L-amino acid sensing by the extracellular Ca2+-sensing receptor

The extracellular calcium (Ca(2+)(o))-sensing receptor (CaR) recognizes and responds to (i.e., "senses") Ca(2+)(o) as its principal physiological ligand. In the present studies, we document that the CaR is activated not only by extracellular calcium ions but also by amino acids, establishing its capacity to sense nutrients of two totally different classes. l-Amino acids, especially aromatic amino acids, including l-phenylalanine and l-tryptophan, stereoselectively mobilized Ca(2+) ions in the presence of the CaR agonists, Ca(2+)(o), gadolinium (Gd(3+)(o)), and spermine in fura-2-loaded human embryonic kidney (HEK-293) cells stably transfected with the human CaR. l-amino acid-dependent effects were observed above, but not below, a threshold level of Ca(2+)(o) of approximately 1.0 mM. l-Amino acids, particularly aromatic amino acids, also stereoselectively enhanced the sensitivity of the CaR to its agonists, Ca(2+)(o) and spermine. Branched-chain amino acids were almost inactive, and charged amino acids, including arginine and lysine, were much less effective than aromatic and other amino acids. l-amino acid mixtures emulating the amino acid composition of fasting human plasma reproduced the effects of high concentrations of individual l-amino acids on Ca(2+) mobilization and enhanced the sensitivity of the CaR to Ca(2+)(o). The data presented herein identify the CaR as a molecular target for aromatic and other l-amino acids. Thus, the CaR can integrate signals arising from distinct classes of nutrients: mineral ions and amino acids. The actions of l-amino acids on the CaR may provide explanations for several long recognized but poorly understood actions of dietary protein on calcium metabolism.     

Functional desensitization of the extracellular calcium-sensing receptor is regulated via distinct mechanisms: role of G protein-coupled receptor kinases, protein kinase C and beta-arrestins

The extracellular calcium-sensing receptor (CaR) senses small fluctuations of the extracellular calcium (Ca(2+)(e)) concentration and translates them into potent changes in parathyroid hormone secretion. Dissecting the regulatory mechanisms of CaR-mediated signal transduction may provide insights into the physiology of the receptor and identify new molecules as potential drug targets for the treatment of osteoporosis and/or hyperparathyroidism. CaR can be phosphorylated by protein kinase C (PKC) and G protein-coupled receptor kinases (GRKs), and has been shown to bind to beta-arrestins, potentially contributing to desensitization of CaR, although the mechanisms by which CaR-mediated signal transduction is terminated are not known. We used a PKC phosphorylation site-deficient CaR, GRK and beta-arrestin overexpression or down-regulation to delineate CaR-mediated desensitization. Fluorescence-activated cell sorting was used to determine whether receptor internalization contributed to desensitization. Overexpression of GRK 2 or 3 reduced Ca(2+)(e)-dependent inositol phosphate accumulation by more than 70%, whereas a GRK 2 mutant deficient in G alpha(q) binding (D110A) was without major effect. Overexpression of GRK 4-6 did not reduce Ca(2+)(e)-dependent inositol phosphate accumulation. Overexpression of beta-arrestin 1 or 2 revealed a modest inhibitory effect on Ca(2+)(e)-dependent inositol phosphate production (20-30%), which was not observed for the PKC phosphorylation site-deficient CaR. Agonist-dependent receptor internalization (10-15%) did not account for the described effects. Thus, we conclude that PKC phosphorylation of CaR contributes to beta-arrestin-dependent desensitization of CaR coupling to G proteins. In contrast, GRK 2 predominantly interferes with G protein-mediated inositol-1,4,5-trisphosphate formation by binding to G alpha(q).     

A syndrome of hypocalciuric hypercalcemia caused by autoantibodies directed at the calcium-sensing receptor

