Apical cell membrane

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. 

Fig. 22.4. Model of aciclovir and Val-aciclovir transport. The prodrug Val-aciclovir is taken up at the apical cell membrane via the di/tri-peptide transporter, hydrolyzed intracelIularly in the enterocyte, with basal exit of valine and aciclovir. (Adapted from Smith et a. [78].)...

Vitamin C (VC, L-ascorbic acid) is known to be essential for many enzymatic reactions. Sodium-dependent VC transporters (SVCT), SVCT1 and SVCT2, were recently identified and reported to be localised in the apical cell membrane of AECs in the lung of adult rats. These results suggest that SVCT proteins could transport the reduced form of VC from the airway/alveolar surface liquid into respiratory epithelial cells [106],... 

Compounds can cross biological membranes by two passive processes, transcellu-lar and paracellular mechanisms. For transcellular diffusion two potential mechanisms exist. The compound can distribute into the lipid core of the membrane and diffuse within the membrane to the basolateral side. Alternatively, the solute may diffuse across the apical cell membrane and enter the cytoplasm before exiting across the basolateral membrane. Because both processes involve diffusion through the lipid core of the membrane the physicochemistry of the compound is important. Paracellular absorption involves the passage of the compound through the aqueous-filled pores. Clearly in principle many compounds can be absorbed by this route but the process is invariably slower than the transcellular route (surface area of pores versus surface area of the membrane) and is very dependent on molecular size due to the finite dimensions of the aqueous pores. 

Carbonic anhydrase-medlated Na+/H+ exchange In proximal convoluted tubule. Na+/H+exchange across apical cell membranes Is shown by the open circle. Carbonic anhydrase (CA) is present in a membrane-bound form in the apical membrane and a soluble form within the cytoplasm. The Na+/K+-ATPase is shown by the filled circle at the basolateral membrane. 

Na+-K+-2Ct cotransport In thick ascending limbs. This transport protein is shown by the open circle on the apical cell membrane. Although K+ enters the cell on the cotransporter, little net K+ reabsorption occurs because much of the K+ is recycled back to the urine from the cell. 

Sodium reabsorption continues in the distal convoluted tubule, which accounts for some 6 to 8% of the transport of sodium. The entry of Na+ across the apical cell membrane is mediated by Na+-Cl cotransport (Fig. 21.4). This protein is a distinct gene product that differs from the Na -K+-2C1 cotransporter in thick ascending limbs. 

Na+-Ct cotransport in distal convoluted tubules. This transport protein, shown by the open circle on the apical cell membrane, does not require K+ for its function. It is a different gene product than the Na+-K+-2Chcotransporter. Na+-CI cotransport is limited largely, if not entirely, to the distal convoluted tubules. 

Principal and intercalated cells of the collecting ducts. Principal (top) cells reabsorb Na+ and the secreted K+. Na+ entry across apical cell membranes is mediated by a Na+ channel. Na+ exit across basolateral cell membranes is effected by the NaVK+-ATPase, shown by the filled circle in the principal cell. The rates of Na+ reabsorption and K+ secretion are regulated by aldosterone. Intercalated (bottom) cells reabsorb K+ and HCOa and secretes H+. K+ entry and H+secretion are mediated by an H+/K ATPase, which is shown by the filled circle in the apical cell membrane of the intercalated cell. 

The mechanism by which Na" is reabsorbed in coupled exchange with and K+ in the collecting duct has been discussed previously that is, Na+-driven K+ secretion is partially under mineralocorticoid control. Aldosterone and other compounds with mineralocorticoid activity bind to a specific mineralocorticoid receptor in the cytoplasm of late distal tubule cells and of principal cells of the collecting ducts. This hormone-receptor complex is transported to the cell nucleus, where it induces synthesis of multiple proteins that are collectively called aldosterone-induced proteins. The precise mechanisms by which these proteins enhance Na+ transport are incompletely understood. However, the net effect is to increase Na" entry across apical cell membranes and to increase basolateral membrane Na+-K+-ATPase activity and synthesis. 

The Class I cells occur in several Orders of the Class Insecta. These cells characteristically occur clustered in intersegmental membranes, and are rarely scattered individually (e.g. Percy, 1979). The apical cell membrane of Class I... 

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... 

Figure9.6 Secondary active Na cotransport system in an apical cell membrane, which is driven by the Na /K -ATPase in the basolateral cell membrane.

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.

Encoded by the MUCI gene on chromosome I and a derivative human antigen, EMA is a transmembrane glycoprotein of the breast mucin complex, and its expression is increased in carcinomas.Unlike normal breast, in which EMA is present on the apical cell membrane, neoplasms demonstrate EMA on the entire circumference of the cell membrane. Increased amounts of the large glycoprotein interfere with cell-to-cell and cell-to-matrix adhesion in neoplastic cells. The utility of EMA antibody is in the detection of epithelial differentiation, as a supplement to the cytokera-tins. Spindle cell, small cell, and large cell neoplasms may on rare occasion be stained with EMA but may be only focally positive for cytokeratins. 437... 

Implantation and subsequent placentation is a unique mammalian form of reproduction. A fertilized mammalian egg autonomously develops into a blastocyst, which must be successfully implanted in the uterus to develop further into a fetus. Initial adhesion of the embryo to the uterus occurs via the apical cell membrane of two polarized epithelial cells, the trophoblast of blastocysts, and surface epithehal cells of the endometrium. This adhesion is unique because it is apical-apical adhesion between two epithelial cells, whereas generally apical cell surfaces of epithelia are nonadhesive. 

Confocal immunofluorescent images of tracheal cells using the anti-Chc (red) and the anti a-Sjjectiin (green) and anti-Crb (green) antibodies. The a-Sp>ectrin marks cell membranes and Crb indicates apical cell membranes. (A) In stage 17 wild-type embryos Che (red) is distributed towards the apical cell membrane. (B,C) In stage 17 f/G4-driven UAS-RNAi-erF knock-down embryos and crb null mutant embryos, Che showed intracellular mis-localization in tracheal cells (arrows). (D) Tracheal Crb overexpression led to intensive Che co-localization with Crb (arrows). Scale bars=10 im. 

Guanyhn, a peptide involved in the activation of gu-anylate cyclase C is exclusively confined to the Clara cells. In cattle, guinea pig and rat, Cetin et al. (1995) found guanylin immunoreactivity in a distinct population of secretory granules mostly loca-Used underneath the apical cell membrane. 

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