Calcium-sensing receptor and calcimimetic agents

Rare genetic disorders that impair parathyroid hormone synthesis, secretion, or bioactivity provide insights into the diagnostic utility of different parathyroid hormone assays

PURPOSE OF REVIEW

Parathyroid hormone (PTH) is the major peptide hormone regulator of blood calcium homeostasis. Abnormal PTH levels can be observed in patients with various congenital and acquired disorders, including chronic kidney disease (CKD). This review will focus on rare human diseases caused by PTH mutations that have provided insights into the regulation of PTH synthesis and secretion as well as the diagnostic utility of different PTH assays.

RECENT FINDINGS

Over the past years, numerous diseases affecting calcium and phosphate homeostasis have been defined at the molecular level that are responsible for reduced or increased serum PTH levels. The underlying genetic mutations impair parathyroid gland development, involve the PTH gene itself, or alter function of the calcium-sensing receptor (CaSR) or its downstream signaling partners that contribute to regulation of PTH synthesis or secretion. Mutations in the pre sequence of the mature PTH peptide can, for instance, impair hormone synthesis or intracellular processing, while amino acid substitutions affecting the secreted PTH(1-84) impair PTH receptor (PTH1R) activation, or cause defective cleavage of the pro-sequence and thus secretion of a pro- PTH with much reduced biological activity. Mutations affecting the secreted hormone can alter detection by different PTH assays, thus requiring detailed knowledge of the utilized diagnostic test.

SUMMARY

Rare diseases affecting PTH synthesis and secretion have offered helpful insights into parathyroid biology and the diagnostic utility of commonly used PTH assays, which may have implications for the interpretation of PTH measurements in more common disorders such as CKD.

Role of calcium-sensing receptor in regulating activation susceptibility of postovulatory aging mouse oocytes

The mechanisms underlying postovulatory oocyte aging (POA) remain largely unknown. The expression of the calcium-sensing receptor (CaSR) in mouse oocytes and its role in POA need to be explored. Our objective was to observe CaSR expression and its role in the susceptibility to activating stimuli (STAS) in POA mouse oocytes. The results showed that, although none of the newly ovulated oocytes were activated, 40% and 94% of the oocytes recovered 19 and 25 h after human chorionic gonadotropin (hCG) injection were activated, respectively, after ethanol treatment. The level of the CaSR functional dimer protein in oocytes increased significantly from 13 to 25 h post hCG. Thus, the CaSR functional dimer level was positively correlated with the STAS of POA oocytes. Aging in vitro with a CaSR antagonist suppressed the elevation of STAS, and cytoplasmic calcium in oocytes recovered 19 h post hCG, whereas aging with a CaSR agonist increased STAS, and cytoplasmic calcium of oocytes recovered 13 h post hCG. Furthermore, the CaSR was more important than the Na-Ca2+ exchanger in regulating oocyte STAS, and T- and L-type calcium channels were inactive in aging oocytes. We conclude that the CaSR is involved in regulating STAS in POA mouse oocytes, and that it is more important than the other calcium channels tested in this connection.

Role of calcium-sensing receptor in regulating spontaneous activation of postovulatory aging rat oocytes

Mechanisms for postovulatory aging (POA) of oocytes and for spontaneous activation (SA) of rat oocytes are largely unknown. Expression of calcium-sensing receptor (CaSR) in rat oocytes and its role in POA remain unexplored. In this study, expression of CaSR in rat oocytes aging for different times was detected by immunofluorescence microscopy, and western blotting and the role of CaSR in POA was determined by observing the effects of regulating its activity on SA susceptibility and cytoplasmic calcium levels. The results showed that CaSR was expressed in rat oocytes. Oocytes recovered 19 h post human chorionic gonadotropin (hCG) injection were more susceptible to SA and expressed more functional CaSR than oocytes recovered 13 h after hCG injection, although both expressed the same level of total CaSR protein. Treatment with CaSR antagonist significantly suppressed cytoplasmic calcium elevation and SA of oocytes. Activation of Na-Ca2+ exchanger with NaCl inhibited SA to a greater extent than suppression of CaSR with NPS-2143, suggesting that calcium sources other than CaSR-controlled channels contributed to the elevation of cytoplasmic calcium. Treatment with T- or L-type calcium channel blockers significantly reduced SA. Suppression of all calcium channels tested reduced SA to minimum. It is concluded that the level of CaSR functional dimer protein, but not that of the total CaSR protein, was positively correlated with the SA susceptibility during POA of rat oocytes confirming that CaSR is involved in POA regulation. Blocking multiple calcium channels might be a better choice for efficient control of SA in rat oocytes.

