The calcium-sensing receptor is required for normal calcium homeostasis independent of parathyroid hormone

The extracellular calcium-sensing receptor (CaR; alternate gene names, CaR or Casr) is a membrane-spanning G protein–coupled receptor. CaR is highly expressed in the parathyroid gland, and is activated by extracellular calcium (Ca²⁺_o). Mice homozygous for null mutations in the CaR gene (CaR^–/–) die shortly after birth because of the effects of severe hyperparathyroidism and hypercalcemia. A wide variety of functions have been attributed to CaR. However, the lethal CaR-deficient phenotype has made it difficult to dissect the direct effect of CaR deficiency from the secondary effects of hyperparathyroidism and hypercalcemia. We therefore generated parathyroid hormone–deficient (PTH-deficient) CaR^–/– mice (Pth^–/–CaR^–/–) by intercrossing mice heterozygous for the null CaR allele with mice heterozygous for a null Pth allele. We show that genetic ablation of PTH is sufficient to rescue the lethal CaR^–/– phenotype. Pth^–/–CaR^–/– mice survive to adulthood with no obvious difference in size or appearance relative to control Pth^–/– littermates. Histologic examination of most organs did not reveal abnormalities. These Pth^–/–CaR^–/– mice exhibit a much wider range of values for serum calcium and renal excretion of calcium than we observe in control littermates, despite the absence of any circulating PTH. Thus, CaR is necessary for the fine regulation of serum calcium levels and renal calcium excretion independent of its effect on PTH secretion.

The extracellular calcium-sensing receptor (CaR) is a plasma membrane–bound G protein–coupled receptor that is activated by extracellular calcium (Ca²⁺_o). CaR is expressed in multiple tissues including parathyroid gland, thyroid, kidney, intestine, bone, bone marrow, brain, skin, pancreas, lung, and heart (1–9). Activation of parathyroid gland CaR regulates Ca²⁺_o homeostasis by suppressing parathyroid hormone (PTH) secretion. Mutations in the human CASR gene have been shown to cause familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism (NSHPT) (10), and autosomal dominant hypocalcemia (11, 12). These Mendelian disorders are all characterized by an altered response to Ca²⁺_o.

In 1995, Ho et al. used homologous recombination to develop a mouse model with a targeted disruption in the CaR gene (also known as Casr) in order to examine the role of CaR in calcium homeostasis and elucidate the mechanism by which inherited human CASR defects cause disease (13). Mice heterozygous for deficient CaR alleles (CaR^+/–) mimic the phenotype of patients with FHH and exhibit modest elevations in serum calcium levels with relative hypocalciuria. Mice homozygous for null CaR alleles (CaR^–/–) exhibit a phenotype similar to human NSHPT. CaR^–/– mice display severe hyperparathyroidism and hypercalcemia, parathyroid hyperplasia, bone abnormalities, growth retardation, and death shortly after birth (13). Most of these abnormalities are presumed to be due to the effects of hyperparathyroidism and hypercalcemia. However, both the wide tissue distribution of CaR expression and reports of possible roles for CaR in processes not directly related to calcium homeostasis (including neuronal transmission, cellular proliferation, lymphocyte differentiation, and hormonal secretion) raise the possibility that some aspects of the phenotype in these mice (and in humans with NSPHT) may be due to an essential role for CaR signaling in these processes (14–17).

Low dietary calcium intake and hypocalcemia are major stimuli for PTH secretion leading to parathyroid gland hyperplasia (18, 19). The observed parathyroid gland hyperplasia in patients and mice with homozygous mutations in CaR suggests that the parathyroid gland enlargement seen during calcium deficiency may not be due to overstimulation of the gland but to a direct effect of understimulation of CaR. CaR transcripts have been identified in cartilage and bone marrow as well as in osteoclast and osteoblast cells (20–22). It has been hypothesized that local changes in Ca²⁺_o acting through CaR may serve a physiological role in regulating the differentiation and function of chondrocytes, osteoblasts, and osteoclasts and therefore may have a direct effect on bone function independent of PTH (23–25).

