The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Prostate cancer metastatic to bone has higher expression of the calcium-sensing receptor (CaSR) than primary prostate cancer

The calcium-sensing receptor (CaSR) is the principal regulator of the secretion of parathyroid hormone and plays key roles in extracellular calcium (Ca²⁺_o) homeostasis. It is also thought to participate in the development of cancer, especially bony metastases of breast and prostate cancer. However, the expression of CaSR has not been systematically analyzed in prostate cancer from patients with or without bony metastases. By comparing human prostate cancer tissue sections in microarrays, we found that the CaSR was expressed in both normal prostate and primary prostate cancer as assessed by immunohistochemistry (IHC). We used two methods to analyze the expression level of CaSR. One was the pathological score read by a pathologist, the other was the positivity% obtained from the Aperio positive pixel count algorithm. Both of the methods gave consistent results. Metastatic prostate cancer tissue obtained from bone had higher CaSR expression than primary prostate cancer (P <0.05). The expression of CaSR in primary prostate cancers of patients with metastases to tissues other than bone was not different from that in primary prostate cancer of patients with or without bony metastases (P >0.05). The expression of CaSR in cancer tissue was not associated with the stage or status of differentiation of the cancer. These results suggest that CaSR may have a role in promoting bony metastasis of prostate cancer, hence raising the possibility of reducing the risk of such metastases with CaSR-based therapeutics.

Activation of the gut calcium-sensing receptor by peptide agonists reduces rapid elevation of plasma glucose in response to oral glucose load in rats

The calcium-sensing receptor (CaSR) is expressed in various tissues, including the gastrointestinal tract. To investigate the role of gut CaSR on glycemic control, we examined whether single oral administration of CaSR agonist peptides affected the glycemic response in rats. Glucose tolerance tests were performed under oral or duodenal administration of various CaSR agonist peptides (γGlu-Cys, protamine, and poly-d-lysine hydrobromide) in conscious rats. Involvement of CaSR was determined by using a CaSR antagonist. Signaling pathways underlying CaSR agonist-modified glycemia were investigated using gut hormone receptor antagonists. The gastric emptying rate after the administration of CaSR agonist peptides was measured by the phenol red recovery method. Oral and duodenal administration of CaSR agonist peptides attenuated glycemic responses under the oral glucose tolerance test, but the administration of casein did not. The promotive effect on glucose tolerance was weakened by luminal pretreatment with a CaSR antagonist. Treatment with a 5-HT3 receptor antagonist partially diminished the glucose-lowering effect of peptides. Furthermore, the gastric emptying rate was decreased by duodenal administration of CaSR agonist peptides. These results demonstrate that activation of the gut CaSR by peptide agonists promotes glucose tolerance in conscious rats. 5-HT3 receptor and the delayed gastric emptying rate appear to be involved in the glucose-lowering effect of CaSR agonist peptides. Thus, activation of gut CaSR by dietary peptides reduces glycemic responses so that gut CaSR may be a potential target for the improvement of postprandial glycemia.

[Pharmaceutical therapy of bone metabolism disorders in chronic kidney disease mineral bone disorder (CKD-MBD) with special respect to antiresorptive substances]

Disturbances in bone and mineral turnover are common complications in patients with impaired renal function. Besides an increased risk for cardiovascular events they promote skeletal events, such as bone pain and fractures. Evidence for the antifracture efficacy of antiresorptive and osteoanabolic treatment strategies has only been demonstrated for patients with osteoporosis. The use of osteotropic drugs in patients with impaired renal function requires large randomized placebo-controlled trials.

Age-dependent parathormone levels and different CKD-MBD treatment practices of dialysis patients in Hungary--results from a nationwide clinical audit

Background

Achieving target levels of laboratory parameters of bone and mineral metabolism in chronic kidney disease (CKD) patients is important but also difficult in those living with end-stage kidney disease. This study aimed to determine if there are age-related differences in chronic kidney disease-mineral and bone disorder (CKD-MBD) characteristics, including treatment practice in Hungarian dialysis patients.

Methods

Data were collected retrospectively from a large cohort of dialysis patients in Hungary. Patients on hemodialysis and peritoneal dialysis were also included. The enrolled patients were allocated into two groups based on their age (<65 years and ≥65 years). Characteristics of the age groups and differences in disease-related (epidemiology, laboratory, and treatment practice) parameters between the groups were analyzed.

Results

A total of 5008 patients were included in the analysis and the mean age was 63.4±14.2 years. A total of 47.2% of patients were women, 32.8% had diabetes, and 11.4% were on peritoneal dialysis. Diabetes (37.9% vs 27.3%), bone disease (42.9% vs 34.1%), and soft tissue calcification (56.3% vs 44.7%) were more prevalent in the older group than the younger group (p<0.001 for all). We found an inverse relationship between age and parathyroid hormone (PTH) levels (p<0.001). Serum PTH levels were lower in patients with diabetes compared with those without diabetes below 80 years (p<0.001). Diabetes and age were independently associated with serum PTH levels (interaction: diabetes × age groups, p=0.138). Older patients were more likely than younger patients to achieve laboratory target ranges for each parameter (Ca: 66.9% vs 62.1%, p<0.001; PO₄: 52.6% vs 49.2%, p<0.05; and PTH: 50.6% vs 46.6%, p<0.01), and for combined parameters (19.8% vs 15.8%, p<0.001). Older patients were less likely to receive related medication than younger patients (66.9% vs 79.7%, p<0.001).

Conclusions

The achievement of laboratory target ranges for bone and mineral metabolism and clinical practice in CKD depends on the age of the patients. A greater proportion of older patients met target criteria and received less medication compared with younger patients.

Effects of pamidronate and calcitriol on the set point of the parathyroid gland in postmenopausal hemodialysis patients with secondary hyperparathyroidism

BACKGROUND/AIMS

Secondary hyperparathyroidism may worsen after the administration of pamidronate in postmenopausal hemodialysis (HD) patients. The aim of this study was to evaluate the short-term effect of coadministration of calcitriol and pamidronate on dynamic parathyroid hormone (PTH) secretion.

METHODS

Fifteen postmenopausal women undergoing regular HD with serum intact PTH levels of >200 pg/ml were enrolled. The PTH-ionized calcium (iCa) curve was evaluated by the response to hypo- and hypercalcemia induced with 1 and 4 mEq/l of dialysate calcium, respectively. Parameters were compared after pamidronate was administered and after coadministration of pamidronate and calcitriol. Changes in serum levels of maximal serum PTH (PTHmax), basal PTH (PTHbase) and minimal PTH (PTHmin) were evaluated.

