Critical role of the epithelial Ca²⁺ channel TRPV5 in active Ca²⁺ reabsorption as revealed by TRPV5/calbindin-D_28K knockout mice

The epithelial Ca²⁺ channel TRPV5 facilitates apical Ca ²⁺ entry during active Ca²⁺ reabsorption in the distal convoluted tubule. In this process, cytosolic Ca²⁺ remains at low nontoxic concentrations because the Ca²⁺ 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 double-knockout mice of TRPV5 and calbindin-D_28K (TRPV5 ^-/-, calbindin-D_28K^-/-, and TRPV5 ^-/-/calbindin-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 Ca ²⁺ 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 hypercalciuria. The similarity between TRPV5^-/-/calbindin-D_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 D₃ levels accompanied the enhanced expression of the Ca²⁺ transporters. Intestinal Ca²⁺ absorption and expression of calbindin-D_9K and TRPV6, as well as serum parameters of the calbindin-D_28K^-/- 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.