Antibodies to cell surface receptors can cause endocrine dysfunction by mimicking or blocking the actions of their respective hormones. We sought patients with autoantibodies to the extracellular calcium (Ca(2+)(o))-sensing receptor (CaR), which sets the normal level of blood calcium, that mimic the genetic disorder, familial hypocalciuric hypercalcemia, caused by heterozygous inactivating mutations of the CaR. Four individuals from two kindreds were identified with PTH-dependent hypercalcemia, who had other autoimmune manifestations: one with sprue and antigliadin and antiendomyseal antibodies and three with antithyroid antibodies. Three of the patients also had relative or absolute hypocalciuria. The patients' sera contained antibodies that reacted with the cell surface of bovine parathyroid cells in a manner similar to an authentic polyclonal anti-CaR antibody, stained bands on Western analysis of sizes similar to those labeled by the anti-CaR antiserum, and reacted with several synthetic peptides derived from sequences within the CaR's extracellular amino terminus. The patients' sera also stimulated PTH release from dispersed human parathyroid cells compared with the effect of sera from normocalcemic control subjects. This stimulation could be blocked by preabsorbing serum with membranes from CaR-transfected, but not nontransfected, human embryonic kidney (HEK293) cells. Finally, in two of the patients, antibodies affinity-purified using a synthetic peptide from within the CaR's extracellular domain inhibited high Ca(2+)(o)-stimulated, CaR-mediated accumulation of inositol phosphates and activation of mitogen-activated protein kinase in CaR-transfected HEK293 cells. DNA sequencing revealed no mutations within the index patients' CaR genes in the two families. Therefore, a biochemical phenotype of PTH-dependent hypercalcemia resembling that caused by heterozygous inactivating mutations of the CaR in familial hypocalciuric hypercalcemia can be observed in patients with antibodies to the CaR's extracellular domain that stimulate PTH release, probably by inhibiting activation of the CaR by Ca(2+)(o). Autoimmune hypocalciuric hypercalcemic is an acquired disorder of Ca(2+)(o) sensing that should be differentiated from that caused by inactivating mutations of the CaR.     

Relationship between parathyroid calcium-sensing receptor expression and potency of the calcimimetic, cinacalcet, in suppressing parathyroid hormone secretion in an in vivo murine model of primary hyperparathyroidism

Cinacalcet HCl, an allosteric modulator of the calcium-sensing receptor (CaR), has recently been approved for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease on dialysis, due to its suppressive effect on parathyroid hormone (PTH) secretion. Although cinacalcet's effects in patients with primary and secondary hyperparathyroidism have been reported, the crucial relationship between the effect of calcimimetics and CaR expression on the parathyroid glands requires better understanding. To investigate its suppressive effect on PTH secretion in primary hyperparathyroidism, in which hypercalcemia may already have stimulated considerable CaR activity, we investigated the effect of cinacalcet HCl on PTH-cyclin D1 transgenic mice (PC2 mice), a model of primary hyperparathyroidism with hypo-expression of CaR on their parathyroid glands. A single administration of 30 mg/kg body weight (BW) of cinacalcet HCl significantly suppressed serum calcium (Ca) levels 2 h after administration in 65- to 85-week-old PC2 mice with chronic biochemical hyperparathyroidism. The percentage reduction in serum PTH was significantly correlated with CaR hypo-expression in the parathyroid glands. In older PC2 mice (93-99 weeks old) with advanced hyperparathyroidism, serum Ca and PTH levels were not suppressed by 30 mg cinacalcet HCl/kg. However, serum Ca and PTH levels were significantly suppressed by 100 mg/kg of cinacalcet HCl, suggesting that higher doses of this compound could overcome severe hyperparathyroidism. To conclude, cinacalcet HCl demonstrated potency in a murine model of primary hyperparathyroidism in spite of any presumed endogenous CaR activation by hypercalcemia and hypo-expression of CaR in the parathyroid glands.     

Mitogenic action of calcium-sensing receptor on rat calvarial osteoblasts

The parathyroid calcium-sensing receptor (CaR) plays a nonredundant role in systemic calcium homeostasis. In bone, Ca(2+)(o), a major extracellular factor in the bone microenvironment during bone remodeling, could potentially serve as an extracellular first messenger, acting via the CaR, that stimulates the proliferation of preosteoblasts and their differentiation to osteoblasts (OBs). Primary digests of rat calvarial OBs express the CaR as assessed by RT-PCR, Northern, and Western blot analysis, and immunocolocalization of the CaR with the OB marker cbfa-1. Real-time PCR revealed a significant increase in CaR mRNA in 5- and 7-d cultures compared with 3-d cultures post harvesting. High Ca(2+)(o) did not affect the expression of CaR mRNA during this time but up-regulated cyclin D (D1, D2, and D3) genes, which are involved in transition from the G1 to the S phase of the cell cycle, as well as the early oncogenes, c-fos and early growth response-1; high Ca(2+)(o) did not, however, alter IGF-I expression, a mitogenic factor for OBs. The high Ca(2+)(o)-dependent increase in the proliferation of OBs was attenuated after transduction with a dominant-negative CaR (R185Q), confirming that the effect of high Ca(2+)(o) is CaR mediated. Stimulation of proliferation by the CaR involves the Jun-terminal kinase (JNK) pathway, as high Ca(2+)(o) stimulated the phosphorylation of JNK in a CaR-mediated manner, and the JNK inhibitor SP600125 abolished CaR-induced proliferation. Our data, therefore, show that the parathyroid/kidney CaR expressed in rat calvarial OBs exerts a mitogenic effect that involves activation of the JNK pathway and up-regulation of several mitogenic genes.     