Hypercalcemia: Pathophysiology, Clinical Signs, and Emergent Treatment

Hypercalcemia is uncommonly encountered in veterinary patients. When it does occur, the effects can be severe, resulting in significant morbidity and mortality if not recognized and addressed in a timely manner. Causes of hypercalcemia are varied and include pituitary-dependent and pituitary-independent causes. A diagnosis of hypercalcemia should be made based on documentation of ionized hypercalcemia. The mainstay of emergency treatment usually involves aggressive IV fluid diuresis, the use of diuretics, and, often, glucocorticoids. The use of bisphosphonates has become increasingly more common in veterinary medicine.

Diffuse parathyroid hormone expression in parathyroid tumors argues against important functional tumor subclones

OBJECTIVE

Primary hyperparathyroidism is usually characterized by a monoclonal parathyroid tumor secreting excess parathyroid hormone (PTH). The main regulator of PTH secretion is calcium and the calcium-PTH set point is shifted in parathyroid tumor cells. We sought to investigate the relationship between tumor PTH and PTH mRNA expression and clinical presentation as well as the regulatory factors including phosphate, vitamin D, and fibroblast growth factor 23.

DESIGN

A total of 154 parathyroid tumors were analyzed by PTH immunohistochemistry and chromogenic in situ hybridization of PTH mRNA. A subset of samples (n = 34) was analyzed using quantitative real-time PCR.

RESULTS

Low tumor PTH mRNA level was significantly associated with low tumor PTH immunoreactivity (P = 0.026), but the two did not correlate with regard to histological distribution within individual tumors. Tumors displaying reduced PTH mRNA levels as compared with normal rim were significantly larger (P = 0.013) and showed higher expression of the calcium-sensing receptor (CASR) (P = 0.046). Weaker tumor PTH mRNA level was significantly associated with higher concentration of circulating 25-hydroxyvitamin D (P = 0.005). No significant correlation was seen between PTH immunoreactivity and patient biochemistry. Tumor weight was strongly associated with circulatory concentrations of calcium and PTH.

CONCLUSIONS

No areas with apparently higher PTH expression were identified, perhaps suggesting that hyper functioning parathyroid tumor subclones should be rare. Circulating 25-hydroxyvitamin D levels may influence tumor PTH expression in vivo. If PTH immunoreactivity reflects the tumor calcium-PTH set point, our data imply that the main determinant of disease severity should be tumor weight.

Management of hypercalcemia

Hypercalcemia is a common metabolic condition encountered in both inpatient and outpatient settings. Physicians can more successfully diagnose and treat patients when armed with a thorough understanding of normal calcium metabolism and how it can be disrupted by disease. In virtually all cases, the cause of an elevated serum calcium concentration can be quickly ascertained through the use of selected diagnostic tests. Effective treatment options can then be implemented.

The PRIMARA study: a prospective, descriptive, observational study to review cinacalcet use in patients with primary hyperparathyroidism in clinical practice

OBJECTIVE

Medical management of primary hyperparathyroidism (PHPT) is important in patients for whom surgery is inappropriate. We aimed to describe clinical profiles of adults with PHPT receiving cinacalcet.

DESIGN

A descriptive, prospective, observational study in hospital and specialist care centres.

