Calcium-receptor-regulated parathyroid and renal function

Autosomal dominant hypercalciuric hypocalcaemia: the calcium-sensing receptor in renal calcium homeostasis and the impact of renal transplantation

Calcium-sensing receptors (CaSRs) are G protein-coupled receptors that help maintain Ca2+ concentrations, modulating calciotropic hormone release (parathyroid hormone (PTH), calcitonin and 1,25-dihydroxyvitamin D) by direct actions in the kidneys, gastrointestinal tract and bone. Variability in population calcium levels has been attributed to single nucleotide polymorphisms in CaSR genes, and several conditions affecting calcium and phosphate homeostasis have been attributed to gain- or loss-of-function mutations. An example is autosomal dominant hypercalciuric hypocalcaemia, because of a missense mutation at codon 128 of chromosome 3, as reported in our specific case and her family. As a consequence of treating symptomatic hypocalcaemia as a child, this female subject slowly developed progressive end-stage kidney failure because of nephrocalcinosis and nephrolithiasis. After kidney transplantation, she remains asymptomatic, with decreased vitamin D and elemental calcium requirements, stable fluid and electrolyte homeostasis during intercurrent illnesses and has normalised urinary calcium and phosphate excretion, reducing the likelihood of hypercalciuria-induced graft impairment. We review the actions of the CaSR, its role in regulating renal Ca2+ homeostasis along with the impact of a proven gain-of-function mutation in the CaSR gene resulting in autosomal dominant hypercalciuric hypocalcaemia before and after kidney transplantation.

Radiofrequency Ablation of Parathyroid Glands to Treat a Patient With Hypercalcemia Caused by a Novel Inactivating Mutation in CaSR

OBJECTIVE

We identified a novel inactivating mutation in the calcium-sensing receptor (CaSR) gene in a patient with refractory hypocalciuric hypercalcemia and analyzed its function. The effectiveness of radiofrequency ablation of the parathyroid glands to treat hypercalcemia caused by this mutation was explored.

METHODS

Clinical data of patients before and after radiofrequency ablation were retrospectively analyzed. The CaSR mutation (D99N) found in the patient was studied in cell lines. HEK-293 cells were transfected with plasmids containing wild-type (WT) or mutant CaSR genes (D99N and W718X). Expression levels of the respective CaSR proteins were measured, and their functions were assessed by examining the effect of NPS R-568 (a CaSR agonist) on intracellular Ca2+ oscillations and that of exogenous parathyroid hormone (PTH) on intracellular cyclic adenosine monophosphate (cAMP) levels.

RESULTS

The effectiveness of pharmacological treatment was poor, whereas radiofrequency ablation of the parathyroid glands resulted in controlled blood calcium and PTH levels in the patient. In cell lines, upon NPS R-568 administration, the amplitude of intracellular Ca2+ oscillations in the D99N group was lower than that in the WT group and higher than that in the W718X group. Upon administration of PTH, intracellular cAMP levels in the D99N group were higher than those in the WT group and lower than those in the W718X group.

CONCLUSION

The homozygous mutation D99N reduced CaSR activity and caused more severe hypocalciuric hypercalcemia. For patients with this type of hypercalcemia and poor response to pharmacological treatments, radiofrequency ablation of the parathyroid glands may be a suitable treatment option.

Effects of continuous and intermittent parathyroid hormone administration on midpalatal suture expansion in rats

OBJECTIVES

The aim of this study was to investigate the effects of continuous parathyroid hormone (cPTH) and intermittent parathyroid hormone (iPTH) on bone formation and bone resorption in midpalatal suture during maxillary expansion.

METHODS

Forty-eight male SD rats were randomly divided into four groups (n = 12 each), including the control, the expansion (E), the E + cPTH, and the E + iPTH. A thermosensitive controlled-release hydrogel was synthesized for cPTH administration. All animals were sacrificed after seven days. Microcomputed tomography, histochemical staining and real-time PCR were used to investigate the bone remodeling of midpalatal suture. Serum chemistry was adopted to evaluate the systemic condition of experimental animals.

RESULTS

The suture width was increased by the expansion, and further elevated by cPTH and iPTH administration. Both regimes improved bone volume fraction and trabecular thickness of suture bone region. Moreover, both cPTH and iPTH decreased SOST expression and enhanced the expression of β-catenin and Col-I. cPTH increased RANKL expression, inhibited OPG expression, and resulted in an increment of osteoclasts, while iPTH had no influence on osteoclastogenesis. The serum calcium concentration was enhanced by PTH administration.

CONCLUSION

Both cPTH and iPTH promote midpalatal suture expansion by enhancing bone formation, probably via SOST downregulation and the resulting β-catenin activation. Our results demonstrated that PTH administration may have potential to be an adjunctive approach for maxillary expansion treatment.

Gestational Gigantomastia Complicated by PTHrP-Mediated Hypercalcemia

CONTEXT

Gestational gigantomastia is an uncommon condition characterized by abnormal and excessive growth of breast tissue during an otherwise uncomplicated pregnancy. Gestational gigantomastia may be accompanied by hypercalcemia, which in some cases has been associated with elevated serum levels of PTHrP. The source of the PTHrP in these cases has been suggested to be the enlarged breasts.

OBJECTIVE

To describe the rapid resolution of hypercalcemia and normalization of serum PTHrP after elective termination of pregnancy, indicating that the placenta was the source of the PTHrP.

DESIGN

A retrospective analysis of clinical and biochemical data over a 2-year interval and review of literature.

SETTING

An academic medical center.

PATIENT

A 33-year-old G8P4 female who presented at week 8 of pregnancy with gestational gigantomastia and subsequently developed marked hypercalcemia at week 13. Serum levels of PTH were suppressed but circulating PTHrP was elevated. There was no history of hypercalcemia or significant breast growth during previous pregnancies.

INTERVENTION

Hypercalcemia was poorly responsive to IV saline, prednisone, calcitonin, and cinacalcet. She requested termination of pregnancy at week 20.

RESULTS

Serum levels of calcium, PTH, and PTHrP normalized within 48 hours of termination of pregnancy.

CONCLUSION

The rapid resolution of hypercalcemia after termination of pregnancy, despite persistent gigantomastia, provides evidence for a pathologic role of the placenta in the excess production of PTHrP, possibly through an as yet uncharacterized placenta-breast hormonal axis.