RESULTS

Pamidronate therapy resulted in a decrease in predialysis basal plasma iCa (p < 0.05) and an increase in PTHmax (p < 0.01), PTHbase (p < 0.01) and PTHmin (p < 0.01). The change in serum iCa and PTH was reversed after the coadministration of calcitriol and pamidronate.

CONCLUSION

Our study demonstrated that pamidronate therapy is associated with a reduced plasma iCa and increased PTH secretion. These adverse effects may be reversed by calcitriol. These findings suggest that in considering pamidronate treatment in postmenopausal patients with osteoporosis receiving HD, it might be safer to add calcitriol to prevent the increased PTH secretion.

Pharmacodynamics of cinacalcet over 48 hours in patients with controlled secondary hyperparathyroidism: useful data in clinical practice

CONTEXT

Cinacalcet induces immediate changes in serum PTH levels, but the pharmacodynamic effect throughout the daily dosing interval in controlled patients is unknown. Also, in patients with reduced PTH, it is unknown what happens in the first 24 hours after withdrawal.

OBJECTIVE

Our aim was to describe the effect over 48 hours of cinacalcet in hemodialysis patients with controlled secondary hyperparathyroidism.

DESIGN

This was a phase 4, open-label, single-arm, single-dose, single-center clinical trial.

SETTING

The study was conducted at a public hospital (Hospital Perpetuo Socorro, Alicante, Spain).

PATIENTS

We included 10 patients on cinacalcet for 6 months or longer with intact PTH (iPTH) levels 100-400 pg/mL [8 men, mean age of 66 years (range 39-82 years)], chronically treated with 30 mg (n = 6), 60 mg (n = 3), or 90 mg (n = 1) of cinacalcet.

INTERVENTION

A single dose (30-90 mg) was administered at baseline.

MAIN OUTCOME MEASURES

iPTH (Duo Kit Scantibodies and Elecsys Roche), PTH 1-84, ionized calcium, phosphorus (P), and calcitonin were determined at baseline and at 1, 3, 6, 12, 24, and 48 hours.

RESULTS

There was a significant reduction in iPTH between 1 and 6 hours, and values returned to baseline at 24 hours [maximum mean (95% confidence interval) percent change from baseline: -50%(-34; -66) at 3 hours]. A transient increase in calcitonin and a decrease in P were also observed, with no changes in calcium. At 48 hours, there was a significant increase in iPTH [+51% (26; 76)] and P. Changes in PTH were similar with the 3 determination methods.

CONCLUSIONS

In hemodialysis patients with secondary hyperparathyroidism controlled by cinacalcet, a transient (1-6 hours) reduction in PTH and P and an increase in calcitonin are observed after each daily dose, with return to baseline at 24 hours. After calcimimetics discontinuation, PTH was significantly increased at 48 hours. The assay used to measure PTH does not influence relative changes induced by cinacalcet.

Calcium-sensing receptor and apoptosis in parathyroid hyperplasia of patients with secondary hyperparathyroidism

Objective

This retrospective study investigated the characteristics of secondary hyperparathyroidism (SHPT) by examining the presence of the calcium-sensing receptor (CaSR) and the rate of cell proliferation and apoptosis in parathyroid glands.

Methods

Eighteen diffuse and 57 nodular hyperplastic parathyroid glands from 24 patients with SHPT were compared with 14 primary adenomas and 33 normal parathyroid glands using immunohistochemical staining of CaSR and a marker of proliferative activity (Ki67 antigen). Apoptosis was measured using the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labelling assay.

Results

The mean ± SE labelling index (LI) of CaSR (12.8% ± 1.5%) in nodular hyperplasia was significantly lower than that in normal parathyroid glands (26.8% ± 0.8%), whereas the mean ± SE LI of CaSR in diffuse hyperplasia was similar to that in normal parathyroid glands (23.3% ± 1.8%). The mean ± SE LI of Ki67 antigen was significantly higher in primary adenoma and nodular hyperplasia than in normal parathyroid glands.

Conclusion

These results indicate that downregulation of CaSR, and a higher rate of proliferation over apoptosis, could contribute to the pathological progression of SHPT.

Differential gene expression by oxyphil and chief cells of human parathyroid glands

CONTEXT

Parathyroid oxyphil cells, whose function is unknown, are thought to be derived from chief cells. Oxyphil cells increase in number in parathyroid glands of patients with chronic kidney disease (CKD) and are even more abundant in patients receiving treatment for hyperparathyroidism with calcitriol and/or the calcimimetic cinacalcet.

OBJECTIVE

We examined oxyphil and chief cells of parathyroid glands of CKD patients for differential expression of genes important to parathyroid function.

DESIGN/SETTING/PARTICIPANTS

Parathyroid tissue from CKD patients with refractory hyperparathyroidism was immunostained for gene expression studies.

MAIN OUTCOME MEASURE

Immunostaining for PTH, PTHrP, calcium-sensing receptor, glial cells missing 2, vitamin D receptor, 25-hydroxyvitamin D-1α-hydroxylase, and cytochrome c was quantified and expression reported for oxyphil and chief cells.

RESULTS

Expression of all proteins analyzed, except for the vitamin D receptor, was higher in oxyphil cells than in chief cells.

CONCLUSION

Human parathyroid oxyphil cells express parathyroid-relevant genes found in the chief cells and have the potential to produce additional autocrine/paracrine factors, such as PTHrP and calcitriol. Additional studies are warranted to define the secretory properties of these cells and clarify their role in parathyroid pathophysiology.

Efficacy and safety of cinacalcet for the treatment of secondary hyperparathyroidism in patients with advanced chronic kidney disease before initiation of regular dialysis

AIM

To evaluate the compassionate use of cinacalcet for the management of secondary hyperparathyroidism in patients who are not on dialysis.

METHODS

Patients with stage 4-5 chronic kidney disease (CKD) who were not on dialysis, had an intact parathyroid hormone (iPTH) level greater than 300 pg/mL, and had not responded satisfactorily to treatment with phosphate binders and vitamin D were prospectively studied. Patients received 6 months of compassionate treatment with cinacalcet, which was initiated at a dose of 30 mg/day orally and flexibly dosed thereafter based on iPTH levels.

