Critical role of the epithelial Ca2+ channel TRPV5 in active Ca2+ reabsorption as revealed by TRPV5/calbindin-D28K knockout mice

The epithelial Ca2+ channel TRPV5 facilitates apical Ca2+ entry during active Ca" reabsorption in the distal convoluted tubule. In this process, cytosolic Ca" remains at low nontoxic concentrations because the Ca" influx is buffered rapidly by calbindin-D-28K. Subsequently, Ca2+ that is bound to calbindin-D-28K is shuttled toward the basolateral Ca2+ extrusion systems. For addressing the in vivo role of TRPV5 and calbindin-D-28K in the maintenance of the Ca2+ balance, single- and doubleknockout mice of TRPV5 and calbindin-D-28K (TRPV5(-/-), calbindin-D-28K(-/-), and TRPV5(-/-)/caIbindin-D-28K(-/-)) were characterized. These mice strains were fed two Ca2+ diets (0.02 and 2% wt/wt) to investigate the influence of dietary Ca2+ content on the Ca2+ balance. Urine analysis indicated that TRPV5(-/-)/calbindin-D-28K(-/-) mice exhibit on both diets hypercalciuria compared with wild-type mice. Ca2+ excretion in TRPV5(-/-)/calbindin-D(28K)(-/-)mice was not significantly different from TRPV5(-/-) mice, whereas calbindin-D-28K(-/-) mice did not show hypercalciura. The similarity between TRPV5(-/-)/calbindinD(28K)(-/-) and TRPV5(-/-) mice was supported further by an equivalent increase in renal calbindin-D-9K expression and in intestinal Ca2+ hyperabsorption as a result of upregulation of calbindin-D-9K and TRPV6 expression in the duodenum. Elevated serum parathyroid hormone and 1,25-dihydroxyvitamin D-3 levels accompanied the enhanced expression of the Ca2+ transporters. Intestinal Ca" absorption and expression of calbindin-DIK and TRPV6, as well as serum parameters of the calbindin-D28K(-/-) mice, did not differ from those of wild-type mice. These results underline the gatekeeper function of TRPV5 being the rate-limiting step in active Ca2+ reabsorption, unlike calbindin-D-28K, which possibly is compensated by calbindin-D-9K.