METHODS

For patients with PHPT, aged 23-92 years, starting cinacalcet treatment for the first time, information was collected on dosing pattern, biochemistry and adverse drug reactions (ADRs). Initial cinacalcet dosage and subsequent dose changes were at the investigator's discretion.

RESULTS

Of 303 evaluable patients with PHPT, 134 (44%) had symptoms at diagnosis (mostly bone pain (58) or renal stones (50)). Mean albumin-corrected serum calcium (ACSC) at baseline was 11.4 mg/dl (2.9 mmol/l). The reasons for prescribing cinacalcet included: surgery deemed inappropriate (35%), patient declined surgery (28%) and surgery failed or contraindicated (22%). Mean cinacalcet dose was 43.9 mg/day (s.d., 15.8) at treatment start and 51.3 mg/day (31.8) at month 12; 219 (72%) patients completed 12 months treatment. The main reason for cinacalcet discontinuation was parathyroidectomy (40; 13%). At 3, 6 and 12 months from the start of treatment, 63, 69 and 71% of patients, respectively, had an ACSC of ≤10.3 mg/dl vs 9.9% at baseline. Reductions from baseline in ACSC of ≥1 mg/dl were seen in 56, 63 and 60% of patients respectively. ADRs were reported in 81 patients (27%), most commonly nausea. A total of 7.6% of patients discontinued cinacalcet due to ADRs.

CONCLUSIONS

Reductions in calcium levels of ≥1 mg/dl was observed in 60% of patients 12 months after initiation of cinacalcet, without notable safety concerns.

Transcriptional responses of olive flounder (Paralichthys olivaceus) to low temperature

The olive flounder (Paralichthys olivaceus) is an economically important flatfish in marine aquaculture with a broad thermal tolerance ranging from 14 to 23°C. Cold-tolerant flounder that can survive during the winter season at a temperature of less than 14°C might facilitate the understanding of the mechanisms underlying the response to cold stress. In this study, the transcriptional response of flounder to cold stress (0.7±0.05°C) was characterized using RNA sequencing. Transcriptome sequencing was performed using the Illumina MiSeq platform for the cold-tolerant (CT) group, which survived under the cold stress; the cold-sensitive (CS) group, which could barely survive at the low temperature; and control group, which was not subjected to cold treatment. In all, 29,021 unigenes were generated. Compared with the unigene expression profile of the control group, 410 unigenes were up-regulated and 255 unigenes were down-regulated in the CT group, whereas 593 unigenes were up-regulated and 289 unigenes were down-regulated in the CS group. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses revealed that signal transduction, lipid metabolism, digestive system, and signaling molecules and interaction were the most highly enriched pathways for the genes that were differentially expressed under cold stress. All these pathways could be assigned to the following four biological functions for flounder that can survive under cold stress: signal response to cold stress, cell repair/regeneration, energy production, and cell membrane construction and fluidity.

Cinacalcet: pharmacological and clinical aspects

The calcium sensing receptor (CaSR) is expressed in cells secreting calcium-regulating hormones, in cells involved in calcium transport and in many other tissues, with an as yet not completely defined role. In parathyroid cells, the CaSR stimulation inhibits parathyroid hormone (PTH) secretion, synthesis and parathyroid cell proliferation. Cinacalcet belongs to calcimimetic type II compounds that can interact with CaSR, increasing its affinity for calcium. Clinical studies have proved cinacalcet to be effective in reducing calcium and PTH levels in primary hyperparathyroidism and in reducing PTH, calcium and phosphate in patients with secondary hyperparathyroidism owing to chronic renal failure, with a relatively safe profile, the only reported adverse events being hypocalcaemia and gastrointestinal symptoms. However, though calcimimetics do represent a real advancement in the field of the treatment of PTH secretion disturbances, there is a need for clinical trials, which should aim to demonstrate that a better control of biochemical parameters is also matched with better clinical outcomes.