Ca(2+) receptor from brain to gut: common stimulus, diverse actions.

An extracellular Ca(2+)-sensing receptor (CaR) plays central roles in Ca(2+) homeostasis by regulating parathyroid hormone (PTH)secretion and renal Ca(2+) handling. The CaR is also expressed in intestine and bone, where its functions in mineral metabolism are not yet well defined. The receptor is also present in various types of cells seemingly uninvolved in systemic mineral ion homeostasis (such as neuronal and glial cells in the brain and various epithelial cells), where its actions are poorly understood but might involve the regulation of local ionic homeostasis and/or diverse cellular processes, such as cellular differentiation and proliferation.     

Physiology and pathophysiology of the extracellular calcium-sensing receptor

The system governing extracellular calcium (Ca2+o) homeostasis maintains near constancy of Ca2+o so as to ensure continual availability of calcium ions for their numerous intracellular and extracellular roles. In contrast to the intracellular ionized calcium concentration (Ca2+i), which varies substantially during intracellular signaling via this key second messenger, Ca2+o remains nearly invariant. Yet there must be a mechanism that senses small changes in Ca2+o so as to set into motion the homeostatic responses that return Ca2+o to its normal level. The recent identification and molecular cloning of the mechanism through which parathyroid cells and a number of other cell types sense Ca2+o, a G protein-coupled Ca2+o-sensing receptor (CaR), has proven unequivocally that extracellular calcium ions serve in an informational capacity. The CaR permits Ca2+o to function in a hormone-like role as an extracellular first messenger through which parathyroid, kidney, and other cells communicate with one another via the CaR. The identification of inherited human hypercalcemic and hypocalcemic disorders arising from inactivating and activating mutations of the CaR, respectively, has provided additional proof of the essential, nonredundant role of the CaR in mineral ion homeostasis. Moreover, CaR-active drugs are currently in clinical trials for the treatment of primary and uremic hyperparathyroidism, disorders in which there are acquired, tissue-specific reductions in CaR expression and, in turn, defective Ca2+o-sensing by pathological parathyroid cells. No doubt further studies of Ca2+o-sensing by the CaR will reveal additional functions of Ca2+o, not only as a systemic "hormone" but also in local, paracrine, and autocrine signaling through this novel Ca2+o-sensing receptor.     

Parathyroid gland calcium receptor mRNA levels are unaffected by chronic renal insufficiency or low dietary calcium in rats

Extracellular ionized calcium (Ca(2+)) is the primary physiological regulator of parathyroid hormone (PTH) secretion and the G protein-coupled receptor (CaR) that mediates this response has been cloned from bovine and human parathyroid glands. The Ca(2+) set-point for the regulation of PTH secretion is right-shifted in primary hyperparathyroidism (1°HPT), but whether there is a similar shift in 2°HPT is unclear. Additionally, the molecular defects associated with such changes in the set-point remain uncharacterized. These experiments were designed to determine (1) if changes in set-point occur in rats with 2°HPT induced by chronic renal insufficiency (CRI) or dietary Ca deficiency, and (2) whether any changes in set-point are mirrored by changes in steady-state mRNA levels for the parathyroid CaR. CaR mRNA levels were quantified in pairs of glands from individual rats using a solution hybridization assay. Blood urea nitrogen and PTH levels were ～ 4-fold higher in rats with CRI induced by 5/6 nephrectomy 7 weeks earlier. Rats with CRI were also significantly hypocalcemic and hyperphosphatemic. The setpoint was unchanged in CRI rats and CaR mRNA levels were also unaffected. Normal rats fed a 0.02% Ca diet for 6 weeks were markedly hypocalcemic, and had 10- and 15-fold increases in plasma PTH and 1,25-dihydroxyvitamin D(3) levels, respectively. Technical problems prevented assessment of the set-point in these animals, but parathyroid gland CaR mRNA levels were identical in both dietary groups. Thus, neither alterations in mRNA levels for the CaR nor changes in the set-point play demonstrable roles in the pathogenesis of 2°HPT in these models.     

