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Active Intestinal Calcium Transport in the Absence of Transient Receptor Potential Vanilloid Type 6 and Calbindin-D_9k

Department of Biochemistry and Molecular Biology (B.S.B., D.A., A.P., P.D., S.C.), University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, New Jersey 07103; Institute of Biochemistry and Molecular Medicine (M.H.), University of Berne, CH-3012 Berne, Switzerland; Department of Medicine (J.-B.P., Y.J.), University of Alabama, Birmingham, Alabama 35294; Laboratory of Cardiovascular Genomics (G.T.O.), Ewha Woman’s University, Seoul 120-750, Korea; Laboratory of Veterinary Biochemistry and Molecular Biology (E.-B.J.), College of Veterinary Medicine, Chungbuk National University, Cheongju, Chungbuk 361-763, Korea; and Laboratory of Experimental Medicine and Endocrinology (L.L., R.B., G.C.), Katholieke Universiteit Leuven, Leuven B-3000, Belgium

Address all correspondence and requests for reprints to: Dr. Sylvia Christakos, Department of Biochemistry and Molecular Biology, UMDNJ-New Jersey Medical School E609, 185 South Orange Avenue, Newark, New Jersey 07103. E-mail: christak{at}umdnj.edu.

To study the role of the epithelial calcium channel transient receptor potential vanilloid type 6 (TRPV6) and the calcium-binding protein calbindin-D_9k in intestinal calcium absorption, TRPV6 knockout (KO), calbindin-D_9k KO, and TRPV6/calbindin-D_9k double-KO (DKO) mice were generated. TRPV6 KO, calbindin-D_9k KO, and TRPV6/calbindin-D_9k DKO mice have serum calcium levels similar to those of wild-type (WT) mice (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-D_9k KO mice, respectively (n = 8–22 per group; P > 0.1, WT vs. calbindin-D_9k 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-D_9k 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-D_9k, thus challenging the dogma that TRPV6 and calbindin-D_9k are essential for vitamin D-induced active intestinal calcium transport.

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