Physiology and pathophysiology of the calcium-sensing receptor in the kidney

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Calcium channel inhibitor and extracellular calcium improve aminoglycoside-induced hair cell loss in zebrafish

Aminoglycosides are commonly used antibiotics for treatment of gram-negative bacterial infections, however, they might act on inner ear, leading to hair-cell death and hearing loss. Currently, there is no targeted therapy for aminoglycoside ototoxicity, since the underlying mechanisms of aminoglycoside-induced hearing impairments are not fully defined. This study aimed to investigate whether the calcium channel blocker verapamil and changes in intracellular & extracellular calcium could ameliorate aminoglycoside-induced ototoxicity in zebrafish. The present findings showed that a significant decreased number of neuromasts in the lateral lines of zebrafish larvae at 5 days' post fertilization after neomycin (20 μM) and gentamicin (20 mg/mL) exposure, which was prevented by verapamil. Moreover, verapamil (10-100 μM) attenuated aminoglycoside-induced toxic response in different external calcium concentrations (33-3300 μM). The increasing extracellular calcium reduced hair cell loss from aminoglycoside exposure, while lower calcium facilitated hair cell death. In contrast, calcium channel activator Bay K8644 (20 μM) enhanced aminoglycoside-induced ototoxicity and reversed the protective action of higher external calcium on hair cell loss. However, neomycin-elicited hair cell death was not altered by caffeine, ryanodine receptor (RyR) agonist, and RyR antagonists, including thapsigargin, ryanodine, and ruthenium red. The uptake of neomycin into hair cells was attenuated by verapamil and under high external calcium concentration. Consistently, the production of reactive oxygen species (ROS) in neuromasts exposed to neomycin was also reduced by verapamil and high external calcium. Significantly, zebrafish larvae when exposed to neomycin exhibited decreased swimming distances in reaction to droplet stimulus when compared to the control group. Verapamil and elevated external calcium effectively protected the impaired swimming ability of zebrafish larvae induced by neomycin. These data imply that prevention of hair cell damage correlated with swimming behavior against aminoglycoside ototoxicity by verapamil and higher external calcium might be associated with inhibition of excessive ROS production and aminoglycoside uptake through cation channels. These findings indicate that calcium channel blocker and higher external calcium could be applied to protect aminoglycoside-induced listening impairments.

The calcium-sensing receptor has only a parathyroid hormone-dependent role in the acute response of renal phosphate transporters to phosphate intake

The kidney controls systemic inorganic phosphate (Pi) levels by adapting reabsorption to Pi intake. Renal Pi reabsorption is mostly mediated by sodium-phosphate cotransporters NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3) that are tightly controlled by various hormones including parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23). PTH and FGF23 rise in response to Pi intake and decrease NaPi-IIa and NaPi-IIc brush border membrane abundance enhancing phosphaturia. Phosphaturia and transporter regulation occurs even in the absence of PTH and FGF23 signaling. The calcium-sensing receptor (CaSR) regulates PTH and FGF23 secretion, and may also directly affect renal Pi handling. Here, we combined pharmacological and genetic approaches to examine the role of the CaSR in the acute phosphaturic response to Pi loading. Animals pretreated with the calcimimetic cinacalcet were hyperphosphatemic, had blunted PTH levels upon Pi administration, a reduced Pi-induced phosphaturia, and no Pi-induced NaPi-IIa downregulation. The calcilytic NPS-2143 exaggerated the PTH response to Pi loading but did not abolish Pi-induced downregulation of NaPi-IIa. In mice with a dominant inactivating mutation in the Casr (CasrBCH002), baseline NaPi-IIa expression was higher, whereas downregulation of transporter expression was blunted in double CasrBCH002/PTH knockout (KO) transgenic animals. Thus, in response to an acute Pi load, acute modulation of the CaSR affects the endocrine and renal response, whereas chronic genetic inactivation, displays only subtle differences in the downregulation of NaPi-IIa and NaPi-IIc renal expression. We did not find evidence that the CaSR impacts on the acute renal response to oral Pi loading beyond its role in regulating PTH secretion.NEW & NOTEWORTHY Consumption of phosphate-rich diets causes an adaptive response of the body leading to the urinary excretion of phosphate. The underlying mechanisms are still poorly understood. Here, we examined the role of the calcium-sensing receptor (CaSR) that senses both calcium and phosphate. We confirmed that the receptor increases the secretion of parathyroid hormone involved in stimulating urinary phosphate excretion. However, we did not find any evidence for a role of the receptor beyond this function.

Functional analysis reveals calcium-sensing receptor gene regulating cell-cell junction in renal tubular epithelial cells

PURPOSE

Calcium-sensing receptor (CASR) influences the expression pattern of multiple genes in renal tubular epithelial cells. The objective of this inquiry was to explore the molecular mechanisms of CASR in renal tubular epithelial cells and nephrolithiasis.

METHODS

HK-2 cells were transfected with lentiviruses carrying either CASR (named CASR) or an empty vector negative control (named NC), as well as shRNA intended to target CASR (named shCASR) or its corresponding negative control (named shNC). CCK-8 assay was used to detect the effect of CASR on the proliferation of HK-2 cells. RNA-Sequencing was applied to explore potential pathways regulated by CASR in HK-2 cells.

RESULTS

PCR and western blot results showed that CASR expression was significantly increased in CASR cells and was decreased in shCASR cells when compared to their corresponding negative control, respectively. CCK-8 assay revealed that CASR inhibited the proliferation of HK-2 cells. RNA-Sequencing results suggested that the shCASR HK-2 cells exhibited a significant up-regulation of 345 genes and a down-regulation of 366 genes. These differentially expressed genes (DEGs) were related to cell apoptosis and cell development. In CASR HK-2 cells, 1103 DEGs primarily functioned in mitochondrial energy metabolism, and amino acid metabolism. With the Venn diagram, 4 DEGs (Clorf116, ENPP3, IL20RB, and CLDN2) were selected as the hub genes regulated by CASR. Enrichment analysis revealed that these hub genes were involved in cell-cell junction, and epithelial cell development.

CONCLUSIONS

In summary, our investigation has the potential to offer novel perspectives on CASR regulating cell-cell junction in HK-2 cells.

Urinary Acidification Does Not Explain the Absence of Nephrocalcinosis in a Mouse Model of Familial Hypomagnesaemia with Hypercalciuria and Nephrocalcinosis (FHHNC)

Patients with mutations in Cldn16 suffer from familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) which can lead to renal insufficiency. Mice lacking claudin-16 show hypomagnesemia and hypercalciuria, but no nephrocalcinosis. Calcium oxalate and calcium phosphate are the most common insoluble calcium salts that accumulate in the kidney in the case of nephrocalcinosis, however, the formation of these salts is less favored in acidic conditions. Therefore, urine acidification has been suggested to limit the formation of calcium deposits in the kidney. Assuming that urine acidification is causative for the absence of nephrocalcinosis in the claudin-16-deficient mouse model, we aimed to alkalinize the urine of these mice by the ablation of the subunit B1 of the vesicular ATPase in addition to claudin-16. In spite of an increased urinary pH in mice lacking claudin-16 and the B1 subunit, nephrocalcinosis did not develop. Thus, urinary acidification is not the only factor preventing nephrocalcinosis in claudin-16 deficient mice.

Calcium trafficking and gastrointestinal physiology following an acute lipopolysaccharide challenge in pigs

The influence of systemic immune activation on whole-body calcium (Ca) trafficking and gastrointestinal tract (GIT) physiology is not clear. Thus, the study objectives were to characterize the effects of lipopolysaccharide (LPS) on Ca pools and GIT dynamics to increase understanding of immune-induced hypocalcemia, ileus, and stomach hemorrhaging. Twelve crossbred pigs [44 ± 3 kg body weight (BW)] were randomly assigned to 1 of 2 intramuscular treatments: (1) control (CON; 2 mL saline; n = 6) or (2) LPS (40 µg LPS/kg BW; n = 6). Pigs were housed in metabolism stalls to collect total urine and feces for 6 h after treatment administration, at which point they were euthanized, and various tissues, organs, fluids, and digesta were weighed, and analyzed for Ca content. Data were analyzed with the MIXED procedure in SAS 9.4. Rectal temperature and respiration rate increased in LPS relative to CON pigs (1.4 °C and 32%, respectively; P ≤ 0.05). Inflammatory biomarkers such as circulating alkaline phosphatase, aspartate aminotransferase, and total bilirubin increased in LPS compared with CON pigs whereas albumin decreased (P ≤ 0.02). Plasma glucose and urea nitrogen decreased and increased, respectively, after LPS (43% and 80%, respectively; P < 0.01). Pigs administered LPS had reduced circulating ionized calcium (iCa) compared to CON (15%; P < 0.01). Considering estimations of total blood volume, LPS caused an iCa deficit of 23 mg relative to CON (P < 0.01). Adipose tissue and urine from LPS pigs had reduced Ca compared to CON (39% and 77%, respectively; P ≤ 0.05). There did not appear to be increased Ca efflux into GIT contents and no detectable increases in other organ or tissue Ca concentrations were identified. Thus, while LPS caused hypocalcemia, we were unable to determine where circulating Ca was trafficked. LPS administration markedly altered GIT dynamics including stomach hemorrhaging, diarrhea (increased fecal output and moisture), and reduced small intestine and fecal pH (P ≤ 0.06). Taken together, changes in GIT physiology suggested dyshomeostasis and alimentary pathology. Future research is required to fully elucidate the etiology of immune activation-induced hypocalcemia and GIT pathophysiology.

