Membrane intestinal basolateral

IREG1 (McKie et al., 2000) Transport of iron across intestinal basolateral membrane 7 Iron-deficiency anaemia... 

The cells lining the lumen of the intestine are polarized, that is they have two distinct sides or domains which have different lipid and protein compositions. The apical or brush border membrane facing the lumen is highly folded into microvilli to increase the surface area available for the absorption of nutrients. The rest of the plasma membrane, the basolateral surface, is in contact with neighboring cells and the blood capillaries (Fig. 5). Movement between adjacent epithelial cells is prevented by the formation of tight junctions around the cells near the apical domain. Thus any nutrient molecules in the lumen of the intestine have to pass through the cytosol of the epithelial cell in order to enter the blood. 

Cl Cheeseman. GLUT2 is the transporter for fructose across the rat intestinal basolateral membrane. Gastroenterology 105 1050—1056, 1993. 

Cl Cheeseman. Role of intestinal basolateral membrane in absorption of nutrients. Am J Physiol 263 R482-R488, 1992. 

Some recent studies on vitamin transport using membrane vesicles include those of vitamin B6 by rat kidney brush border membranes (Bowman et al, 1990), ascorbic acid by teleost intestinal brush border membranes (Mafha et ai, 1993), biotin by human kidney brush border membranes (Baur and Baumgartner, 1992), pantothenate by human placental brush border membranes (Grassl, 1992), folate and riboflavin by rabbit intestinal brush border membranes (Said and Mohammadkhani, 1993a,b Said et al, 1993), and thiamine by rat small intestine basolateral membranes (Laforenza et al, 1993). Bile acid transport in human placental, rat ileal, and rabbit small intestinal brush border membrane vesicles (Dumaswala et al, 1993 Gong et al, 1991 Kramer et al, 1993) and the effect of vitamin D status... 

Lubiprostone, a drug used for treating obstipation, has been claimed to be an activator of C1C-2. This is based on a single paper showing activation by lubiprostone of currents thought to represent C1C-2. These currents, however, differ starkly from typical C1C-2 currents. Furthermore, C1C-2 is located in basolateral membranes of the intestine. This localization is incompatible with the hypothesis that its activation increases intestinal chloride and fluid secretion. Thus, the claim that lubiprostone is a Cl- channel activator must be subject to considerable doubt. 

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. 

The membrane surface facing the lumen is called the apical surface, and the membrane surface on the side facing blood is called the basolateral surface. The intestinal cells are joined at the tight junctions [63,75]. These junctions have pores that can allow small molecules (MW < 200 Da) to diffuse through in aqueous solution. In the jejunum, the pores are 7-9 A in size. In the ileum the junctions are tighter, and pores are 3-4 A in size (i.e., dimensions of mannitol) [63]. 

Additional epithelial aqueous pathways of significantly smaller radius (<3 A) have also been documented utilizing both equivalent pore and circuit theory [25], These pathways may correspond to specific channels through lipid membranes as opposed to paracellular pathways. Osmotically activated ion channels [35] and even specific water channels [36] have been characterized in renal epithelia. In intestinal epithelia, mucosal chloride channels have been studied in secreting crypt cells, and basolateral potassium channels in colonic epithelia serve cellular ion and volume homeostatic functions. 

S Chapelle, M GiUes-BaiUien. Phospholipids and cholesterol in brush border and basolateral membranes from rat intestinal mucosa. Biochim Biophys Acta 753 269-271, 1983. 

Figure 8.3 A model of iron transport across the intestine. Reduction of ferric complexes to the ferrous form is achieved by the action of the brush border ferric reductase. The ferrous form is transported across the brush border membrane by the proton-coupled divalent cation transporter (DCT1) where it enters an unknown compartment in the cytosol. Ferrous iron is then transported across the basolateral membrane by IREG1, where the membrane-bound copper oxidase hephaestin (Hp) promotes release and binding of Fe3+ to circulating apotransferrin. Except for hephaestin the number of transmembrane domains for each protein is not shown in full. Reprinted from McKie et al., 2000. Copyright (2000), with permission from Elsevier Science.

Proulx [30] summarized the published lipid compositions of BBM isolated from epithelial cells from pig, rabbit, mouse and rat small intestines. Table 3.1 shows the lipid make-up for the rat, averaged from five reported studies [30], On a molar basis, cholesterol accounts for about 50% of the total lipid content (37% on a weight basis). Thus, the cholesterol content in BBM is higher than that found in kidney epithelial (MDCK) and brain endothelial cells (Table 3.1). Slightly different BBM lipid distribution was reported by Alcorn et al. [31] here, the outer (luminal) leaflet of the BBM was seen to be rich in sphingomyelin content, while the inner leaflet (cytosol) was rich in PE and PC. Apical (brush border) and basolateral lipids are different in epithelia. The basolateral membrane content (not reported by... 

The cause is defective transport of dibasic amino acids by the proximal tubule and intestine. The transport defect occurs at the basolateral rather than the luminal membrane. Hyperammonemia reflects a deficiency of intra-mitochondrial ornithine. An effective treatment is oral citrulline supplementation, which corrects the hyperammonemia by allowing replenishment of the mitochondrial pool of ornithine. 

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