Cinacalcet for the treatment of primary hyperparathyroidism

Primary hyperparathyroidism (HPT) is the leading cause of hypercalcemia in the outpatient setting, and it is treated primarily by parathyroidectomy. There are few nonsurgical treatment options for patients who do not wish to have surgery, who have failed surgery, or who have contraindications to surgery. Cinacalcet increases the sensitivity of parathyroid calcium-sensing receptors to extracellular calcium, thereby reducing serum calcium levels. We conducted a retrospective chart review from 2004 to 2006 to investigate the efficacy of cinacalcet in reducing serum total calcium, ionized calcium, and parathyroid hormone (PTH) in patients with primary HPT. Patients were started on cinacalcet if they met at least one indication for parathyroidectomy, which includes T score less than -2.5 standard deviations from the mean, serum calcium 1 mg/dL above the upper limit of normal, 24-hour urine calcium above 400 mg/dL, age less than 50 years, or a creatinine clearance that is 30% below age- and sex-matched controls. The primary outcome was normalization of serum calcium. A total of 18 patients with primary HPT were started on cinacalcet: 16 men and 2 women with a mean age of 70 years. Mean baseline serum calcium was 10.60 +/- .53 mg/dL; ionized serum calcium, 1.45 +/- .07 mmol/L; and serum PTH, 141 +/- 78 pg/mL. After treatment with cinacalcet, the mean serum calcium decreased to 9.46 +/- .34 mg/dL, ionized calcium decreased to 1.26 +/- .06 mmol/L, and PTH decreased to 108 +/- 64.5 pg/mL. Ninety-four percent of the patients on cinacalcet had normal total serum calcium, 81% had normal serum ionized calcium, whereas only 25% had a normal serum PTH level. Cinacalcet normalizes serum calcium in most patients while only modestly reducing serum PTH levels.

Alanine to serine polymorphism at position 986 of the calcium-sensing receptor associated with coronary heart disease, myocardial infarction, all-cause, and cardiovascular mortality

BACKGROUND

Disorders of calcium homeostasis have been implicated in atherosclerosis. The calcium-sensing receptor (CASR) is crucial to the regulation of calcium metabolism. An alanine (A) to serine (S) polymorphism at codon 986 (A986S) of the CASR gene has been associated with higher calcium and osteoporosis; the association with coronary artery disease (CAD) has not been studied.

METHODS AND RESULTS

We investigated this polymorphism in individuals with CAD (n = 2561), including survivors of myocardial infarction (MI) (n = 1358) compared to 698 controls without angiographic CAD. Compared to AA homozygotes, the prevalence of CAD [multivariate odds ratio 1.25; 95% confidence interval (CI) 1.02-1.54] and previous MI (multivariate odds ratio 1.33; 95% CI 1.06-1.68) was increased in carriers of at least one S-allele. With each S-allele, the prevalence of CAD and MI increased 1.22-fold (95% CI 1.02-1.47) and 1.30-fold (95% CI 1.06-1.60), respectively. Fully adjusted hazard ratios for total and cardiovascular mortality per one S-allele were 1.24 (95% CI 1.05-1.46) and 1.38 (95% CI 1.13-1.67), respectively. In carriers of at least one S-allele, the adjusted hazard ratios for all-cause and cardiovascular death were 1.25 (95% CI 1.04-1.51) and 1.48 (95% CI 1.18-1.86), respectively. These associations were independent of cardiovascular risk factors, calcium and phosphate. The S-allele was associated with higher calcium (P < 0.001) and PTH (P < 0.02), and lower phosphate (P < 0.003) in CAD patients and controls.

CONCLUSION

Serine at position 986 of CASR may be an independent genetic predictor of angiographic CAD, previous MI, and cardiovascular mortality.

Cinacalcet chloride is efficient and safe in renal transplant recipients with posttransplant hyperparathyroidism

BACKGROUND

Persistent hyperparathyroidism (HPT) is observed in approximately 50% of kidney transplant recipients one year after transplantation. It may result in hypercalcemia, hypophosphatemia, bone demineralization, vascular calcification, lithiasis, and participate in chronic allograft nephropathy. We evaluated the use of the calcimimetic cinacalcet chloride to correct chronic hypercalcemia in posttransplant HPT, in a prospective single-center study.

