Epithelium apical membranes

ENaC is located in the apical membrane of polarized epithelial cells where it mediates Na+ transport across tight epithelia [3], The most important tight epithelia expressing ENaC include the distal nephron of the kidney, the respiratory epithelium, and the distal colon. The basic function of ENaC in polarized epithelial cells is to allow vectorial transcellular transport of Na+ ions. This transepithelial Na+ transport through a cell involves... 

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR), a chloride (CF) channel characterised by chloride permeability and secretion, and also by the regulation of other epithelial ion channels (Eidelman et al, 2001). Mutations in the CFTR gene lead to an impaired or absent Cl conductance in the epithelial apical membrane, which leads to defective Cl secretion and absorption across the epithelium. Genistein (Illek et al, 1995 Weinreich et al, 1997) and other flavonoids (Illek and Fisher, 1998) have been shown, in different animal and tissue models, to activate wild-type CFTR and CFTR mutants by (Eidelman et al, 2001 Roomans, 2001 Suaud et al, 2002) ... 

WS Marshall, JW Hanrahan. (1991). Anion channels in the apical membrane of mammalian corneal epithelium primary cultures. Invest Ophthalmol Vis Sci 32 1562-1568. 

Kitchens, D.L., K. Dawson, and L. Reuss. 1990. Zinc blocks apical membrane anion exchange in gallbladder epithelium. Amer. Jour. Physiol. 258 (Gastrointestinal and Liver Physiology 21) G745-G752. 

In the renal epithelium, many uptake transporters are localized in the basolateral membrane and efflux transporters are localized in the apical membrane (Figure 15.2). As a result, vectorial transport of endogenous substances and of drugs from the blood into the urine is achieved. The important uptake transporters are members of the SLC22 family of solute carriers, especially the family members OCT2 and OAT2, which are highly expressed in human kidney. 

Passive transcellular transport across the intestinal epithelium involves three discrete steps (1) uptake across the apical membrane, (2) diffusion through the cytoplasm, and (3) efflux across the basolateral membrane. Occasionally, drug molecules without favorable physicochemical properties traverse the intestinal epithelium using endogenous membrane transporters.6-8 In addition, the intestinal mucosa, with its numerous drug-metabolizing enzymes and efflux transporters, such as P-glycoprotein (Pgp), functions as a biochemical barrier.9... 

After i.c.v. injection, the rate of elimination from the CNS compartment is dominated by cerebrospinal fluid dynamics. The CSF, which is secreted by the choroid plexus epithelium across the apical membrane, circulates along the surface and convexities of the brain in a rostral to caudal direction. It is reabsorbed by bulk flow into the peripheral bloodstream at the arachnoid vUh within both cranial and spinal arachnoid spaces [62]. Of note is that the turnover rate of total CSF volume is species dependent and varies between approximately... 

The taste cells are situated in the lingual epithelium with the apical membrane exposed to the mucosal surface of the oral cavity and the basal surface in contact with the nerve [interstitial fluid] [FIGURE 10]. Within the basolateral surface are the nerves which respond to the chemestiietic stimulants, i.e. direct nerve stimulation. The microvilli at the apical membrane contain receptor proteins which respond to sweeteners, some bitters and possibly coolants. The olfactory cells are bipolar neurons with dendritic ends containing cilia exposed to the surface and axons linked to the brain, where they synapse in the olfactory bulb. The transfer of information from this initial stimulus-receptor interaction to the brain processing centers involves chentical transduction steps in the membrane and within the receptor cells. The potential chemical interactions at the cell membrane and within the cell are schematically outlined in FIGURE 10. 

FIGURE 10. A) Schematic of taste cell embedded in epithelium. Shown are the apical membrane which contains the microvilli and the basal membrane which synapses with the nerve B) Stimulus can interact with a receptor protein embedded in membrane which activates G-protein, with the membrane or directly with an ion channel protein C) GTP stimulation of phosphatase for formation of cAMP or IP3 and opening of ion channel (Adapted from ref. 2). 

Miyamoto, Y., et al. 1991. Taurine uptake in apical membrane vesicles from the bovine retinal pigment epithelium. Invest Ophthalmol Vis Sci 32 2542. 

The secretion of CSF and ECF is essentially driven by an osmotic gradient created by the Na+/K+-ATPase, expressed in the abluminal membrane of the BBB endothelium and the apical membrane of the choroid plexus epithelium, which produces water movement into brain ECF and produces volume secretion. 

Mechanism of action Acetazolamide inhibits carbonic anhydrase, located intracellularly and on the apical membrane of the proximal tubular epithelium (Figure 23.4). [Note Carbonic anhydrase catalyzes the reaction of C02 and H20 leading to H+ and HC03 (bicarbonate)]. The decreased ability to exchange Na+ for H+ in the presence of acetazolamide results in a mild diuresis. Additionally, HC03 is retained in the lumen with marked elevation in urinary pH. The loss of HCO3- causes a hyperchloremic... 

Achler C,Filmer D, Merte C, Drenckhahn D. 1989. Role of microtubules in polarized delivery of apical membrane proteins to the brush border of the intestinal epithelium. J. Cell Biol. 109 179-89... 

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