[The principles of studying ion transport in the kidney tubules]

The present overview indicates that the various nephron segments take part in the Na+Cl- reabsorption, the primary driving force for it is the (Na+, K+)-ATPase, which is localized in the basolateral membrane. The various segments have different modalities of Na+ uptake: in is by Na+/H+ exchange in the proximal tubule, by Na+/2Cl-/K+ co-transport in TAL, Na+Cl- cotransport in the distal tubule, and via Na+ channels in the principal cell of the collecting duct. In the proximal tubule bulk reabsorption occurs, but very small ionic gradients are built up, there for the transport here is so economical. In the TAL transport is already less economical, however ionic gradients are built up by this nephron segment inasmuch as Na+Cl- is reabsorbed and water cannot follow (the urinary concentrating mechanism). The distal tubule is concerned with defined control of Na+ and K+ excretion. Transport at this side is expensive, but very steep ionic gradients can be built up. The control is mediated by several hormones amongst which ADH and aldosterone are the most important ones.