with anhydrous alcohol, glacial acetic acid, chloroform, and diethyl ether. It consists chiefly of the glycerides of ricinoleic acid [141 -22-0], and isoricinoleic acid [73891-08-4], found in the small intestine. The seed contains a highly... 

Castor oil is a cathartic only after Hpolysis in the small intestine Hberating ricinoleic acid. Ricinoleic acid inhibits the absorption of water and electrolytes. It is commonly used for preparation of the large bowel for diagnostic procedures. 

Lactulose. 4-O-P -D-Galactopyranosyl-4-D-fmctofuranose [4618-18-2] (Chronolac) (12) may be made from lactose using the method described in Reference 9. It is a synthetic disaccharide that is not hydroly2ed by gastrointestinal enzymes in the small intestine, but is metabolized by colonic bacteria to short-chain organic acids. The increased osmotic pressure of these nonabsorbable organic acids results in an accumulation of fluid in the colon. Lactulose may not be tolerated by patients because of an extremely sweet taste. It frequently produces flatulence and intestinal cramps. 

Acarbose is a nonabsorbable a-glucosidase inhibitor which blocks the digestion of starch, sucrose, and maltose. The digestion of complex carbohydrates is delayed and occurs throughout the small intestine rather than in the upper part of the jejunum. Absorption of glucose and other monosaccharides is not affected. Acarbose is adrninistered orally three times a day and chewed with the first mouthful of food. 

The largest use of endoscopic techniques is in the examination of the gastrointestinal tract. Upper intestinal endoscopy is the examination of the esophagus, stomach, and proximal duodenum. Colonoscopy is the examination of the colon, large intestine, and in some cases the distal parts of the small intestine. Cholangiopancreatography is the examination of the biUary tree and pancreas. 

Metabolism. Absorption, distribution, metaboHsm, and excretion of thioglycolic acid have been reviewed (20). In summary,. -thioglycolic acid was absorbed significantly after appHcation to the skin of rabbits. After intravenous injection, the greatest counts of radioactivity were found in the kidneys, lungs, and spleen of monkey and in the small intestine and kidneys of rat. Most of the radioactivity was rapidly excreted in the urine in the form of inorganic sulfate and neutral sulfur. 

Absorption, Transport, and Excretion. The vitamin is absorbed through the mouth, the stomach, and predominantly through the distal portion of the small intestine, and hence, penetrates into the bloodstream. Ascorbic acid is widely distributed to the cells of the body and is mainly present in the white blood cells (leukocytes). The ascorbic acid concentration in these cells is about 150 times its concentration in the plasma (150,151). Dehydroascorbic acid is the main form in the red blood cells (erythrocytes). White blood cells are involved in the destmction of bacteria. 

Food vitamin B 2 appears to bind to a saUvary transport protein referred to as the R-protein, R-binder, or haptocorrin. In the stomach, R-protein and the intrinsic factor competitively bind the vitamin. Release from the R-protein occurs in the small intestine by the action of pancreatic proteases, leading to specific binding to the intrinsic factor. The resultant complex is transported to the ileum where it is bound to a cell surface receptor and enters the intestinal cell. The vitamin is then freed from the intrinsic factor and bound to transcobalamin II in the enterocyte. The resulting complex enters the portal circulation. 

Intestinal Fluke. The intestinal fluke, Fascio/opsis huski which can reach a length of 8 cm, Hves attached to the wall of the small intestine. 

Both the adult and the larval cysticerci (bladderworm) of Taenia solium (pork tapeworm) are able to Hve in humans the parasite is found sporadically in uncooked pork. In the stomach, the larva is digested out of the pork flesh it then grows and attaches to the wall of the small intestine. Maturity is reached in 5—12 weeks. The adult is 5 m long, and untreated adult worms may survive for 25 years. 

The use of a bioadhesive, polymeric dosage form for sustained dehvery raises questions about swallowing or aspirating the device. The surface area is small, and patient comfort should be addressed by designing a small (less than 2 cm ), thin (less than 0.1 mm (4 mil) thick) device that conforms to the mucosal surface. The buccal route may prove useful for peptide or protein dehvery because of the absence of protease activity in the sahva. However, the epithelium is relatively tight, based on its electrophysiological properties. An average conductance in the dog is 1 mS/cm (57) as compared to conductances of about 27 and 10 mS/cm in the small intestine and nasal mucosa, respectively (58,59) these may be classified as leaky epitheha. 

FIGURE 2.16 pH versus enzymatic activity. The activity of enzymes is very sensitive to pH. The pH optimum of an enzyme is one of its most important characteristics. Pepsin is a protein-digesting enzyme active in the gastric fluid. Trypsin is also a proteolytic enzyme, but it acts in the more alkaline milieu of the small intestine. Lysozyme digests the cell walls of bacteria it is found in tears. 

FIGURE 24.3 (a) A duct at the junction of the pancreas and duodenum secretes pancreatic juice into the duodenum, the first portion of the small intestine, (b) Hydrolysis of triacylglycerols by pancreatic and intestinal lipases. Pancreatic lipases cleave fatty acids at the C-1 and C-3 positions. Resulting monoacylglycerols with fatty acids at C-2 are hydrolyzed by intestinal lipases. Fatty acids and monoacylglycerols are absorbed through the intestinal wall and assembled into lipoprotein aggregates termed chylomicrons (discussed in Chapter 25). 

FIGURE 24.4 In the small intestine, fatty acids combine with bile salts in mixed micelles, which deliver fatty acids to epithelial cells that cover the intestinal villi. Triacylglycerols are formed within the epithelial cells. 

AQP10 has only been identified in the small intestine so far and is thought to play a role in hormonal secretion. AQP11 is expressed in kidney, liver, testis and brain, but no function has been found so far. AQP12 has been identified in pancreatic acinar cells, where it is thought to facilitate the release of digestive enzymes into the pancreatic duct. 

Cholera toxin-induced diarrhoea 2. Increased AEA levels after administration of cholera toxin to mice, a model of diarrhoea, exert anti-secretory action in the small intestine 2. Inhibitors of cellular reuptake... 

Motilin is a 22-amino acid peptide hormone, secreted by the enterochromaffin cells of the small intestine,... 

Vitamin B12 is special in as far as its absorption depends on the availability of several secretory proteins, the most important being the so-called intrinsic factor (IF). IF is produced by the parietal cells of the fundic mucosa in man and is secreted simultaneously with HC1. In the small intestine, vitamin B12 (extrinsic factor) binds to the alkali-stable gastric glycoprotein IF. The molecules form a complex that resists intestinal proteolysis. In the ileum, the IF-vitamin B 12-complex attaches to specific mucosal receptors of the microvilli as soon as the chymus reaches a neutral pH. Then either cobalamin alone or the complex as a whole enters the mucosal cell. 

Although vitamin K is a fat soluble vitamin, only little stores are found in the body which have to be refilled permanently via dietary input. The role of vitamin K derived from bacteria in the colon is controversely discussed, as the concentration of biliary acids for the resorption the fatsoluble vitamin K is very low in the colon. In addition, only diseases of the small intestine lead to a deficit in vitamin K concentration which cannot be restored by K2 production of colonic bacteria. However, watersoluble vitamin Ks can be resorbed by the colonic mucosa. Maybe because of the little stores for vitamin K, the process of vitamin K-dependent carboxylation of proteins is part of a cycle with several steps during which vitamin K normally is regenerated (see Fig. 1) and thus can be used several times. 

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