Bone-eating kidney disease

In the current era of early detection of chronic kidney disease and efficient therapeutic options for management of its complications, skeletal manifestations of renal hyperparathyroidism are increasingly rare. A 31-year-old female patient presented for evaluation of severe pain in the left forearm, right hand, right knee, right hip, and lower back following a fall sustained 3 days prior to presentation. She had a history of end-stage renal disease and received maintenance hemodialysis. Review of the medical records revealed that she had poor compliance with her diet, medications, and dialysis treatments. Laboratory values were significant for marked elevation in serum parathyroid hormone level (1735 pg/mL), as well as hyperphosphatemia and normal serum calcium levels. Interestingly, X-ray images showed generalized severe demineralization of the extremities with the pelvic computed tomography scan revealing presence of diffuse brown tumors. In addition, she had insufficiency fractures of the extremities that left her incapacitated and had to be managed conservatively due to her poor functional status and ongoing severe hyperparathyroidism. Unfortunately, the patient refused surgical removal of the parathyroid glands and was hence treated with a high-dose phosphate binder and a calcimimetic agent together with reinforcement of compliance. This case highlights the importance of metabolic assessment of patients presenting with unexpected bone complications and can be used to raise awareness of the physicians on the extreme cases of mineral bone complications secondary to renal disease that are observed rarely.

Use of Cinacalcet and 99mTc-sestamibi Imaging During Pregnancy

A case report is reviewed, in which two nontraditional interventions were used in a patient with gestational primary hyperparathyroidism: cinacalcet and technetium-99m methoxyisobutylisonitrile imaging. The rationale for these decisions is considered in view of the available data.

Polymorphisms in CaSR and CLDN14 Genes Associated with Increased Risk of Kidney Stone Disease in Patients from the Eastern Part of India

Kidney stone disease (KSD) is a major clinical problem imposing a large burden for both healthcare and economy globally. In India, the prevalence of kidney stone disease is rapidly increasing. This study aimed to evaluate the association between genetic defects in vitamin D receptor (VDR), calcium sensing receptor (CaSR) and claudin 14 (CLDN14) genes and kidney stone disease in patients from eastern India. We enrolled 200 consecutive kidney stone patients (age 18-60 years) (cases) and their corresponding sex and age matched 200 normal individuals (controls). To identify genetic variants responsible for KSD, we performed sequence analysis of VDR, CaSR and CLDN14 genes. Four non-synonymous (rs1801725, rs1042636, rs1801726 and rs2228570), one synonymous (rs219780) and three intronic single nucleotide polymorphisms (SNPs) (rs731236, rs219777 and rs219778) were identified. Genotype and allele frequency analysis of these SNPs revealed that, rs1801725 (Ala986Ser), rs1042636 (Arg990Gly) of CaSR gene and rs219778, rs219780 (Thr229Thr) of CLDN14 gene were significantly associated with KSD. Serum calcium levels were significantly higher in subjects carrying 986Ser allele and calcium excretion was higher in subjects bearing 990Gly allele. In conclusion, rs1801725, rs1042636, rs219778 and rs219780 SNPs were associated with kidney stone risk in patients from the eastern part of India.

Autosomal dominant hypocalcemia with Bartter syndrome due to a novel activating mutation of calcium sensing receptor, Y829C

The calcium sensing receptor (CaSR) plays an important role in calcium homeostasis. Activating mutations of CaSR cause autosomal dominant hypocalcemia by affecting parathyroid hormone secretion in parathyroid gland and calcium resorption in kidney. They can also cause a type 5 Bartter syndrome by inhibiting the apical potassium channel in the thick ascending limb of the loop of Henle in the kidney. This study presents a patient who had autosomal dominant hypocalcemia with Bartter syndrome due to an activating mutation Y829C in the transmembrane domain of the CaSR. Symptoms of hypocalcemia occurred 12 days after birth and medication was started immediately. Medullary nephrocalcinosis and basal ganglia calcification were found at 7 years old and at 17 years old. Three hypercalcemic episodes occurred, one at 14 years old and two at 17 years old. The Bartter syndrome was not severe while the serum calcium concentration was controlled, but during hypercalcemic periods, the symptoms of Bartter syndrome were aggravated.

Moderation of hematological and plasma biochemical indices of sub-chronic salt-loaded rats by aqueous extract of the sclerotia of Pleurotus tuberregium (Fr) Sing's: implications for the reduction of cardiovascular risk

ETHNOPHARMACOLOGICAL RELEVANCE

The sclerotia of Pleurotus tuberregium are used in Southern Nigeria for the management of diabetes and hypertension, yet there is scarcity of information in the literature regarding the evaluation of the biochemical basis of its antihypertensive property, as well as the biochemical impact of its administration to the hypertensive. Thus, in this study, the ability of an aqueous extract of the sclerotia of Pleurotus tuberregium to moderate biochemical and hematological indices was investigated in normal and sub-chronic salt-loaded rats.

MATERIALS AND METHODS

The normal and treatment control groups received a diet consisting 100% of the commercial feed, while the test control, reference and test treatment groups received an 8% salt-loaded diet. The extract was orally administered daily at 100 and 200 mg/kg body weight; while the moduretics was administered at 1 mg/kg. The normal and test control groups received appropriate volumes of water by the same route.

RESULTS

On gas chromatographic analysis of the crude aqueous extract, 29 known flavonoids (mainly 47.71% kaempferol and 37.36% quercetin), four saponins (mainly 72.93% avenacin B1 and 26.80% avenacin A1), six hydroxycinnamic acid derivatives (mainly 57.57% p-coumaric and 42.10% caffeic acid), ten carotenoids (mainly 58.44% carotene and 28.16% lycopene) and seven phytosterols (mainly 98.16% sitosterol) were detected. Also detected were nine benzoic acid derivatives (mainly 44.19% ferulic acid and 25.92% rosmarinic acid), six lignans (mainly 70.88% galgravin and 22.69% retusin), three allicins (mainly 71.92% diallyl thiosulphinate and 23.68% methyl allyl thiosulphinate), seven glycosides (mainly 84.86% arbutin and 12.01% ouabain), 31 alkaloids (mainly 48.82% lupanine, 32.26% augustamine) and 24 terpenes (mainly 60.66% limonene and 6.52% geranyl acetate). Compared to test control, the treatment significantly, dose-dependently lowered (P < 0.05) the mean cell volume, atherogenic indices (cardiac risk ratio, atherogenic coefficient and atherogenic index of plasma), plasma alanine and aspartate transaminase activities, mean cell hemoglobin, sodium, bicarbonate, urea, blood urea nitrogen, triglyceride, total-, non-high density lipoprotein-, low density lipoprotein- and very low density lipoprotein cholesterol concentrations, and neutrophils, monocytes and platelets counts of the treated animals. However, it significantly, dose-dependently increased (P < 0.05) the hemoglobin concentration, mean cell hemoglobin, red cells and lymphocytes counts, plasma high density lipoprotein cholesterol, calcium, potassium, chloride, creatinine, albumin and total protein concentrations of the treated animals.

