Active Intestinal Calcium Transport in the Absence of Transient Receptor Potential Vanilloid Type 6 and Calbindin-D9k

Abstract

To study the role of the epithelial calcium channel transient receptor potential vanilloid type 6 (TRPV6) and the calcium-binding protein calbindin-D9k in intestinal calcium absorption, TRPV6 knockout (KO), calbindin-D9k KO, and TRPV6/calbindin-D(9k) double-KO (DKO) mice were generated. TRPV6 KO, calbindin-D9k KO, and TRPV6/calbindin-D9k DKO mice have serum calcium levels similar to those of wild-type (WT) mice ( approximately 10 mg Ca2+/dl). In the TRPV6 KO and the DKO mice, however, there is a 1.8-fold increase in serum PTH levels (P < 0.05 compared with WT). Active intestinal calcium transport was measured using the everted gut sac method. Under low dietary calcium conditions there was a 4.1-, 2.9-, and 3.9-fold increase in calcium transport in the duodenum of WT, TRPV6 KO, and calbindin-D9k KO mice, respectively (n = 8-22 per group; P > 0.1, WT vs. calbindin-D9k KO, and P < 0.05, WT vs. TRPV6 KO on the low-calcium diet). Duodenal calcium transport was increased 2.1-fold in the TRPV6/calbindin-D9k DKO mice fed the low-calcium diet (P < 0.05, WT vs. DKO). Active calcium transport was not stimulated by low dietary calcium in the ileum of the WT or KO mice. 1,25-Dihydroxyvitamin D3 administration to vitamin D-deficient null mutant and WT mice also resulted in a significant increase in duodenal calcium transport (1.4- to 2.0-fold, P < 0.05 compared with vitamin D-deficient mice). This study provides evidence for the first time using null mutant mice that significant active intestinal calcium transport occurs in the absence of TRPV6 and calbindin-D9k, thus challenging the dogma that TRPV6 and calbindin-D9k are essential for vitamin D-induced active intestinal calcium transport.

Endurance swimming stimulates transepithelial calcium transport and alters the expression of genes related to calcium absorption in the intestine of rats

“…Relative contributions of Ca v and parvalbumin to the total calcium absorption remain to be investigated. It was speculated that their roles may become more significant when TRPV6 and/or calbindin-D 9k are absent or saturated as in the TRPV6/ calbindin-D 9k double knockout mice in which active calcium transport occurred at a normal rate (5).…”

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects

“…They are colocalized in the intestine (similar to VDR they are expressed in all segments of the small and large intestine) and their expression is strongly correlated to transcellular calcium absorption efficiency (85,380,420,459). However, studies in Trpv6 and calbindin-D 9k (S100g) KO mice show that 1,25(OH) 2 D 3 -mediated calcium transport is similar to WT in the absence of TRPV6 or calbindin, suggesting compensation by other calcium channels and other calcium binding proteins yet to be identified (9,37). Although bone mass is comparable in the WT and Trpv6 KO mice under conditions of normal dietary calcium, when dietary calcium is low, excessive bone turnover and impaired mineralization have been observed in the Trpv6 KO mice, suggesting a role for TRPV6 under low dietary calcium conditions (262) (see model FIGURE 3).…”

Vitamin D: current status and perspectives

“…If the transport mechanisms are still unclear (44), they include calbindin-D 9k , a calcium-binding protein induced by 125VTD which facilitates the diffusion of calcium through the cell interior toward the basolateral membrane, and TRPV6, a membrane calcium channel responsible for the first step in calcium absorption in the intestine. TRPV6 is colocalized with calbindin-D 9k and induced by 125VTD (45). 125VTD also stimulates the intestinal expression of NPT2B, which activates absorption of phosphate.…”