Transport of inorganic and organic substances in the renal proximal tubule.

The transport through the epithelial cell layer of the renal proximal tubule proceeds in principle by passive paracellular and active transcellular transport. The active transcellular transport is mostly secondary active. This means it proceeds coupled with the flux of Na+ ions, whereby the transcellular gradient of sodium, created by the (Na+ + K+)-ATPase, located at the contraluminal cell side, provides the main driving force. Once in the cell the substances leave the other cell side by a Na+ -independent, but carrier-mediated transport system. Using microperfusion and electrophysiological techniques as well as brush border membrane vesicle preparation the Na+ -H+ countertransport and the Na+-cotransport of amino acids, phosphate, sulfate, thiosulfate, bile acids, aliphatic-aromatic monocarboxylic acids (lactate) and dicarboxylic acids was studied. Special emphasis will be given to the bidirectional transport of thiosulfate as well as to the specificity of the monocarboxylic acid and dicarboxylic acid transport system.