Renal magnesium handling: new insights in understanding old problems

Deletion of claudin-10 (Cldn10) in the thick ascending limb impairs paracellular sodium permeability and leads to hypermagnesemia and nephrocalcinosis

In the kidney, tight junction proteins contribute to segment specific selectivity and permeability of paracellular ion transport. In the thick ascending limb (TAL) of Henle's loop, chloride is reabsorbed transcellularly, whereas sodium reabsorption takes transcellular and paracellular routes. TAL salt transport maintains the concentrating ability of the kidney and generates a transepithelial voltage that drives the reabsorption of calcium and magnesium. Thus, functionality of TAL ion transport depends strongly on the properties of the paracellular pathway. To elucidate the role of the tight junction protein claudin-10 in TAL function, we generated mice with a deletion of Cldn10 in this segment. We show that claudin-10 determines paracellular sodium permeability, and that its loss leads to hypermagnesemia and nephrocalcinosis. In isolated perfused TAL tubules of claudin-10-deficient mice, paracellular permeability of sodium is decreased, and the relative permeability of calcium and magnesium is increased. Moreover, furosemide-inhibitable transepithelial voltage is increased, leading to a shift from paracellular sodium transport to paracellular hyperabsorption of calcium and magnesium. These data identify claudin-10 as a key factor in control of cation selectivity and transport in the TAL, and deficiency in this pathway as a cause of nephrocalcinosis.

Laxative abuse, eating disorders, and kidney stones: a case report and review of the literature

Kidney stones are listed among the complications of eating disorders; however, very few cases have been reported. We present an additional case of nephrolithiasis associated with laxative abuse, including detailed results of the patient's urine metabolic profiles, in a patient with idiopathic hypercalciuria. We review the literature and provide an explanation for the paucity of cases of nephrolithiasis associated with these disorders. Despite low urine volumes resulting from extracellular fluid volume depletion and hypocitraturia resulting from hypokalemia, both of which would tend to favor the formation of kidney stones, most patients with eating disorders are likely to be protected from stone formation by the hypocalciuric effect of extracellular fluid volume depletion and increased proximal tubular sodium reabsorption. However, patients with underlying idiopathic hypercalciuria who develop eating disorders may be at increased risk of stone formation in the setting of low urine volume and therefore high supersaturation of calcium oxalate and phosphate.

Drug-induced alterations in Mg2+ homoeostasis

Magnesium (Mg2+) balance is tightly regulated by the concerted actions of the intestine, bone and kidneys. This balance can be disturbed by a broad variety of drugs. Diuretics, modulators of the EGFR (epidermal growth factor receptor), proton pump inhibitors, antimicrobials, calcineurin inhibitors and cytostatics may all cause hypomagnesaemia, potentially leading to tetany, seizures and cardiac arrhythmias. Conversely, high doses of Mg2+ salts, frequently administered as an antacid or a laxative, may lead to hypermagnesaemia causing various cardiovascular and neuromuscular abnormalities. A better understanding of the molecular mechanisms underlying the adverse effects of these medications on Mg2+ balance will indicate ways of prevention and treatment of these adverse effects and could potentially provide more insight into Mg2+ homoeostasis.

[Renal physiology]

The kidneys are responsible for the urinary excretion of uremic toxins and the regulation of several body systems such as intra and extracellular volume status, acid-base status, calcium and phosphate metabolism or erythropoiesis. They adapt quantitative and qualitative composition of the urine to keep these systems in balance. The flow of plasma is filtered in the range of 120 mL/min, and depends on the systemic and renal hemodynamics which is subject to self-regulation. The original urine will then be modified in successive segments of the nephron. The proximal nephron is to lead the massive reabsorption of water and essential elements such as sodium, bicarbonates, amino-acids and glucose. The distal nephron includes the distal convoluted tubule, the connector tube and the collecting duct. Its role is to adapt the quality composition of urine to the needs of the body.

In vivo behavior of biodegradable Mg-Nd-Y-Zr-Ca alloy

The aim of the this study is to evaluate the in vivo behavior of Mg-1.5%Nd-0.5%Y-0.5%Zr implants with and without 0.4%Ca in comparison with inert Ti-6Al-4V reference implants. This was carried out by implanting cylindrical disks at the back midline of Wister male rats within the subcutaneous layer of the skin for up to 12 weeks. The degradation of magnesium-based implants in terms of hydrogen gas bubble formation was evaluated by radiography assessment; corrosion rate was analyzed by visual examination and weight loss measurements. The physiological response of the rats post-implantation was obtained by evaluating their wellbeing behavior and blood biochemical analysis including serum Mg, blood urea nitrogen, and serum creatinine. In addition, histological analyses of the soft tissue around the implants were carried out to assess local lesions relating to the implants such as inflammation, tissue necrosis, granulation, mineralization, and tumor development. The results obtained clearly indicate that apart from the normal degradation characteristics and subsequent formation of hydrogen gas bubbles, the in vivo behavior of Mg implants was adequate and comparable to that of Ti-6Al-4V reference alloy. In addition, it was evident that the corrosion degradation of the magnesium alloys was strongly related to the location of the implant within the animal's body. The addition of 0.4%Ca improves the biodegradation corrosion resistance of the tested magnesium implants.