RESULTS

Twenty-six patients with a mean age±standard deviation (SD) of 58.8±16.1 years were enrolled in the study and included in the statistical analysis. The mean percentage change in iPTH levels from baseline after 6 months of treatment was -67.9±17.0%, with 92.3% (95% confidence interval (CI), 75.9-97.9) of patients showing an iPTH level within the limits recommended by Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines. The mean serum calcium concentrations had decreased significantly at the end of the study (-8.0±6.9%), while the mean serum phosphorus concentration had significantly increased (+8.3±17.0%).

CONCLUSION

Our results suggest that cinacalcet may be a useful alternative for the treatment of secondary hyperparathyroidism in pre-dialysis patients who are unresponsive to other treatments. The hypocalcemia and hyperphosphatemia reported in previous studies may not occur if a moderate dose of calcimimetics is used in patients with marginal glomerular filtration rates, especially if combined with vitamin D analogues and calcium-based phosphate binders.

Kokumi substances, enhancers of basic tastes, induce responses in calcium-sensing receptor expressing taste cells

Recently, we reported that calcium-sensing receptor (CaSR) is a receptor for kokumi substances, which enhance the intensities of salty, sweet and umami tastes. Furthermore, we found that several γ-glutamyl peptides, which are CaSR agonists, are kokumi substances. In this study, we elucidated the receptor cells for kokumi substances, and their physiological properties. For this purpose, we used Calcium Green-1 loaded mouse taste cells in lingual tissue slices and confocal microscopy. Kokumi substances, applied focally around taste pores, induced an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in a subset of taste cells. These responses were inhibited by pretreatment with the CaSR inhibitor, NPS2143. However, the kokumi substance-induced responses did not require extracellular Ca(2+). CaSR-expressing taste cells are a different subset of cells from the T1R3-expressing umami or sweet taste receptor cells. These observations indicate that CaSR-expressing taste cells are the primary detectors of kokumi substances, and that they are an independent population from the influenced basic taste receptor cells, at least in the case of sweet and umami.

The role of the calcium-sensing receptor in human disease

Following the discovery of the calcium-sensing receptor (CaSR) in 1993, its pivotal role in disorders of calcium homeostasis such as Familial Hypocalciuric Hypercalcemia (FHH) was quickly demonstrated. Since then, it has become clear that the CaSR has immense functional versatility largely through its ability to activate many different signaling pathways in a ligand- and tissue-specific manner. This allows the receptor to play diverse and crucial roles in human physiology and pathophysiology, both in calcium homeostasis and in tissues and biological processes unrelated to calcium balance. This review covers current knowledge of the role of the CaSR in disorders of calcium homeostasis (FHH, neonatal severe hyperparathyroidism, autosomal dominant hypocalcemia, primary and secondary hyperparathyroidism, hypercalcemia of malignancy) as well as unrelated diseases such as breast and colorectal cancer (where the receptor appears to play a tumor suppressor role), Alzheimer's disease, pancreatitis, diabetes mellitus, hypertension and bone and gastrointestinal disorders. In addition, it examines the use or potential use of CaSR agonists or antagonists (calcimimetics and calcilytics) and other drugs mediated through the CaSR, in the management of disorders as diverse as hyperparathyroidism, osteoporosis and gastrointestinal disease.

Paricalcitol versus cinacalcet plus low-dose vitamin D therapy for the treatment of secondary hyperparathyroidism in patients receiving haemodialysis: results of the IMPACT SHPT study

BACKGROUND

Optimal treatment for secondary hyperparathyroidism (SHPT) has not been defined. The IMPACT SHPT (ClinicalTrials.gov identifier: NCT00977080) study assessed whether dose-titrated paricalcitol plus supplemental cinacalcet only for hypercalcaemia is superior to cinacalcet plus low-dose vitamin D in controlling intact parathyroid hormone (iPTH) levels in patients with SHPT on haemodialysis.

METHODS

In this 28-week, multicentre, open-label Phase 4 study, participants were randomly selected to receive paricalcitol or cinacalcet plus low-dose vitamin D. Randomization and analyses were stratified by mode of paricalcitol administration [intravenous (IV) or oral]. The primary efficacy end point was the proportion of subjects who achieved a mean iPTH value of 150-300 pg/mL during Weeks 21-28.

RESULTS

Of 272 subjects randomized, 268 received one or more dose of study drug; 101 in the IV and 110 in the oral stratum with two or more values during Weeks 21-28 were included in the primary analysis. In the IV stratum, 57.7% of subjects in the paricalcitol versus 32.7% in the cinacalcet group (P = 0.016) achieved the primary end point. In the oral stratum, the corresponding proportions of subjects were 54.4% for paricalcitol and 43.4% for cinacalcet (P = 0.260). Cochran-Mantel-Haenszel analysis, controlling for stratum, revealed overall superiority of paricalcitol (56.0%) over cinacalcet (38.2%; P = 0.010) in achieving iPTH 150-300 pg/mL during Weeks 21-28. Hypercalcaemia occurred in 4 (7.7%) and 0 (0%) of paricalcitol-treated subjects in the IV and oral strata, respectively. Hypocalcaemia occurred in 46.9% and 54.7% of cinacalcet-treated subjects in the IV and oral strata, respectively.

CONCLUSION

Paricalcitol versus cinacalcet plus low-dose vitamin D provided superior control of iPTH, with low incidence of hypercalcaemia.

Cinacalcet HCl prevents development of parathyroid gland hyperplasia and reverses established parathyroid gland hyperplasia in a rodent model of CKD

BACKGROUND

Secondary hyperparathyroidism (sHPT) represents an adaptive response to progressively impaired control of calcium, phosphorus and vitamin D in chronic kidney disease (CKD). It is characterized by parathyroid hyperplasia and excessive synthesis and secretion of parathyroid hormone (PTH). Parathyroid hyperplasia in uremic rats can be prevented by calcium-sensing receptor (CaSR) activation with the calcimimetic cinacalcet (Sensipar®/Mimpara®); however, it is unknown, how long the effects of cinacalcet persist after withdrawal of treatment or if cinacalcet is efficacious in uremic rats with established sHPT.

METHODS

We sought to determine the effect of cinacalcet discontinuation in uremic rats and whether cinacalcet was capable of influencing parathyroid hyperplasia in animals with established sHPT.