Familial hypocalciuric hypercalcemia and other disorders with resistance to extracellular calcium.

Cloning of the CaR has increased understanding of the normal control of mineral ion homeostasis and has clarified the pathophysiology of PTH-dependent hypercalcemia. Cloning of the CaR has enabled identification of FHH and NSHPT as inherited conditions with generalized resistance to Ca2+o, which is caused in many cases by inactivating mutations in the CaR gene. In most kindreds with FHH, there is resetting of Ca2+o to a mildly elevated level that does not require an increase in the circulating level of PTH above the normal range to maintain it. FHH is not accompanied by the usual symptoms, signs, and complications of hypercalcemia. The kidney participates in the genesis of the hypercalcemia in FHH by avidly reabsorbing Ca2+; consequently, there is no increased risk of forming urinary calculi in most cases. Generally, there is no compelling rationale for attempting to lower the level of Ca2+o in these patients to a nominal normal level. In contrast, in primary hyperparathyroidism, the Ca2+o resistance is limited to the pathologic parathyroid glands, and the rest of the body suffers the consequences of high circulating levels of calcium, PTH, or both. In this condition, removal of the offending parathyroid glands is often the treatment of choice. Parathyroidectomy may also be appropriate in disorders with generalized resistance to Ca2+o owing to inactivating CaR mutations in the following special circumstances: in selected families with FHH in which there is unusually severe hypercalcemia, frankly elevated PTH levels, or atypical features such as hypercalciuria; in cases of NSHPT with severe hypercalcemia and hyperparathyroidism; and in the occasional mild case of homozygous FHH owing to CaR mutations that confer mild-to-moderate resistance to Ca2+o that escapes clinical detection in the neonatal period. As discussed elsewhere in this issue, selective calcimimetic CaR activators are being tested in clinical trials, which potentiate the activation of the CaR by Ca2+o, thereby resetting the elevated set point for Ca2+o-regulated PTH release in primary and secondary hyperparathyroidism toward normal. It is hoped that these agents may become an effective medical therapy for the acquired Ca2+o resistance in primary and secondary hyperparathyroidism and perhaps for that present in the unusual cases of FHH and NSHPT, resetting the "calciostat" downward and thereby reducing Ca2+o and PTH toward normal.     

Regulation of the calcium-sensing receptor. Influence of secondary hyperparathyroidism

The parathyroid glands have a great sensitivity to small changes in the extracellular ionic calcium. The calcium-sensing receptor (CaR) is a G protein-coupled receptor that responds to extracellular ionic calcium changes activating several intracellular signalling systems (phospholipases C, A2 and D) finally inhibiting the PTH secretion. In addition to calcium, there are some other agonists and modulators such as the Mg2+, spermine, amyloid beta-peptides, a variety of aminoacids, especially aromatic aminoacids and ionic strength. In the uraemia, the sensitivity of the parathyroid glands to calcium is altered and higher values of calcium are necessary to suppress the PTH. In the secondary hyperparathyroidism the CaR expression is reduced. It has been found a negative correlation between cellular proliferation and the expression of the CaR in hyperplasic glands. Despite it is a calcium receptor, the expression of the CaR does not seem to be regulated by calcium and there is some controversy about the role of calcitriol regulating its expression. On the other hand, the phosphorous induces hyperplasia of the parathyroid gland increasing the cellular proliferation and a decrease of the CaR expression.     

Drugs inhibiting parathyroid hormone (PTH) secretion by control of the calcium receptor (calcimimetics)--effect on the set point of calcium-regulated PTH secretion

Calcimimetics are positive allosteric modulators that activate the parathyroid calcium receptor (CaR) and thereby immediately suppress parathyroid hormone (PTH) secretion. Preclinical studies have demonstrated that calcimimetics inhibit PTH secretion and parathyroid gland hyperplasia and ameliorates bone qualities in rats with chronic renal insufficiency. Clinical trials with cinacalcet hydrochloride, a calcimimetic compound, have shown that calcimimetics possess lowering effects not only on serum PTH levels but also on serum phosphorus levels in dialysis patients with secondary hyperparathyroidism (2HPT). Thus, calcimimetics have considerable potential as an innovative medical approach to manage 2HPT. In this review, the similarities are extrapolated between the pharmacological effect of calcimimetics on the set point of Ca-regulated PTH secretion and clinical observations in affected subjects with activating CaR mutations.     