METHODS

Nine patients with persistent hypercalcemia (>2.6 mmol/L) and stable graft function were treated with cinacalcet (30 mg/day, thereafter adapted to obtain normal serum Ca levels) for six months. Their immunosuppressive schedule included mycophenolate mofetil (MMF), steroids, and cyclosporine A (4), tacrolimus (4), or sirolimus (2).

RESULTS

Serum Ca levels significantly decreased from 2.75+/-0.15 to 2.59+/-0.10, 2.42+/-0.29 and 2.44+/-0.25 mmol/L by one, two, and six months, respectively (P<0.02, Wilcoxon test for paired data, for all the data points). Parathyroid hormone (PTH) serum levels decreased from 171+/-102 to 134+/-63 pg/ml by two months (P<0.05) and stabilized thereafter (148+/-99 pg/ml at six months; NS). No changes in glomerular filtration rate (49.8+/-18.6 and 51.3+/-19 ml/min at initiation and six months, respectively) and no variation in serum concentration of the immunosuppressive drugs were observed. Three patients withdrew the treatment because gastrointestinal intolerance.

CONCLUSION

Cinacalcet allows the correction of hypercalcemia with no interference in immunosuppressive treatment or renal function. However, whether the increased intolerance observed was due to the association of cinacalcet chloride with other drugs required in renal transplantation (e.g., MMF) needs to be assessed.

Complications of chronic kidney disease: anemia, mineral metabolism, and cardiovascular disease

This article focuses on the importance of three major complications of chronic kidney disease: (1) anemia, (2) calcium-phosphorus regulation and bone disease, and (3) cardiovascular risk profiling and treatment. The arguments for early and effective intervention have been amply made with respect to these three complications. Substantive trial data are sorely need to provide the definitive evidence that effective treatment of these complications results in better outcomes.

Improved Strategies for the Treatment of Renal Osteodystrophy

Renal osteodystrophy (ROD) encompasses several distinct skeletal complications resulting from the metabolic abnormalities associated with chronic kidney disease (CKD). Other manifestations of altered calcium and phosphorus homeostasis include an increased risk of cardiovascular mortality and morbidity. Hyperphosphatemia and secondary hyperparathyroidism (HPT) are associated with these adverse events, and ultimately can lead to ROD. Dietary restriction of phosphorus, use of phosphate binding agents, and vitamin D therapy have been the mainstay of therapy for HPT. Recently, several new therapeutic agents have become available for the management of HPT. The nonelemental phosphate binder, sevelamer hydrochloride (Renagel®) has provided an alternative to calcium-containing binders for the management of hyperphosphatemia. This agent also decreases calcium load and minimizes the risk of hypercalcemia and skin and soft tissue calcifications. Research in the area of vitamin D receptors has lead to the development of vitamin D analogs including paricalcitol (Zemplar®) and doxercalciferol (Hectorol®) as alternative agents for HPT. Potential benefits of these analogs include a lower risk of hypercalcemia and hyperphosphatemia compared with calcitriol, although further evaluation is warranted as these agents are used more in clinical practice. Calcimimetics are also in the pipeline as potential agents for management of HPT by inhibition of parathyroid hormone secretion. These new developments in the management of metabolic disorders of CKD provide pharmacotherapeutic alternatives to improve patient outcomes and prevent/manage ROD. The challenge is to determine appropriate guidelines for use of these agents at all stages of CKD.

Relationship between the serum parathyroid hormone and magnesium levels in continuous ambulatory peritoneal dialysis (CAPD) patients using low-magnesium peritoneal dialysate

BACKGROUND

Patients on continuous ambulatory peritoneal dialysis (CAPD) have increased risk of low-turnover bone disease and relative hypoparathyroidism. Recently, it has been believed that magnesium plays an important role in regulating secretion of parathyroid hormone (PTH). The aim of this study was to evaluate the relationship between serum PTH and serum magnesium as a factor increasing the frequency of relative hypoparathyroidism.