In order to address some of these questions, we attempted to rescue the CaR-deficient mice by genetic means. Earlier unsuccessful efforts included crossing the targeted CaR allele onto collagenase-resistant Col1a1 mice (26), osteoclast-deficient Op/Op mice (27), and transgenic mice in which expression of wild-type CaR is targeted to the parathyroid gland(26). Here, we describe the results of our efforts to separate the direct effect of CaR deficiency from the confounding effects of hyperparathyroidism and hypercalcemia by examining the role of CaR in a mouse model that lacks PTH. Homozygous PTH-deficient mice (Pth^–/–) display diminished cartilage matrix mineralization and reduced metaphyseal osteoblasts and trabecular bone during the fetal and neonatal period, but these findings normalize in adulthood (28). We bred CaR^+/– mice with mice heterozygous for a null Pth allele (Pth^+/–) and crossed the double heterozygous offspring to generate mice lacking both PTH and CaR (Pth^–/–CaR^–/–). We made direct comparisons between the phenotypes of CaR^–/– mice and CaR-expressing sex-matched littermates, on Pth^–/– backgrounds, in order to assess the role of CaR independent of the effects of PTH.

Rescue of lethal CaR-deficient phenotype in the absence of PTH. Pth^–/–CaR^–/– mice were born in predicted Mendelian ratios. The lethal phenotype of CaR-deficient mice previously described by Ho et al. (13) was not observed in animals also deficient for PTH. Of the over 40 animals generated that were homozygous for null alleles at both the PTH and CaR loci, all but two pregnant females survived well into adulthood, appeared healthy, and exhibited no visible abnormalities.

Reduced fertility of Pth^–/– mice. During attempts to establish a breeding colony, we noticed very reduced pregnancy and offspring rates in cages of Pth^–/– mice regardless of CaR genotype. Five Pth^–/– females died (including two Pth^–/–CaR^–/– females) during pregnancy or while lactating. We hypothesized that hypocalcemia during pregnancy and lactation was the cause of death and therefore maintained all Pth^–/– mice on a high-calcium (2%), high–vitamin D diet. To assay the affect of the high-calcium diet on serum calcium levels, two groups of Pth^–/–CaR^+/+ control animals were maintained on either a regular or a high-calcium diet for 2 weeks and then sacrificed so that serum could be obtained. There was no difference in the serum calcium levels of Pth^–/–CaR^+/+ mice on regular and high-calcium diets (data not shown). All animals continued to have reduced rates of reproduction, but the high-calcium diet appeared to prevent maternal death during pregnancy and nursing.

Phenotypic features of Pth^–/–CaR^–/– versus control mice. We made comparisons between Pth^–/–CaR^–/– mice and control littermates that had either the Pth^–/–CaR^+/+ or Pth^–/–CaR^+/– genotype. Pth^–/–CaR^–/– mice were indistinguishable from their control littermates with respect to size, body weight, and behavior (Figures 1 and 2). We observed no differences between CaR-deficient mice and control littermates in light microscopic and histologic analyses of tissues from multiple organs, including gonads, thymus, brain, kidney, lung, and liver (data not shown).

Figure 1

Gross appearance of (a) 23-day-old CaR–/–, CaR+/– and CaR+/+ mice and (b) 10-week-old Pth–/–CaR–/–, Pth–/–CaR+/–, and Pth–/–CaR+/+ mice.

Figure 2

Body weight of Pth–/–CaR–/– and control mice as a function of age. Pth–/–CaR–/– mice (gray bars) and control sex-matched littermates (black bars) were weighed between 3.5 and 12 weeks of age. Each column represents the mean ± SD derived from 8–15 measurements.

Histologic examination of parathyroid glands. Examination of the parathyroid glands from Pth^–/–CaR^–/– mice revealed a marked increase in the glandular size and hypercellularity in three out of three animals examined (Figure 3). This is consistent with earlier observations in CaR^–/– mice (14). Thus, parathyroid gland hyperplasia in CaR-deficient mice can occur independently of the presence of PTH. Fifty percent of the control animals (one of two Pth^–/–CaR^+/– mice and one of two Pth^–/–CaR^+/+ mice) also exhibited some degree of hyperplasia.

