Membrane vesicle intestinal luminal

In particular, the use of intestinal brush border membranes has offered much potential for the detailed investigation of absorptive phenomena. It is likely that the absorption of lipid nutrients including fat-soluble vitamins for example, which may intercalate transiently into the bilayer and affect the physical properties of the membrane, will concurrently influence the entry of other substances. Alternatively, interactions between nutrients in the gut might be expected to mutually influence absorption of these substances. Also, there may be competition between nutrients for the same transport system. Membrane vesicles are very much suited for studying this type of problem and their use has helped resolve, for example, the mechanism whereby lecithins inhibit cholesterol uptake (Merrill et al, 1987). Again, the use of isolated brush order membranes has been conducive to a better understanding of the mechanisms of interaction between lipid micelles or lipid vesicles and the intestinal luminal membrane. 

Ascorbate is absorbed from the lumen of the human intestine by sodium-ascorbate cotransport in enterocytes, as has been shown by measuring transport activities in luminal (brush-border) membrane vesicles (Malo and Wilson, 2000). This phenomenon couples ascorbate uptake to the concentration gradient of sodinm ion across the plasma membrane that is maintained by sodium/potassium-ATPase. It is likely because of the limited capacity of enterocytes for sodium-ascorbate cotransport that large oral doses of ascorbate are absorbed less completely than are small doses. Absorption sites for ascorbate are found along the entire length of the small intestine (Malo and Wilson, 2000). 

Figure 4 Tnmscdiuiar transport in me proximal and diuai intestine 01 the suckling mu (A) In die proximal intestine, macro molecules may be selectively absorbed in coaled pits along luminal membranes (M) and mis ported via coated vesicles to tbe basolanral...

Figure 4 Transcellular transport in the proximal and distal intestine of the suckling rat. (A) In the proximal intestine, macromolecules may be selectively absorbed in coated pits along luminal membranes (M) and transported via coated vesicles to the basolateral surface (arrows). Alternatively, selection may occur in tubular compartments (T) or from endosomal vesicles (V). (B) In the distal intestine selective absorption may occur in coated pits at the luminal membrane (M) or from the numerous tubules (T) of the elaborate endocytic complex. (From Ref. 18.)...

Besides investigations with whole tissue or isolated cells, studies with vesicles isolated from luminal or basal-lateral membranes of small intestine or kidney gave... 

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