METHODS

We analyzed the data of 56 patients who had been on CAPD for more than 6 months without any significant problems. No patient had been previously treated with vitamin D or aluminum hydroxide. The patients had used peritoneal dialysate with the magnesium concentration of 0.5 mEq/L. Biochemical parameters, such as BUN, creatinine, alkaline phosphatase bony isoenzyme, total protein, albumin, total calcium, ionized calcium and intact parathyroid hormone level were measured.

RESULTS

The mean serum magnesium level was 1.99 +/- 0.36 mEq/L. Among total 56 patients, 15 patients (26.8%) showed hypermagnesemia (serum magnesium > 2.2 mEq/L) and 5 patients (8.9%) showed hypomagnesemia (serum magnesium < 1.6 mEq/L). Among all 56 patients, serum iPTH (intact PTH) level was not correlated with serum magnesium level. However, it was inversely correlated with serum total calcium and ionized calcium levels, respectively (r = -0.365, p = 0.006; r = -0.515 p < 0.001). Among 49 patients whose serum iPTH level was less than 300 pg/mL, serum iPTH level was inversely correlated with serum magnesium level (r = -0.295, p = 0.039) and inversely correlated with serum total calcium and ionized calcium levels, respectively (r = -0.546, p < 0.001; r = -0.572 p < 0.001). Among 49 patients whose serum iPTH level was less than 300 pg/mL, lower iPTH group (serum iPTH < 120 pg/mL) showed higher serum magnesium level (p = 0.037), higher serum total calcium level (p < 0.001) and lower bone isoenzyme of alkaline phosphatase level (p < 0.001) than those of higher iPTH group (120 pg/mL < or = serum iPTH < 300 pg/mL).

CONCLUSION

Among the CAPD patients whose serum iPTH level was less than 300 pg/mL, there was a significant inverse correlation between serum iPTH level and serum magnesium level. This study indicates that not only serum calcium level but also serum magnesium level are important in the regulation of serum iPTH levels of CAPD patients who have been dialyzed by low-magnesium peritoneal dialysate.

Calciphylaxis is usually non-ulcerating: risk factors, outcome and therapy

BACKGROUND

Calciphylaxis, historically considered rare, seems to be increasing in frequency. In our single center, 36 new cases have accumulated in seven years. The majority of these cases were non-ulcerating, which we believe to be early disease, in contradistinction to the vast majority of published cases that presented with ulcers.

METHODS

Prospective data were collected on all patients with calciphylaxis. As well, a case control study, with two controls per patient, was performed on patients presenting with non-ulcerating plaques.

RESULTS

The incidence of calciphylaxis in dialysis patients increased with a rate of 4.5/100 patient-years in the past three years. Eighty percent of cases presented with non-ulcerating subcutaneous plaques in the calves, easily confused with cellulitis. In those patients presenting with plaques only, the mortality rate was 33% at six months. Once ulceration develops, the mortality rate increased to above 80%. Bone scan was positive in 97% of patients. Steroid therapy appeared to be beneficial in some patients. Peritoneal dialysis, female sex and diabetes were risk factors. In the case control study of patients presenting with plaques only, serum phosphate (OR 2.6; 95% CI 1.05 to 6.45, P = 0.038) and Ca x P product (OR 1.46; 95% CI 1.02 to 20, P = 0.038) predicted the disease, as did being on calcium salts + vitamin D (OR 4.05; 95% CI 1.14 to 14.5, P = 0.03).

CONCLUSIONS

Calciphylaxis is no longer rare. It is usually nonulcerating and can be diagnosed clinically in all patients. These patients have a high mortality, especially once ulceration occurs. Calcium salts plus vitamin D, as well as serum Ca x P product and high serum P increase the chance of the diseases. Therefore, the disease may be preventable. Steroids may be of benefit to some patients.