Acute-onset hypomagnesemia-induced hypocalcemia caused by the refractoriness of bones and renal tubules to parathyroid hormone

Chronic hypomagnesemia is closely associated with hypocalcemia, which is caused by impaired parathyroid hormone (PTH) secretion or the refractoriness of bone and renal tubules to PTH. The dominant mechanism of acute-onset, hypomagnesemia-induced hypocalcemia is currently unclear. An 83-year-old man who had undergone chemotherapy with carboplatin for prostate cancer suffered from acute diarrhea and finger paresthesia. Laboratory data confirmed hypocalcemia as well as hypomagnesemia. Urinary calcium levels were not measured. However, the urinary fractional excretion of Mg (FE(Mg)) was elevated. Despite elevated PTH levels, the renal tubular maximal reabsorption rate of phosphate to GFR (TmP/GFR) was elevated, and bone formation and resorption markers were suppressed. A magnesium loading test revealed a clear magnesium deficiency. After administration of magnesium, bone marker levels were increased, and TmP/GFR was reduced to normal levels, despite the persistent elevation of PTH. Serum calcium levels eventually increased to approximately the reference range. Clinical histories and these observations both suggest that when patients with hypomagnesemia-induced hypocalcemia rapidly lose magnesium through complications such as diarrhea, the primary cause may be the refractoriness of bone and renal tubules to PTH, rather than impaired PTH secretion.

Evaluation of essential trace and toxic elements in biological samples of normal and night blindness children of age groups 3-7 and 8-12 years

The most common cause of blindness in developing countries is vitamin A deficiency. The World Health Organization estimates 13.8 million children to have some degree of visual loss related to vitamin A deficiency. The causes of night blindness in children are multifactorial, and particular consideration has been given to childhood nutritional deficiency, which is the most common problem found in underdeveloped countries. Such deficiency can result in physiological and pathological processes that in turn influence biological samples composition. Vitamin and mineral deficiency prevents more than two billion people from achieving their full intellectual and physical potential. This study was designed to compare the levels of Zn, Mg, Ca, K, Na, As, Cd, and Pb in scalp hair, blood, and urine of night blindness children age ranged 3-7 and 8-12 years of both genders, comparing them to sex- and age-matched controls. A microwave-assisted wet acid digestion procedure was developed as a sample pretreatment, for the determination of As, Ca, Cd, K, Pb, Mg, Na, and Zn in biological samples of night blindness children. The proposed method was validated by using conventional wet digestion and certified reference samples of hair, blood, and urine. The concentrations of trace and toxic elements were measured by atomic absorption spectrophotometer prior to microwave-assisted acid digestion. The results of this study showed that the mean values of As, Cd, Na, and Pb were significantly higher in scalp hair, blood, and urine samples of male and female night blindness children than in referents (p < 0.001), whereas the concentrations of Zn, Ca, K, and Mg were lower in the scalp hair and blood but higher in the urine samples of night blindness children. These data present guidance to clinicians and other professional investigating deficiency of essential mineral elements in biological samples (scalp hair and blood) of night blindness children.

Intravenous administration of magnesium and potassium solution lowers energy levels and increases success rates electrically cardioverting atrial fibrillation

BACKGROUND

External biphasic electrical cardioversion (CV) is a standard treatment option for patients suffering from acute symptoms of atrial fibrillation (AF). Nevertheless, CV is not always successful, and thus strategies to increase the success rate are desirable.

OBJECTIVE

The purpose of this study was to evaluate the effect of intravenously administered K/Mg solution on the biphasic CV energy threshold and success rate to restore sinus rhythm (SR) in patients with AF.

METHODS

The study consisted of 170 patients with persistent AF. The patients were randomly assigned to undergo biphasic CV either with (n = 84) or without (n = 86) pretreatment with K/Mg solution. An energy step-up protocol of 75, 100, and 150 W (J) was used.

RESULTS

Biphasic CV of AF was effective in 81 (96.4%) patients in the pretreatment and 74 (86.0%) patients in the control group (P = 0.005). The effective energy level required to achieve SR was significantly lower in the pretreated group (140.8 ± 26.9 J vs 182.5 ± 52.2 J, P = 0.02). No K/Mg-solution-associated side effects such as hypotension or bradycardia were observed.