Contribution of thick ascending limb and distal convoluted tubule to glucose-induced hypercalciuria in healthy controls

Carbohydrates increase kidney stone risk and increase urine calcium and magnesium. We hypothesize that the effects of glucose as an allosteric modulator of calcium-sensing receptors may mediate this effect. Six healthy subjects were on a low-sodium diet before consuming 100 g of glucose beverage. Timed fasting (3) and postglucose (6) urine and blood samples were collected every 30 min. Urine composition and serum markers were measured and microvesicular abundance of tubular transport proteins (NHE3, NKCC2, NCC, and TRPV5) were quantified. Postglucose, serum glucose, and insulin rose rapidly with a parallel increase in calcium and magnesium excretion and no change in fractional excretion of sodium. Both serum parathyroid hormone (PTH) and urine TRPV5 fell in the postglucose periods. The rise in the calcium and magnesium excretion likely occurred primarily in the thick ascending limb where they are both under control of the calcium-sensing receptor. The fall in PTH and TRPV5 support the role of glucose as an allosteric modulator of calcium-sensing receptor.NEW & NOTEWORTHY Sugar increases urine calcium and magnesium as well as kidney stone and bone disease risk. Our study provided new insights into the underlying mechanism as we gave healthy subjects an oral glucose load and used newer tools such as fractional excretion of lithium, serum parathyroid hormone, and microvesicular abundance of tubular transport proteins to characterize the mechanism and identify the thick ascending limb with possible calcium-sensing receptor mediation as a likely contributor to this mechanism.

Hypermagnesemia in Clinical Practice

Hypermagnesemia is a relatively uncommon but potentially life-threatening electrolyte disturbance characterized by elevated magnesium concentrations in the blood. Magnesium is a crucial mineral involved in various physiological functions, such as neuromuscular conduction, cardiac excitability, vasomotor tone, insulin metabolism, and muscular contraction. Hypomagnesemia is a prevalent electrolyte disturbance that can lead to several neuromuscular, cardiac, or nervous system disorders. Hypermagnesemia has been associated with adverse clinical outcomes, particularly in hospitalized patients. Prompt identification and management of hypermagnesemia are crucial to prevent complications, such as respiratory and cardiovascular negative outcomes, neuromuscular dysfunction, and coma. Preventing hypermagnesemia is crucial, particularly in high-risk populations, such as patients with impaired renal function or those receiving magnesium-containing medications or supplements. Clinical management of hypermagnesemia involves discontinuing magnesium-containing therapies, intravenous fluid therapy, or dialysis in severe cases. Furthermore, healthcare providers should monitor serum magnesium concentration in patients at risk of hypermagnesemia and promptly intervene if the concentration exceeds the normal range.

Metabolic changes in kidney stone disease

Kidney stone disease (KSD) is one of the earliest medical diseases known, but the mechanism of its formation and metabolic changes remain unclear. The formation of kidney stones is a extensive and complicated process, which is regulated by metabolic changes in various substances. In this manuscript, we summarized the progress of research on metabolic changes in kidney stone disease and discuss the valuable role of some new potential targets. We reviewed the influence of metabolism of some common substances on stone formation, such as the regulation of oxalate, the release of reactive oxygen species (ROS), macrophage polarization, the levels of hormones, and the alternation of other substances. New insights into changes in substance metabolism changes in kidney stone disease, as well as emerging research techniques, will provide new directions in the treatment of stones. Reviewing the great progress that has been made in this field will help to improve the understanding by urologists, nephrologists, and health care providers of the metabolic changes in kidney stone disease, and contribute to explore new metabolic targets for clinical therapy.

Haplotype of CaSR gene is associated with risk of renal stone disease in West Indian population

Calcium is the most abundant metabolite involved in the stone matrix. The CaSR gene controls calcium homeostasis, and genetic variation in the CaSR gene could lead to the development of renal stone disease. Therefore, the current study has been designed to assess the association of genetic variants of CaSR gene polymorphisms with renal stone disease. A single-centric prospective study has been carried out on a total of 300 participants (150 cases and 150 controls). Serum levels of calcium, creatinine, parathyroid hormone, and 24 h urine metabolites were measured. Two polymorphisms, rs1801725 and rs1042636, of the CaSR gene, have been genotyped for each participant. T test, binary logistic regression, and Chi-square analysis were used for statistical analysis. Renal stone patients had significantly higher levels of serum parathyroid hormone, creatinine, and 24-h urine metabolites in comparison to the controls. CaSR gene variants, rs1801725 (GG) and rs1042636 (AA), both have shown significant association with renal stone disease. In addition, individuals having specific genotypes along with metabolic abnormalities such as hypercalcemia and hyperparathyroidism are found to be at a higher significant risk of developing the renal stone disease. In the present study, the haplotype of the CaSR gene has shown an association with renal stone disease. Individuals with hyperparathyroidism and hypercalcemia and risk genotype have a higher susceptibility to developing renal stone disease.

Hypoparathyroidism: Genetics and Diagnosis

This narrative report summarizes diagnostic criteria for hypoparathyroidism and describes the clinical presentation and underlying genetic causes of the nonsurgical forms. We conducted a comprehensive literature search from January 2000 to January 2021 and included landmark articles before 2000, presenting a comprehensive update of these topics and suggesting a research agenda to improve diagnosis and, eventually, the prognosis of the disease. Hypoparathyroidism, which is characterized by insufficient secretion of parathyroid hormone (PTH) leading to hypocalcemia, is diagnosed on biochemical grounds. Low albumin-adjusted calcium or ionized calcium with concurrent inappropriately low serum PTH concentration are the hallmarks of the disease. In this review, we discuss the characteristics and pitfalls in measuring calcium and PTH. We also undertook a systematic review addressing the utility of measuring calcium and PTH within 24 hours after total thyroidectomy to predict long-term hypoparathyroidism. A summary of the findings is presented here; results of the detailed systematic review are published separately in this issue of JBMR. Several genetic disorders can present with hypoparathyroidism, either as an isolated disease or as part of a syndrome. A positive family history and, in the case of complex diseases, characteristic comorbidities raise the clinical suspicion of a genetic disorder. In addition to these disorders' phenotypic characteristics, which include autoimmune diseases, we discuss approaches for the genetic diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Classical and Nonclassical Manifestations of Primary Hyperparathyroidism

This narrative review summarizes data on classical and nonclassical manifestations of primary hyperparathyroidism (PHPT). It is based on a rigorous literature search, inclusive of a Medline search for systematic reviews from 1940 to December 2020, coupled with a targeted search for original publications, covering four databases, from January 2013-December 2020, and relevant articles from authors' libraries. We present the most recent information, identify knowledge gaps, and suggest a research agenda. The shift in the presentation of PHPT from a predominantly symptomatic to an asymptomatic disease, with its varied manifestations, has presented several challenges. Subclinical nephrolithiasis and vertebral fractures are common in patients with asymptomatic disease. The natural history of asymptomatic PHPT with no end organ damage at diagnosis is unclear. Some observational and cross-sectional studies continue to show associations between PHPT and cardiovascular and neuropsychological abnormalities, among the different disease phenotypes. Their causal relationship is uncertain. Limited new data are available on the natural history of skeletal, renal, cardiovascular, neuropsychological, and neuromuscular manifestations and quality of life. Normocalcemic PHPT (NPHPT) is often diagnosed without the fulfillment of rigorous criteria. Randomized clinical trials have not demonstrated a consistent long-term benefit of parathyroidectomy (PTX) versus observation on nonclassical manifestations. We propose further refining the definition of asymptomatic disease, into two phenotypes: one without and one with evidence of target organ involvement, upon the standard evaluation detailed in our recommendations. Each of these phenotypes can present with or without non-classical manifestations. We propose multiple albumin-adjusted serum calcium determinations (albumin-adjusted and ionized) and exclusion of all secondary causes of high parathyroid hormone (PTH) when establishing the diagnosis of NPHPT. Refining the definition of asymptomatic disease into the phenotypes proposed will afford insights into their natural history and response to interventions. This would also pave the way for the development of evidence-based guidance and recommendations. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Oral expressions and functional analyses of the extracellular calcium-sensing receptor (CaSR) in chicken