Pathophysiological mechanisms of vascular calcification in end-stage renal disease

Vascular calcification has been clearly defined as a risk factor for cardiovascular mortality in the general population and is highly prevalent in end-stage renal disease (ESRD), where it is associated with a number of markers of increased mortality such as left ventricular hypertrophy. The pattern of calcification in ESRD is characterized by mineral deposition in the tunica media, in contrast to non-ESRD populations, where calcification of atheromatous plaque predominates. This difference may have important clinical implications. The pathophysiological mechanisms underlying both types of vascular calcification remain to be clarified; however, current evidence suggests that they are active processes rather than passive mineral precipitation, and the presence in the vasculature of cells expressing an osteoblastic phenotype may be of central importance. In ESRD, the presence of secondary and tertiary hyperparathyroidism, disordered calcium and phosphate homeostasis, and the use of vitamin D- and calcium-based treatments in its therapy may all contribute to vascular calcification. These issues and the impact on other current and future therapies have great importance for clinical nephrology, and a better understanding of vascular calcification through a focused research effort is essential.

Prevention of uremic bone disease using calcimimetic compounds

The discovery, characterization, and cloning of the calcium-sensing receptor (CaR) in 1993 was soon followed by the creation of a new type of drug, the calcimimetics-NPS R-568 and NPS R-467-which are small phenylalkylamine derivative compounds that act as CaR agonists and increase the sensitivity of the CaR to activation by extracellular calcium (Ca2+). As expected, these compounds turned out to have a significant effect on the Ca2+/parathyroid hormone (PTH) relationship, resulting in a dramatically greater suppression of the PTH level than would otherwise occur at the actual extracellular Ca2+ levels. Renal osteodystrophy (RO) due to secondary hyperparathyroidism (HPT) in chronic renal failure was an obvious target for studying the effects of NPS R-568. In a study on experimental animals, the results clearly showed that this first generation of calcimimetics, NPS R-568, had an acute dose-dependent and short-lived suppressive effect on PTH secretion from the parathyroid glands. A similar effect was found in patients with chronic renal failure and secondary HPT. At the same time, the calcimimetics induced a slight degree of hypocalcemia. Such a significant suppressive effect on PTH secretion would be expected to result in therapeutic potential for a preventive or therapeutic effect on the RO accompanying chronic uremia. Administration would probably be in close concert with present strategies, phosphate binders and vitamin D analogs. A wide distribution of CaRs have now been demonstrated in the body, and an important question is how calcimimetics will affect the function of different tissues and organs when used for long-term treatment or prevention of secondary HPT and RO. Although relatively few experimental and clinical investigations have been completed, they clearly confirm the suppressive effect of calcimimetics on PTH secretion. In rats with experimental chronic renal failure, a significant and beneficial effect on the prevention of RO has been demonstrated. The effect of calcimimetic compounds is presently being evaluated in humans. Besides induction of hypocalcemia, the adverse effects in these mainly short-term studies have been few. Future studies with calcimimetics will further define the physiology and pathophysiology of the CaR and the long-term benefit of calcimimetic compounds in patients with chronic renal failure.

Is there an optimal parathyroid hormone level in end-stage renal failure: the lower the better?

Skeletal resistance to parathyroid hormone is well defined in patients with chronic renal failure. In recent years, with the increased frequency of development of adynamic bone disease, it has been recognized that secondary hyperparathyroidism must exist as a 'trade off' mechanism to maintain skeletal bone remodeling in this patient population. An optimal level of intact parathyroid hormone to maintain the normal skeletal bone turnover is believed to be between 2.0 and 2.5 times the upper limit of normal parathyroid hormone. It has very recently been argued that the optimal parathyroid hormone level for maintenance of skeletal bone remodeling may be insufficient to prevent the extraskeletal complications of coronary artery calcifications, calcific valvular heart disease, and cardiac death. To provide optimal health care for these patients several new treatments have been developed, including use of new vitamin D analogs, calcimimetic agents, and noncalcium-based phosphorus binders. It is anticipated that with lower suppression of parathyroid hormone by these vitamin D analogs, intermittent suppression of parathyroid hormone with calcimimetic agents, and the use of noncalcium phosphorus binders (Renageltrade) by regulating serum calcium, the resultant phosphorus concentrations will provide an optimal parathyroid hormone activity to maintain skeletal bone remodeling, while preventing extraskeletal complications.