CONCLUSIONS

All these results support the use of the plant in traditional health care, for the management of hypertension, and highlight the cardio-protective potential of the sclerotia, whilst suggesting that its antihypertensive activity may be mediated through alteration of plasma levels of sodium and potassium, or increases in muscle tone brought about by changes in plasma calcium levels.

Surgeons save bones: an algorithm for orthopedic surgeons managing secondary fracture prevention

Postmenopausal osteoporosis has a big impact on health care budget worldwide, which are expected to double by 2050. In spite of severe medical and socioeconomic consequences from fragility fractures, there are insufficient efforts in optimizing osteoporotic treatment and prevention. Undertreatment of osteoporosis is a well known phenomenon, particularly in elderly patients. Treatment rates remain low across virtually all patient, provider, and hospital-level characteristics, even after fragility fractures. In-hospital initiation is one of the options to increase treatment rates and improve osteoporosis management. However, multiple factors contribute to the failure of initiating appropriate treatment of osteoporosis in patients with fragility fractures. These include a lack of knowledge in osteoporosis and an absence of a comprehensive treatment guideline among family physicians and orthopedic surgeons. Furthermore, orthopedic surgeons are hardly willing to accept their responsibility for osteoporosis treatment due to the fact that they are usually not familiar with the initiation of specific drug treatments. The presented algorithm offers trauma surgeons and orthopedic surgeons a safe and simple guided pathway of treating osteoporosis in postmenopausal women appropriately after fragility fractures based on the current literature. From our point of view, this algorithm is useful for almost all cases and the user can expect treatment recommendations in more than 90 % of all cases. Nevertheless, some patients may require specialized review by an endocrinologist. The proposed algorithm may help to increase the rate of appropriate osteoporosis treatment hence reducing the rates of fragility fractures.

Cinacalcet attenuates hypercalcemia observed in mice bearing either Rice H-500 Leydig cell or C26-DCT colon tumors

Excessive secretion of parathyroid hormone-related protein (PTHrP) by tumors stimulates bone resorption and increases renal tubular reabsorption of calcium, resulting in hypercalcemia of malignancy. We investigated the ability of cinacalcet, an allosteric modulator of the calcium-sensing receptor, to attenuate hypercalcemia by assessing its effects on blood ionized calcium, serum PTHrP, and calcium-sensing receptor mRNA in mice bearing either Rice H-500 Leydig cell or C26-DCT colon tumors. Cinacalcet effectively decreased hypercalcemia in a dose- and enantiomer-dependent manner; furthermore, cinacalcet normalized phosphorus levels, but did not affect serum PTHrP. Ribonuclease protection assay results demonstrated presence of PTHrP receptor, but not calcium-sensing receptor mRNA in C26-DCT tumors. The mechanism by which cinacalcet lowered serum calcium was investigated in parathyroidectomized rats (i.e., without PTH) made hypercalcemic by PTHrP. Cinacalcet attenuated PTHrP-mediated elevations in blood ionized calcium, which were accompanied by increased plasma calcitonin. Taken together these results suggest that the cinacalcet-mediated decrease in serum calcium is not the result of a direct effect on tumor cells, but rather is the result of increased calcitonin release. In summary, cinacalcet effectively reduced tumor-mediated hypercalcemia and corrected hypophosphatemia in mice. Further investigation of cinacalcet for treatment of hypercalcemia of malignancy is warranted.

Effect of aqueous extract of the leaves of Acalypha wilkesiana 'Godseffiana' Muell Arg (Euphorbiaceae) on the hematology, plasma biochemistry and ocular indices of oxidative stress in alloxan induced diabetic rats

ETHNOPHARMACOLOGICAL RELEVANCE

The leaves of Acalypha wilkesiana are used in Southern Nigeria for the management of hypertension and diabetes mellitus. In this study, the effect of aqueous extract of the leaves of Acalypha wilkesiana on the hematology, plasma biochemistry and ocular indices of oxidative stress was investigated in alloxan induced diabetic rats.

METHODOLOGY

Diabetes mellitus was induced by injection of alloxan (80 mg/kg body weight), via the tail vein. The extract was administered orally at 100, 200 and 300 mg/kg (both to normal and diabetic rats), and metformin at 50mg/kg.

RESULTS

On gas chromatographic analysis of the extract, twenty nine known flavonoids were detected, consisting mainly of 29.77% apigenin, 14.97% quercetin, 11.12% naringenin, 10.62% kaempferol, 9.05% (-)-epicatechin and 4.04% (+)-catechin. Tannic acid and β-sitosterol were also detected. Compared to test control, the treatment lowered (significantly, P < 0.05) plasma glucose, triglyceride, conjugated bilirubin levels, atherogenic index of plasma, plasma alanine transaminase activity, total protein and total bilirubin, aspartate transaminase activity and unconjugated bilirubin, plasma urea, blood urea nitrogen and ocular malondialdehyde contents, lymphocyte and monocyte counts, and not significantly, plasma very low density lipoprotein cholesterol, but increased (significantly, P < 0.05) plasma calcium contents, total white cell and platelet counts, mean cell volume and ocular ascorbic acid content, and (though not significantly) plasma high density lipoprotein cholesterol level, red cell and neutrophil counts.

CONCLUSIONS

This study showed that the extract was hypoglycemic, positively affected the hemopoietic system and integrity and function (dose dependently) of the liver and kidney of the diabetic rats; improved the lipid profile and had no deleterious effect on red cell morphology. It also protected against oxidative stress in ocular tissues. This study also revealed the presence of pharmacologically active compounds in the leaf extract. All of these highlights the cardioprotective potential of the leaves of Acalypha wilkesiana and support its use in traditional health practices for the management of diabetes mellitus.

Pharmacological evidences for the stimulation of calcium-sensing receptors by nifedipine in gingival fibroblasts

OBJECTIVE

To investigate pharmacologically whether CaSRs are involved in the Ca(2+) antagonist-induced [Ca(2+)]i elevation in gingival fibroblasts.

MATERIALS AND METHODS

Gin-1 cells, normal human gingival fibroblasts, were used as the material. The [Ca(2+)] i was measured with fura-2/AM, a Ca(2+)-sensitive fluorescent dye.