In vertebrates, the extracellular calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis by sensing slight changes in extracellular Ca²⁺. CaSR is also expressed in mammals including rodent taste cells and is involved in sensing kokumi, a rich, savory quality that enhances the intensities of salty, sweet, and umami tastes. In this study, we focused on chicken CaSR (cCaSR) since calcium is an essential nutrient that is necessary for making eggshell and for the extremely rapid initial growth of bones. First we confirmed that cCaSR is expressed in taste cells. Next we cloned the cCaSR gene from kidney and transiently transfected human embryonic kidney 293 T (HEK293T) cells with the recombinant cCaSR, or empty vector and looked for the agonists and allosteric modulators (including kokumi substances) of cCaSR by Ca²⁺ imaging. We found that cCaSR was activated by extracellular Ca²⁺ and Mg²⁺ in a dose dependent manner. Several L-amino acids and kokumi substances such as glutathione enhanced the response of cCaSR. In addition, NPS2143 as a negative allosteric modulator of human CaSR negatively modulated the response of cCaSR. These results suggest that cCaSR can sense extracellular Ca²⁺ and Mg²⁺ as well as positive and negative allosteric modulators. Taken together, the results imply that CaSR might be a multifunctional receptor for calcium, amino acids, and kokumi substances in chicken. The present finding that functional CaSR is expressed in the chicken oral tissues will allow us to further elucidate the physiological role of CaSR in the chickens' taste sense, and to create new feeds that will contribute to the poultry industry.

Metabolic Alkalosis Pathogenesis, Diagnosis, and Treatment: Core Curriculum 2022

Metabolic alkalosis is a widespread acid-base disturbance, especially in hospitalized patients. It is characterized by the primary elevation of serum bicarbonate and arterial pH, along with a compensatory increase in Pco2 consequent to adaptive hypoventilation. The pathogenesis of metabolic alkalosis involves either a loss of fixed acid or a net accumulation of bicarbonate within the extracellular fluid. The loss of acid may be via the gastrointestinal tract or the kidney, whereas the sources of excess alkali may be via oral or parenteral alkali intake. Severe metabolic alkalosis in critically ill patients-arterial blood pH of 7.55 or higher-is associated with significantly increased mortality rate. The kidney is equipped with sophisticated mechanisms to avert the generation or the persistence (maintenance) of metabolic alkalosis by enhancing bicarbonate excretion. These mechanisms include increased filtration as well as decreased absorption and enhanced secretion of bicarbonate by specialized transporters in specific nephron segments. Factors that interfere with these mechanisms will impair the ability of the kidney to eliminate excess bicarbonate, therefore promoting the generation or impairing the correction of metabolic alkalosis. These factors include volume contraction, low glomerular filtration rate, potassium deficiency, hypochloremia, aldosterone excess, and elevated arterial carbon dioxide. Major clinical states are associated with metabolic alkalosis, including vomiting, aldosterone or cortisol excess, licorice ingestion, chloruretic diuretics, excess calcium alkali ingestion, and genetic diseases such as Bartter syndrome, Gitelman syndrome, and cystic fibrosis. In this installment in the AJKD Core Curriculum in Nephrology, we will review the pathogenesis of metabolic alkalosis; appraise the precipitating events; and discuss clinical presentations, diagnoses, and treatments of metabolic alkalosis.

Autosomal Dominant Hypocalcemia Type 1: A Systematic Review

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism due to activating variants of the calcium-sensing receptor gene (CASR). Inherited or de novo activating variants of the CASR alter the set point for extracellular calcium, resulting in inadequate parathyroid hormone (PTH) secretion and inappropriate renal calcium excretion leading to hypocalcemia and hypercalciuria. Conventional therapy includes calcium and activated vitamin D, which can worsen hypercalciuria, resulting in renal complications. A systematic literature review, using published reports from 1994 to 2021, was conducted to catalog CASR variants, to define the ADH1 clinical spectrum, and to determine the effect of treatment on patients with ADH1. There were 113 unique CASR variants reported, with a general lack of genotype/phenotype correlation. Clinical data were available in 191 patients; 27% lacked symptoms, 32% had mild/moderate symptoms, and 41% had severe symptoms. Seizures, the most frequent clinical presentation, occurred in 39% of patients. In patients with blood and urine chemistries available at the time of diagnosis (n = 91), hypocalcemia (99%), hyperphosphatemia (59%), low PTH levels (57%), and hypercalciuria (34%) were observed. Blood calcium levels were significantly lower in patients with severe symptoms compared with asymptomatic patients (6.8 ± 0.7 versus 7.6 ± 0.7 mg/dL [mean ± SD]; p < 0.0001), and the age of presentation was significantly lower in severely symptomatic patients (9.1 ± 15.0 versus 19.3 ± 19.4 years; p < 0.01). Assessments for complications including nephrocalcinosis, nephrolithiasis, renal impairment, and brain calcifications in 57 patients on conventional therapy showed that 75% had at least one complication. Hypercalciuria was associated with nephrocalcinosis, nephrolithiasis, renal impairment, or brain calcifications (odds ratio [OR] = 9.3; 95% confidence interval [CI] 2.4-37.2; p < 0.01). In 27 patients with urine calcium measures before and after starting conventional therapy, the incidence of hypercalciuria increased by 91% (p < 0.05) after therapy initiation. ADH1 is a condition often associated with severe symptomatology at presentation with an increase in the risk of renal complications after initiation of conventional therapy. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Why Does Inflammation Result in Resorptive Bone Loss? What the Study of Burns Teaches Us

Burn injury serves as an example of a condition with a robust systemic inflammatory response. The elevation of circulating interleukins (IL)-1β and -6 in children and adolescents with severe burn injury upregulates the parathyroid calcium-sensing receptor (CaSR), resulting in hypocalcemic hypoparathyroidism accompanied by urinary calcium wasting. This effect protects the body from the hypercalcemia that results from bone resorption, liberating calcium into the circulation. Extracellular calcium can exacerbate and prolong the inflammatory response by stimulating mononuclear cell chemokine production as well as the NLRP3 inflammasome of the innate immune system, resulting in increased IL-1 production by monocytes and macrophages. Interestingly, the CaSR upregulation in response to inflammatory cytokines disappears with age, potentially trapping calcium from bone resorption in the circulation, allowing it to contribute to increased inflammation and possibly increased calcium deposition in small arteries, such as the coronaries, as conditions with increased chronic inflammation, such as spinal cord injury, osteoarthritis, and rheumatoid arthritis have an incidence of cardiovascular disease and coronary artery calcium deposition significantly higher than the unaffected age-matched population.

Reduced affinity of calcium sensing-receptor heterodimers and reduced mutant homodimer trafficking combine to impair function in a model of familial hypocalciuric hypercalcemia type 1

Heterozygous loss-of-function mutation of the calcium sensing-receptor (CaSR), causes familial hypocalciuric hypercalcemia type 1 (FHH1), a typically benign condition characterized by mild hypercalcemia. In contrast, homozygous mutation of this dimer-forming G-protein coupled receptor manifests as the lethal neonatal severe hyperparathyroidism (NSHPT). To investigate the mechanisms by which CaSR mutations lead to these distinct disease states, we engineered wild-type (WT) and an exon 5-deficient disease-causing mutation, and transfected expression constructs into human embryonic kidney (HEK) cells. WT protein was mainly membrane-expressed whereas the mutant CaSR protein (mCaSR) was confined to the cytoplasm. Co-expression of WT CaSR directed mCaSR to the cell membrane. In assays of CaSR function, increases in extracellular [Ca2+] ([Ca2+]o) increased intracellular [Ca2+] ([Ca2+]i) in cells expressing WT CaSR while the response was reduced in cells co-expressing mutant and WT receptor. Untransfected cells or those expressing mCaSR alone, showed minimal, equivalent responses to increased [Ca2+]o. Immunoprecipitation experiments confirmed an association between mutant and wild-type CaSR. The affinity of the WT CaSR for calcium was three times greater than that of the heterodimer. The maximal functional response to [Ca]o was dependent on localization of CaSR to the membrane level and independent of homo- or heterodimerizations. In summary, these results suggest that heterodimerization of WT and mCaSR receptors, rescues the trafficking defect of the mutant receptors and also reduces the affinity of the WT-mutant heterodimer for [Ca]o. In contrast, the homozygous mutants do not produce functional receptors on cell membrane. These data indicate how substantial differences between signaling of hetero- and homodimeric mutants may lead to profound differences in the severity of disease in heterozygous and homozygous carriers of these mutations.