RESULTS

Discontinuation of cinacalcet resulted in reversal of the beneficial effects on serum PTH and parathyroid hyperplasia. In rats with established sHPT, cinacalcet decreased serum PTH and mediated regression of parathyroid hyperplasia. The cinacalcet-mediated decrease in parathyroid gland size was accompanied by increased expression of the cyclin-dependent kinase inhibitor p21. Prevention of cellular proliferation with cinacalcet occurred despite increased serum phosphorus and decreased serum calcium.

CONCLUSIONS

The animal data provided suggest established parathyroid hyperplasia can be reversed by modulating CaSR activity with cinacalcet and that continued treatment may be necessary to maintain reductions in PTH.

New mutation in the CASR gene in a family with familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT)

A loss of calcium-sensing receptor (CASR) function due to inactivating mutations can cause familial hypocalciuric hypercalcemia (FHH) or neonatal severe hyperparathyroidism (NSHPT). NSHPT represents the most severe expression of FHH and courses as a life-threatening condition. The aim of this study was to identify and characterize a CASR mutation in a female infant brought to the health service due to dehydration, apathy, lack of breast feeding and severe hypercalcemia. Molecular analysis was performed on genomic DNA of the index case and her parents. A novel homozygous mutation (p.E519X) in CASR was identified in the proband; both mother and father had the same mutation in heterozygous state, confirming their FHH condition. The mutation results in a truncated and inactive protein due to the lack of transmembrane and intracellular domains. The identification of this novel CASR gene mutation established the basis of hypercalcemia in this family and allowed a genetic counseling.

Paricalcitol versus cinacalcet plus low-dose vitamin D for the treatment of secondary hyperparathyroidism in patients receiving haemodialysis: study design and baseline characteristics of the IMPACT SHPT study

Background. Paricalcitol and cinacalcet are common therapies for patients on haemodialysis with secondary hyperparathyroidism (SHPT). We conducted a multi-centre study in 12 countries to compare the safety and efficacy of paricalcitol and cinacalcet for the treatment of SHPT.

Methods. Patients aged ≥18 years with Stage 5 chronic kidney disease receiving maintenance haemodialysis and with intact parathyroid hormone (iPTH) 300–800 pg/mL, calcium 8.4–10.0 mg/dL (2.09–2.49 mmol/L) and phosphorus ≤6.5 mg/dL (2.09 mmol/L) were randomized within two strata defined by the mode of paricalcitol administration to treatment with paricalcitol- (intra-venous, US and Russian sites, IV stratum; oral, non-US and non-Russian sites, oral stratum) or cinacalcet-centred therapy. The primary endpoint is the proportion of patients in each treatment group who achieve a mean iPTH value of 150–300 pg/mL during Weeks 21–28 of treatment. Assuming efficacy response rates of 36 and 66% for cinacalcet and paricalcitol, respectively, and a 20% discontinuation rate, 124 subjects in each stratum were estimated to provide 81% power to detect a 30% absolute difference in the primary endpoint.

Results. Of 746 patients screened, 272 (mean age, 63 years; mean iPTH, 509 pg/mL) were randomized. Mean duration of haemodialysis at baseline was 3.7 years. Comorbidities included hypertension (90.4%), Type 2 diabetes (40.4%), congestive heart failure (17.3%), coronary artery disease (34.6%) and gastrointestinal disorders (75%).

Conclusions. The study participants are representative of a multinational cohort of patients on haemodialysis with elevated iPTH. The study results will provide valuable information on the best available treatment of SHPT in patients on haemodialysis.

Transcriptome analysis of the duodenum in Wistar rats fed a trypsin inhibitor derived from squid viscera

To investigate the effects of oral administration of a trypsin inhibitor (TI), normal Wistar rats were fed a TI derived from squid (Todarodes pacificus) for 10 weeks and gene expression profiles in the duodenum, pancreas, liver, and muscle were then analyzed using DNA microarrays. Although no significant changes could be observed in growth, food intake, tissue weight, or blood tests among the tissues tested, the duodenum showed the most remarkable changes in the global gene expression profile. Significant up-regulation of mRNAs encoding gastrin, gastrokine, cholecystokinin and somatostatin in the duodenum was validated by qPCR analysis. In gene ontology (GO) analysis of the up-regulated differentially expressed genes (DEGs), GO terms related to keratinization and innate mucosal defense were enriched (p < 0.001) in the category of biological processes in addition to assumable terms such as regulation of secretion and response to nutrients, vesicle-mediated transport, and so forth. In the same analysis, calcium ion binding was listed at the deepest hierarchy in the category of molecular function. These results indicate that the duodenum responds to TI treatment by a wider range of physiological processes than previously assumed such as keratinocyte differentiation and innate mucosal defense, in which calcium plays a crucial role.

Early initiation of cinacalcet for the treatment of secondary hyperparathyroidism in hemodialysis patients: a three-year clinical experience

Despite the availability of standard therapy (vitamin D sterols and phosphate binders) for the treatment of secondary hyperparathyroidism (SHPT) in hemodialyzed (HD) patients, a significant percentage of patients still fail to achieve targets recommended by the Kidney Disease Outcomes Quality Initiative (K/DOQI) of the National Kidney Foundation for parathyroid hormone (PTH), calcium, and phosphorus. The calcimimetic cinacalcet (CN) has been shown to be an effective treatment for SHPT, significantly reducing serum PTH while simultaneously lowering calcium, phosphorus, and calcium-phosphorus product levels, thus increasing the proportion of patients achieving the K/DOQI targets for bone mineral parameters. The aim of this study was to evaluate if early treatment with CN had beneficial effects in HD patients with mild-to-moderate SHPT in whom conventional treatments had failed to achieve NKF-K/DOQI targets for PTH, serum-corrected calcium, and phosphorus while minimizing the risk of paradoxical hypercalcemia and/or hyperphosphatemia. Clinical practice data were collected monthly, starting from 6 months prior to, and up to 36 months after, the start of CN therapy. CN was started at a dose of 30 mg daily or every other day, and titrated thereafter to achieve intact PTH (iPTH) <300 pg/mL. The dose of concomitant vitamin D and phosphate binders were also adjusted in order to achieve K/DOQI targets. Data from 32 patients were collected, 28 of whom had been treated with CN for at least 36 months at the time of data analysis. At baseline, patients had serum iPTH >300 pg/mL (570 ± 295 pg/mL) and/or serum-corrected calcium >9.5 mg/dL. CN induced significant decreases in iPTH, calcium, and calcium-phosphorus product with respect to baseline levels. The percentage of patients within K/DOQI target levels at baseline, 12, 24, and 36 months was 0, 81.2, 83.3, and 86.2% for iPTH; 34.4, 65.6, 86.6, and 89.6% for serum-corrected calcium; 40.6, 56.2, 69.6, and 72.4% for phosphorus; and 37.5, 62.5, 80, and 82.7% for calcium-phosphorus product. The mean dose of CN at the end of the observation period was 38 mg/day. The mean dose of concomitant medication (calcitriol, Al-containing phosphate binders, and sevelamer) decreased from baseline to 36 months. Early treatment with CN in HD patients with SHPT increases the proportion of patients achieving and maintaining K/DOQI targets with a low dose of CN (38 mg/day). These results suggest that the metabolic control obtained with low-dose CN administered early in the course of SHPT can be maintained or increased over time.