RESULTS

At first, we confirmed the existence of CaSRs in these cells by showing that [Ca(2+)] i was elevated by high concentrations of extracellular Ca(2+) and by prototypic agonists of the CaSR such as gentamicin. The action of gentamicin was antagonized by inhibitors of phospholipase C (PLC), inositol trisphosphate (IP(3)) receptors, NSCCs, and, importantly, by the CaSR antagonist, NPS2390. Furthermore, the action of gentamicin was potentiated by activators of PLC and protein kinase C (PKC). This confirmed the pathway components mediating Ca(2+) responses to a known agonist of the CaSR. We then investigated whether nifedipine (an L-type Ca(2+) channel blocker) stimulates CaSRs to elevate [Ca(2+)] i via a similar mechanism. Nifedipine Ca(2+) responses were dose-dependently blocked by NPS2390 and by the same inhibitors of PLC, IP(3) receptors, and NSCCs that disrupted the action of gentamicin. Calphostin C (a PKC inhibitor) and TMB-8 (an inhibitor of Ca(2+) release from stores) also inhibited the nifedipine-induced [Ca(2+)] i elevation.

CONCLUSION

These findings suggest that CaSRs are involved in the nifedipine-induced [Ca(2+)] i elevation in gingival fibroblasts.

The regulation of parathyroid hormone secretion and synthesis

Secondary hyperparathyroidism classically appears during the course of chronic renal failure and sometimes after renal transplantation. Understanding the mechanisms by which parathyroid hormone (PTH) synthesis and secretion are normally regulated is important in devising methods to regulate overactivity and hyperplasia of the parathyroid gland after the onset of renal insufficiency. Rapid regulation of PTH secretion in response to variations in serum calcium is mediated by G-protein coupled, calcium-sensing receptors on parathyroid cells, whereas alterations in the stability of mRNA-encoding PTH by mRNA-binding proteins occur in response to prolonged changes in serum calcium. Independent of changes in intestinal calcium absorption and serum calcium, 1α,25-dihydroxyvitamin D also represses the transcription of PTH by associating with the vitamin D receptor, which heterodimerizes with retinoic acid X receptors to bind vitamin D-response elements within the PTH gene. 1α,25-Dihydroxyvitamin D additionally regulates the expression of calcium-sensing receptors to indirectly alter PTH secretion. In 2°HPT seen in renal failure, reduced concentrations of calcium-sensing and vitamin D receptors, and altered mRNA-binding protein activities within the parathyroid cell, increase PTH secretion in addition to the more widely recognized changes in serum calcium, phosphorus, and 1α,25-dihydroxyvitamin D. The treatment of secondary hyperparathyroidism by correction of serum calcium and phosphorus concentrations and the administration of vitamin D analogs and calcimimetic agents may be augmented in the future by agents that alter the stability of mRNA-encoding PTH.

Familial hypocalciuric hypercalcemia revealed by chondrocalcinosis

BACKGROUND

Calcium pyrophosphate dihydrate crystal deposition disease (CPPD-CDD) has been associated to hypercalcemia. Familial hypocalciuric hypercalcemia (FHH) is a rare but important consideration in the differential diagnosis of hypercalcemia. This autosomal dominantly inherited condition is characterized by elevated plasma calcium levels, relative or absolute hypocalciuria, and normal to moderately elevated plasma PTH level. The disease is caused by inactivating mutations in the calcium-sensing receptor gene.

CASE REPORT

We describe a 77-year-old Italian man with arthritis secondary to CPPD-CDD and hypercalcemia. Clinical and biochemical data (s-Ca: 2.94 mmol/L; PTH: 5.9 pmol/L; 24 h urinary calcium: 69.6 mg; calcium/creatinine clearance: 0.004) suggested the diagnosis of FHH. Mild hypocalciuric hypercalcemia was also found in five of seven relatives confirming the diagnosis, of these one showed chondrocalcinosis.

CONCLUSIONS

It is important to screen for FHH using fractional urinary excretion of calcium in subjects with CPPD-CDD associated to hypercalcemia, this approach may prevent unnecessary parathyroidectomy.

Activation of the calcium sensing receptor stimulates gastrin and gastric acid secretion in healthy participants

In 17 adults on a fixed metabolic diet, an 11-day course of cinacalcet increased serum gastrin and basal gastric acid output, but not maximal gastric acid output, compared with a placebo. These findings indicate that the calcium sensor receptor plays a role in the regulation of gastric acid.

INTRODUCTION

Gastric acid secretion is a complex process regulated by neuronal and hormonal pathways. Ex vivo studies in human gastric tissues indicate that the calcium sensing receptor (CaR), expressed on the surface of G and parietal cells, may be involved in this regulation. We sought to determine whether cinacalcet, a CaR allosteric agonist, increases serum gastrin and gastric acid secretion.

METHODS

Seventeen healthy adults with normal gastric acid output were placed on an 18-day metabolic diet. On day 8 (baseline), participants were given cinacalcet (15 then 30 mg/day) or placebo for 11 days. Changes in gastric acid output, serum gastrin, and other measures were compared in the two groups.

RESULTS

Changes in serum gastrin and basal acid output (adjusted for baseline body weight) were significantly more positive in the cinacalcet group compared with placebo (P = 0.004 and P = 0.039 respectively). Change in maximal acid output was similar in the two groups (P = 0.995). As expected, cinacalcet produced significant decreases in serum PTH (P < 0.001) and ionized calcium levels (P = 0.032), and increases in serum phosphorus levels (P = 0.001) and urinary calcium (P = 0.023).

CONCLUSIONS

This study provides in vivo evidence that activation of the CaR increases serum gastrin levels and basal gastric acid secretion in healthy adults.

Use of cinacalcet HCl to achieve the recommended targets of bone metabolism in a patient with therapy-resistant renal hyperparathyroidism

We report on a patient with end-stage renal disease and severe progressive secondary hyperparathyroidism, whose condition failed to respond to conventional pharmacologic or surgical interventions. Although immunotherapy produced a partial response, it failed to decrease serum parathyroid hormone to the levels recommended by the National Kidney Foundation Kidney Disease Outcomes Quality Initiative clinical practice guidelines. Treatment with a new calcimimetic agent, cinacalcet HCl (Mimpara, Amgen, Munich, Germany), resulted in a rapid decline in elevated parathyroid hormone levels, near normalization of other laboratory markers of bone metabolism, improvement in mobility and skeletal pain caused by renal osteodystrophy, and an increase in body weight.