The importance of kidney calcium handling in the homeostasis of extracellular fluid calcium

Extracellular fluid calcium concentration must be maintained within a narrow range in order to sustain many biological functions, encompassing muscle contraction, blood coagulation, and bone and tooth mineralization. Blood calcium value is critically dependent on the ability of the renal tubule to reabsorb the adequate amount of filtered calcium. Tubular calcium reabsorption is carried out by various and complex mechanisms in 3 distinct segments: the proximal tubule, the cortical thick ascending limb of the loop of Henle, and the late distal convoluted/connecting tubule. In addition, calcium reabsorption is tightly controlled by many endocrine, paracrine, and autocrine factors, as well as by non-hormonal factors, in order to adapt the tubular handling of calcium to the metabolic requirements. The present review summarizes the current knowledge of the mechanisms and factors involved in calcium handling by the kidney and, ultimately, in extracellular calcium homeostasis. The review also highlights some of our gaps in understanding that need to be addressed in the future.

Reduced blood circulating calcium level is an outstanding biomarker for preeclampsia among 48 types of human diseases

BACKGROUND

Calcium ion (Ca2+) is essential for human physiology by regulating various signal transduction pathways inside all cells and in the blood circulation.

AIM

We compared circulating Ca2+ levels in the healthy control against 48 different types of human diseases.

DESIGN

A total of 144 201 independent test results of Ca2+ levels from 48 clinically defined diseases and 141 679 independent test results of Ca2+ from healthy individuals who came to the hospital for annual physical examination were retrieved during the past 5 years.

METHODS

Ca2+ was determined by the standard 'Arsenazo III method' in the clinical laboratory of Affiliated Hospital of Qingdao University. We analyzed all data using RStudio V.1.3.1073 and python libraries 3.8.

RESULTS

All 48 types of diseases had decreased Ca2+ levels than the healthy control based on either mean or median values. Patients suffering from preeclampsia had the lowest Ca2+ levels among all 48 diseases. The perfect area under the curve, sensitivity, and specificity values of 1.0, 0.96 and 0.96 indicated that Ca2+ was an excellent biomarker for diagnosing preeclampsia. Extremely low Ca2+ was present in patients suffering kidney-related diseases. Since the correlation between each disease on the statistical features is proportional to their vector distance, the two-component analysis revealed that preeclampsia, sepsis and chronic obstructive pulmonary disease were closely related among 48 diseases.

CONCLUSIONS

All human diseases were associated with reduced circulating Ca2+ levels, where the decreased Ca2+ was a perfect biomarker for preeclampsia. Kidney-related conditions were related to over-down-regulation of Ca2+ levels. The resemblance of preeclampsia to sepsis and chronic obstructive pulmonary disease based on two-component analysis suggested that the three unrelated diseases might share a similar mechanism of the circulating Ca2+ regulation.

Serum protein and electrolyte imbalances are associated with chemotherapy induced neutropenia

Introduction

Cancer and its treatment using various chemotherapeutic agents can have many adverse side effects. These side effects often result in significant changes in haematological and biochemical composition of blood. As a result, the regular monitoring of serum biochemical and haematological changes plays an important role in management of disease. The present study aimed to determine the relationship between haematological and biochemical changes in neutropenic cancer patients following chemotherapy. Specifically we evaluated the association between neutrophil count and serum proteins and electrolytes.

Methods

For this purpose we analysed retrospectively collected laboratory results from two independent patient cohorts. Each cohort was divided into a control group consisting of patients with normal haematological parameters and a study group which included patients with reduced neutrophil counts. Neutropenic patients (study group) were cancer patients on chemotherapy.

Results and conclusion

Blood samples of cancer patients in study group showed reduction in haemoglobin, neutrophils and platelets. Neutropenic group showed a significant reduction in serum albumin, total protein, calcium, and potassium. Our results show that patients with severe neutropenia had pronounced changes in serum protein and electrolytes and increased incidence of abnormal serum protein and electrolyte level. The changes in the neutrophil counts showed a positive correlation with the changes in serum protein and electrolyte levels. A similar trend was seen in both the patient cohorts: the discovery set (176 patients) and the validation set (200 patients). Taken together our results suggest that chemotherapy-induced neutropenia is associated with dysregulation in haemoglobin, platelets, serum proteins and electrolytes.

Impaired Mineral Ion Metabolism in a Mouse Model of Targeted Calcium-Sensing Receptor (CaSR) Deletion from Vascular Smooth Muscle Cells

BACKGROUND

Impaired mineral ion metabolism is a hallmark of CKD-metabolic bone disorder. It can lead to pathologic vascular calcification and is associated with an increased risk of cardiovascular mortality. Loss of calcium-sensing receptor (CaSR) expression in vascular smooth muscle cells exacerbates vascular calcification in vitro. Conversely, vascular calcification can be reduced by calcimimetics, which function as allosteric activators of CaSR.

METHODS

To determine the role of the CaSR in vascular calcification, we characterized mice with targeted Casr gene knockout in vascular smooth muscle cells ( ^SM22α CaSR ^{Δflox/Δflox} ).

RESULTS

Vascular smooth muscle cells cultured from the knockout (KO) mice calcified more readily than those from control (wild-type) mice in vitro. However, mice did not show ectopic calcifications in vivo but they did display a profound mineral ion imbalance. Specifically, KO mice exhibited hypercalcemia, hypercalciuria, hyperphosphaturia, and osteopenia, with elevated circulating fibroblast growth factor 23 (FGF23), calcitriol (1,25-D₃), and parathyroid hormone levels. Renal tubular α-Klotho protein expression was increased in KO mice but vascular α-Klotho protein expression was not. Altered CaSR expression in the kidney or the parathyroid glands could not account for the observed phenotype of the KO mice.

CONCLUSIONS

These results suggest that, in addition to CaSR's established role in the parathyroid-kidney-bone axis, expression of CaSR in vascular smooth muscle cells directly contributes to total body mineral ion homeostasis.

The circadian rhythm of calcium and bone homeostasis in Maasai

OBJECTIVES

Ethnic groups differ in prevalence of calcium-related diseases. Differences in the physiology and the endogenous circadian rhythm (CR) of calcium and bone homeostasis may play a role. Thus, we aimed to investigate details of CR pattern in calcium and bone homeostasis in East African Maasai.

METHODS

Ten clinically healthy adult Maasai men and women from Tanzania were examined. Blood samples were collected every 2nd hour for 24 h. Serum levels of total calcium, albumin, parathyroid hormone (PTH), 25(OH)D, creatinine, C-terminal telopeptide (CTX), bone-specific alkaline phosphatase (BSAP), procollagen type 1 N-terminal propeptide (P1NP), and osteocalcin were measured. Circadian patterns were derived from graphic curves of medians, and rhythmicity was assessed with Fourier analysis.

RESULTS

PTH-levels varied over the 24 h exhibiting a bimodal pattern. Nadir level corresponded to 65% of total 24-h mean. CTX and P1NP showed 24-h variations with a morning nadir and nocturnal peak with nadir levels corresponding to 23% and 79% of the 24-h mean, respectively. Albumin-corrected calcium level was held in a narrow range and alterations were corresponding to alterations in PTH. There was no distinct pattern in 24-h variations of 25(OH)D, creatinine, osteocalcin, or BSAP.

CONCLUSIONS

All participants showed pronounced 24-h variations in PTH and bone turnover markers CTX and P1NP. These findings support that Maasai participants included in this study have typical patterns of CR in calcium and bone homeostasis consistent with findings from other ethnic populations.