Biochemical analyses. We measured serum and urine calcium, phosphorus, and creatinine levels (Table 1). There was a trend toward hypercalcemia and hypercalciuria in Pth^–/–CaR^–/– mice relative to controls, but the differences in these serum and urine measurements did not reach statistical significance. Further examination of the data and P values suggested that increasing the sample size would not increase the significance. Although the mean values for serum calcium, the urine calcium/urine creatinine ratio, and the fractional excretion of calcium were not significantly different between Pth^–/–CaR^–/– mice and control mice, the variability in these values was much greater in Pth^–/–CaR^–/– mice than in controls (Figure 3). The variation in the serum calcium levels of the Pth^–/–CaR^–/– mice (SD = 16.523) was significantly greater than the variation in controls (SD = 0.470, P < 0.0001) (Table 1 and Figure 4a). Similarly, the variation in the urine calcium/urine creatinine ratio of the Pth^–/–CaR^–/– mice (SD = 1.178) was significantly greater than the variation in controls (SD = 0.3887, P < 0.001) (Table 1 and Figure 4b), and the variation in the fractional excretion of calcium for Pth^–/–CaR^–/– animals was significantly greater (P < 0.0001) than that of controls as well (Table 1 and Figure 4c).

Figure 4

Histologic appearance of parathyroid gland from control mice (a) and Pth–/–CaR–/– sex-matched littermates (b). Sections were prepared from adult mice for light microscopy and stained with H&E.

Alterations in intestinal and renal calcium transporter expression. We assayed the mRNA expression of CaT1 (30) and CaT2/ECaC (31, 32) in the duodenum and in the kidney by quantitative real-time PCR. Results were normalized to control animal expression levels of 100%. We compared expression in RNA samples from three pairs of sex-matched littermates maintained on high-calcium diets and three pairs of sex-matched littermates on regular diets. We performed assays in duplicate for each RNA sample. CaT1 and CaT2/ECaC expression was consistently higher in the kidneys of CaR-deficient mice (65% and 60% higher, respectively; data from both diets) (Figure 5). These differences reached statistical significance with P = 0.015 and P = 0.0025, respectively, in paired t tests.

Figure 5

Expression levels of CaT1 and CaT2/ECaC are altered in Pth–/–CaR–/– mice relative to control littermates. The mRNA expression levels of CaT1 (black bars) and CaT2/ECaC (white bars) in the duodenum and kidney were assessed by real-time quantitative PCR and normalized against GAPDH mRNA levels. We compared expression in RNA samples from three pairs of sex-matched littermates maintained on high-calcium diets (2% calcium, top) and three pairs of sex-matched littermates on regular diets (0.81% calcium, bottom). We performed assays in duplicate for each RNA sample. Renal CaT1 and CaT2/ECaC expression were consistently higher in CaR-deficient mice, regardless of diet (mean increased expression of 65% and 60%, respectively). These differences reached statistical significance with P = 0.015 and P = 0.0025, respectively, in paired t tests. Intestinal CaT1 expression was highly variable. In animals fed high-calcium diets as well as those fed regular diets, the overall trend was toward increased CaT1 expression in the CaR-deficient animals, but this trend did not reach statistical significance either when the groups were analyzed separately according to diet or when all six pairs were analyzed together.

Intestinal CaT1 expression has been shown to be downregulated in animals fed high-calcium diets (33). In the mice we studied that were fed high-calcium diets, intestinal CaT1 expression was highly variable. In both the mice fed high-calcium diets and those fed regular diets, the overall trend was toward increased CaT1 expression in the CaR-deficient animals, but this did not reach statistical significance either when the groups were analyzed separately according to diet or when all six pairs were analyzed together (Figure 5).

Skeletal phenotype of Pth^–/–CaR^–/– mice. We compared bones from CaR-deficient and CaR-expressing littermates (all Pth^–/–) maintained on high-calcium diets. Histologic analysis of femoral and tibial sections from Pth^–/–CaR^–/– mice revealed no apparent differences as compared with control littermates (data not shown). However, light microscopy of H&E-stained sections of vertebral bone from Pth^–/–CaR^–/– mice revealed decreased marrow space, suggesting that BMD was increased in the spines of these mice (Figure 6, b and d). Analysis of BMD measurements showed that female Pth^–/–CaR^–/– mice, but not male Pth^–/–CaR^–/– mice, have a statistically significant increase in vertebral BMD as compared with sex-matched control littermates (mean = 0.083 g/cm² and SD = 0.016 in CaR-expressing mice, mean = 0.103 g/cm² and SD = 0.24 in CaR-deficient mice, P = 0.043 in a one-tailed t test). This observation supports the hypothesis that CaR has a role in normal bone function. Measurement of osteoclast number per bone volume showed a trend toward a reduction in the number of osteoclasts in the spine per millimeter of bone volume in the Pth^–/–CaR^–/– mice but did not reach statistical significance. Measurements of BFR and MAR, performed in three pairs of littermates, did not show any consistent changes that could be attributed to CaR absence or presence.