Decreased expression of caveolin-1 and altered regulation of mitogen-activated protein kinase in cultured bovine parathyroid cells and human parathyroid adenomas

Caveolins are key components of caveolae membranes. The calcium-sensing receptor (CaR) resides within caveolin-rich membrane domains in bovine parathyroid (PT) cells. Recent studies reported reduced CaR expression, and abnormal calcium-sensing in PT tumors. To examine this altered CaR signaling, we investigated ERK activation after CaR stimulation in human and bovine PT cells. In freshly prepared bovine PT cells, high extracellular calcium (Ca(2+)(0)) stimulates ERK1/2 phosphorylation, and activated ERK1/2 colocalizes with caveolin-1 at the plasma membrane but fails to translocate to the nucleus, and cell proliferation is low. In cultured bovine PT cells, CaR and caveolin-1 levels are reduced; activated ERK1/2 localizes in the cell periphery at 10 min and in the perinuclear and nuclear regions at 60 min after exposure to high Ca(2+)(0), and cell proliferation is increased. In PT cells from adenomas, there are high levels of caveolin-2, variably reduced caveolin-1, and hyperactivation of ERK1/2, which colocalizes with caveolin-1 in some cells, but localizes in the cytosol and nucleus in others. Finally, caveolin-1 negative human PT cells exhibit reduced suppressibility of PTH secretion by high Ca(2+)(0). Thus, CaR and caveolin-1 colocalize in PT cells, and reduced levels of caveolin-1 could participate in the abnormal cellular function and proliferation of cultured bovine PT cells and PT adenomas.     

Parathyroid expression of calcium-sensing receptor protein and in vivo parathyroid hormone-Ca(2+) set-point in patients with primary hyperparathyroidism

A reduced expression of calcium-sensing receptor (CaR) messenger ribonucleic acid and protein accompanied by abnormalities in parathyroid cell proliferation and PTH secretion are present in primary hyperparathyroidism. We studied the expression of CaR protein by immunohistochemistry in 36 sporadic parathyroid adenomas and investigated the relationship between CaR expression and several preoperative clinical parameters, including the set-point of Ca(2+)-regulated PTH secretion (measured in vivo). The adenomas were classified in 4 categories according to the intensity of immunohistochemical staining: 5 (14%) showed a CaR staining intensity similar to that of normal parathyroid ( ), 10 (27%) showed moderate staining (++), 16 (45%) showed weak staining (+), and 5 (14%) were negative (-). The intensity of CaR staining was not related to preoperative serum Ca(2+), PTH levels or adenoma volume. Twenty-nine patients underwent preoperatively the calcium infusion test to evaluate the PTH-Ca(2+) set-point. Individual values of PTH-Ca(2+) set-point ranged from 1.38-1.93 mmol/L and were significantly correlated with basal Ca(2+) levels (r = 0.96; P: = 0. 0001) and adenoma volume (r = 0.5; P: = 0.01). The mean PTH-Ca(2+) set-point values were significantly different in the 4 groups of patients classified according to immunohistochemical staining intensity of their adenoma (P: = 0.025; F = 3.78); the mean PTH-Ca(2+) set-point was significantly higher in the groups classified as negative than in those classified as weak or moderate. No correlation was observed between the PTH-Ca(2+) set-point and basal PTH levels or between the percent maximal PTH inhibition and adenoma volume and basal PTH or Ca(2+) levels. In summary, our data suggest that there is a relationship between apparent CaR protein expression and PTH-Ca(2+) set-point abnormality, suggesting that a reduced receptor content might have an important role in the pathogenesis of primary hyperparathyroidism.     

Calcium ions as extracellular, first messengers

Summary Ca²⁺ concentration in the extracellular fluids ([Ca²⁺]_o) is essential for a number of vital processes from bone formation to blood clotting. For this reason, it is necessary that [Ca²⁺]_o must be strictly controlled. Mammalian species have developed a complex homeostatic system that includes parathyroid glands, kidney and bone. The extracellular Ca²⁺-sensing receptor (CaR) is an essential component of this system, regulating parathyroid hormone secretion, calcium excretion by the kidney and bone remodeling. Initially identified from bovine parathyroid glands (1), within the five years following its identification CaR presence has rapidly been extended to organs where the link with mineral ion metabolism has not been elucidated (i.e., brain, stomach, eye, skin, and many other epithelial cells) (see 2 for review). This review will address the discovery of a novel class of ion-sensing receptors, receptor-effector coupling, and the roles of the CaR inside and outside the Ca²⁺_o homeostatic systems.     