Hypercalcemia, Acute Kidney Injury, and Metabolic Alkalosis

Calcium regulation is tightly controlled in the body. Multiple causes of hypercalcemia have been studied including primary hyperparathyroidism, hypercalcemia of malignancy, and chronic granulomatous disorders. Among the less studied causes is calcium-alkali syndrome. Here, we discuss a case of hypercalcemia secondary to calcium-alkali syndrome, presenting with hypercalcemia, metabolic alkalosis, and acute kidney injury as a result of ingestion of a large amount of calcium supplements. Hypercalcemia can result in impaired collecting duct system sensitivity to antidiuretic hormone, afferent arteriole constriction, and activation of calcium sensor receptors in multiple tissues. The net effect is an increase in calcium reabsorption with a salt and water diuresis which leads to volume depletion, acute kidney injury, and metabolic alkalosis.

Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis Due to CLDN16 Gene Mutations: Novel Findings in Two Cases with Diverse Clinical Features

Biallelic loss of function mutations in the CLDN16 gene cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), and chronic kidney disease. Here we report two cases of FHHNC with diverse clinical presentations and hypercalcemia in one as a novel finding. Pt#1 initially presented with urinary tract infection and failure to thrive at 5.5 months of age to another center. Bilateral nephrocalcinosis, hypercalcemia (Ca: 12.2 mg/dl), elevated parathyroid hormone (PTH) level, and hypercalciuria were detected. Persistently elevated PTH with high/normal Ca levels led to subtotal-parathyroidectomy at the age of 2.5. However, PTH levels remained elevated with progressive deterioration in renal function. At 9-year-old, she was referred to us for evaluation of hyperparathyroidism and, hypomagnesemia together with hypercalciuria, elevated PTH with normal Ca levels, and medullary nephrocalcinosis were detected. Compound heterozygosity of CLDN16 variants (c.715G>A, p.G239R; and novel c.360C>A, p.C120*) confirmed the diagnosis. Pt#2 was a 10-month-old boy, admitted with irritability and urinary crystals. Hypocalcemia, hypophosphatemia, elevated PTH and ALP, low 25(OH)D levels, and radiographic findings of rickets were detected. However, additional findings of hypercalciuria and bilateral nephrocalcinosis were inconsistent with the nutritional rickets. Low/normal serum Mg levels suggested the diagnosis of FHHNC which was confirmed genetically as a homozygous missense (c.602G > A; p.G201E) variant in CLDN16. Yet, hypocalcemia and hypomagnesemia persisted in spite of treatment. In conclusion, FHHNC may present with diverse clinical features with mild hypomagnesemia leading to secondary hyperparathyroidism with changing Ca levels from low to high. Early and accurate clinical and molecular genetic diagnosis is important for proper management.

An optogenetic approach for regulating human parathyroid hormone secretion

Parathyroid hormone (PTH) plays crucial role in maintaining calcium and phosphorus homeostasis. In the progression of secondary hyperparathyroidism (SHPT), expression of calcium-sensing receptors (CaSR) in the parathyroid gland decreases, which leads to persistent hypersecretion of PTH. How to precisely manipulate PTH secretion in parathyroid tissue and underlying molecular mechanism is not clear. Here, we establish an optogenetic approach that bypasses CaSR to inhibit PTH secretion in human hyperplastic parathyroid cells. We found that optogenetic stimulation elevates intracellular calcium, inhibits both PTH synthesis and secretion in human parathyroid cells. Long-term pulsatile PTH secretion induced by light stimulation prevented hyperplastic parathyroid tissue-induced bone loss by influencing the bone remodeling in mice. The effects are mediated by light stimulation of opsin expressing parathyroid cells and other type of cells in parathyroid tissue. Our study provides a strategy to regulate release of PTH and associated bone loss of SHPT through an optogenetic approach.

Parathyroid hormone (PTH) plays a role in maintaining calcium and phosphorus homeostasis, and in secondary hyperparathyroidism excess PTH secretion contributes to bone loss. Here the authors report an optogenetic approach to inhibit PTH secretion in human hyperplastic parathyroid cells, and prevented hyperplastic parathyroid tissue-induced bone loss in mice.

Role of claudins in idiopathic hypercalciuria and renal lithiasis

Paracellular transport in the kidney is mediated by a family of proteins located in the tight junctions called claudins which confers its ionic selectivity. Claudin-2 is highly expressed in the proximal tubule and descending limb of Henle and mediate paracellular reabsorption of sodium and calcium cations. In the thick ascending limb of Henle (TALH) calcium is reabsorbed by a paracellular channel formed by Claudin-16 and-19. Claudin-16 mediates cationic permeability while Claudin-19 increases the cationic selectivity of Claudin-16 by blocking anionic permeability. On the other hand, Claudin 14, that is also located in TALH, inhibits the paracellular permeability of Claudin-16 to calcium. Recent wide genomic association analysis studies have detected four common synonymous variants (genetic polymorphisms of a single nucleotide, SNPs) at the locus of Claudin-14 gene that were significantly associated with the presence of renal lithiasis. Another study of wide genomic association and nephrolithiasis was carried out in the general population but including chromosome X, where claudin-2 gene is located. They detected nine SNPs that had a significant association with renal lithiasis risk. A greater knowledge of the paracellular pathway controlled by claudins and its regulation will allow us to develop future new treatments for idiopathic hypercalciuria and renal lithiasis.

Density-Dependent Differentiation of Tonsil-Derived Mesenchymal Stem Cells into Parathyroid-Hormone-Releasing Cells

Mesenchymal stem cells (MSCs) can differentiate into endoderm lineages, especially parathyroid-hormone (PTH)-releasing cells. We have previously reported that tonsil-derived MSC (T-MSC) can differentiate into PTH-releasing cells (T-MSC-PTHCs), which restored the parathyroid functions in parathyroidectomy (PTX) rats. In this study, we demonstrate quality optimization by standardizing the differentiation rate for a better clinical application of T-MSC-PTHCs to overcome donor-dependent variation of T-MSCs. Quantitation results of PTH mRNA copy number in the differentiated cells and the PTH concentration in the conditioned medium confirmed that the differentiation efficiency largely varied depending on the cells from each donor. In addition, the differentiation rate of the cells from all the donors greatly improved when differentiation was started at a high cell density (100% confluence). The large-scale expression profiling of T-MSC-PTHCs by RNA sequencing indicated that those genes involved in exiting the differentiation and the cell cycle were the major pathways for the differentiation of T-MSC-PTHCs. Furthermore, the implantation of the T-MSC-PTHCs, which were differentiated at a high cell density embedded in hyaluronic acid, resulted in a higher serum PTH in the PTX model. This standardized efficiency of differentiation into PTHC was achieved by initiating differentiation at a high cell density. Our findings provide a potential solution to overcome the limitations due to donor-dependent variation by establishing a standardized differentiation protocol for the clinical application of T-MSC therapy in treating hypoparathyroidism.

Occult Renal Calcifications in Patients with Normocalcemic Primary Hyperparathyroidism and Their Association with the Parathyroid Hormone-Vitamin D Axis

Normocalcemic primary hyperparathyroidism (NPHPT) is characterized by elevated serum levels of parathyroid hormone (PTH) with persistently normal serum calcium concentrations after excluding secondary causes of hyperparathyroidism. Urolithiasis and/or nephrocalcinosis may occur in hypercalcemic PHPT, but little is known about these complications in NPHPT. Objectives. To identify occult urolithiasis and nephrocalcinosis in asymptomatic patients with NPHPT and evaluate biochemical markers as risk predictors for the development of renal calcification (RC). Methods. Cross-sectional analysis of 34 patients with no history of urolithiasis and/or nephrocalcinosis. The diagnosis of NPHPT was as follows: elevated serum PTH (reference range: 15-65 pg/mL), normal albumin-corrected serum calcium, normal urinary calcium excretion, serum 25(OH)D >30 ng/mL, eGFR (CKD-EPI) > 60 mL/min/1.73 m², without intestinal disease, and not on medications such as thiazide diuretics, lithium, bisphosphonates, or denosumab. Patients were categorized according to the presence or absence of RC identified by renal imaging. Their clinical and biochemical characteristics were then compared. Results. The patients had a mean age of 67.97 ± 10.45 years, predominantly postmenopausal women (88.2%); serum PTH, 119.67 ± 64.44 pg/mL; 25(OH)D, 39.00 ± 8.88 ng/dL; 1.25(OH))₂D, 74.53 ± 26.37 pg/mL; corrected serum calcium, 9.34 ± 0.62 mg/dL; and 24-hour urinary calcium, 134.87 ± 79.68 mg/day. RC was identified in 26.5% of the patients. There was no difference in anthropometric and clinical parameters, renal function, 25(OH)D, and urinary pH in patients with or without RC. Patients with RC had higher PTH values (176.22 vs. 99.32 pg/mL, P = 0.001), 1.25(OH) 2D (96.83 vs. 62.36 pg/mL, P = 0.005), and 24-hour urinary calcium (181.9 vs. 117.94 mg/day, P = 0.037). Conclusion. Occult renal calcifications are common in NPHPT and are associated with increased serum PTH, 1.25(OH))₂D, and 24 h urinary calcium.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