The Mendelian disorders of abnormal calcium homeostasis that are caused by CaR mutations are all characterized by an altered response to Ca²⁺_o by the parathyroid gland and kidneys. However, CaR is expressed in multiple tissues in addition to the parathyroid glands and kidney. It has been unclear what contribution, if any, CaR mutations in these tissues make to the overall phenotypes of CaR^–/– mice and the human disorders FHH and NSHPT. Furthermore, it has been difficult to determine which abnormalities caused by inherited defects in CaR are direct effects of the CaR alterations and which are secondary to hypercalcemia and hyperparathyroidism.

In this study, we developed a CaR-deficient mouse model that survives into adulthood. By eliminating PTH, we separated the detrimental effects of hyperparathyroidism and severe hypercalcemia from the direct effects of CaR deficiency. In order to compare the effects of CaR deficiency rather than the compound effects of two mutant genes, we made all of our comparisons between Pth^–/–CaR^–/– mice and PTH-deficient control sex-matched littermates (as opposed to wild-type mice). The Pth^–/–CaR^–/– mice were viable and fertile, although females had difficulty maintaining pregnancies and nursing pups when not on a calcium-enriched diet. Our results show that the major phenotypic features characteristic of CaR^–/– mice are the consequence of impaired suppression of PTH secretion and the resulting elevation in serum calcium levels and not the direct effect of CaR deficiency. This work further supports the hypothesis that the primary role of CaR is to suppress PTH secretion.

All of the Pth^–/–CaR^–/– mice had grossly enlarged parathyroid glands. Ca²⁺_o inhibits parathyroid cell proliferation. However, through studies of cyclin D1 transgenic mice, Imanishi et al. have shown that parathyroid hyperplasia can occur independently of altered Ca²⁺_o-mediated control of PTH secretion (34). Our finding that CaR-deficient mice show parathyroid hyperplasia in the absence of PTH demonstrates that parathyroid gland hyperplasia also does not depend on the overproduction of PTH per se. The enlarged parathyroid glands also observed in some of the control mice may be secondary to sustained hypocalcemia during the fetal period. Calcium is actively transported across the placenta from mother to fetus, resulting in a higher fetal calcium concentration (35–37). The parathyroid glands of a fetus in a calcium-stressed Pth^–/– mother would likely be exposed to a calcium level below normal, leading to secondary parathyroid hyperplasia.

Despite the fact that CaR-deficient mice are viable in the absence of PTH, our results indicate a role for CaR in maintaining calcium homeostasis independent of PTH. The serum calcium levels of PTH-deficient mice are much less tightly regulated in the absence of CaR. The large amount of variability between clinically significant high and low measured serum calcium values suggests that serum calcium levels in Pth^–/–CaR^–/– mice fluctuate widely. Similarly, urinary calcium excretion varied widely in mice lacking CaR. The mechanism by which CaR helps to maintain serum calcium levels within a narrow range in the absence of PTH is unclear, but this model should be useful for more detailed studies of CaR function in intestine, kidney, bone, and other homeostatic tissues. Our work indicates that CaR is necessary for the tight regulation of serum calcium levels independent of its effect on PTH secretion and that CaR is involved in fine tuning renal calcium excretion in a PTH-independent manner.

The trend in the Pth^–/–CaR^–/– animals was towards hypercalcemia and hypercalciuria. We therefore examined the expression levels of two calcium entry channels involved in the maintenance of calcium homeostasis, CaT1 (30) and CaT2/ECaC (31, 32, 38). CaT1 is expressed throughout the human gastrointestinal tract and mediates intestinal calcium absorption (30, 33, 39). CaT2/ECaC is kidney specific and localized to the apical membrane of the distal convoluted tubule (32). CaT1 and CaT2/ECaC levels in the kidney and duodenum were increased in the Pth^–/–CaR^–/– mice, although the trend was less consistent in the duodenum than in the kidney. The increases in calcium transporter expression in the kidney and duodenum may have an impact on calcium balance and contribute to the greater variability in renal calcium excretion and serum calcium concentrations observed in the CaR-deficient mice.

