Mucosal cell

Calcium is absorbed from the intestine by facilitated diffusion and active transport. In the former, Ca " moves from the mucosal to the serosal compartments along a concentration gradient. The active transport system requires a cation pump. In both processes, a calcium-binding protein (CaBP) is thought to be required for the transport. Synthesis of CaBP is activated by 1,25-DHCC. In the active transport, release of Ca " from the mucosal cell into... 

FIGURE 10.16 The H+,lO-ATPase of gastric mucosal cells mediates proton transport into the stomach. Potassimn ions are recycled by means of an associated K /Cl cotransport system. The action of these two pnmps results in net transport of and Cl into the stomach. 

Enterochromaffin cells are interspersed with mucosal cells mainly in the stomach and small intestine. In the blood, serotonin is present at high concentrations in platelets, which take up serotonin from the plasma by an active transport process. Serotonin is released on platelet activation. In the central nervous system, serotonin serves as a transmitter. The main serotonin-containing neurons are those clustered in form of the Raphe nuclei. Serotonin exerts its biological effects through the activation of specific receptors. Most of them are G-protein coupled receptors (GPCRs) and belong to the 5-HTr, 5-HT2-, 5-HT4-, 5-HTs-, 5-HT6-, 5-HT7-receptor subfamilies. The 5-HT3-receptor is a ligand-operated ion channel. 

Lymphocytes, inflammatory cells, intestinal mucosal cells, cartilage cells and bone precursor cells Inhibition of proliferation... 

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. 

There are regional asymmetries in membranes. Some, such as occur at the villous borders of mucosal cells, are almost macroscopicaUy visible. Others, such as those at gap junctions, tight junctions, and synapses, occupy much smaller regions of the membrane and generate correspondingly smaller local asymmetries. 

There are two main classes of proteolytic digestive enzymes (proteases), with different specificities for the amino acids forming the peptide bond to be hydrolyzed. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, fi"om the ends of polypeptides. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from rhe free carboxyl terminal, and aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. 

In addition to its role in regulating calcium homeostasis, vitamin D is required for the intestinal absorption of calcium. Synthesis of the intracellular calciumbinding protein, calbindin, required for calcium absorption, is induced by vitamin D, which also affects the permeability of the mucosal cells to calcium, an effect that is rapid and independent of protein synthesis. 

Although iron deficiency is a common problem, about 10% of the population are genetically at risk of iron overload (hemochromatosis), and elemental iron can lead to nonen2ymic generation of free radicals. Absorption of iron is stricdy regulated. Inorganic iron is accumulated in intestinal mucosal cells bound to an intracellular protein, ferritin. Once the ferritin in the cell is saturated with iron, no more can enter. Iron can only leave the mucosal cell if there is transferrin in plasma to bind to. Once transferrin is saturated with iron, any that has accumulated in the mucosal cells will be lost when the cells are shed. As a result of this mucosal barrier, only about 10% of dietary iron is normally absorbed and only 1-5% from many plant foods. 

PENG Y s, PENG Y M, MCGEE D L and ALBERTS D s (1994) Carotenoids, tocopherols and retinoids in human buccal mucosal cells intra- and inter-individual variability and storage stability. J Clin Nutr 59(3) 636-43. 

A wide diversity of herbal remedies have purported abilities to stimulate defense functions. Complexes of carbohydrate and lignin, which are present in some herbs, modulate enteric immune functions (Kiyohara et al, 2000), and the changes in cytokine secretion (Matsumoto and Yamada, 2000) can trigger systemic responses. The polysaccharides present in other herbal medicines augment production of immunoglobulin (Ig) A by the Peyer s patches in the small intestine (Sakushima et al, 1997 Yu et al, 1998). The responses of the enteric immune system to lectins are variable (Pusztai 1993), and can elicit systemic responses (Lavelle et al, 2000). Other phytochemicals provide protection by inducing detoxification pathways in mucosal cells (Williamson et al, 1998). 

Hiraishi, H., Terano, A., Ota, S., Ivey, K.J. and Sugjmoto, T. (1987). Oxygen metabolite-induced cytotoxicity to cultured rat gastric mucosal cells. Am. J. Physiol. 253, G40-G48. 

Hiraishi, H., Razandi, M., Terano, A. and Ivey, K.J. (1990). Antioxidant defenses of culture gastric mucosal cells against toxic oxygen metabolites. Role of glutathione redox cycle and endogenous catalase. Gastroenterology 98, A544. 

Kobayashi, K., Sakuma, H. and Arakawa, T. (1991). Role of leukotrienes in damage caused by active oxygen species in cultured gastric mucosal cells. Gastroenterology 100, A99. 

Prostaglandins, one of the most important epithelial growth factors, inhibit gastric acid secretion and have numerous mucosal protective effects, the most important of which include the stimulation of both mucus and phospholipid production, promotion of bicarbonate secretion, and increased mucosal cell turnover. Damage to the mucosal defense system is the primary method by which HP or NSAIDs cause peptic ulcers. 

As with UC, the immune activation seen in CD involves the release of many proinflammatory cytokines. Cytokines thought to play major roles in CD are derived from T-helper type 1 cells and include interferon-y, TNF-a, and IL-1, IL-6, and IL-12. TNF-a is a major contributor to the inflammatory process seen in CD. Its physiologic effects include activation of macrophages, procoagulant effects in the vascular endothelium, and increases in production of matrix metallo-proteinases in mucosal cells.9,15 Excessive production of both... 

Iron is another vital nutrient in the development of functioning erythrocytes it is essential for the formation of hemoglobin. Lack of iron leads to a decrease in hemoglobin synthesis and ultimately red blood cells. Normal homeostasis of iron transport and metabolism is depicted in Fig. 63-2.7 Approximately 1 to 2 mg of iron is absorbed through the duodenum each day, and the same amount is lost via blood loss, desquamation of mucosal cells, or menstruation. 

E. histolytica invades mucosal cells of colonic epithelium, producing the classic flask-shaped ulcer in the submucosa. The trophozoite toxin has a cytocidal effect on cells. If the trophozoite gets into the portal circulation, it will be carried to the liver, where it produces abscess and periportal fibrosis. Liver abscesses are more common in men than women and are rarely seen in children. Amebic ulcerations can affect the perineum and genitalia, and abscesses may occur in the lung and brain. 

The risk of colon cancer appears to be inversely related to calcium and folate intake. Calciums protective effect may be related to a reduction in mucosal cell proliferation rates or through its binding to bile salts in the intestine, whereas dietary folate helps in maintaining normal bowel mucosa. Additional micronutrient deficiencies have been demonstrated through several studies to increase colorectal cancer risk and include selenium, vitamin C, vitamin D, vitamin E, and 3-carotene however, the benefit of dietary supplementation does not appear to be substantial.11... 

JPF Bai, GL Amidon. Structural specificity of mucosal-cell transport and metabolism of peptide drugs. J Pharm Sci 9 969-978, 1992. 

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