CONCLUSION

Administration of K/Mg solution positively influences the success rate of CV in patients with persistent AF. Furthermore, significantly less energy is required to successfully restore SR and therefore K/Mg pretreatment may facilitate SR restoration in patients undergoing CV for AF.

[Electrolyte disorders]

Disorders of electrolyte balance are frequent and pathophysiologically complex. Sodium is responsible for a large part of the osmolarity of extracellular fluids. Therefore, pathological concentrations of serum sodium reflect the relation between sodium and water in the extracellular compartment rather than the total body sodium content. The causes of hypo- or hypernatremia can only be deduced if total body volume status is considered. Patients with hyponatremia and volume deficit should receive sodium chloride solution while patients with this disorder in the presence of volume overload need strict water restriction. In certain cases additional specific pharmacotherapy directed at the effects of antidiuretic hormone may be considered. Potassium and calcium are extracellular regulatory ions; their concentrations do not relevantly contribute to osmolarity and water distribution but to electrophysiologically relevant transmembrane potentials. These ions are influenced by active membrane transporters and regulated by several hormones. The rather small extracellular pools are overfilled or depleted by alterations of intake and excretion. In addition, several inborn or acquired defects of transmembrane transporters may severely alter their extracellular concentrations. Therapy needs to consider the specific mechanisms that led to the electrolyte disorder including modification of intake, excretion or extra-intracellular distribution.

Hypoxia induces an increase in intracellular magnesium via transient receptor potential melastatin 7 (TRPM7) channels in rat hippocampal neurons in vitro

TRPM7, a divalent cation channel, plays an important role in neurons damaged from cerebral ischemia due to permitting intracellular calcium overload. This study aimed to explore whether magnesium was transported via a TRPM7 channel into the intracellular space of rat hippocampal neurons after 1 h of oxygen-glucose deprivation (OGD) and acute chemical ischemia (CI) by using methods of the Mg(2+) fluorescent probe Mag-Fura-2 to detect intracellular magnesium concentration ([Mg(2+)](i)) and flame atomic absorption spectrometry to measure extracellular magnesium concentration ([Mg(2+)](o)). The results showed that the neuronal [Mg(2+)](i) was 1.51-fold higher after 1 h of OGD at a basal level, and the increase of neuronal [Mg(2+)](i) reached a peak after 1 h of OGD and was kept for 60 min with re-oxygenation. Meanwhile, the [Mg(2+)](o) decreased after 1 h of OGD and recovered to the pre-ischemic level within 15 min after re-oxygenation. In the case of CI, the [Mg(2+)](i) peak immediately appeared in hippocampal neurons. This increase of [Mg(2+)](i) declined by removing extracellular magnesium in OGD or CI. Furthermore, by using Gd(3+) or 2-aminoethoxydiphenyl borate to inhibit TRPM7 channels, the [Mg(2+)](i) increase, which was induced by OGD or CI, was attenuated without altering the basal level of [Mg(2+)](i). By silencing TRPM7 with shRNA in hippocampal neurons, it was found that not only was the increase of [Mg(2+)](i) induced by OGD or CI but also the basal levels of [Mg(2+)](i) were attenuated. In contrast, overexpression of TRPM7 in HEK293 cells exaggerated both the basal levels and increased [Mg(2+)](i) after 1 h of OGD/CI. These results suggest that anoxia induced the increase of [Mg(2+)](i) via TRPM7 channels in rat hippocampal neurons.

CNNM2, encoding a basolateral protein required for renal Mg2+ handling, is mutated in dominant hypomagnesemia

Familial hypomagnesemia is a rare human disorder caused by renal or intestinal magnesium (Mg(2+)) wasting, which may lead to symptoms of Mg(2+) depletion such as tetany, seizures, and cardiac arrhythmias. Our knowledge of the physiology of Mg(2+) (re)absorption, particularly the luminal uptake of Mg(2+) along the nephron, has benefitted from positional cloning approaches in families with Mg(2+) reabsorption disorders; however, basolateral Mg(2+) transport and its regulation are still poorly understood. Here, by using a candidate screening approach, we identified CNNM2 as a gene involved in renal Mg(2+) handling in patients of two unrelated families with unexplained dominant hypomagnesemia. In the kidney, CNNM2 was predominantly found along the basolateral membrane of distal tubular segments involved in Mg(2+) reabsorption. The basolateral localization of endogenous and recombinant CNNM2 was confirmed in epithelial kidney cell lines. Electrophysiological analysis showed that CNNM2 mediated Mg(2+)-sensitive Na(+) currents that were significantly diminished in mutant protein and were blocked by increased extracellular Mg(2+) concentrations. Our data support the findings of a recent genome-wide association study showing the CNNM2 locus to be associated with serum Mg(2+) concentrations. The mutations found in CNNM2, its observed sensitivity to extracellular Mg(2+), and its basolateral localization signify a critical role for CNNM2 in epithelial Mg(2+) transport.