[Clinical and biological forms of secondary hyperparathyroidism in dialysis patients]

The diagnosis and treatment of hyperparathyroidism (HPT) are not yet well standardized in chronic renal failure patients. The aim of this study was to identify the main types of HPT on the basis of clinical and biological findings in a haemodialysis population. Between 2004 and 2010, all patients undergoing haemodialysis were observed and treated using the same strategy: conventional therapy with vitamin D supplements, phosphate binders, dialysate calcium adjusted to serum parathyroid hormone (PTH) level and calcitriol analogues (CA), along with regular bone marker analysis. Wherever required, cinacalcet (CC) was administered and parathyroidectomy (PTX) was performed. Of the 520 patients, 158 were classified as having HPT (30%) with a serum PTH level greater than 300 pg/mL. From this population, we identified five main types of HPT: (1) HPT with 'no bone impact' had normal or low bone marker levels (n=28, 17.7%); (2) 'secondary' HPT had elevated bone marker levels, but showed favorable response to CT (n=59, 37.7%); (3) 'tertiary' HPT was accompanied with hypercalcemia and required CC or PTX in case of CT failure (n=11, 6.9%); (4) 'mixed' HPT could not be completely treated with CT and required CC or PTX (n=57, 36%); (5) 'resistant' HPT did not show hypercalcemia, but required PTX after CT and CC failure (n=3, 1.8%). CC was prescribed in 51% cases, CA in 76%, and PTX in 7% of cases. We typified HPT on the basis of physiopathology and stages of HPT progression. Further studies on HPT that focus on bone marker levels are required to establish well-defined treatment strategies. In our study, HPT cases did not show uniform findings in Hémodialyse (HD) patients because of the variation in the stages of the disease at the time of diagnosis.

Secondary hyperparathyroidism: pathogenesis, disease progression, and therapeutic options

Secondary hyperparathyroidism (SHPT) is a challenge frequently encountered in the management of patients with chronic kidney disease (CKD). Downregulation of the parathyroid vitamin D and calcium-sensing receptors represent critical steps that lead to abnormalities in mineral metabolism: high phosphate, low calcium, and vitamin D deficiency. These imbalances result in parathyroid hyperplasia and contribute to vascular calcification. New studies have established a central role for fibroblast growth factor 23 (FGF-23) in the regulation of phosphate-vitamin D homeostasis. FGF-23 concentration increases in CKD and contributes to SHPT. Achieving current targets for the key mineral parameters in the management of SHPT set by the Kidney Disease Improving Global Outcomes (KDIGO) guidelines can be challenging. This review summarizes the current understanding and evidence supporting strategies for SHPT treatment in CKD patients. Treatment should include a combination of dietary phosphorus restriction, phosphate binders, vitamin D sterols, and calcimimetics. Parathyroidectomy is effective in suitable candidates refractory to medical therapy and the standard against which new approaches should be measured. Future strategies may focus on the stimulation of apoptotic activity of hyperplastic parathyroid cells.

Novel calcium sensing receptor ligands: a patent survey

INTRODUCTION

In the parathyroid gland, the calcium sensing receptor responds to small changes in circulating levels of Ca(2+), and consequently stimulates or inhibits the secretion of parathyroid hormone (PTH). Thus, ligands potentiating the action of calcium (calcimimetics) lead to decreased PTH secretion and can thus be useful for the treatment of hyperparathyroidism. On the other hand, ligands which antagonize the action of calcium (calcilytics) stimulate PTH secretion, favoring bone tissue regeneration.

AREAS COVERED

This review first discusses the rapid development of calcimimetics (only one of which has been approved for the treatment of hyperparathyroidism) followed by that of calcilytics (none of which has as yet been approved for the treatment of osteoporosis). Peer-reviewed articles generated by these patents are also surveyed.

EXPERT OPINION

The rapid progress in developing a clinically approved calcimimetic has not been matched by an identical success in finding an orally available calcilytic useful for the treatment of osteoporosis. However, the growing importance of osteoporosis as a debilitating disease is a stimulating factor in discovering such compounds.

Vitamin D in solid organ transplantation with special emphasis on kidney transplantation

Within the past decades, vitamin D was identified as having additional physiological functions far beyond calcium homeostasis and bone metabolism. Stimulated by the discovery of the vitamin D receptor in a broad range of tissues as well as the expression of 1α-hydroxylase, the enzyme responsible for the activation of vitamin D, it became evident that the actions of vitamin D are not restricted to cells involved in mineral and bone metabolism. In fact, it affects proliferation, differentiation, and function of a large number of different cell types including cells of the immune system. Vitamin D receptor agonists were found to exert immunosuppressive effects on the adaptive immune system, thus being able to mediate immunologic tolerance. However, they promote the innate immune system and thereby improve the ability of the host to combat invading pathogens. This review summarizes our current understanding of vitamin D as an immunomodulatory agent with special emphasis on its clinical implications in the transplant setting.