Expression and function of the calcium-sensing receptor in juxtaglomerular cells

Calcium-sensing receptors sense and translate micromolar changes of extracellular calcium into changes in intracellular calcium. Renin, a component of the renin-angiotensin system, is synthesized by, stored in, and released from the juxtaglomerular cells through a cAMP-dependent pathway. Increased intracellular calcium inhibits the adenylyl cyclase isoform type V, cAMP formation, and renin release from juxtaglomerular cells. We hypothesized that calcium-sensing receptors are expressed in juxtaglomerular cells and mediate changes in intracellular calcium and renin release. To test this we used primary cultures of isolated mouse juxtaglomerular cells in which we ran RT-PCR, Western blots, and immunofluorescence. RT-PCR showed a positive band at the expected 151 bp consistent with calcium-sensing receptor. Western blots showed a 130- to 150-kDa band confirming the calcium-sensing receptor in juxtaglomerular cells. Immunofluorescence and confocal microscopy using 2 different antibodies against the calcium-sensing receptor in juxtaglomerular cells showed positive fluorescence in the juxtaglomerular cells, which also had positive labeling for renin. To test whether calcium-sensing receptors regulate renin release, juxtaglomerular cells were incubated with a calcium-sensing receptor agonist, the calcimimetic cinacalcet-HCl, at concentrations of 50 and 1000 nmol/L in 0.25 mmol/L of calcium medium. Cinacalcet-HCl decreased juxtaglomerular cell cAMP formation to 47.3+/-6.8% and 44.2+/-9.7% of basal, respectively (P<0.001), and decreased renin release from 541.9+/-86.2 to 364.6+/-64.1 (P<0.05) and 279.6+/-56.9 (P<0.005) ng of angiotensin I per milliliter per hour per milligram of protein, respectively. We conclude that juxtaglomerular cells express the calcium-sensing receptor and that their activation leads to inhibition of adenylyl cyclase-V activity, decreasing cAMP formation and suppressing renin release.

Gene polymorphism association studies in dialysis: bone and mineral metabolism

Abnormalities in bone and mineral metabolism have a significant impact on morbidity and mortality among patients with end-stage renal disease (ESRD). In addition to confounding environmental factors, genetic susceptibility factors may also influence the occurrence and severity of these abnormalities and account for interindividual variability among patients. Indeed, polymorphisms involving genes of the calcium/parathyroid hormone/calcitriol axis have been associated with bone and mineral metabolism abnormalities. This review summarizes studies involving polymorphisms of candidate genes and their effects on the development of complications related to bone and mineral metabolism abnormalities among patients with ESRD.

Primary hyperparathyroidism and the presence of kidney stones are associated with different haplotypes of the calcium-sensing receptor

INTRODUCTION

Three single-nucleotide polymorphisms in the calcium-sensing receptor gene (CASR) encoding the missense substitutions A986S, R990G, and Q1011E have been associated with normal variation in extracellular calcium homeostasis, both individually and in haplotype combination. The aim of this study was to examine haplotype associations in primary hyperparathyroidism (PHPT).

PATIENTS AND METHODS

Patients with sporadic PHPT (n = 237) were recruited from endocrine clinics and healthy controls (n = 433) from a blood donor clinic, and levels of serum calcium, albumin, and PTH were measured. In PHPT patients, urinary calcium/creatinine clearances and bone mineral density at spine and femoral neck were measured and the presence of kidney stones and vertebral fractures identified. The CASR single-nucleotide polymorphisms were haplotyped by allele-specific sequencing.

RESULTS

Four haplotypes (ARQ, SRQ, AGQ, and ARE) of eight were observed, in keeping with significant linkage disequilibrium, but haplotype frequencies did not show significant Hardy-Weinberg disequilibrium. The SRQ haplotype was more common in PHPT (125 of 474 alleles) than in controls (170 of 866 alleles, P = 0.006) and showed a significant (P = 0.006) gene-dosage effect. There was no significant association between haplotype and bone mineral density or fractures, but association with kidney stones was significant (P = 0.0007). In the stone-forming subgroup, the SRQ haplotype was underrepresented and AGQ overrepresented. Patients bearing the AGQ haplotype had an odds ratio of 3.8 (95% confidence interval, 1.30-11.3) for presentation with renal stones compared with the rest.

CONCLUSION

Our data indicate that the CASR SRQ haplotype is significantly associated with PHPT in our population. Within the PHPT patient population, the AGQ haplotype is significantly associated with kidney stones.

Minimizing bone abnormalities in children with renal failure

Renal osteodystrophy (ROD), a metabolic bone disease accompanying chronic renal failure (CRF), is a major clinical problem in pediatric nephrology. Growing and rapidly remodeling skeletal systems are particularly susceptible to the metabolic and endocrine disturbances in CRF. The pathogenesis of ROD is complex and multifactorial. Hypocalcemia, phosphate retention, and low levels of 1,25 dihydroxyvitamin D(3) related to CRF result in disturbances of bone metabolism and ROD. Delayed diagnosis and treatment of bone lesions might result in severe disability. Based on microscopic findings, renal bone disease is classified into two main categories: high- and low-turnover bone disease. High-turnover bone disease is associated with moderate and severe hyperparathyroidism. Low-turnover bone disease includes osteomalacia and adynamic bone disease. The treatment of ROD involves controlling serum calcium and phosphate levels, and preventing parathyroid gland hyperplasia and extraskeletal calcifications. Serum calcium and phosphorus levels should be kept within the normal range. The calcium-phosphorus product has to be <5 mmol(2)/L(2) (60 mg(2)/dL(2)). Parathyroid hormone (PTH) levels in children with CRF should be within the normal range, but in children with end-stage renal disease PTH levels should be two to three times the upper limit of the normal range. Drug treatment includes intestinal phosphate binding agents and active vitamin D metabolites. Phosphate binders should be administered with each meal. Calcium carbonate is the most widely used intestinal phosphate binder. In children with hypercalcemic episodes, sevelamer, a synthetic phosphate binder, should be introduced. In children with CRF, ergocalciferol (vitamin D(2)), colecalciferol (vitamin D(3)), and calcifediol (25-hydroxyvitamin D(3)) should be used as vitamin D analogs. In children undergoing dialysis, active vitamin D metabolites alfacalcidol (1alpha-hydroxy-vitamin D(3)) and calcitriol (1,25 dihydroxyvitamin D(3)) are applied. In recent years, a number of new drugs have emerged that hold promise for a more effective treatment of bone lesions in CRF. This review describes the current approach to the diagnosis and treament of ROD.

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.

Calcium-sensing receptor gene A986S polymorphism and bone mass in hypertensive women

BACKGROUND

The function of the calcium-sensing receptor (CaSR) is to maintain serum calcium concentration within a narrow physiological range. Two types of mutations have been described: activating that causes hypocalcemia and inactivating, which leads to hypercalcemia. The objective was to assess the effect of CaSR gene A986S polymorphism on the lumbar spine bone mass, calcium metabolism parameters and markers of bone remodeling in hypertensive women.

METHODS

The study included 48 patients (mean age 59 +/- 10 years) with mild-to-moderate hypertension, according to JNC VI and who did not present any associated diseases. We determined the following in all patients: Ca, P, Mg, PTHi, 25-vitamin D, 1,25-vitamin D, osteocalcin, deoxypyridinoline in urine, 24-h urine calcium. A bone densitometry of the lumbar spine was also performed. CaSR gene A986S polymorphism was also studied in all patients by PCR.

