Intestinal apical cell membrane

After the break down of proteins by proteolytic enzymes, the pancreas, and brush border peptidases, the di- and trip-eptides are absorbed through the epithelial cell membrane. Many studies have shown that intact di- and tripeptides are absorbed across the epithelial cell membrane by active transport via specific carrier systems. The absorption process is mediated by the hydrogen-coupled peptide transporter (PEPTl) located in the intestinal apical cell membrane. Because there are 20 amino acids, there may be 400... 

The third mucosal layer is that lining the entire length of the small intestine and which represents a continuous sheet of epithelial cells. These epithelial cells (or enterocytes) are columnar in shape, and the luminal cell membrane, upon which the microvilli reside, is called the apical cell membrane. Opposite this membrane is the basal (or basolateral) plasma membrane, which is separated from the lamina propria by a basement membrane. A sketch of this cell is shown in Fig. 5. The primary function of the villi is absorption. 

Figure 9.7 Intestinal peptide transport. Peptides are taken upinto enterocytes together with H+ ions. The proton gradient is maintained via an Na + /H+ antiport system in the apical cell membrane. The Na+ gradient is guaranteed by the Na + /I<+-ATPase in the basolateral cell membrane.

Figure 2 Comparison of intestinal epithelial cells in culture and in situ. (A) Human colon Caco-2 cells grown in culture for 16 days on a semiporous filter. (B) Epithelial layer of rat jejunum. AP, apical or luminal membrane B, basal or abluminal membrane BM, basement membrane G, goblet cell LS, lateral space mv, microvilli Nu, nucleus TJ, tight junction. Bars equal 10 pm. 

Y. Kato, M. Sugiura, T. Sugiura, T. Wakayama, Y. Kubo, D. Kobayashi, Y. Sai, I. Tamai, S. Iseki, and A. Tsuji. Organic cation/camitine transporter octn2 (slc22a5) is responsible for carnitine transport across apical membranes of small intestinal epithelial cells in mouse. Mol Pharmacol 70 829-837 (2006). 

The successful application of in vitro models of intestinal drug absorption depends on the ability of the in vitro model to mimic the relevant characteristics of the in vivo biological barrier. Most compounds are absorbed by passive transcellular diffusion. To undergo tran-scellular transport a molecule must cross the lipid bilayer of the apical and basolateral cell membranes. In recent years, there has been a widespread acceptance of a technique, artificial membrane permeation assay (PAMPA), to estimate intestinal permeability.117118 The principle of the PAMPA is that, diffusion across a lipid layer, mimics transepithelial permeation. Experiments are conducted by applying a drug solution on top of a lipid layer covering a filter that separates top (donor) and bottom (receiver) chambers. The rate of drug appearance in the bottom wells should reflect the diffusion across the lipid layer, and by extrapolation, across the epithelial cell layer. 

In intestinal epithelial cells, glucose and certain amino acids are accumulated by symport with Na+, down the Na+ gradient established by the Na+K+ ATPase of the plasma membrane (Fig. 11-44). The apical surface of the intestinal epithelial cell is covered with microvilli, long thin projections of the plasma membrane... 

Brayden, D.J. and A.W. Baird. 2004. Apical membrane receptors on intestinal M cells potential targets for vaccine delivery. Adv Drug Deliv Rev 56 721. 

The epithelium is supported underneath by lamina propria and a layer of smooth muscle called muscularis mucosa (3-10 cells thick). These three layers, i.e., the epithelium, lamina propria, and muscularis mucosa, together constitute the intestinal mucosa.On the apical surface, the epithelium along with lamina propria projects to form villi. The cell membranes of epithelial cells that comprise the villi contain uniform microvilli, which give the cells a fuzzy border, collectively called a brush border. These structures, although greatly increase the absorptive surface area of the small intestine, provide an additional enzymatic barrier since the intestinal digestive enzymes are contained in the brush border. In addition, on the top of the epithelial layer lies another layer, the UWL, as previously described. The metabolic and biochemical components of the epithelial barrier will be discussed. 

Inui, K.I. Yamamoto, M. Saito, H. Transepithehal transport of oral cephalosprins by monolayers of intestinal epithehal-cell line Caco-2 secific transport-system in apical and basolateral membranes. J. Pharmacol. Exp. Ther. 1992, 261, 195-201. 

Matsumoto S, Saito H, Inui K. TransceUular transport of oral cephalosporins in human intestinal epithehal cells, Caco-2 interaction with dipeptide transport systems in apical and basolateral membranes. J Pharmacol Exp Ther 1994 270(2) 498-504. 

A key feature of the liver is the antidromic blood and bile flow system (Fig. lc). Blood enters the lobules in the periphery, passes the sinusoids, and is drained off into the central vein. Bile is secreted by the hepatocytes into the apical bile canaliculi, which form a network organized parallel as well as perpendicular to the sinusoidal vessels. Bile flows to the bile ducts in the periportal field and is finally drained off into the gall bladder and the small intestine. The bile canalicular network is not composed of conventional vessels with endothelial cells. It is formed by the apical membrane of hepatocytes and thereby is a consequence of the polar structure of these cells which have an apical pole (facing the bile canaliculus) and a basolateral side (facing the sinusoid). The direct contact to bile leads to a high vulnerability of hepatocytes. Destruction of the apical hepatocyte membrane may lead to bile acids entering the cell, hepatocyte killing, and inflammation. 

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