Clinical pharmacokinetic and pharmacodynamic profile of cinacalcet hydrochloride

Cinacalcet hydrochloride (cinacalcet) is a calcimimetic approved for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease (CKD) receiving dialysis and for the treatment of hypercalcaemia in patients with parathyroid carcinoma. Following oral administration, peak plasma concentrations of cinacalcet occur within 2-6 hours. The absolute bioavailability is 20-25%, and administration of cinacalcet with low- or high-fat meals increases exposure (area under the plasma concentration-time curve from time zero to infinity [AUC(infinity)]) 1.5- to 1.8-fold. Cinacalcet has no significant interaction with calcium carbonate or sevelamer hydrochloride, phosphate binders commonly used in the treatment of patients with CKD receiving dialysis. The terminal elimination half-life is 30-40 hours, and steady-state concentrations are achieved within 7 days. The pharmacokinetics of cinacalcet are dose proportional over the dose range of 30-180 mg. The pharmacokinetic profile of cinacalcet is not notably affected by varying degrees of renal impairment. The pharmacokinetics of cinacalcet are comparable between healthy subjects, patients with primary hyperparathyroidism and patients with secondary hyperparathyroidism with reduced renal function (including those patients with secondary hyperparathyroidism receiving dialysis). Additionally, the pharmacokinetics of cinacalcet are similar in patients with secondary hyperparathyroidism receiving haemodialysis and patients with secondary hyperparathyroidism receiving peritoneal dialysis. Mild hepatic impairment does not affect the pharmacokinetics of cinacalcet, whereas moderate or severe hepatic impairment increases the exposure (AUC(infinity)) by approximately 2- and 4-fold, respectively. Age, sex, bodyweight and race do not notably affect the pharmacokinetics of cinacalcet. Cinacalcet is extensively metabolized by multiple hepatic cytochrome P450 (CYP) enzymes (primarily 3A4, 2D6 and 1A2) with <1% of the parent drug excreted in the urine. Dose adjustments of cinacalcet may be necessary, and parathyroid hormone (PTH) and serum calcium concentrations should be closely monitored if a patient initiates or discontinues therapy with a strong CYP3A4 inhibitor (e.g. ketoconazole, erythromycin, itraconazole). Cinacalcet is a strong inhibitor of CYP2D6; therefore, dose adjustment of concomitant medications that are predominantly metabolized by CYP2D6 and have a narrow therapeutic index (e.g. flecainide, vinblastine, thioridazine and most tricyclic antidepressants) may be required. Cinacalcet does not appreciably inhibit or induce the activities of CYP3A4, 1A2, 2C9 or 2C19. An inverse relationship exists between plasma PTH and cinacalcet concentrations. PTH concentrations are greatest before dose administration when the cinacalcet concentration is lowest (24 hours after the previous day's dose). Nadir PTH levels occur approximately 2-3 hours after dosing.

Lanthanum activates calcium-sensing receptor and enhances sensitivity to calcium

BACKGROUND

The aim of this study was to investigate whether nanomolar concentrations of lanthanum could influence the calcium-sensing receptor (CaSR) response.

METHODS

Embryonic kidney (HEK-293) cells transiently transfected with the human CaSR were used to test the ability of lanthanum to activate the CaSR, either alone or in combination with calcium. CaSR activation was measured by flow cytometry. Parathyroid glands from 4-month-old male Wistar rats with normal renal function (n = 60) were also cultured ex vivo with different concentrations of lanthanum to measure parathyroid hormone (PTH) secreted to the medium and PTH mRNA.

RESULTS

The maximal CaSR activation induced by 1 muM lanthanum chloride (LaCl(3)) was similar to that induced by 16 mM calcium chloride (CaCl(2) 16 mM: 294 +/- 14%; LaCl(3) 1 muM: 303 +/- 11%). Lanthanum half effective concentration (EC(50)) was 77.28 nM, lower than the 2.30 mM obtained for calcium, supporting the concept that this metal is a strong agonist of the CaSR. Moreover, lanthanum was also able to enhance CaSR sensitivity to calcium. The presence of 1 nM LaCl(3) significantly left-shifted the CaSR response curve, changing the EC(50) value for calcium from 2.30 mM (calcium alone) to 1.26 mM (calcium + 1 nM lanthanum). The parathyroid glands cultured with lanthanum showed a trend to secrete less PTH compared to the control glands: 1.51 +/- 0.23 (control), 0.91 +/- 0.17 (La 100 nM) and 1.04 +/- 0.18 (La 400 nM) [(pg/h)/(pg/h), mean +/- SEM] (ANOVA P = 0.0145). A similar trend was also observed in PTH synthesis measured by PTH mRNA levels.

CONCLUSIONS

These in vitro findings demonstrate that lanthanum, at nanomolar concentrations, is an agonist of the CaSR able to activate it in the absence of calcium. In addition, it can also enhance CaSR sensitivity to calcium, modulating PTH synthesis and secretion.

Parathyroid hormone, a uremic toxin

Despite the innovations in the treatment of secondary hyperparathyroidism, there are uremic patients with marked elevation in PTH levels. Uremic toxicity is in part attributable to the excess of circulating PTH. It has been known for many years that PTH may induce changes in cell calcium, a key intracellular signal required for normal cell function. The effect of PTH in dialysis patients is not limited to bone; the diversity of biologic effects of PTH is summarized in this review. In addition, the present review addresses other issues: (i) the presence of different circulating PTH fragments in uremic patients, (ii) the PTH assays currently utilized to measure circulating PTH, and (iii) the fact that some of the PTH effects seen in uremic patients may be due to the interaction of C-terminal PTH fragment with putative C-terminal PTH receptors.

Involvement of the calcium-sensing receptor in human taste perception

By human sensory analyses, we found that various extracellular calcium-sensing receptor (CaSR) agonists enhance sweet, salty, and umami tastes, although they have no taste themselves. These characteristics are known as "kokumi taste" and often appear in traditional Japanese cuisine. Although GSH is a typical kokumi taste substance (taste enhancer), its mode of action is poorly understood. Here, we demonstrate how the kokumi taste is enhanced by the CaSR, a close relative of the class C G-protein-coupled receptors T1R1, T1R2, and T1R3 (sweet and umami receptors). We identified a large number of CaSR agonist gamma-glutamyl peptides, including GSH (gamma-Glu-Cys-Gly) and gamma-Glu-Val-Gly, and showed that these peptides elicit the kokumi taste. Further analyses revealed that some known CaSR agonists such as Ca(2+), protamine, polylysine, L-histidine, and cinacalcet (a calcium-mimetic drug) also elicit the kokumi taste and that the CaSR-specific antagonist, NPS-2143, significantly suppresses the kokumi taste. This is the first report indicating a distinct function of the CaSR in human taste perception.