RESULTS

Genotype frequency was 69% for AA, 27% for AS and 4% for SS, with a prevalence of 82% for allele A and 18% for allele S. Patients with a lack of allele S had lower levels of p (3.5 +/- 0.5 vs. 4 +/- 0.4, p = .034). No differences in calcium plasma levels, urinary calcium excretion and bone mass were observed.

CONCLUSIONS

We found no clinical significance in the parameters studied of the CaSR gene A986S polymorphism in hypertensive women.

The calcium-sensing receptor and related diseases

The calcium-sensing receptor (CASR) adjusts the extracellular calcium set point regulating PTH secretion and renal calcium excretion. The receptor is expressed in several tissues and is also involved in other cellular functions such as proliferation, differentiation and other hormonal secretion. High extracellular calcium levels activate the receptor resulting in modulation of several signaling pathways depending on the target tissues. Mutations in the CASR gene can result in gain or loss of receptor function. Gain of function mutations are associated to Autossomal dominant hypocalcemia and Bartter syndrome type V, while loss of function mutations are associated to Familial hypocalciuric hypercalcemia and Neonatal severe hyperparathyroidism. More than one hundred mutations were described in this gene. In addition to calcium, the receptor also interacts with several ions and polyamines. The CASR is a potential therapeutic target to treatment of diseases including hyperparathyroidism and osteoporosis, since its interaction with pharmacological compounds results in modulation of PTH secretion.

Insulin-like growth factors (IGF) I and II utilize different calcium signaling pathways in a primary human parathyroid cell culture model

BACKGROUND

In most cell types, influx of calcium (Ca2+) induces a growth or secretory response. The opposite occurs in parathyroid (PTH), cells where there is an inverse relationship between intracellular Ca2+ concentration and PTH secretion. We have examined the effects of calcium channel and metabolism modulators on insulin-like growth factors (IGFs) in a parathyroid cell culture model.

METHODS

Cell cultures were prepared from 9 patients undergoing operation for hyperparathyroidism. Following adhesion, the cells were transferred to serum-free medium and dosed with IGF I, II +/- ethyleneglycol-bis(beta-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA), nifedipine, nickel, 2-aminoethoxy-diphenylborate (2-APB), or dantrolene. Proliferation (96 hours) was assessed by measuring tritiated thymidine incorporation and PTH release (1 and 3 hours) assayed by IRMA.

RESULTS

Both IGF I and II increased DNA synthesis to 162.8% +/- 10.6% (SEM) and 131.1% +/- 7.7%, respectively (P < 0.05). EGTA at 0.2 mmol (ionized Ca2+ 0.2 mmol) did not affect the response to both IGFs. EGTA at 2 mmol (ionized Ca2+ 0 mmol) reduced the DNA synthesis of IGF I and II to 29% and 26%, respectively (P < 0.05). Nifedipine and nickel (nonspecific Ca2+ channel blocker) were equally potent in negating the mitogenic effects of both IGFs. 2-APB (IP3R blocker) reduced the basal DNA synthesis to 51.3% +/- 8.4% but had no effect on either IGF. Dantrolene (ryanodine receptor blocker) negated IGF II induced mitogenisis (74.2% +/- 6.7%) and partially inhibited IGF I mitogenesis (123% +/- 6%) (P < 0.05). The rate of PTH secretion was greater after IGF II stimulation than after IGF I stimulation.

CONCLUSIONS

IGFs I and II induce mitogenesis by different calcium signaling pathways. These data suggest that parathyroid cells may utilize different calcium signaling pathways to distinguish growth factors and serum calcium changes.

Dissimilar PTH, gastrin, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, primary hyperparathyroidism, and normal subjects: a useful tool for differential diagnosis

We analyzed gastrin, PTH, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, mild primary hyperparathyroidism, and normal controls. We observed diverse hormonal responses that may help in the differential diagnosis of these conditions.

INTRODUCTION

Hypocalciuric hypercalcemia (HH) is consequent to calcium-sensing receptor (CaSR) genetic mutations or anti-CaSR antibodies. CaSR is expressed in parathyroid tissue, thyroid C cells, and gastrin-secreting cells, where it has been suggested that on calcium and/or amino acid allosteric activation, promotes gastrin secretion.

MATERIALS AND METHODS

We evaluated gastrin, PTH, and calcitonin responses to oral calcium (1 g) and peptones (10 g) in 10 patients with HH (mean age, 58.5 +/- 10.3 years; F/M = 9/1), 15 patients with primary hyperparathyroidism (PH; mean age, 60.4 +/- 8.3 years; F/M = 11/4), and 30 healthy controls (mean age, 60.3 +/- 8.1 years). Statistical analyses for differences during oral loading tests were calculated with ANOVA for repeated measurements and comparisons between two groups were performed with Student's t-test.

RESULTS

PTH response to peptones was markedly increased in patients with PH compared with flat responses in controls and HH patients (p < 0.05). Gastrin increase after oral calcium was absent in HH and PH subjects (p < 0.05 versus controls), and gastrin responses to peptones were blunted in HH and PH subjects compared with controls (p < 0.05). PTH drop and calcitonin increase after calcium load observed in controls were absent in HH and PH subjects (p < 0.05).

CONCLUSIONS

The marked difference in PTH response elicited by peptones observed in PH compared with subjects with HH may help in the differential diagnosis of these conditions without genetic studies. Peptones may stimulate CaSR-controlled hormones as an allosteric regulatory pathway. CaSR abnormalities may help to explain the different calcium- and peptones-induced hormonal responses observed in PH and HH compared with normal subjects.

An update on acid secretion

Gastric acid secretion is a complex process that requires hormonal, neuronal, or calcium-sensing receptor activation for insertion of pumps into the apical surface of the parietal cell. Activation of any or all these pathways causes the parietal cell to secrete concentrated acid with a pH at or close to 1. This acidic fluid combines with enzymes that are secreted from neighbouring chief cells and passes out of the gland up through a mucous gel layer covering the surface of the stomach producing a final intragastric pH of less than 4 during the active phase of acid secretion. Defects in either the mucosal barrier or in the regulatory mechanisms that modulate the secretory pathways will result in erosion of the barrier and ulcerations of the stomach or esophagus. The entire process of acid secretion relies on activation of the catalytic cycle of the gastric H+,K+-ATPase, resulting in the secretion of acid into the parietal cell canaliculus, with K+ being the important and rate-limiting ion in this activation process. In addition to K+ as a rate limiter for acid production, Cl- secretion via an apical channel must also occur. In this review we present a discussion of the mechanics of acid secretion and a discussion of recently identified transporter proteins and receptors. Included is a discussion of some of the recent candidates for the apical K' recycling channel, as well as two recently identified apical proteins (NHE-3, PAT-1), and the newly characterized calcium-sensing receptor (CaSR). We hope that this review will give additional insight into the complex process of acid secretion.

