[New concepts of tubular calcium transport in the kidney: clinical implications].

The kidney plays an important role in calcium homeostasis. In this review we discuss new concepts in tubular calcium transport, related proteins and the clinical implications of these new findings. Most calcium reabsorption occurs in the proximal tubules via the passive paracellular pathway, but calcium reabsorption also occurs in the thick ascending limb of Henle's loop (50% via the transcellular pathway). Finally, at the level of the distal convoluted tubule and connecting tubule calcium is reabsorbed via the active transcellular route. The Calcium-sensing receptor, localised along the thick ascending limb of Henle's loop, regulates the urinary calcium excretion in response to changes in extracellular calcium concentration. The Epithelial Calcium Channel 1 is a highly Ca2+-sensitive channel that is predominantly present in the connecting tubule. The Calbindin D(28k) is a cytoplasmatic protein expressed in the distal tubule, it is involved both in transcellular calcium diffusion and in the control of intracellular calcium concentration. Heterozygous mutations in the gene for the calcium sensing receptor, which result in a loss of function by the receptor, are associated with familial hypocalciuric hypercalcaemia. Mutations involving a gain of function have been associated with hypocalcaemia with normal serum parathyroid hormone concentration. Bartter's syndrome, caused by a dysfunction of thick ascending limb cells, is associated with calcium wasting. On the contrary, Gitelman's syndrome, caused by a dysfunction of the distal tubule, is characterised by hypocalciuria and hypomagnesemia.