Calcimimetics in the chronic kidney disease-mineral and bone disorder

Mineral and bone disorders (MBD) are both an early and very common complication of chronic kidney disease (CKD). It is now accepted that they represent a significant risk factor, explaining the high cardiovascular morbidity and mortality in CKD patients. During the last decade, we have been witnessing many advances in the nomenclature, classification, pathophysiology, diagnosis, and treatment of CKD and some of its complications, such as CKD-MBD. The identification of the calcium-sensing receptor (CaSR) involvement in the pathogenesis of primary and secondary hyperparathyroidism (SHPT) and the availability of a new class of drugs called calcimimetics are two outstanding examples. Cinacalcet, the only available calcimimetic, has been shown to be a very effective therapeutic tool in CKD-MBD. Many clinical trials with cinacalcet in hemodialysis patients with SHPT have shown a reduction in parathyroid hormone, calcium (Ca), phosphate (P) and Ca x P product levels, allowing far greater success in reaching therapeutic goals as recommended by international guidelines. Additionally, some studies have shown that the use of cinacalcet may improve other aspects of CKD-MBD, reducing the risk of vascular calcification and parathyroidectomy, among others. Prospective studies on dialysis patients, with hard endpoint data, are currently underway. This review summarizes the most significant aspects of calcimimimetics based on both experimental and clinical results, underlining their possibilities not only for the treatment of isolated SHPT but also for other CKD-MBD related conditions.

Novel regulatory aspects of the extracellular Ca2+-sensing receptor, CaR

The capacity to sense and adapt to changes in environmental cues is of paramount importance for every living organism. From yeast to man, cells must be able to match cellular activities to growth environment and nutrient availability. Key to this process is the development of membrane-bound systems that can detect modifications in the extracellular environment and to translate these into biological responses. Evidence gathered over the last 15 years has demonstrated that many of these cell surface "sensors" belong to the G protein-coupled receptor superfamily. Crucial to our understanding of nutrient sensing in mammalian species has been the identification of the extracellular Ca(2+)/cation-sensing receptor, CaR. CaR was the first ion-sensing molecule identified in man and genetic studies in humans have revealed the importance of the CaR in mineral ion metabolism. Latter, it has become apparent that the CaR also plays an important role outside the Ca(2+) homeostatic system, as an integrator of multiple environmental signals for the regulation of many vital cellular processes, from cell-to-cell communication to secretion and cell survival/cell death. Recently, novel aspects of receptor function reveal an unexpected role for the CaR in the regulation of growth and development in utero.

The calcimimetic AMG 641 abrogates parathyroid hyperplasia, bone and vascular calcification abnormalities in uremic rats

Calcimimetics and vitamin D sterols reduce serum parathyroid hormone (PTH) in patients with secondary hyperparathyroidism receiving dialysis, a disease state associated with parathyroid hyperplasia, vascular calcification, bone disease, and increased mortality. The aim of this study was to determine the effects of the research calcimimetic AMG 641 (Amgen, Inc., Thousand Oaks, CA) or calcitriol (Sigma Aldrich Corporation, St. Louis, MO) on vascular calcification in a rodent model of progressive uremia with accompanying secondary hyperparathyroidism induced by dietary adenine. Treatment effects on parathyroid gland hyperplasia and bone loss were also investigated. Rats were treated daily with vehicle, calcitriol (10 ng), AMG 641 (3 mg/kg), or no treatment during the 4 week period the animals were fed adenine. The uremia-induced increases in serum PTH levels were significantly attenuated by both AMG 641 (>90%) and calcitriol (approximately 50%). AMG 641 significantly reduced calcium-phosphorus product (CaxP) and significantly attenuated the development of both parathyroid hyperplasia and vascular calcification. In addition, AMG 641 prevented the defects in trabecular bone volume, trabecular number, and bone mineralization, as well as increases in trabecular spacing in this rodent model of secondary hyperparathyroidism. Calcitriol (10 ng/rat) decreased osteoid surface/bone surface, but had no effects on other bone parameters, or parathyroid hyperplasia (likely due to the lower PTH suppressive effect of calcitriol at the dose used in this study). However, this dose of calcitriol significantly exacerbated vascular calcification. These results suggest that calcimimetics can reduce the development of vascular calcification, parathyroid hyperplasia and bone abnormalities associated with secondary hyperparathyroidism.

The calcium-sensing receptor regulates parathyroid hormone gene expression in transfected HEK293 cells

Background

The parathyroid calcium receptor determines parathyroid hormone secretion and the response of parathyroid hormone gene expression to serum Ca²⁺ in the parathyroid gland. Serum Ca²⁺ regulates parathyroid hormone gene expression in vivo post-transcriptionally affecting parathyroid hormone mRNA stability through the interaction of trans-acting proteins to a defined cis element in the parathyroid hormone mRNA 3'-untranslated region. These parathyroid hormone mRNA binding proteins include AUF1 which stabilizes and KSRP which destabilizes the parathyroid hormone mRNA. There is no parathyroid cell line; therefore, we developed a parathyroid engineered cell using expression vectors for the full-length human parathyroid hormone gene and the human calcium receptor.

Results

Co-transfection of the human calcium receptor and the human parathyroid hormone plasmid into HEK293 cells decreased parathyroid hormone mRNA levels and secreted parathyroid hormone compared with cells that do not express the calcium receptor. The decreased parathyroid hormone mRNA correlated with decreased parathyroid hormone mRNA stability in vitro, which was dependent upon the 3'-UTR cis element. Moreover, parathyroid hormone gene expression was regulated by Ca²⁺ and the calcimimetic R568, in cells co-transfected with the calcium receptor but not in cells without the calcium receptor. RNA immunoprecipitation analysis in calcium receptor-transfected cells showed increased KSRP-parathyroid hormone mRNA binding and decreased binding to AUF1. The calcium receptor led to post-translational modifications in AUF1 as occurs in the parathyroid in vivo after activation of the calcium receptor.

Conclusion

The expression of the calcium receptor is sufficient to confer the regulation of parathyroid hormone gene expression to these heterologous cells. The calcium receptor decreases parathyroid hormone gene expression in these engineered cells through the parathyroid hormone mRNA 3'-UTR cis element and the balanced interactions of the trans-acting factors KSRP and AUF1 with parathyroid hormone mRNA, as in vivo in the parathyroid. This is the first demonstration that the calcium receptor can regulate parathyroid hormone gene expression in heterologous cells.