We observed no detrimental bone abnormalities in Pth^–/–CaR^–/– mice. However, our measurements found that female Pth^–/–CaR^–/– mice have elevated BMD in the spine. Since multiple hypotheses were tested in the course of the various studies described in this paper, we acknowledge the possibility that this result may represent a “false positive,” particularly given the P value of 0.043, which is just under the generally accepted value of 0.05. However, this finding is consistent with complementary findings from other studies. Genetic factors have been shown to account for about 50–85% of the variation in bone mass, and CaR has been examined as a possible susceptibility locus for osteoporosis. An A986S CaR polymorphism has been reported to have an effect on serum calcium, circulating PTH concentrations, and BMD in healthy Caucasian girls (40, 41). (A second group found no significant difference in the distribution of the A986S polymorphism between groups of postmenopausal and healthy control women and concluded that CaR had no effect on BMD [42]). Other investigators have shown that high Ca²⁺_o stimulates CaR in chondrogenic cell lines, leading to increased production of mineral matrix (43).

We have demonstrated that the lethal CaR-deficient phenotype is dependent on PTH. In the absence of PTH, serum calcium levels and urine calcium excretion are much less tightly regulated in CaR-deficient mice than in mice with CaR. These mice provide a model for more detailed examination of the role of CaR in a wide variety of physiological functions that cannot be addressed by studying mice deficient only in CaR. These findings also have implications for understanding the role of CaR in regulating bone function independently of its effect on PTH secretion.

This work was supported by grants F32 DK09937 (to C.H. Kos), DK 52479 (to M.R. Pollak), and DK 48330 (to T.A. Guise) from the NIH/National Institute of Diabetes, Digestive, and Kidney Diseases as well as by grants from the MRC/Canadian Institutes for Health Research (to A.C. Karaplis and to D. Goltzman) and the Burroughs-Wellcome fund (to M.R. Pollak).

See the related Commentary beginning on page 945.

Conflict of interest: The authors have declared that no conflict of interest exists.

Nonstandard abbreviations used: calcium-sensing receptor (CaR); extracellular calcium (Ca²⁺_o); parathyroid hormone (PTH); familial hypocalciuric hypercalcemia (FHH); neonatal severe hyperparathyroidism (NSHPT); fractional excretion of calcium (FECa); calcium transporter type 1 (CaT1); calcium transporter type 2 (CaT2/ECaC); bone mineral density (BMD); tartrate-resistant acid phosphatase (TRAP); mineral apposition rate (MAR); bone formation rate (BFR).