Hypercalcaemic and hypocalcaemic conditions due to calcium-sensing receptor mutations

The extracellular calcium (Ca2+o)-sensing receptor (CaSR) enables the parathyroid glands and other CaSR-expressing cells involved in calcium homeostasis, such as the kidney and bone, to sense alterations in the level of Ca2+o and to respond with changes in function that are directed at normalizing the blood calcium concentration. Several disorders of Ca2+o sensing arise from inherited or acquired abnormalities that 'reset' the serum calcium concentration upwards or downwards. Heterozygous inactivating mutations of the CaSR produce a benign form of hypercalcaemia, termed 'familial hypocalciuric hypercalcaemia', while homozygous mutations produce a much more severe hypercalcaemic disorder resulting from marked hyperparathyroidism, called 'neonatal severe hyperparathyroidism'. Activating mutations cause a hypocalcaemic syndrome of varying severity, termed 'autosomal-dominant hypocalcaemia or hypoparathyroidism' as well as Bartter's syndrome type V. Calcimimetic CaSR activators and calcilytic CaSR antagonists have also been developed with potential for use in the treatment of these disorders.     

维生素D不足、矿物质代谢异常对慢性肾病心血管系统的影响及处理

1,25-二羟维生素D3合成减少、肾脏对磷廓清不足、血钙失调、继发性甲状旁腺功能亢进和骨病是慢性肾脏疾病的特征,临床观察性研究证实维生素D及维生素D受体激活剂治疗与血液透析患者的生存率获益有关,并独立于甲状旁腺激素、血钙和磷,可能的解释是维生素D及维生素D受体激活剂对心血管系统产生了有利影响。现就矿物质代谢紊乱、维生素D不足对慢性肾脏疾病患者心血管系统的影响及相关处理进行简要综述。     

Expression and functional assessment of an alternatively spliced extracellular Ca2+-sensing receptor in growth plate chondrocytes

The extracellular Ca(2+)-sensing receptor (CaR) plays an essential role in mineral homeostasis. Studies to generate CaR-knockout (CaR(-/-)) mice indicate that insertion of a neomycin cassette into exon 5 of the mouse CaR gene blocks the expression of full-length CaRs. This strategy, however, allows for the expression of alternatively spliced CaRs missing exon 5 [(Exon5(-))CaRs]. These experiments addressed whether growth plate chondrocytes (GPCs) from CaR(-/-) mice express (Exon5(-))CaRs and whether these receptors activate signaling. RT-PCR and immunocytochemistry confirmed the expression of (Exon5(-))CaR in growth plates from CaR(-/-) mice. In Chinese hamster ovary or human embryonic kidney-293 cells, recombinant human (Exon5(-))CaRs failed to activate phospholipase C likely due to their inability to reach the cell surface as assessed by intact-cell ELISA and immunocytochemistry. Human (Exon5(-))CaRs, however, trafficked normally to the cell surface when overexpressed in wild-type or CaR(-/-) GPCs. Immunocytochemistry of growth plate sections and cultured GPCs from CaR(-/-) mice showed easily detectable cell-membrane expression of endogenous CaRs (presumably (Exon5(-))CaRs), suggesting that trafficking of this receptor form to the membrane can occur in GPCs. In GPCs from CaR(-/-) mice, high extracellular [Ca(2+)] ([Ca(2+)](e)) increased inositol phosphate production with a potency comparable with that of wild-type GPCs. Raising [Ca(2+)](e) also promoted the differentiation of CaR(-/-) GPCs as indicated by changes in proteoglycan accumulation, mineral deposition, and matrix gene expression. Taken together, our data support the idea that expression of (Exon5(-))CaRs may compensate for the loss of full-length CaRs and be responsible for sensing changes in [Ca(2+)](e) in GPCs in CaR(-/-) mice.     

Calcium Receptor and Regulation of Parathyroid Hormone Secretion

The cloning of the CaR has made it possible to show definitively that the CaR is a critical mediator of the inhibitory effect of high Cao2+ on PTH secretion and parathyroid cellular proliferation. The receptor may also mediate the suppressive action of high Cao2+ on PTH gene expression, while its involvement in several other known actions of Cao2+ on parathyroid function remain to be examined. Alterations in CaR expression and/or function are clearly involved in hyper- and hypocalcemic disorders caused by inactivating or activating CaR mutations, respectively, and could contribute to the deranged Cao(2+)-sensing in primary and uremic secondary HPT. Finally, CaR activators offer promise as the first truly effective mode of medical therapy for these latter two conditions.