Drug-Related Hypercalcemia

This review focuses on the commonly prescribed medicaments that can be responsible for hypercalcemia, considering the prevalence, the predominant pathophysiological mechanisms, and the optimal medical management of each drug-induced hypercalcemia. Vitamin D supplements and 1α-hydroxylated vitamin D analogues increase intestinal calcium absorption, renal calcium reabsorption as well as bone resorption. In patients with hypoparathyroidism receiving recombinant human PTH, transient hypercalcemia can occur because of overtreatment, usually during acute illness. Thiazide-induced hypercalcemia is mainly explained by enhanced renal proximal calcium reabsorption, changing preexistent asymptomatic normocalcemic or intermittently hypercalcemic hyperparathyroidism into the classic hypercalcemic hyperparathyroidism. Lithium causes hypercalcemia mainly by drug-induced hyperparathyroidism.

Effects of Sodium Ferulate on Cardiac Hypertrophy Are via the CaSR-Mediated Signaling Pathway

As a common complication of many cardiovascular diseases, cardiac hypertrophy is characterized by increased cardiac cell volume, reorganization of the cytoskeleton, and the reactivation of fetal genes such as cardiac natriuretic peptide and β-myosin heavy chain. Cardiac hypertrophy is a distinguishing feature of some cardiovascular diseases. Our previous study showed that sodium ferulate (SF) alleviates myocardial hypertrophy induced by coarctation of the abdominal aorta, and these protective effects may be related to the inhibition of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) signaling pathways. This study investigated the inhibitory effect and mechanism of SF on myocardial hypertrophy in spontaneously hypertensive rats (SHRs). The effects of SF on cardiac hypertrophy were evaluated using echocardiographic measurement, pathological analysis, and detection of atrial natriuretic peptide (ANP) and β-myosin heavy chain (β-MHC) expression. To investigate the mechanisms underlying the anti-hypertrophic effects of SF, the calcium-sensing receptor (CaSR), calcineurin (CaN), nuclear factor of activated T cells 3 (NFAT3), zinc finger transcription factor 4 (GATA4), protein kinase C beta (PKC-β), Raf-1, extracellular signal-regulated kinase 1/2 (ERK 1/2), and mitogen-activated protein kinase phosphatase-1 (MKP-1) were detected by molecular biology techniques. Treatment with SF ameliorated myocardial hypertrophy in 26-week-old SHRs. In addition, it downregulated the levels of ANP, β-MHC, CaSR, CaN, NFAT3, phosphorylated GATA4 (p-GATA4), PKC-β, Raf-1, and p-ERK 1/2; and upregulated the levels of p-NFAT3 and MKP-1. These results suggest that the effects of SF on cardiac hypertrophy are related to regulation of the CaSR-mediated signaling pathway.

Urothelial Calcium-Sensing Receptor Modulates Micturition Function via Mediating Detrusor Activity and Ameliorates Bladder Hyperactivity in Rats

The urothelium displays mechano- and chemosensory functions via numerous receptors and channels. The calcium-sensing receptor (CaSR) detects extracellular calcium and modulates several physiological functions. Nonetheless, information about the expression and the role of CaSR in lower urinary tract has been absent. We aimed to determine the existence of urothelial CaSR in urinary bladder and its effect on micturition function. We utilized Western blot to confirm the expression of CaSR in bladder and used immunofluorescence to verify the location of the CaSR in the bladder urothelium via colocalization with uroplakin III A. The activation of urothelial CaSR via the CaSR agonist, AC-265347 (AC), decreased urinary bladder smooth muscle (detrusor) activity, whereas its inhibition via the CaSR antagonist, NPS-2143 hydrochloride (NPS), increased detrusor activity in in vitro myography experiments. Cystometry, bladder nerve activities recording, and bladder surface microcirculation detection were conducted to evaluate the effects of the urothelial CaSR via intravesical administrations. Intravesical AC inhibited micturition reflex, bladder afferent and efferent nerve activities, and reversed cystitis-induced bladder hyperactivity. The urothelial CaSR demonstrated a chemosensory function, and modulated micturition reflex via regulating detrusor activity. This study provided further evidence of how the urothelial CaSR mediated micturition and implicated the urothelial CaSR as a potential pharmacotherapeutic target in the intervention of bladder disorders.

A Rare Case of Patiromer Induced Hypercalcemia

Patiromer is a calcium (Ca)-potassium (K) exchange resin approved for the treatment of hyperkalemia. Disorders of Ca or acid base balance were not reported in pre-approval clinical trials. We present a case of a patient with chronic kidney disease (CKD) with an unusual picture of hypercalcemia, metabolic alkalosis and hypokalemia upon intensification of patiromer dosing. A 56-year-old white man with CKD stage 4 (baseline creatinine 2.8 mg/dL) due to type 1 diabetes mellitus, proteinuria (1.5 g/g) and persistently high serum potassium 5.9 mEq/L attributed to type 4 renal tubular acidosis was evaluated in clinic. Due to high risk of CKD progression, patiromer 8.4 g daily, followed by 16.8 g daily was prescribed to enable renin angiotensin aldosterone system (RAAS) inhibitor. After 5 months of being on patiromer 16.8 g daily, routine laboratory tests revealed serum potassium 2.5 mEq/L, serum calcium 12.8 mg/dL and carbon dioxide 34 mEq/L. Patiromer was discontinued and thorough investigation held was negative for other causes of hypercalcemia. Five days after patiromer discontinuation, serum calcium returned to normal. The role of secondary hyperparathyroidism in this case remains unclear. We, therefore recommend cautious vigilance of patients receiving patiromer and undergoing dose escalation.

Effects of diets differing in dietary cation-anion difference and calcium concentration on calcium homeostasis in neutered male sheep

Feeding low dietary cation-anion difference (DCAD) diets is one strategy to prevent milk fever in cows. The mechanism of action, as well as whether the calcium (Ca) supply of such diets combined with this feeding regimen should meet the requirements, is still unclear. Small ruminants are commonly used as models for cows. The goal of the present study was to demonstrate basic effects of DCAD against a background of different Ca supplies in a sheep model. Twenty-three castrated male East Friesian milk sheep, aged 11 to 12 mo, were randomly assigned to 4 different feeding groups. The ration of each group was either high (highDCAD) or low in DCAD (lowDCAD) combined with adequate (nCa) or restricted Ca supply (lowCa). At baseline, serum and urine were collected from all sheep and a peripheral quantitative computed tomography of the left metatarsus was performed. After a 14-d adaptation period to the different diets, the experiment started (d 0). Urine, feces, and serum were collected on d 0, 4, 7, 14, and 22, and peripheral quantitative computed tomography was performed on d 0 and 22. On d 22, the sheep were killed and sampled for functional studies. LowDCAD was significantly associated with lower urine pH, higher urinary Ca excretion, higher ionized Ca in blood, and higher serum Ca concentrations. Blood pH and bone parameters did not differ significantly between groups. It is unclear from which compartment the high amounts of Ca excreted with urine in the lowDCAD groups originated. Interestingly, lowDCAD resulted in higher renal mRNA abundance of parathyroid hormone receptor but unaffected mRNA abundance of Ca transporters. As neither renal abundance of these transporters nor Ca excretion were influenced by dietary Ca supply, our results support the hypothesis that increased urinary Ca observed with low DCAD diets represents a loss rather than an excretion of surplus Ca.

Therapy with PTH 1-34 or calcitriol and calcium in diverse etiologies of hypoparathyroidism over 27 years at a single tertiary care center

OBJECTIVE

Hypoparathyroidism has heterogeneous genetic and acquired etiologies with a broad spectrum of severity. Herein we describe the clinical outcomes of the largest cohort of hypoparathyroid patients reported to date, who were followed over 27-years.

DESIGN

Pooled analysis of current and past studies describing the differential responses to PTH 1-34 injections vs conventional therapy among the varied hypoPT etiologies.

METHODS

192 participants (ages 2-74 years) with hypoparathyroidism who received either calcitriol and calcium or PTH 1-34 by subcutaneous injection.