Regulation of bone remodeling: impact of novel therapies

The kidneys serve as both an endocrine organ and as a target of endocrine action, with the aim of controlling mineral and water balance. Hormones and other key metabolites regulate mineral homeostasis by altering gene function directly or by initiating a sequence of events, leading ultimately to a change in enzyme function. Two of these hormones, parathyroid hormone (PTH) and calcitriol (the active form of vitamin D), interact in multiple tissues in the body to regulate the flux of calcium and phosphorus between extra- and intracellular compartments. Changes in the concentration of PTH and vitamin D, or the interaction of these with other factors, lead to the aberrant regulation of calcium and phosphorus. Among other effects, this aberrant regulation leads to pathologic changes in bone metabolism. The pathology of renal bone disease varies along a spectrum from disorders of low turnover to those of high turnover. This spectrum reflects the results of therapeutic intervention, hormone balances, and other causes. Effective management of renal bone disease therefore requires thorough evaluation of relevant risk factors, measurement of biochemical markers of bone remodeling, and determination of the physical status of bone tissue either by bone mineral density or bone biopsy. Subsequent therapeutic intervention with newer vitamin D compounds, novel phosphate binders, calcimimetics, and the use of alternative dialysis modalities offer hope in normalizing bone remodeling and mineral balance. The human skeleton functions in two capacities: the storage of minerals and structural support of the body. It is the only tissue that behaves as both a major source and a sink of calcium (Ca) and phosphorus (P). Healthy bone is a composite of a collagenous matrix embedded with crystals of hydroxyapatite. On the surface of bone and within the calcified matrix are specialized cells that build and maintain the tissue, and facilitate the movement of Ca and P into and out of serum. Bone undergoes remodeling in response to either damage from mechanical strain or as part of the normal cycle of bone renewal. The process involves distinct steps of cellular activation, bone resorption, and subsequent bone formation. It is a relatively slow process that takes several months, and at any one time occurs at many different sites along the bone surface. Systemic factors, such as PTH and vitamin D, regulate the resorption and formation of bone, and thus the systemic movement of Ca and P. However, during conditions of stress and disease, other factors may also play a role. In patients with chronic renal disease, the balance of Ca and P is profoundly disturbed. This disruption and the compensatory changes that occur in response alter the normal processes of bone metabolism.

Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia

The calcium-sensing receptor (CASR) is a plasma membrane G protein coupled receptor that is expressed in the parathyroid hormone (PTH) producing chief cells of the parathyroid gland and the cells lining the kidney tubule. By virtue of its ability to sense small changes in circulating calcium concentration ([Ca(2+)](o)) and to couple this information to intracellular signaling pathways that modify PTH secretion or renal cation handling, the CASR plays an essential role in maintaining mineral ion homeostasis. Inherited abnormalities of the CASR gene located on chromosome 3p13.3-21 can cause either hypercalcemia or hypocalcemia depending upon whether they are inactivating or activating, respectively. Heterozygous loss-of-function mutations give rise to familial (benign) hypocalciuric hypercalcemia (FHH) in which the lifelong hypercalcemia is asymptomatic. The homozygous condition manifests itself as neonatal severe hyperparathyroidism (NSHPT), a rare disorder characterized by extreme hypercalcemia and the bony changes of hyperparathyroidism which occur in infancy. The disorder autosomal dominant hypocalcemia (ADH) is due to gain-of-function mutations in the CASR gene. ADH may be asymptomatic or present with neonatal or childhood seizures. A common polymorphism in the intracellular tail of the CASR, Ala to Ser at position 986, has a modest effect on the serum calcium concentration in healthy individuals.