Stimulating parathyroid cell proliferation and PTH release with phosphate in organ cultures obtained from patients with primary and secondary hyperparathyroidism for a prolonged period

The pathogenesis of primary hyperparathyroidism (I degrees -HPT) and secondary hyperparathyroidism (II degrees -HPT) remains to be elucidated. To characterize their pathophysiology, we investigated the effects of calcium and phosphate on cell proliferation and PTH release in an organ culture of parathyroid tissues. Dissected parathyroid tissues obtained from patients with I degrees -HPT (adenoma) or II degrees -HPT (nodular hyperplasia) were precultured on a collagen-coated membrane for 1-4 week. After changing the medium for one containing various concentrations of phosphate, PTH release and [(3)H]thymidine incorporation were studied. In contrast to dispersed parathyroid cells cultured in a monolayer, calcium decreased PTH release in a concentration-dependent manner in parathyroid tissues. Furthermore, when parathyroid tissues obtained from II degrees -HPT were precultured for 1-4 weeks, PTH release and parathyroid cell proliferation were significantly increased in high-phosphate medium. These phosphate effects were also observed to a lesser extent in parathyroid tissues obtained from I degrees -HPT, but there was no significant difference between I degrees -HPT and II degrees -HPT. Microarray analyses revealed that mRNA levels of PTH, CaSR, and VDR were well preserved, and several growth factors (e.g. TGF-beta1-induced protein) were abundantly expressed in II degrees -HPT. Using organ cultures of hyperparathyroid tissues, in which PTH release and CaSR are well preserved for a prolonged period, we have demonstrated that phosphate stimulates parathyroid cell proliferation not only in II degrees -HPT but also in I degrees -HPT. Although the mechanism responsible for phosphate-induced cell proliferation remains to be elucidated, our in vitro findings suggest that both parathyroid tissues preserve to some extent a physiological response system to hyperphosphatemia as observed in normal parathyroid cells.

Calcium and signal transduction

Cell signaling is an essential process in which a variety of external signals, defined as first messengers, are translated inside the cells into specific responses, which are mediated by a less numerous group of second messengers. The exchange of signals became a necessity when the transition from monocellular to pluricellular life brought with it the division of labor among the cells of the organisms: unicellular organisms do not depend on the mutual exchange of signals, as they essentially only compete with each other for nutrients. Calcium (Ca²⁺) was selected during evolution as second messenger, because its chemistry made it a much more flexible ligand than the other abundant cations in the primordial environment (Na⁺, K⁺, Mg²⁺ ). Ca²⁺ can accept binding sites of irregular geometries and is thus ideally suited to be a carrier of biological information. The Ca²⁺ signal has properties that set it apart from those of all other biological messengers: they will be reviewed in this contribution. Among them, the ambivalent character of the Ca²⁺ signal is the most important: while essential to the viability of the cells, it can also easily become a conveyor of doom.

Does vitamin D receptor and calcium receptor activation therapy play a role in the histopathologic alterations of parathyroid glands in refractory uremic hyperparathyroidism?

BACKGROUND AND OBJECTIVES

Vitamin D receptor activation by vitamin D sterols and calcium-sensing receptor stimulation by cinacalcet are the most powerful treatments of secondary hyperparathyroidism. This study was aimed to assess a possible association between histopathologic changes of parathyroid tissue and treatment modality.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Studies were performed on 82 parathyroids of 22 adult white hemodialysis patients undergoing first parathyroidectomy. The type of hyperplasia and the distribution of chief and oxyphil cells, expressed as oxyphil/chief cell ratio, were assessed. Three groups could be studied according to treatment modality: group A consisted of 6 patients who were treated with cinacalcet, intravenous calcitriol, and phosphate binders; group B consisted of 6 patients who were treated with intravenous calcitriol and phosphate binders, and group C consisted of 10 patients who were treated with phosphate binders alone.

RESULTS

Sixty-eight (82.9%) out of 82 glands removed showed nodular hyperplasia. It was more frequent in groups A and B than in group C. A stepwise forward logistic regression model showed that the probability of nodular hyperplasia was higher in patients who were on calcitriol and/or cinacalcet therapy, in female gender and in patients with a higher body mass index. Oxyphil/chief cell ratio also was significantly different among the three groups. Cinacalcet treatment was the only predictor of this ratio.

CONCLUSIONS

An association was found between calcitriol and/or cinacalcet therapy and a high prevalence of nodular hyperplasia, and between cinacalcet therapy and high oxyphil/chief cell ratio. The meaning of the observed associations remains uncertain.

Extracellular calcium as an integrator of tissue function

The past several decades of research into calcium signaling have focused on intracellular calcium (Ca(i)(2+)), revealing both exquisite spatial and dynamic control of this potent second messenger. Our understanding of Ca(i)(2+) signaling has benefited from the evolution of cell culture methods, development of high affinity fluorescent calcium indicators (both membrane-permeant small molecules and genetically encoded proteins), and high-resolution fluorescence microscopy. As our understanding of single cell calcium dynamics has increased, translational efforts have attempted to push calcium signaling studies back into tissues, organs and whole animals. Emerging results from these more complicated, diffusion-limited systems have begun to define a role for extracellular calcium (Ca(o)(2+)) as an agonist, spurred by the cloning and characterization of a G protein-coupled receptor activated by Ca(o)(2+) (the calcium sensing receptor, CaR). Here, we review the current state-of-the art for measurement of Ca(o)(2+) fluctuations, and the evidence that fluctuations in Ca(o)(2+) can act as primary signals regulating cell function. Current results suggest that Ca(o)(2+) in bone and epidermis may act as a chemotactic homing signal, targeting cells to the appropriate tissue locations prior to initiation of the differentiation program. Ca(i)(2+) signaling-mediated Ca(o)(2+) fluctuations in interstitial spaces may integrate cell signaling responses in multicellular networks through activation of CaR. Appreciation of the importance of Ca(o)(2+) fluctuations in coordinating cell function will likely spur identification of additional, niche-specific Ca(2+) sensors, and provide unique insights into the regulation of multicellular signaling networks.

The role of the calcium-sensing receptor in the pathophysiology of secondary hyperparathyroidism

The calcium-sensing receptor (CaR), a seven-transmembrane domain receptor belonging to the G protein-coupled receptor family, is responsible for calcium-mediated signalling initiated at the surface of parathyroid cells that controls the synthesis and secretion of parathyroid hormone (PTH). Expression of the CaR is downregulated in animal models of uraemia and in patients with secondary hyperparathyroidism (SHPT). Cinacalcet is a type II calcimimetic agent that acts as an allosteric modulator of CaR signalling. It has been shown in clinical studies to improve control of serum PTH levels and in preclinical studies to attenuate SHPT disease progression and parathyroid hyperplasia. Cinacalcet represents the first of this novel class of agents and a major advance in the treatment of SHPT.