RESULTS

Among the 4 main etiologic categories of hypoparathyroidism (autoimmune polyglandular failure type 1, activating mutation of the calcium receptor, surgical, and idiopathic hypoparathyroidism), we reveal significant differences in PTH 1-34 dose requirements, prevalence of nephrocalcinosis, biomarkers of mineral homeostasis, and pharmacodynamic profiles. Serum 1,25-dihydroxyvitamin D₃ increased significantly (P < 0.001) and 25-hydroxyvitamin D levels decreased during PTH 1-34 injections compared to calcitriol therapy (P < 0.01). Post-surgical patients achieved consistently lower urine calcium excretion over long-term PTH 1-34 therapy compared to conventional therapy (p < 0.001), but this was not achieved in the other etiologies. At study entry, patients had a high prevalence of renal insufficiency and nephrocalcinosis which were directly related to the duration of hypoparathyroidism (P < 0.03). Renal function remained stable during participation in our studies for both PTH 1-34 and conventional therapies.

CONCLUSIONS

We conclude that the effects and dose-response of PTH 1-34 treatment differ according to the etiology of hypoparathyroidism. Postsurgical hypoPT maintained mean serum calcium levels in the mid- to low-normal range while concurrently maintaining normal mean urine calcium during long-term twice-daily PTH 1-34 therapy.

Secondary Hyperparathyroidism in Chronic Kidney Disease: Pathophysiology and Management

Serum calcium concentration is the main determinant of parathyroid hormone (PTH) release. Defect in the activation of vitamin D in the kidneys due to chronic kidney disease (CKD) leads to hypocalcemia and hyperphosphatemia, resulting in a compensatory increase in parathyroid gland cellularity and parathyroid hormone production and causing secondary hyperparathyroidism (SHP). Correction and maintenance of normal serum calcium and phosphate are essential to preventing SHP, hungry bone disease, cardiovascular events, and anemia development. Understanding the pathophysiology of PTH and possible therapeutic agents can reduce the development and associated complications of SHP in patients with CKD. Medical interventions to control serum calcium, phosphate, and PTH such as vitamin D analogs, calcium receptor blockers, and parathyroidectomy are needed in some CKD patients. In this review, we discuss the pathophysiology, clinical presentation, and management of SHP in CKD patients.

Late diagnosis of chronic hypocalcemia due to autoimmune hypoparathyroidism

Hypoparathyroidism is most often the result of postsurgical damage to the parathyroid glands but may occasionally be autoimmune hypoparathyroidism. In the latter context, activating antibodies directed against the calcium-sensing receptor (CaSR) have been described. We hereby present the case of a patient suffering from chronic recurrent muscle cramps and paresthesia, presenting for a seizure due to hypocalcaemia. After eliminating the possibility of a genetic disorder, we searched for autoimmune hypoparathyroidism as there was no obvious cause of hypoparathyroidism. The search for anti-CaSR antibodies was positive. There was no argument for autoimmune polyendocrine syndrome type 1 so we concluded that it was isolated autoimmune hypoparathyroidism caused by activating antibodies to the CaSR. The patient was treated with vitamin D and calcium supplementation. The search for complications of hypoparathyroidism and hypercalciuria revealed basal ganglia calcification. The patient's hypocalcaemia is now being kept under control with oral supplementation.

Effects of intravenous magnesium sulfate on serum calcium-regulating hormones and plasma and urinary electrolytes in healthy horses

Intravenous magnesium sulfate (MgSO₄) is used in equine practice to treat hypomagnesemia, dysrhythmias, neurological disorders, and calcium dysregulation. MgSO₄ is also used as a calming agent in equestrian events. Hypercalcemia affects calcium-regulating hormones, as well as plasma and urinary electrolytes; however, the effect of hypermagnesemia on these variables is unknown. The goal of this study was to investigate the effect of hypermagnesemia on blood parathyroid hormone (PTH), calcitonin (CT), ionized calcium (Ca²⁺), ionized magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl^-) and their urinary fractional excretion (F) after intravenous administration of MgSO₄ in healthy horses. Twelve healthy female horses of 4–18 years of age and 432–600 kg of body weight received a single intravenous dose of MgSO₄ (60 mg/kg) over 5 minutes, and blood and urine samples were collected at different time points over 360 minutes. Plasma Mg²⁺ concentrations increased 3.7-fold over baseline values at 5 minutes and remained elevated for 120 minutes (P < 0.05), Ca²⁺ concentrations decreased from 30–60 minutes (P < 0.05), but Na⁺, K⁺ and Cl^- concentrations did not change. Serum PTH concentrations dropped initially to rebound and remain elevated from 30 to 60 minutes, while CT concentrations increased at 5 minutes to return to baseline by 10 minutes (P < 0.05). The FMg, FCa, FNa, FK, and FCl increased, while urine osmolality decreased from 30–60 minutes compared baseline (P < 0.05). Short-term experimental hypermagnesemia alters calcium-regulating hormones (PTH, CT), reduces plasma Ca²⁺ concentrations, and increases the urinary excretion of Mg²⁺, Ca²⁺, K⁺, Na⁺ and Cl^- in healthy horses. This information has clinical implications for the short-term effects of hypermagnesemia on calcium-regulation, electrolytes, and neuromuscular activity, in particular with increasing use of Mg salts to treat horses with various acute and chronic conditions as well as a calming agent in equestrian events.

Heterogeneity of G protein activation by the calcium-sensing receptor

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that plays a fundamental role in extracellular calcium (Ca2+e) homeostasis by regulating parathyroid hormone release and urinary calcium excretion. The CaSR has been described to activate all four G protein subfamilies (Gαq/11, Gαi/o, Gα12/13, Gαs), and mutations in the receptor that cause hyper/hypocalcaemia, have been described to bias receptor signalling. However, many of these studies are based on measurements of second messengers or gene transcription that occurs many steps downstream of receptor activation and can represent convergence points of several signalling pathways. Therefore, to assess CaSR-mediated G protein activation directly, we took advantage of a recently described NanoBiT G protein dissociation assay system. Our studies, performed in HEK293 cells stably expressing CaSR, demonstrate that Ca2+e stimulation activates all Gαq/11 family and several Gαi/o family proteins, although Gαz was not activated. CaSR stimulated dissociation of Gα12/13 and Gαs from Gβ-subunits, but this occurred at a slower rate than that of other Gα-subunits. Investigation of cDNA expression of G proteins in three tissues abundantly expressing CaSR, the parathyroids, kidneys and pancreas, showed Gα11, Gαz, Gαi1 and Gα13 genes were highly expressed in parathyroid tissue, indicating CaSR most likely activates Gα11 and Gαi1 in parathyroids. In kidney and pancreas, the majority of G proteins were similarly expressed, suggesting CaSR may activate multiple G proteins in these cells. Thus, these studies validate a single assay system that can be used to robustly assess CaSR variants and biased signalling and could be utilised in the development of new pharmacological compounds targeting CaSR.

Phosphocalcic metabolism and the role of vitamin D, vitamin K2, and nattokinase supplementation

Calcium is involved in bone metabolism, regulation of nerve signaling, and release of neurotransmitters. Phosphorus is a structural component of ATP, participates in metabolic energy regulation, and ensures stability to biological membranes and cells. Vitamin D and vitamin K are important for intestinal absorption and renal excretion of calcium and phosphorus. Vitamin D plays a regulatory role in bone formation, carbohydrate metabolism, immune responses, and cardiovascular regulation. Research has linked vitamin D deficiency to the development of diabetes mellitus, hypertension, cancer, and osteoporosis. Vitamin K has been associated with a reduced risk of osteoporosis, cancer, and cardiovascular diseases (due to improved vascular elasticity). This review highlights the importance of vitamins D and K in the metabolism of calcium and phosphorus and explores various molecular mechanisms that help maintain the system's mineral homeostasis. Moreover, the paper reviews the enzyme nattokinase's role in thrombotic prevention due to its fibrinolytic activity.