1. Malaisse, WJ, et al. Possible participation of an islet B-cell calcium sensing receptor in insulin release. Endocrine. 1999. 11:293-300.
   View this article via: PubMed CrossRef Google Scholar
2. Rutten, MJ, et al. Identification of a functional Ca2+-sensing receptor in normal human gastric mucous epithelial cells. Am. J. Physiol. 1999. 277:G662-G670.
   View this article via: PubMed Google Scholar
3. Riccardi, D, et al. Localization of the extracellular Ca2+-sensing receptor and PTH/PTHrP receptor in rat. Am. J. Physiol. 1996. 271:F951-F956.
   View this article via: PubMed Google Scholar
4. Ruat, M, Molliver, M, Snowman, A, Snyder, S. Calcium sensing receptor: molecular cloning in rat and localization to nerve terminals. Proc. Natl. Acad. Sci. U. S. A. 1995. 92:3161-3165.
   View this article via: PubMed CrossRef Google Scholar
5. Garrett, JE, et al. Calcitonin-secreting cells of the thyroid express an extracellular calcium receptor gene. Endocrinology. 1995. 136:5202-5211.
   View this article via: PubMed CrossRef Google Scholar
6. Bikle, DD, Ratnam, A, Mauro, T, Harris, J, Pillai, S. Changes in calcium responsiveness and handling during keratinocyte differentiation. Potential role of the calcium receptor. J. Clin. Invest. 1996. 97:1085-1093.
   View this article via: JCI PubMed CrossRef Google Scholar
7. House, MG, et al. Expression of an extracellular calcium-sensing receptor in human and mouse bone marrow cells. J. Bone Miner. Res. 1997. 12:1959-1970.
   View this article via: PubMed CrossRef Google Scholar
8. Aida, K, Koishi, S, Tawata, M, Onaya, T. Molecular cloning of a putative Ca(2+)-sensing receptor cDNA from human kidney. Biochem. Biophys. Res. Commun. 1995. 214:524-529.
   View this article via: PubMed CrossRef Google Scholar
9. Tu, CL, Oda, Y, Bilke, DD. Effects of a calcium receptor activator on the cellular response to calcium in human keratinocytes. J. Invest. Dermatol. 1999. 113:340-345.
   View this article via: PubMed CrossRef Google Scholar
10. Pollak, MR, et al. Mutations in the human Ca(2+)-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Cell. 1993. 75:1297-1303.
    View this article via: PubMed CrossRef Google Scholar
11. Pollak, M, et al. Autosomal dominant hypocalcaemia caused by Ca2+-sensing receptor gene mutation. Nat. Genet. 1994. 8:303-307.
    View this article via: PubMed CrossRef Google Scholar
12. Pearce, SH, et al. A familial syndrome of hypocalcemia with hypercalciuria due to mutations in the calcium-sensing receptor. N. Engl. J. Med. 1996. 335:1115-1122.
    View this article via: PubMed CrossRef Google Scholar
13. Ho, C, et al. A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Nat. Genet. 1995. 11:389-394.
    View this article via: PubMed CrossRef Google Scholar
14. Bruce, JI, et al. Molecular and functional identification of a Ca2+ (polyvalent cation)-sensing receptor in rat pancreas. J. Biol. Chem. 1999. 274:20561-20568.
    View this article via: PubMed CrossRef Google Scholar
15. Cheng, I, et al. Expression of an extracellular calcium-sensing receptor in rat stomach. Gastroenterology. 1999. 116:118-126.
    View this article via: PubMed CrossRef Google Scholar
16. Chattopadhyay, N, et al. Calcium-sensing receptor in the rat hippocampus: a developmental study. Brain Res. Dev. Brain Res. 1997. 100:13-21.
    View this article via: PubMed CrossRef Google Scholar
17. Hennings, H, et al. Calcium regulation of growth and differentiation of mouse epidermal cells in culture. Cell. 1980. 19:245-254.
    View this article via: PubMed CrossRef Google Scholar
18. Chen, H, et al. Effect of low calcium diet on the ultrastructure of the rat parathyroid gland. Okajimas Folia Anat. Jpn. 2001. 78:153-159.
    View this article via: PubMed Google Scholar
19. Brandi, ML. Molecular mechanisms of parathyroid hyperplasia and neoplasia. Horm. Res. 1997. 47:194-198.
    View this article via: PubMed Google Scholar
20. Kanatani, M, Sugimoto, T, Kanzawa, M, Yano, S, Chihara, K. High extracellular calcium inhibits osteoclast-like cell formation by directly acting on the calcium-sensing receptor existing in osteoclast precursor cells. Biochem. Biophys. Res. Commun. 1999. 261:144-148.
    View this article via: PubMed CrossRef Google Scholar
21. Kameda, T, et al. Calcium-sensing receptor in mature osteoclasts, which are bone resorbing cells. Biochem. Biophys. Res. Commun. 1998. 245:419-422.
    View this article via: PubMed CrossRef Google Scholar
22. Garner, SC, Pi, M, Tu, Q, Quarles, LD. Rickets in cation-sensing receptor-deficient mice: an unexpected skeletal phenotype. Endocrinology. 2001. 142:3996-4005.
    View this article via: PubMed CrossRef Google Scholar