Purification, crystallization and preliminary crystallographic analysis of the CBS pair of the human metal transporter CNNM4

This work describes the purification and preliminary crystallographic analysis of the CBS-pair regulatory domain of the human ancient domain protein 4 (ACDP4), also known as CNNM4. ACDP proteins represent the least-studied members of the eight different types of magnesium transporters that have been identified in mammals to date. In humans the ACDP family includes four members: CNNM1-4. CNNM1 acts as a cytosolic copper chaperone and has been associated with urofacial syndrome, whereas CNNM2 and CNNM4 have been identified as magnesium transporters. Interestingly, mutations in the CNNM4 gene have clinical consequences that are limited to retinal function and biomineralization and are considered to be the cause of Jalili syndrome, which consists of autosomal recessive cone-rod dystrophy and amelogenesis imperfecta. The truncated protein was overexpressed, purified and crystallized in the orthorhombic space group C222. The crystals diffracted X-rays to 3.6 Å resolution using synchrotron radiation. Matthews volume calculations suggested the presence of two molecules in the asymmetric unit, which were likely to correspond to a CBS module of the CBS pair of CNNM4.

Diuretics in clinical practice. Part II: electrolyte and acid-base disorders complicating diuretic therapy

IMPORTANCE OF THE FIELD

As with all potent therapeutic agents, the use of diuretic compounds has been linked with several adverse effects that may reduce quality of life and patient compliance and, in some cases, may be associated with considerable morbidity and mortality. Among the various types of adverse effects, disturbances of electrolyte and acid-base balance are perhaps the most common, and some of them are the aetiological factors of other side effects (i.e., hypokalaemia causing ventricular arrhythmias or glucose intolerance). The mechanism and site of action and, therefore, the pharmacological effects of each diuretic class largely determine the specific electrolyte or acid-base abnormalities that will accompany the use of each diuretic agent.

AREAS COVERED IN THE REVIEW

This article reviews the major electrolyte disturbances (hypokalaemia, hyperkalaemia, hyponatraemia, disorders of magnesium and calcium balance), as well as the acid-base abnormalities complicating the use of the various diuretic agents.

WHAT THE READER WILL GAIN

The reader will gain insights into the pathogenesis of the diuretic-induced electrolyte and acid-base disorders together with considerations for their prevention and treatment.

TAKE HOME MESSAGE

Knowledge of the pharmacologic properties of each diuretic class and appropriate monitoring of patients under diuretic treatment represent the most important strategies to prevent the development of diuretic-related adverse events and their consequences.

Implications of magnesium deficiency in type 2 diabetes: a review

Magnesium is the fourth most abundant cation in the body and plays an important physiological role in many of its functions. It plays a fundamental role as a cofactor in various enzymatic reactions involving energy metabolism. Magnesium is a cofactor of various enzymes in carbohydrate oxidation and plays an important role in glucose transporting mechanism of the cell membrane. It is also involved in insulin secretion, binding, and activity. Magnesium deficiency and hypomagnesemia can result from a wide variety of causes, including deficient magnesium intake, gastrointestinal, and renal losses. Chronic magnesium deficiency has been associated with the development of insulin resistance. The present review discusses the implications of magnesium deficiency in type 2 diabetes.

Renal TRPathies

Many ion channels and transporters are involved in the filtration, secretion, and resorption of electrolytes by the kidney. In recent years, the superfamily of transient receptor potential (TRP) ion channels have received deserved attention because mutated TRP channels are linked to human kidney diseases. This review focuses on two TRP members--TRPC6 and TRPM6--and their functions in the kidney. Gain-of-function mutations in TRPC6 are the cause for progressive kidney failure with urinary protein loss such as FSGS. Thus, TRPC6 is an essential signaling component in a functional slit diaphragm formed by podocytes around the glomerular capillaries. Loss-of-function mutations in TRPM6 are a molecular cause of hypomagnesemia with secondary hypocalcemia, suggesting that TRPM6 is critically involved in transcellular Mg2+ transport in the kidney. Here, we highlight how recent studies analyzing function and expression of these channels in the kidney improve our mechanistic understanding of TRP channel function in general and pave the way to new, promising therapeutic strategies to target kidney diseases such as FSGS and hypomagnesemia with secondary hypocalcemia.

