Basolateral cell membrane surfaces

Figure 22.3 Gastric NIS in rat fetus. Section of oxyntic mucosa from a rat fetus at E19 stained by immunohistochemistry with a polyclonal antiserum raised against the eight C-terminal amino acids of rat-NIS. The site of the antigen-antibody reaction was revealed by FITC-labeled pig anti-rabbit IgG. Staining is intense in the basolateral cell membranes of the epithelial surface cells. Magnification x200.

NIS is located in the basolateral cell membrane of the gastric mucosal surface cells. 

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]. 

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. 

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 12.6 Mechanism of action of mineralocortjcoid receptor antagonists in the collecting tubule. Aldosterone enters the tubular cell by the basolateral surface and binds to a specific mineralocorticoid receptor (MNR) in the cytoplasm. The hormone receptor complex triggers the production of an aldosterone-induced protein (AlP) by the cell nucleus (NUC). The AIP acts on the sodium ion channel (ic) to augment the transport of Na+across the basolateral membrane and in to the cell. An increase in AIP activity leads to the recruitment of dormant sodium ion channels and Na pumps (P) in the cell membrane. AIP also leads to the synthesis of new channels and pumps within the cell. The increase in Na+conductance causes electrical changes in the luminal membrane that favour the excretion of intracellular cations, such as K+and H-h. Spironolactone competes with aldosterone for the binding site on the MNR and forms a complex which does not excite the production of AIP by the nucleus.

Figure 13.4 The colloidal silica method used to enrich the apical and basolateral plasma membranes recovered from cells growing on a surface. Adapted by A. Rahbar from Spector and Leinwand (1998).

A number of OATs function as dicarboxylate antiporters and are responsible for the movement of organic anions across cell membranes (Anzai et al., 2006 Sekine et al., 2006 Robertson and Rankin, 2006 You, 2004). OATl is expressed largely on the basolateral membrane of proximal tubule cells in the kidney and to a lesser degree in choroid plexus epithelium and brain neurons (Bahn et al., 2005). OATS shares similar tissue distribution and subcellular orientation in the kidney and choroid plexus but is also expressed in brain capillary endothelial cells. By contrast, OAT2 has greater expression in the liver and OAT4 is expressed on the apical surface of renal tubules and serves as a reabsorptive pathway for organic anions. 

Figure 21.1 Placement of a functional sodium iodide symporter (NIS) at the cell s basolateral surface is a complex process involving a number of molecular biological steps. The gene is first translated into mRNA. The mRNA is then translated into a protein, which must then be properly folded, glycosylated and otherwise modified, and successfully trafficked to and inserted into the cell s basolateral plasma membrane.

Although cadherins do not exclusively cluster, the formation of adherens junctions is important for several tissue functions [18]. In epithelial tissues cadherins mediate zonula adherens junctions, which partition the cell membrane into apical and basolateral surfaces. In neuronal cells they mechanically stabilize synaptic junctions by mediating adhesion between the presynaptic and postsynaptic cells. In cardiac myocytes they establish intercalated discs, which both stabilize gap junctional coupling and transmit contractile forces. 

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