Vitamin D and renal calcium transport.

The kidney plays an important role in maintaining calcium balance in plasma and extracellular fluid. 1,25 Dihydroxyvitamin D3, which regulates intestinal calcium absorption, bone calcium resorption, and renal calcium reabsorption, is synthesized in the renal proximal tubules. Vitamin D-dependent calcium reabsorption from urine occurs in renal distal tubules. New information, obtained from both intestinal and kidney cells, enables us to propose a model of the mechanism of vitamin D-dependent calcium transport in renal distal tubule cells. This model involves several vitamin D-regulated processes and proteins. Briefly, 1,25 dihydroxyvitamin D3 stimulates calcium uptake at the distal tubule luminal membrane. A cytosolic vitamin D-dependent calcium binding protein (calbindin-D) sequesters calcium, which allows more calcium to enter the cell and more efficient diffusion of calcium across the cell. The ATP-dependent plasma membrane Ca2+ pump in the basolateral membrane pumps calcium out of the distal tubule cells into plasma.