Mechanisms of renal calcium transport.

The kidneys play a key role in the integrated regulation of calcium homeostasis. Calcium absorption takes place throughout the nephron. Proximal tubules, thick ascending limbs of Henle's loop, and distal tubules are the major sites of calcium absorption. The mechanisms of absorption vary significantly from one segment to another, as does the extent of hormonal regulation. At one extreme is the considerable reabsorption by proximal tubules that proceeds primarily, if not entirely, by a paracellular pathway that is not regulated by hormones or drugs. In thick ascending limbs, calcium absorption occurs through a combination of transcellular and paracellular routes. The active, transcellular component is regulated by parathyroid hormone (PTH) and calcitonin, whereas the passive, paracellular route is governed by the extent of concomitant sodium absorption. At the other extreme is the distal tubule, where calcium absorption is entirely transcellular and is regulated by PTH,1,25[OH(2)] vitamin D(3), calcitonin, and by calcium-sparing drugs such as thiazide-type diuretics. The present review focuses on recent insights into the mechanisms of transcellular calcium movement and highlights the discovery of an epithelial calcium channel, ECaC, that may mediate calcium entry in distal tubules.