Management of musculoskeletal complications in endstage renal disease: an update

People treated with prolonged hemodialysis now survive longer and increasingly develop musculoskeletal complications. Significant advances have been made in the management of musculoskeletal manifestations in patients undergoing hemodialysis for endstage renal disease. Noncalcium-based phosphate binders are preferred for the management of hyperphosphatemia. Vitamin D therapy is based on intact parathyroid hormone levels. Newer Vitamin D sterols and calcimimetic agents seem to control secondary hyperparathyroidism without increasing the calcium-phosphorous product. Use of biocompatible dialyzers for hemodialysis may delay articular manifestations of beta-2 microglobulin amyloidosis. Beta-2 microglobulin adsorption columns when used along with hemodialysis may lead to lower beta-2 microglobulin levels and improvement in articular manifestations of beta-2 microglobulin amyloidosis. In this review, we focus on the latest advancements in the management of secondary hyperparathyroidism and beta-2 microglobulin amyloidosis.

Calcium and salinity sensing by the thick ascending limb: a journey from mammals to fish and back again

The roles of the CaSR in endocrine, epithelial, CNS, and other cells have been reviewed previously [17-19, 20, 27-30, 31-33]. This brief review focuses on the roles of the CaSR in the thick ascending limb of Henle (TAL), and is written in honor of my mentor and long-term friend and colleague, Thomas E. Andreoli, on the occasion of his retirement. My early studies of TAL function with Tom Andreoli were the inspiration for this work.

The effects of salmon calcitonin-induced hypocalcemia on bone metabolism in ovariectomized rats

The ovariectomized rat has proved to be a most useful model for preclinical testing of potential therapies for osteoporosis. We describe the immediate effects of a single treatment with salmon calcitonin (sCT) on calcium homeostasis and bone turnover markers in 6-month-old sham and ovariectomized (ovx) rats at 15 days postovariectomy. Rats were fasted for 24 h prior to and following administration of 0.3 microg/kg body weight sCT. Blood specimens were collected at 0 (pretreatment), 2, 4, and 8 h. Urine samples were collected during the intervening periods. sCT treatment produced a decrease in blood ionized calcium at 2 h posttreatment in sham and ovx rats (P < 0.001), which was exaggerated in the ovx rats (P < 0.001). Increased parathyroid hormone (PTH) levels (P < 0.001) accompanied the hypocalcemia in ovx rats. Furthermore, PTH levels were significantly higher in ovx rats compared with sham rats for the same ionized calcium range of 1.275-1.300 mmol/l (P < 0.05). sCT treatment in sham rats increased urine hydroxyproline (UHyp) at 6 h posttreatment (P < 0.01). In conclusion, the calcitonin-induced hypocalcemia and secondary hyperparathyroidism was more pronounced in the ovariectomized rats, consistent with the actions of calcitonin in states of increased bone turnover induced by estrogen deficiency. This study highlights the importance of considering the actions of PTH and estrogen status when interpreting changes in calcium homeostasis and bone turnover following treatment with calcitonin in rodent models and provides further evidence for a potential role of estrogen in parathyroid function.

pH dependence of extracellular calcium sensing receptor activity determined by a novel technique

BACKGROUND

Increasing evidence points to the role of the extracellular Calcium Sensing Receptor (CaSR) as a multimodal receptor responding to diverse physiologic stimuli, such as extracellular divalent and polyvalent cations, amino acids, and ionic strength. Within the kidney, these stimuli converge on the CaSR to coordinate systemic calcium and water homeostasis. In this process, the impact of urinary pH changes on the activity of the CaSR has not yet been defined. We therefore performed the present study to analyze the pH sensitivity of the CaSR.

METHODS

To assess the activation state of the CaSR, we developed a new method based on the functional coupling between CaSR activity and gating of calcium sensitive potassium currents mediated by SK4 potassium channels. Two-electrode voltage clamping was used to determine whole cell currents in Xenopus oocytes heterologously expressing rat CaSR and rat SK4 potassium channels.

RESULTS

Coexpression of CaSR and SK4 gave rise to potassium currents that were dependent on CaSR-mediated intracellular calcium release, and thereby corresponded to the activation state of the CaSR. In presence of extracellular calcium, ambient alkalinization above pH 7.5 increased CaSR activity. Evaluation of the CaSR calcium sensitivity at various ambient proton concentrations revealed that this effect was due to a sensitization of the CaSR towards extracellular calcium.

CONCLUSION

Coexpression with SK4 potassium channels provides a fast and sensitive approach to evaluate CaSR activity in Xenopus oocytes. As disclosed by this novel technique, CaSR activity is regulated by extracellular pH.

Association of Genetic Variants in the Calcium-Sensing Receptor with Risk of Colorectal Adenoma

Objective: Evidence suggests that calcium prevents colorectal cancer, possibly mediated through the calcium-sensing receptor (CASR). We assessed the associations between CASR gene variants and risk for colorectal adenoma, a cancer precursor. We further investigated gene-diet interactions between the CASR variants and calcium intake on adenoma risk.

Methods: Individuals with advanced distal adenomas (n = 716) and controls with a negative sigmoidoscopy exam (n = 729) were randomly selected from participants in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Three nonsynonymous variants in the intracellular signaling region of CASR (A986S, R990G, Q1011E) were analyzed by Taqman.

Results: Compared with the most common diplotype (haplotype pair), the odds ratios for advanced adenoma were 0.80 [95% confidence interval (CI), 0.60-1.06], 0.79 (95% CI, 0.55-1.13), and 0.56 (95% CI, 0.36-0.88) for the other three common diplotypes (&gt;5% frequency). Although calcium intake was inversely associated with adenoma risk, CASR diplotypes did not modify this association. However, the power to investigate interactions was limited.

Conclusion: Variants in the CASR intracellular signaling region were significantly associated with the risk of advanced adenoma.

