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Abstract

Brush border membranes (luminal) and basal-lateral plasma membranes (contraluminal) of rat kidney proximal tubules were isolated by freeflow electrophoresis and their role in transepithelial transport was investigated. Enzymatic analysis revealed that the brush border membranes contain a bicarbonate stimulated ATPase and that the basal-lateral plasma membranes contain a Na⁺-K⁺-ATPase and a calcium stimulated ATPase. These findings suggest that an active, ATPase-mediated step in transepithelial bicarbonate or proton transport is located in the luminal membrane, whereas an active, ATPase-mediated step in transepithelial sodium and calcium transport is located in the contraluminal membrane. Transport studies with membrane vesicles demonstrated that sodium-dependent stereospecific transport systems for sugars, amino acids and phosphate are located in the brush border membrane; the basal-lateral plasma membranes contain sodium-independent transport systems for sugars; amino acids, phosphate and p-aminohippurate. The sodium-dependent systems represent sodium-substrate contransport systems which in the course of transepithelial transport derive energy from the transmembranel electrochemical potential difference of sodium for the intracellular accumulation and active transepithelial transport of sugars, amino acids and phosphate. The brush border membrane contains in addition a Na⁺/H⁺ exchange system which might be involved in the proton secretion of the proximal tubule. In the presence of a sodium gradient the permeability of the luminal membrane vesicles for L-lactate is higher than the permeability of the contraluminal membrane vesicles. This indicates that L-lactate-which is metabolized by the tubular epithelium-enters the tubular cell mainly from the tubular lumen. The role of membranes in the uptake of proteins by the tubular cell was investigated by isolation and biochemical characterization of microvilli, pinocytic vesicles and lysosomes. Pinocytic vesicles were found to be rich in acid phospholipids and glycoproteins which show a more rapid turnover than the proteins of the microvilli. It is concluded that pinocytic vesicles are biochemically defined entities with unique functions which are synthetized during the pinocytic process.