Differences in neutral amino acid and glucose transport between brush border and basolateral plasma membrane of intestinal epithelial cells

Abstract

A comparison of L‐valine and D‐glucose transport was carried out with vesicles of plasma membrane isolated either from the luminal (brush border) or from the contra‐luminal (basolateral) region of small intestinal epithelial cells. The existence of transport systems for both non‐electrolytes was demonstrated by stereospecificity and saturability of uptake, as well as tracer coupling. Transport of L‐valine and D‐glucose differs markedly in the two types of plasma membrane with respect to stimulation by Na⁺. The presence of Na⁺ stimulated initial L‐valine and D‐glucose uptake in brush border, but not in basolateral membrane. Moreover, an electro‐chemical Na⁺ gradient, oriented with the lower potential on the inside, supported accumulation of the non‐electrolytes above medium concentration only in the brush border membrane. L‐Valine and D‐glucose transport also were saturated at lower concentrations in brush border (10‐20 mM) than in basolateral plasma membranes (30‐50 mM). A third difference between the two membranes was found in the effectiveness of known inhibitors of D‐glucose transport. In brush border membranes phlorizin was more potent than phloretin and 2′,3′,4′‐trihydroxy‐4‐methoxy chalcone and cytochalasin B did not inhibit at all. In contrast, with the basolateral plasma membranes the order of potency was changed to phloretin = 2′,3′,4′‐trihydroxy‐4‐methoxy chalcone > cytochalasin B > phlorizin.These results indicate the presence of different types of transport systems for monosaccharides and neutral amino acids in the luminal and contra‐luminal region of the plasma membrane. Active transepithelial transport can be explained on the basis of the different properties of the non‐electrolyte transport systems in the two cellular regions and an electro‐chemical Na⁺ gradient that is dependent on cellular metabolism.