Epithelial cells membranes

The bumetanide-sensitive Na+, K+, 2CF cotransporter (NKCC) mediates the electroneutral uptake of chloride across epithelial cell membranes and is found in both absorptive and secretory epithelia (airways, salivary gland). NKCC exists in two isoforms, the secretory isoform NKCC1, and the absorptive isoform NKCC. 

The hypothesis of the participation of those cholesterol transporters (NPCILI and ABCAl) in the carotenoid transport remains to be confirmed, especially at the in vivo human scale. If the mechanism by which carotenoids are transported through the intestinal epithelial membrane seems better understood, the mechanism of intracellular carotenoid transport is yet to be elucidated. The fatty acid binding protein (FABP) responsible for the intracellular transport of fatty acids was proposed earlier as a potential transporter for carotenoids. FABP would transport carotenoids from the epithelial cell membrane to the intracellular organelles such as the Golgi apparatus where CMs are formed and assembled, but no data have illustrated this hypothesis yet. 

Shen, H. et al. (2001) From interaction of lipidic vehicles with intestinal epithelial cell membranes to the formation and secretion of chylomicronfodv. Drug Del. Rev., 50 S103-S125. 

Apart from causing very well known cardiotoxic effects, phenothiazine derivatives can accumulate in lung epithelial cell membranes and therefore cause severe respiratory disorders. In the study performed by Ito et al. [279] it was found that CPZ (9) inhibited transepithelial Cl transport, mainly due to two mechanisms influence on the beta-adrenergic receptor and inhibition of basolateral potassium channels. The authors of this study also suggested that the recorded effects could result from the electrostatic interactions between the drug molecules and negatively charged components of the inner leaflet of the plasma membrane. 

From Figure 1.3 it can be seen that in order to reach the underlying blood capillaries to be absorbed, the drug must pass through at least two epithelial membrane barriers (the apical and basolateral epithelial cell membranes) and also the endothelial membrane of the capillaries. In some cases, for example in stratified epithelia such as that found in the skin and buccal mucosa, the epithelial barrier comprises a number of cell layers rather than a single epithelial cell. Thus the effective barrier to drag absorption is not diffusion across a single membrane as described above, but diffusion across the entire epithelial and endothelial barrier, which may comprise several membranes and cells in series. 

Passively absorbed compounds diffuse either through the cell itself (transcellular pathway) or in between cells (paracellular pathway). The lipid bilayers of which the mucosal and basolateral epithelial cell membranes are composed of, define the primary transcellular diffusion resistance to solute transport across the intestinal barrier. Transcellular permeabihty, particularly of lipophilic solutes, depends on their partitioning between intestinal membrane and aqueous compartments (Fig. 1). 

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... 

In diseases of the small intestine, active secretion caused by cyclic nucleotide stimulation can result in a large volume of water and electrolytes moving into the lumen. Additionally, enteric neuron activation of mast cells can increase intestinal capillary permeability and promote passive fluid secretion. Diseases that increase intestinal permeability can result in passive secretion of protein-rich fluid into the intestinal lumen. Active secretion of electrolytes and water is a feature of many diarrheal disorders and can be stimulated by bacterial enterotoxins. Several bacterial enterotoxins interact with intestinal epithelial cell membrane adenylate cyclase or guanylate cyclase, resulting in increased cAMP or cGMP. These, in turn, activate basolateral chloride channels, resulting in an increase in the luminal secretion of chloride, accompanied by sodium and followed by water (Gemmell 1984). Bacterial enterotoxins that stimulate cAMP include cholera toxin, Escherichia coli... 

Little is known about the mechanism by which these organisms cause disease. They are known to invade cells but this process is atypical in that the parasites form a vacuole just below the epithelial cell membrane. During infection a variety of changes are seen such as partial villous atrophy, crypt lengthening and inflammation these responses are probably due in part to cell damage that occurs during the growth of the intracellular forms. It has also been proposed that parasite enzymes and/or immune-mediated mechanisms may also be involved. It should be remembered, however, that cryptosporidiosis is resolved by the immune system in healthy patients normally within 3 weeks. 

Vehicle-dependent effects are relevant not only because solvents and suspending agents allow delivery of higher concentrations, but also because the choice of vehicle can alter absorption rate. This may be related to decreased stomach emptying rate (for vegetable oils) or altered partitioning among fluid phases in the GI tract and across the epithelial cell membranes. Lipophilic compounds dissolved in oil will not be absorbed until the oil phase is acted upon by bile salts, converted to chylomicrons, and absorbed. If the compounds are dissolved or suspended in water, they may partition much more rapidly into cells this may be facilitated by rapid absorption of a water-miscible solvent such as alcohol or DMSO. 

The active toxin has two main functional entities, responsible for receptor binding and ion channel activity, respectively. The activated toxin binds to receptors, which seem to be of different types, on the midgut microvilli of the susceptible insects. Different toxins seem to bind to different receptor proteins that may be an enzyme such as aminopeptidase or alkaline phosphatase, or a cadherin-like membrane protein. (The cadherins are proteins that are important in keeping the cells together by mediating Ca+-dependent cell-cell adhesion in animal tissue.) The toxins are anchored to the outer epithelial cell membrane in such a way that the membrane is perforated by pores or channels where ions can freely pass. This model proposes that an influx of water, along with ions, results in swelling and lysis. The epithelium is destroyed and the insect rots. 

In polarized epithelial cells, membrane proteins destined for the apical or basolateral domains of the plasma membrane are sorted in the trans-Golgi network into different transport vesicles (see Figure 17-26). The GPI anchor is the only apical-basolateral sorting signal identified so far. 

A unique representative of the CLC family is the epithelial CLC, GFTR, which is similar to the SUR subunit that regulates Kjj.6 of the Katp channel complexes. The GFTR is a unique CLC that transports CP across epithelial cells membranes and has ATPase activity that is used to drive the protein between open and closed conformations (Figure 16.18). To function, the CFTR requires phosporylation by cAMP-protein kinase (PKA). GFTR mutations lead to cystic fibrosis (Section 16.6). 

Figure 9.15 An example of the use of a prodrug to improve carrier-mediated transport. Valacyclovir, an L-valine ester prodrug, is transported across intestinal epithelial cell membranes by the HPTl and PEPTl transporters, and is then enzymatically hydrolyzed to the antiviral agent acyclovir before being converted to the true active component, acyclovir triphosphate, in cells.

The same PDA fusion methodology was also used to study attaehment, perturbation of the membrane, and entry of vaeeinia virus to the cell membrane. The PDA patch was introduced on epithelial cell membranes and polymerized. Attachment of viral particle to the membranes induced changes in color (blue to red) and emission spectra. It was observed that the lipid composition of the membrane had a profound effect for the interactions of the PDA labeled cells with the virus particles. Membrane domains or lipid rafts rich in cholesterol and sphingomylein favored the interactions with the vaccinia virus. 

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