Intestinal mucosa glucose transport

Osmotic laxative effects are also produced by the polyhydric alcohols, mannitol and sorbitol, which unlike glucose cannot be transported through the intestinal mucosa, as well as by the non-hydrolyzable disaccharide, lactubse. Fermentation of lactulose by colon bacteria results in acidification of bowel contents and microfloral damage. Lactulose is used in hepatic failure in order to prevent bacterial production of ammonia and its subsequent absorption (absorbable NH3 nonabsorbable NH4+), so as to forestall hepatic coma. 

In the mammal, complex polysaccharides which are susceptible to such treatment, are hydrolyzed by successive exposure to the amylase of the saliva, the acid of the stomach, and the disaccharidases (e.g., maltase, invertase, amylase, etc.) by exposure to juices of the small intestine. The last mechanism is very important. Absorption of the resulting monosaccharides occurs primarily in the upper part of the small intestine, from which the sugars are earned to the liver by the portal system. The absorption across die intestinal mucosa occurs by a combination of active transport and diffusion. For glucose, the aclive transport mechanism appears to involve phosphorylation The details are not yet fully understood. Agents which inhibit respiration (e.g., azide, fluoracetic acid, etc.) and phosphorylation (e.g., phlorizin), and those which uncouple oxidation from phosphorylation (e.g., dinitrophenol) interfere with the absorption of glucose. See also Phosphorylation (Oxidative). Once the various monosaccharides pass dirough the mucosa, interconversion of the other... 

Besides, it has been found3 that Silics possesses some antidiarrheal properties that are not related to sorption of microorganisms or their toxins. A Silics medicinal preparation retards secretion of water in isolated intestinal loops of rats in the case of its combined introduction with diarrhea mediators such as sodium desoxycholate, cyclo-adenosine 3 ,5 -monophosphate (c-AMP), and serotonin (Table 5). The decrease of water release into intestinal segment lumens seems to be related to the modifying action of the preparation on mucosa of bowels, because sorption of the stimulators themselves on Silics does not take place. The modifying action of the preparation on the small intestine mucosa is also attributed to the stimulation exerted by Silics on transport of glucose and electrolytes through bowel walls.3... 

Osmotic laxative effects are also produced by the polyhydric alcohols mannitol and sorbitol, which unlike glucose cannot be transported through the intestinal mucosa. 

The biguanides have a special affinity for the mitochondrial membrane, which causes an alteration in electron transport and results in reduced oxygen consumption. Inhibition of the active transport of glucose in the intestinal mucosa, absent activation of glucose transporters, inhibition of gluconeogenesis, and inhibition of fatty... 

Intracellular metabolism of amino acids requires their transport across the cell membrane. Transport of L-amino acids occurs against a concentration gradient and is an active process usually coupled to Na -dependent carrier systems as for transport of glucose across the intestinal mucosa (Chapter 12). At least five transport systems for amino acids (with overlapping specificities) have been identified in kidney and intestine. They transport neutral amino acids, acidic amino acids, basic amino acids, ornithine and cystine, and glycine and proline, respectively. Within a given carrier system, amino acids may compete for transport (e.g., phenylalanine with tryptophan). Na+-independent transport carriers for neutral and lipophilic amino acids have also been described, d-Amino acids are transported by simple diffusion favored by a concentration gradient. 

Fructose is found in many plants and is an important portion of dietary carbohydrate. Most commonly it is ingested as free fructose or sucrose. Fructose is not actively transported by the intestinal mucosa, and variable proportions are converted to glucose in the process of absorption in man, about one-sixth is converted (Ml). Most of the metabolism of fructose occurs in the liver. If a renal threshold for fructose exists, it is very low. 

GLUT 2 Liver Kidney Pancreatic p-cell Serosal surface of Intestinal mucosa cells A high capacity, low affinity transporter. May be used as the glucose sensor in the pancreas. 

Amino acids, glucose and certain other sugars are taken up by carrier-mediated mechanisms which may be either passive or active according to the type of cell. Active transport processes are particularly highly developed in the cells of the intestinal mucosa and kidney tubules which are specialized for the absorption of material and its transport across the cell. Most other types of cell, e.g. liver, muscle and erythrocytes, take up glucose and amino acids by facilitated diffusion rather than by active transport. 

Lennernas s group at Uppsala has performed extensive studies to confirm the validity of this in vivo experimental set-up at assessing the rate and the extent of drug absorption. Recovery of PEG 4000 (a non-absorbable marker) is more than 95%, which indicates that the absorption barrier is intact. In addition, maintenance of functional viability of the mucosa during perfusion has been demonstrated by the rapid transmucosal transport of D-glucose and L-leucine. Estimation of absorption half-lives from the measured Pefr agree well with half-lives derived from oral dose studies in humans (i.e. physiologically realistic half-lives). Human Peff estimates are well correlated with the fraction absorbed in humans, and served as the basis for BCS development, and hence the technique is ultimately the benchmark by which other in situ intestinal perfusion techniques are compared. The model has been extensively used to... 

Glucose is also an important ingredient as it acts as a carrier for the transport of sodium ions, and hence water, across the mucosa of the small intestine, as well as providing the energy necessary for that process. 

Specific carrier systems exist for the transport of some species across the intestinal wall (mucosa), e.g. glucose or amino acids. However, for other species the important properties for good absorption are the presence of a high proportion of a non-ionized form with a high lipid-water partition coefficient and a small atomic or molecular radius. It is generally assumed that ionized species cannot cross the mucosa whereas non-ionized forms equilibrate fairly freely, providing that they have molecular radii of less than about 3 nm, which corresponds to a molecular mass of 6000 Da. For metals the most important region for absorption is the small intestine, where the pH lies between about 6 and 7.4. 

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