[Principles of epithelial transport in the kidney and intestines]

Epithelia of kidney and small intestine consist of one layer of cells which, at their luminal edge, are linked together by terminal bars. Solute transport proceeds either across the cells, which is true of all active transports, or it proceeds paracellularly through the basolateral spaces and terminal bars and is then passive. The driving force for the active transport of a substance is derived either directly from metabolism (primary active transport), or from the gradient of another solute, usually Na+, which in turn is created by primary active transport. In the latter case the transport is referred to as secondary active. The driving forces of passive transport are the electrochemical gradient of the respective substance and solvent drag. The proximal parts of the kidney as well as of the intestine are leaky so that a considerable part of net reabsorption proceeds passively. Their distal parts, however, where the transport is regulated, are tight so that large concentration differences can be created and maintained. Transcellular active transport is only possible if the cells are polar, i.e., the transport characteristics of the luminal cell membrane differ from those of the contraluminal cell membrane. By measuring the cellular electrical potential difference or by measuring transport into isolated plasma membrane vesicles from either cell side the driving forces for the two transport steps, the luminal and contraluminal, have been elucidated. Schemes for the transport steps in the proximal tubule and in the small intestine are given. They show the principal similarity of the transport of many substances in both epithelia.