23. Wang, D, et al. Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes. J. Biol. Chem. 2001. 276:33995-34005.
    View this article via: PubMed CrossRef Google Scholar
24. Yamaguchi, T, et al. Enhanced expression of extracellular calcium sensing receptor in monocyte-differentiated versus undifferentiated HL-60 cells: potential role in regulation of a nonselective cation channel. Calcif. Tissue Int. 2000. 66:375-382.
    View this article via: PubMed CrossRef Google Scholar
25. Lorget, F, et al. High extracellular calcium concentrations directly stimulate osteoclast apoptosis. Biochem. Biophys. Res. Commun. 2000. 268:899-903.
    View this article via: PubMed CrossRef Google Scholar
26. Zhao, W, Byrne, MH, Boyce, BF, Krane, SM. Bone resorption induced by parathyroid hormone is strikingly diminished in collagenase-resistant mutant mice. J. Clin. Invest. 1999. 103:517-524.
    View this article via: JCI PubMed CrossRef Google Scholar
27. Marks, SCJ, Lane, PW. Osteopetrosis, a new recessive skeletal mutation on chromosome 12 of the mouse. J. Hered. 1976. 67:11-18.
    View this article via: PubMed Google Scholar
28. Miao, D, He, B, Karaplis, AC, Goltzman, D. Parathyroid hormone is essential for normal fetal bone formation. J. Clin. Invest. 2002. 109:1173-1182. doi:10.1172/JCI200214817.
    View this article via: JCI PubMed Google Scholar
29. Uy, HL, et al. Tumor necrosis factor enhances parathyroid hormone-related protein-induced hypercalcemia and bone resorption without inhibiting bone formation in vivo. Cancer Res. 1997. 57:3194-3199.
    View this article via: PubMed Google Scholar
30. Peng, JB, et al. Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption. J. Biol. Chem. 1999. 274:22739-22746.
    View this article via: PubMed CrossRef Google Scholar
31. Hoenderop, JG, et al. Molecular identification of the apical Ca2+ channel in 1,25-dihydroxyvitamin D3-responsive epithelia. J. Biol. Chem. 1999. 274:8375-8378.
    View this article via: PubMed CrossRef Google Scholar
32. Peng, JB, et al. A rat kidney-specific calcium transporter in the distal nephron. J. Biol. Chem. 2000. 275:28186-28194.
    View this article via: PubMed Google Scholar
33. Van Cromphaut, SJ, et al. Duodenal calcium absorption in vitamin D receptor-knockout mice: functional and molecular aspects. Proc. Natl. Acad. Sci. U. S. A. 2001. 98:13324-13329.
    View this article via: PubMed CrossRef Google Scholar
34. Imanishi, Y, et al. Primary hyperparathyroidism caused by parathyroid-targeted overexpression of cyclin D1 in transgenic mice. J. Clin. Invest. 2001. 107:1093-1102.
    View this article via: JCI PubMed CrossRef Google Scholar
35. Kovacs, CS, et al. Regulation of murine fetal-placental calcium metabolism by the calcium-sensing receptor. J. Clin. Invest. 1998. 101:2812-2820.
    View this article via: JCI PubMed CrossRef Google Scholar
36. Kovacs, CS. Calcium and bone metabolism in pregnancy and lactation. J. Clin. Endocrinol. Metab. 2001. 86:2344-2348.
    View this article via: PubMed CrossRef Google Scholar
37. Marx, S, Fraser, D, Rapaport, A. Familial hypocalciuric hypercalcemia. Mild expression of the gene in heterozygotes and severe expression in homozygotes. Am. J. Med. 1985. 78:15-22.
    View this article via: PubMed Google Scholar
38. Peng, JB, Hediger, MA. A family of calcium-permeable channels in the kidney: distinct roles in renal calcium handling. Curr. Opin. Nephrol. Hypertens. 2002. 11:555-561.
    View this article via: PubMed CrossRef Google Scholar
39. Wood, RJ, Tchack, L, Taparia, S. 1,25-Dihydroxyvitamin D3 increases the expression of the CaT1 epithelial calcium channel in the Caco-2 human intestinal cell line. BMC Physiol. 2001. 1:11.
    View this article via: PubMed CrossRef Google Scholar
40. Lorentzon, M, Lorentzon, R, Lerner, UH, Nordstrom, P. Calcium sensing receptor gene polymorphism, circulating calcium concentrations and bone mineral density in healthy adolescent girls. Eur. J. Endocrinol. 2001. 144:257-261.
    View this article via: PubMed CrossRef Google Scholar
41. Eckstein, M, et al. Vitamin D and calcium-sensing receptor genotypes in men and premenopausal women with low bone mineral density. Isr. Med. Assoc. J. 2002. 4:340-344.
    View this article via: PubMed Google Scholar
42. Takacs, I, et al. Lack of association between calcium-sensing receptor gene “A986S” polymorphism and bone mineral density in Hungarian postmenopausal women. Bone. 2002. 30:849-852.
    View this article via: PubMed CrossRef Google Scholar
43. Chang, W, Tu, C, Pratt, S, Chen, TH, Shoback, D. Extracellular Ca(2+)-sensing receptors modulate matrix production and mineralization in chondrogenic RCJ3.1C5.18 cells. Endocrinology. 2002. 143:1467-1474.
    View this article via: PubMed CrossRef Google Scholar