Blood ionized calcium is associated with clustered polymorphisms in the carboxyl-terminal tail of the calcium-sensing receptor

Blood ionized calcium (iCa) is a quantitative trait subject to genetic influence. iCa is maintained in a narrow range through the action of the calcium-sensing receptor (CASR) controlling PTH secretion and calcium excretion. A CASR single nucleotide polymorphism (SNP) prevalent in Caucasian populations (A986S) has shown significant association with iCa in a cohort of young women, but association with the neighboring SNPs, R990G and Q1011E, has not been examined. We studied 377 unrelated adults (184 men and 193 women) recruited as healthy adults from a blood donor clinic. The subjects were not taking any medications, nor did they have disorders of calcium metabolism. Relative frequencies for the CASR 986S, 990G, and 1011E minor alleles were 24%, 4%, and 3% respectively. At the A986S locus, subjects with the AA genotype had significantly lower iCa (P = 0.0001) than subjects with one or two S alleles (mean +/- se, 1.221 +/- 0.003 vs. 1.239 +/- 0.003 mmol/liter). For the R990G site, subjects with the RR genotype had higher iCa than those with one copy of the 990G allele (1.230 +/- 0.002 vs. 1.213 +/- 0.007 mmol/liter; P = 0.032). With respect to the 1011 locus, iCa was lower in QQ genotype subjects than in the QE group (1.227 +/- 0.002 vs. 1.255 +/- 0.008 mmol/liter; P = 0.002). After resolution of phase for the doubly heterozygous subjects, analysis was conducted on haplotypes across all three loci. As expected, subjects with SRQ and ARE haplotypes are relatively hypercalcemic, and those with AGQ are hypocalcemic, relative to subjects with the common ARQ haplotype. Multiple regression analysis with clinical covariates (age, sex and menopausal status, creatinine, and PTH) showed that 16.5% of the total variance in iCa may be explained, and the seven CASR haplotypes contribute significantly (P < 0.0001) and substantially (49.1% of the explained variance) to the model, with the following corrected iCa means: ARQ/AGQ, 1.21 +/- 0.01; ARQ/ARQ, 1.22 +/- 0.01; ARQ/SRQ, 1.24 +/- 0.01; SRQ/AGQ, 1.24 +/- 0.03; SRQ/SRQ, 1.25 +/- 0.01; ARQ/ARE, 1.25 +/- 0.01; and SRQ/ARE, 1.27 +/- 0.01. Our data confirm the association between iCa and the A986S locus and suggest that R990G and Q1011E are also predictive. Given the significant between-population variations in frequency of variant alleles in this CASR SNP cluster, tri-locus haplotyping may prove to be more informative in studies of association between variation in CASR and disease.

Role of the calcium-sensing receptor in parathyroid gland physiology

The calcium-sensing receptor (CaSR) represents the molecular mechanism by which parathyroid cells detect changes in blood ionized calcium concentration and modulate parathyroid hormone (PTH) secretion to maintain serum calcium levels within a narrow physiological range. Much has been learned in recent years about the diversity of signal transduction through the CaSR and the various factors that affect receptor expression. Beyond its classic role as a determinant of calcium-regulated PTH secretion, signaling through the CaSR also influences both gene transcription and cell proliferation in parathyroid cells. The CaSR thus serves a broad physiological role by integrating several distinct aspects of parathyroid gland function. The current review summarizes recent developments that enhance our understanding of the CaSR and its fundamental importance in parathyroid gland physiology.

Effect of the calcimimetic NPS R-467 on furosemide-induced nephrocalcinosis in the young rat

BACKGROUND

Furosemide induces nephrocalcinosis in both humans and animals. We showed previously that parathyroidectomy protected against the development of furosemide-induced nephrocalcinosis in young rats, indicating a possible role for parathyroid hormone (PTH) in its pathogenesis. Calcimimetic agents such as NPS R-467 are potent and selective agonists at the calcium-sensing receptor in parathyroid glands and inhibit PTH secretion.

METHODS

To determine whether NPS R-467 could, like parathyroidectomy, prevent furosemide-induced nephrocalcinosis, we studied 35 6-week-old male Sprague-Dawley rats, divided into five groups. Group A served as control, group B received intraperitoneally furosemide (40 mg/kg), groups C, D, and E received furosemide and NPS R-467 intraperitoneally at doses of 10, 20, and 40 micromol/kg, respectively, daily for 8 days. During the last 3 days, animals were placed in metabolic cages for measurement of urine output, food, and water intake. Blood and kidneys were collected on day 8, 60 to 90 minutes after the last doses. Kidney calcium content was measured and nephrocalcinosis scoring (0 to 5) was assessed histologically.

RESULTS

Furosemide increased urine output and fluid intake, and decreased body weight gain similarly in all groups. Serum PTH levels (mean +/- SD) were significantly higher in furosemide-treated control animals (276 +/- 226 pg/mL vs. 64 +/- 21 pg/mL); NPS R-467 induced a dose-dependent decrease in PTH levels (52 +/- 51 pg/mL, 18 +/- 7 pg/mL, and 13 +/- 3 pg/mL in groups C, D, and E, respectively). Plasma Ca(2+) was slightly, but significantly lower in all three NPS R-467 treated groups (5.1 +/- 0.4 mg/dL, 4.8 +/- 0.3 mg/dL, and 4.5 +/- 0.3 mg/dL in groups C, D, and E, respectively) compared to 5.7 +/- 0.1 mg/dL and 5.5 +/- 0.2 mg/dL in groups A and B, respectively. Furosemide treatment induced a substantial increase in kidney calcium content (1819 +/- 664 microg/g dry weight vs. 126 +/- 26 microg/g dry weight) and nephrocalcinosis scoring (5.0 +/- 0.0 vs. 0.0 +/- 0.0). Treatment with NPS R-467 ameliorated the furosemide-induced increase in kidney calcium content (673 +/- 312 microg/g, 361 +/- 188 microg/g, and 563 +/- 291 microg/g) and nephrocalcinosis scoring (2.2 +/- 1.2, 0.7 +/- 0.8, and 1.0 +/- 1.2) in groups C, D, and E, respectively.

CONCLUSION

The calcimimetic agent NPS R-467 prevents the development of hyperparathyroidism and attenuates nephrocalcinosis in the furosemide-treated young rat.

Calcium-sensing receptors

It is now known that variations in extracellular calcium concentration exert diverse physiologic effects in a variety of tissues that are mediated by a calcium-sensing receptor (CaSRs). In parathyroid tissue, the CaSR represents the molecular mechanism by which parathyroid cells detect changes in blood ionized calcium concentration, modulate parathyroid hormone (PTH) secretion accordingly, and thus maintain serum calcium levels within a narrow physiologic range. In the kidney, the CaSR regulates renal calcium excretion and influences the transepithelial movement of water and other electrolytes. More generally, activation of the CaSR represents an important signal transduction pathway in intestine, placenta, brain, and perhaps bone. Some of these actions involve cell cycle regulation, changes that may be relevant to understanding the pathogenesis of parathyroid gland hyperplasia in secondary hyperparathyroidism caused by chronic kidney disease. The CaSR represents an appealing target for therapeutic agents designed to modify parathyroid gland function in vivo, offering the prospect of novel therapies for selected disorders of bone and mineral metabolism. Other receptors capable of responding to extracellular calcium ions also have been identified, but the functional importance of these interactions remains to be determined.