ABSTRACT
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 O
and fuel consumption. The overall O
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 O
/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 H
O 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].