Magnesium excretion in C. elegans requires the activity of the GTL-2 TRPM channel

Systemic magnesium homeostasis in mammals is primarily governed by the activities of the TRPM6 and TRPM7 cation channels, which mediate both uptake by the intestinal epithelial cells and reabsorption by the distal convoluted tubule cells in the kidney. In the nematode, C. elegans, intestinal magnesium uptake is dependent on the activities of the TRPM channel proteins, GON-2 and GTL-1. In this paper we provide evidence that another member of the TRPM protein family, GTL-2, acts within the C. elegans excretory cell to mediate the excretion of excess magnesium. Thus, the activity of GTL-2 balances the activities of the paralogous TRPM channel proteins, GON-2 and GTL-1.

Molecular identification of ancient and modern mammalian magnesium transporters

A large number of mammalian Mg2+ transporters have been hypothesized on the basis of physiological data, but few have been investigated at the molecular level. The recent identification of a number of novel proteins that mediate Mg2+ transport has enhanced our understanding of how Mg2+ is translocated across mammalian membranes. Some of these transporters have some similarity to those found in prokaryocytes and yeast cells. Human Mrs2, a mitochondrial Mg2+ channel, shares many of the properties of the bacterial CorA and yeast Alr1 proteins. The SLC41 family of mammalian Mg2+ transporters has a similarity with some regions of the bacterial MgtE transporters. The mammalian ancient conserved domain protein (ACDP) Mg2+ transporters are found in prokaryotes, suggesting an ancient origin. However, other newly identified mammalian transporters, including TRPM6/7, MagT, NIPA, MMgT, and HIP14 families, are not represented in prokaryotic genomes, suggesting more recent development. MagT, NIPA, MMgT, and HIP14 transporters were identified by differential gene expression using microarray analysis. These proteins, which are found in many different tissues and subcellular organelles, demonstrate a diversity of structural properties and biophysical functions. The mammalian Mg2+ transporters have no obvious amino acid similarities, indicating that there are many ways to transport Mg2+ across membranes. Most of these proteins transport a number of divalent cations across membranes. Only MagT1 and NIPA2 are selective for Mg2+. Many of the identified mammalian Mg2+ transporters are associated with a number of congenital disorders encompassing a wide range of tissues, including intestine, kidney, brain, nervous system, and skin. It is anticipated that future research will identify other novel Mg2+ transporters and reveal other diseases.

Improvement in electrolyte imbalance in critically ill patient after magnesium supplementation - A case report

Hypomagnesaemia is common finding in current medical practice mainly in critically ill, post-operative patients and patients admitted to ICU in tertiary cancer cases. Magnesium has been directly implicated in hypokalemia, hypocalcaemia and dysrrthymias. We report a case of 60 year old patient, suffering from rectal carcinoma for a period of one year with confirmed hypokalemia, hypocalcaemia and hyponatremia. Magnesium supplementation corrected the underlying multiple electrolyte disturbances in the patient thus, establishing a positive correlation of magnesium with sodium, potassium and calcium.

Electrolyte disorders related to EGFR-targeting drugs

It is now clearly established that anti-vascular endothelial growth factor (VEGF) drug class induces hypertension and proteinuria sometimes related to thrombotic microangiopathy and/or various glomerulopathies, according to capillary and glomerular VEGF and VEGF-receptor expressions. As reported in the literature, anti-epidermal growth factor receptor (EGFR) therapies seem to be less nephrotoxic. Indeed, many reports of anti-EGFR nephrotoxicity are tubular dependent such as acute tubular necrosis, electrolyte disorders (hypophosphatemia, hypomagnesemia, etc.) or both. This is explained by elective tubular expression of renal EGF/EGFR. In this paper, we focus on electrolyte disorders related to anti-EGFR treatment and discuss the tubular involvement of these drugs based on their renal expression.

A missense mutation in the Kv1.1 voltage-gated potassium channel-encoding gene KCNA1 is linked to human autosomal dominant hypomagnesemia

Primary hypomagnesemia is a heterogeneous group of disorders characterized by renal or intestinal magnesium (Mg2+) wasting, resulting in tetany, cardiac arrhythmias, and seizures. The kidney plays an essential role in maintaining blood Mg2+ levels, with a prominent function for the Mg2+-transporting channel transient receptor potential cation channel, subfamily M, member 6 (TRPM6) in the distal convoluted tubule (DCT). In the DCT, Mg2+ reabsorption is an active transport process primarily driven by the negative potential across the luminal membrane. Here, we studied a family with isolated autosomal dominant hypomagnesemia and used a positional cloning approach to identify an N255D mutation in KCNA1, a gene encoding the voltage-gated potassium (K+) channel Kv1.1. Kv1.1 was found to be expressed in the kidney, where it colocalized with TRPM6 along the luminal membrane of the DCT. Upon overexpression in a human kidney cell line, patch clamp analysis revealed that the KCNA1 N255D mutation resulted in a nonfunctional channel, with a dominant negative effect on wild-type Kv1.1 channel function. These data suggest that Kv1.1 is a renal K+ channel that establishes a favorable luminal membrane potential in DCT cells to control TRPM6-mediated Mg2+ reabsorption.