An infant with congenital nephrogenic diabetes insipidus presenting with hypercalcemia and hyperphosphatemia

SUMMARY

We report a male infant with congenital nephrogenic diabetes insipidus (NDI) who presented with hypercalcemia and hyperphosphatemia since birth. Serum sodium started to increase at 39 days. Although there was no polyuria, urine osmolality was 71 mOsm/kg, when serum osmolality was 296 mOsm/kg with plasma arginine vasopressin 22.5 pg/mL. He was thus diagnosed as NDI. An undetectable level of urine calcium and unsuppressed intact parathyroid hormone suggested hyperparathyroidism including calcium-sensing receptor mutations that could cause hypercalcemia-induced NDI. Polyuria became apparent after the initiation of i.v. infusion for the treatment of hypernatremia. Low calcium and low sodium formula with hypotonic fluid infusion did not correct hypernatremia, hypercalcemia, or hyperphosphatemia. Hydrochlorothiazide and subsequently added celecoxib effectively decreased urine output and corrected electrolytes abnormalities. Normal serum electrolytes were maintained after the discontinuation of low calcium formula. The genetic analysis revealed a large deletion of the arginine vasopressin receptor-2 (AVPR2) gene but no pathogenic variant in the calcium-sensing receptor (CASR) gene. Whether hypercalcemia and hyperphosphatemia were caused by dehydration alone or in combination with other mechanisms remains to be clarified.

LEARNING POINTS

Congenital NDI can present with neonatal hypercalcemia and hyperphosphatemia. Hypercalcemia and hyperphosphatemia can be treated with low calcium and low sodium formula, hydration, hydrochlorothiazide, and celecoxib. Genetic testing is sometimes necessary in the differentiating diagnosis of hypercalcemia associated with NDI.

Crosstalk between Renal and Vascular Calcium Signaling: The Link between Nephrolithiasis and Vascular Calcification

Calcium (Ca²⁺) is an important mediator of multicellular homeostasis and is involved in several diseases. The interplay among the kidney, bone, intestine, and parathyroid gland in Ca²⁺ homeostasis is strictly modulated by numerous hormones and signaling pathways. The calcium-sensing receptor (CaSR) is a G protein–coupled receptor, that is expressed in calcitropic tissues such as the parathyroid gland and the kidney, plays a pivotal role in Ca²⁺ regulation. CaSR is important for renal Ca²⁺, as a mutation in this receptor leads to hypercalciuria and calcium nephrolithiasis. In addition, CaSR is also widely expressed in the vascular system, including vascular endothelial cells (VECs) and vascular smooth muscle cells (VSMCs) and participates in the process of vascular calcification. Aberrant Ca²⁺ sensing by the kidney and VSMCs, owing to altered CaSR expression or function, is associated with the formation of nephrolithiasis and vascular calcification. Based on emerging epidemiological evidence, patients with nephrolithiasis have a higher risk of vascular calcification, but the exact mechanism linking the two conditions is unclear. However, a dysregulation in Ca²⁺ homeostasis and dysfunction in CaSR might be the connection between the two. This review summarizes renal calcium handling and calcium signaling in the vascular system, with a special focus on the link between nephrolithiasis and vascular calcification.

Thick ascending limb claudins are altered to increase calciuria and magnesiuria in metabolic acidosis

Urinary calcium and magnesium wasting is a characteristic feature of metabolic acidosis, and this study focused on the role of the thick ascending limb of Henle's loop in metabolic acidosis-induced hypercalciuria and hypermagnesiuria because thick ascending limb is an important site of paracellular calcium and magnesium reabsorption. Male Sprague-Dawley rats were used to determine the effects of acid loading (by adding NH4Cl, 7.2 mmol/220 g body wt/day to food slurry for 7 days) on renal expression of claudins and then to evaluate whether the results were reversed by antagonizing calcium-sensing receptor (using NPS-2143). At the end of each animal experiment, the kidneys were harvested for immunoblotting, immunofluorescence microscopy, and quantitative PCR (qPCR) analysis of claudins and the calcium-sensing receptor. As expected, NH4Cl loading lowered urinary pH and increased excretion of urinary calcium and magnesium. In NH4Cl-loaded rats, renal protein and mRNA expression of claudin-16, and claudin-19, were decreased compared with controls. However, claudin-14 protein and mRNA increased in NH4Cl-loaded rats. Consistently, the calcium-sensing receptor protein and mRNA were up-regulated in NH4Cl-loaded rats. All these changes were reversed by NPS-2143 coadministration and were confirmed using immunofluorescence microscopy. Hypercalciuria and hypermagnesiuria in NH4Cl-loaded rats were significantly ameliorated by NPS-2143 coadministration as well. We conclude that in metabolic acidosis, claudin-16 and claudin-19 in the thick ascending limb are down-regulated to produce hypercalciuria and hypermagnesiuria via the calcium-sensing receptor.NEW & NOTEWORTHY This study found that the thick ascending limb of Henle's loop is involved in the mechanisms of hypercalciuria and hypermagnesiuria in metabolic acidosis. Specifically, expression of claudin-16/19 and claudin-14 was altered via up-regulation of calcium-sensing receptor in NH4Cl-induced metabolic acidosis. Our novel findings contribute to understanding the regulatory role of paracellular tight junction proteins in the thick ascending limb.

Molecular distribution and localization of extracellular calcium-sensing receptor (CaSR) and vitamin D receptor (VDR) at three different laying stages in laying hens (Gallus gallus domesticus)

The extracellular calcium-sensing receptor (CaSR) and vitamin D receptor (VDR) play important roles in regulating calcium mobilization, calcium absorption, and calcium homeostasis, and they could be potential therapeutic targets to osteoporosis in laying hens. The present study investigated the molecular distribution of CaSR and VDR and the localization of CaSR in the kidney, proventriculus (true stomach), duodenum, jejunum, ileum, colon, cecum, shell gland, and tibia of laying hens at 3 different laying stages (19, 40, and 55 wk). The results showed that the relative mRNA abundance of CaSR in the kidney, ileum, proventriculus, duodenum, and colon was higher (P < 0.05) than the other tissues at 40 and 55 wk. The relative mRNA abundance of CaSR in the tibia was higher (P < 0.05) at 55 wk than at 40 wk. However, there were no significant differences in the relative protein abundance of CaSR among all tested tissues at peak production or in each tissue at the 3 different laying stages (P > 0.05). The relative mRNA abundance of VDR was higher (P < 0.05) in the small intestine (duodenum, jejunum, and ileum) when compared with other tissues at the 3 different laying stages. The relative protein abundance of VDR in the duodenum was higher (P < 0.05) than that in the proventriculus, colon, and cecum. There were no significant differences in the VDR expression among the tested tissues at the 3 different laying stages (P > 0.05). The immunohistochemical results showed that the positive staining was found widely in each tissue. Moreover, different laying stages did not affect the localization of CaSR except for the tibia tissue. In conclusion, similar to VDR, CaSR was widely expressed not only in the gut but also in the tibia and shell gland in laying hens. The expression level of CaSR and VDR in all tested tissues was unchanged at the different laying stages.

Disorders of Calcium Metabolism: Hypocalcemia and Hypercalcemia

Calcium (Ca⁺²) is a divalent cation that plays a critical role in numerous body functions such as skeletal mineralization, signal transduction, nerve conduction, muscle contraction, and blood coagulation. Ca⁺² metabolism is linked to magnesium (Mg⁺²) and phosphate metabolism. Ca⁺² homeostasis is dependent on intestinal absorption, bone turnover, and renal reabsorption. The hormonal regulators of these processes are the parathyroid hormone (PTH), calcitriol {1,25-dihydroxyvitamin D [1,25(OH)₂D]}, and serum ionized Ca⁺². Cloning of the Ca⁺²-sensing receptor (CaSR) has greatly advanced the understanding of Ca⁺² metabolism. Disorders of Ca⁺² metabolism are easily recognized because Ca⁺² is included in routine chemistry panels. Measurement of ionized Ca⁺² is the preferred way to ascertain the diagnosis of hypocalcemia and hypercalcemia.

Immune checkpoint inhibitor-associated hypercalcaemia

Immune checkpoint inhibitors (CPIs) have recently become a cornerstone for the treatment of different advanced cancers. These drugs have the ability to reactivate the immune system against tumour cells but can also trigger a myriad of side effects, termed immune-related adverse events (irAEs). Although there are numerous reports of CPI-related endocrinopathies, hypercalcaemia as a suspected irAE is not well documented. The mechanisms of CPI hypercalcaemia are not clearly established. However, in our review, four distinct causes emerged: endocrine disease-related, sarcoid-like granuloma, humoral hypercalcaemia due to parathyroid-related hormone and hyperprogressive disease following CPI initiation. Prompt recognition of hypercalcaemia and the institution of therapy can be lifesaving, affording the opportunity to address the underlying aetiology. In this review we discuss the incidence, diagnosis and management of immune-related hypercalcaemia in oncological patients receiving CPI agents.