Renal structure proteins and sodium transport

The regulation of sodium transport in the kidney is important for maintenance of extracellular fluid volume and arterial blood pressure regulation. A wealth of studies on the molecular identification of ion transporters has increased our understanding of how the urinary excretion is fine-tuned to meet homeostatic requirements. In fact the filtered load of human kidneys is impressive. The filtration rate amounts to 180 L/day, which corresponds to 1.7 kg of NaCl. This is approximately 11 times our total extracellular space. The largest part of this filtered load must be absorbed by renal tubules; this is paid by O2 and fuel consumption. The overall O2 consumption of both kidneys is fairly small, with approximately 20 L/day, which corresponds to 6 mol of ATP/day (on the basis of 1 mol O2/6 mol ATP) (1). The major sodium transporters and channels in individual renal tubule segments have now been identified via micropuncture, physiological techniques, and complementary DNAs for all of the key sodium transporters, and channels expressed along the renal tubule have been cloned (2,3). Complementary DNA probes and antibodies are now being used to investigate the molecular basis of renal tubule sodium transport regulation. Approximately 60% to 70% of the filtered load of Na+ and H2O are absorbed in the proximal nephron (Fig. 1). The essential regulatory mechanisms include: the glomerulotubular balance and neural and hormonal control [sympathetic innervation, angiotensin II (AII), endothelin, parathyroid hormone (PTH), and other mediators].