Blood pressure drug therapy and electrolyte disturbances

Antihypertensive pharmacologic treatment may be associated with diverse disturbances of electrolyte homeostasis. These drug-induced disorders are relatively common, typically including hyponatraemia, hypokalaemia, hyperkalaemia, hypomagnesaemia, hypophosphataemia and hypercalcaemia. Diuretics, beta-blockers, angiotensin-converting enzyme inhibitors and angiotensin receptor blockers are particularly likely to cause these complications. Recognised risk factors include high-dosage regimens (especially diuretics), old age, diabetes and impairment of renal function. Strategies to prevent these adverse drug reactions involve careful consideration of risk factors and clinical and laboratory evaluation in the course of treatment.

Huntingtin-interacting proteins, HIP14 and HIP14L, mediate dual functions, palmitoyl acyltransferase and Mg2+ transport

Polyglutamine expansions of huntingtin protein are responsible for the Huntington neurological disorder. HIP14 protein has been shown to interact with huntingtin. HIP14 and a HIP14-like protein, HIP14L, with a 69% similarity reside in the Golgi and possess palmitoyl acyltransferase activity through innate cysteine-rich domains, DHHC. Here, we used microarray analysis to show that reduced extracellular magnesium concentration increases HIP14L mRNA suggesting a role in cellular magnesium metabolism. Because HIP14 was not on the microarray platform, we used real-time reverse transcriptase-PCR to show that HIP14 and HIP14L transcripts were up-regulated 3-fold with low magnesium. Western analysis with a specific HIP14 antibody also showed that endogenous HIP14 protein increased with diminished magnesium. Furthermore, we demonstrate that when expressed in Xenopus oocytes, HIP14 and HIP14L mediate Mg2+ uptake that is electrogenic, voltage-dependent, and saturable with Michaelis constants of 0.87 +/- 0.02 and 0.74 +/- 0.07 mm, respectively. Diminished magnesium leads to an apparent increase in HIP14-green fluorescent protein and HIP14L-green fluorescent fusion proteins in the Golgi complex and subplasma membrane post-Golgi vesicles of transfected epithelial cells. We also show that inhibition of palmitoylation with 2-bromopalmitate, or deletion of the DHHC motif HIP14DeltaDHHC, diminishes HIP14-mediated Mg2+ transport by about 50%. Coexpression of an independent protein acyltransferase, GODZ, with the deleted HIP14DeltaDHHC mutant restored Mg2+ transport to values observed with wild-type HIP14. Although we did not directly measure palmitoylation of HIP14 in these studies, the data are consistent with a regulatory role of autopalmitoylation in HIP14-mediated Mg2+ transport. We conclude that the huntingtin interacting protein genes, HIP14 and HIP14L, encode Mg2+ transport proteins that are regulated by their innate palmitoyl acyltransferases thus fulfilling the characteristics of "chanzymes."

Functional characterization of NIPA2, a selective Mg2+ transporter

We used microarray analysis to identify renal cell transcripts that were upregulated with low magnesium. One transcript, identified as NIPA2 (nonimprinted in Prader-Willi/Angelman syndrome) subtype 2, was increased over twofold relative to cells cultured in normal magnesium. The deduced sequence comprises 129 amino acids with 8 predicted transmembrane regions. As the secondary structure of NIPA2 conformed to a membrane transport protein, we expressed it in Xenopus oocytes and determined that it mediated Mg(2+) uptake with two-electrode voltage-clamp and fluorescence studies. Mg(2+) transport was electrogenic, voltage dependent, and saturable, demonstrating a Michaelis affinity constant of 0.31 mM. Unlike other reported Mg(2+) transporters, NIPA2 was very selective for the Mg(2+) cation. NIPA2 mRNA is found in many tissues but particularly abundant in renal cells. With the use of immunofluorescence, it was shown that NIPA2 protein was normally localized to the early endosomes and plasma membrane and was recruited to the plasma membrane in response to low extracellular magnesium. We conclude that NIPA2 plays a role in magnesium metabolism and regulation of renal magnesium conservation.

Omeprazole and refractory hypomagnesaemia

Omeprazole may cause hypomagnesaemia, along withhypocalcaemia and hypokalaemia

Relationship between low magnesium status and TRPM6 expression in the kidney and large intestine

The body maintains Mg(2+) homeostasis by renal and intestinal (re)absorption. However, the molecular mechanisms that mediate transepithelial Mg(2+) transport are largely unknown. Transient receptor potential melastatin 6 (TRPM6) was recently identified and shown to function in active epithelial Mg(2+) transport in intestine and kidney. To define the relationship between Mg(2+) status and TRPM6 expression, we used two models of hypomagnesemia: 1) C57BL/6J mice fed a mildly or severely Mg(2+)-deficient diet, and 2) mice selected for either low (MgL) or high (MgH) erythrocyte and plasma Mg(2+) status. In addition, the mice were subjected to a severely Mg(2+)-deficient diet. Our results show that C57BL/6J mice fed a severely Mg(2+)-deficient diet developed hypomagnesemia and hypomagnesuria and showed increased TRPM6 expression in kidney and intestine. When fed a Mg(2+)-adequate diet, MgL mice presented hypomagnesemia and hypermagnesuria, and lower kidney and intestinal TRPM6 expression, compared with MgH mice. A severely Mg(2+)-deficient diet led to hypomagnesemia and hypomagnesuria in both strains. Furthermore, this diet induced kidney TRPM6 expression in MgL mice, but not in MgH mice. In conclusion, as shown in C57BL/6J mice, dietary Mg(2+)-restriction results in increased Mg(2+) (re)absorption, which is correlated with increased TRPM6 expression. In MgL and MgH mice, the inherited Mg(2+) status is linked to different TRPM6 expression. The MgL and MgH mice respond differently to a low-Mg(2+) diet with regard to TRPM6 expression in the kidney, consistent with genetic factors contributing to the regulation of cellular Mg(2+) levels. Further studies of these mice strains could improve our understanding of the genetics of Mg(2+) homeostasis.

Hereditary etiologies of hypomagnesemia

Magnesium ions are essential to all living cells. As the second most abundant intracellular cation, magnesium has a crucial role in fundamental metabolic processes such as DNA and protein synthesis, oxidative phosphorylation, enzyme function, ion channel regulation, and neuromuscular excitability. After presenting an overview of magnesium homeostasis, we review the etiologies of hypomagnesemia, with an emphasis on hereditary causes.

Clinical consequences and management of hypomagnesemia

Magnesium deficiency and hypomagnesemia remain quite prevalent, particularly in patients in intensive care units, and may have important clinical consequences. Magnesium should be measured directly in clinical circumstances in which a risk for magnesium deficiency exists and appropriately corrected when found. This commentary reviews the current knowledge of magnesium homeostasis and the risk factors and clinical consequences of magnesium deficiency and outlines approaches to therapy.

Identification and characterization of a novel family of membrane magnesium transporters, MMgT1 and MMgT2

Magnesium is an essential metal, but few selective transporters have been identified at the molecular level. Microarray analysis was used to identify two similar transcripts that are upregulated with low extracellular Mg(2+). The corresponding cDNAs encode proteins of 131 and 123 amino acids with two predicted transmembrane domains. The two separate gene products comprise the family that we have termed "membrane Mg(2+) transporters" (MMgTs), because the proteins reside in the membrane and mediate Mg(2+) transport. When expressed in Xenopus laevis oocytes, MMgT1 and MMgT2 mediate Mg(2+) transport as determined with two-electrode voltage-clamp analysis and fluorescence measurements. Transport is saturable Mg(2+) uptake with Michaelis constants of 1.47 +/- 0.17 and 0.58 +/- 0.07 mM, respectively. Real-time RT-PCR demonstrated that MMgT mRNAs are present in a wide variety of cells. Subcellular localization with immunohistochemistry determined that the MMgT1-hemagglutinin (HA) and MMgT2-V5 fusion proteins reside in the Golgi complex and post-Golgi vesicles, including the early endosomes in COS-7 cells transfected with the respective tagged constructs. Interestingly, MMgT1-HA and MMgT2-V5 were found in separate populations of post-Golgi vesicles. MMgT1 and MMgT2 mRNA increased by about threefold, respectively, in kidney epithelial cells cultured in low-magnesium media relative to normal media and in the kidney cortex of mice maintained on low-magnesium diets compared with those animals consuming normal diets. With the increase in transcripts, there was an apparent increase in MMgT1 and MMgT2 protein in the Golgi and post-Golgi vesicles. These experiments suggest that MMgT proteins may provide regulated pathways for Mg(2+) transport in the Golgi and post-Golgi organelles of epithelium-derived cells.

Impaired basolateral sorting of pro-EGF causes isolated recessive renal hypomagnesemia

Primary hypomagnesemia constitutes a rare heterogeneous group of disorders characterized by renal or intestinal magnesium (Mg(2+)) wasting resulting in generally shared symptoms of Mg(2+) depletion, such as tetany and generalized convulsions, and often including associated disturbances in calcium excretion. However, most of the genes involved in the physiology of Mg(2+) handling are unknown. Through the discovery of a mutation in the EGF gene in isolated autosomal recessive renal hypomagnesemia, we have, for what we believe is the first time, identified a magnesiotropic hormone crucial for total body Mg(2+) balance. The mutation leads to impaired basolateral sorting of pro-EGF. As a consequence, the renal EGFR is inadequately stimulated, resulting in insufficient activation of the epithelial Mg(2+) channel TRPM6 (transient receptor potential cation channel, subfamily M, member 6) and thereby Mg(2+) loss. Furthermore, we show that colorectal cancer patients treated with cetuximab, an antagonist of the EGFR, develop hypomagnesemia, emphasizing the significance of EGF in maintaining Mg(2+) balance.

Crosstalk of tight junction components with signaling pathways

Tight junctions (TJs) regulate the passage of ions and molecules through the paracellular pathway in epithelial and endothelial cells. TJs are highly dynamic structures whose degree of sealing varies according to external stimuli, physiological and pathological conditions. In this review we analyze how the crosstalk of protein kinase C, protein kinase A, myosin light chain kinase, mitogen-activated protein kinases, phosphoinositide 3-kinase and Rho signaling pathways is involved in TJ regulation triggered by diverse stimuli. We also report how the phosphorylation of the main TJ components, claudins, occludin and ZO proteins, impacts epithelial and endothelial cell function.

Cellular magnesium acquisition: an anomaly in embryonic cation homeostasis

The intracellular dominance of magnesium ion makes clinical assessment difficult despite the critical role of Mg(++) in many key functions of cells and enzymes. There is general consensus that serum Mg(++) levels are not representative of the growing number of conditions for which magnesium is known to be important. There is no consensus method or sample source for testing for clinical purposes. High intracellular Mg(++) in vertebrate embryos results in part from interactions of cations which influence cell membrane transport systems. These are functionally competent from the earliest stages, at least transiently held over from the unfertilized ovum. Kinetic studies with radiotracer cations, osmolar variations, media lacking one or more of the four biological cations, Na(+), Mg(++), K(+), and Ca(++), and metabolic poison 0.05 mEq/L NaF, demonstrated that: (1) all four cations influence the behavior of the others, and (2) energy is required for uptake and efflux on different time scales, some against gradient. Na(+) uptake is energy dependent against an efflux gradient. The rate of K(+) loss is equal with or without fluoride, suggesting a lack of an energy requirement at these stages. Ca(++) efflux took twice as long in the presence of fluoride, likely due in part to intracellular binding. Mg(++) is anomalous in that early teleost vertebrate embryos have an intracellular content exceeding the surrounding sea water, an isolated unaffected yolk compartment, and a clear requirement for energy for both uptake and efflux. The physiological, pathological, and therapeutic roles of magnesium are poorly understood. This will change: (1) when (28)Mg is once again generally available at a reasonable cost for both basic research and clinical assessment, and (2) when serum or plasma levels are determined simultaneously with intracellular values, preferably as part of complete four cation profiles. Atomic absorption spectrophotometry, energy-dispersive x-ray analysis, and inductively coupled plasma emission spectroscopy on sublingual mucosal and peripheral blood samples are potential methods of value for coordinated assessments.

TRPM6 and TRPM7--Gatekeepers of human magnesium metabolism

Human magnesium homeostasis primarily depends on the balance between intestinal absorption and renal excretion. Magnesium transport processes in both organ systems - next to passive paracellular magnesium flux - involve active transcellular magnesium transport consisting of an apical uptake into the epithelial cell and a basolateral extrusion into the interstitium. Whereas the mechanism of basolateral magnesium extrusion remains unknown, recent molecular genetic studies in patients with hereditary hypomagnesemia helped gain insight into the molecular nature of apical magnesium entry into intestinal brush border and renal tubular epithelial cells. Patients with Hypomagnesemia with Secondary Hypocalcemia (HSH), a primary defect in intestinal magnesium absorption, were found to carry mutations in TRPM6, a member of the melastatin-related subfamily of transient receptor potential (TRP) ion channels. Before, a close homologue of TRPM6, TRPM7, had been characterized as a magnesium and calcium permeable ion channel vital for cellular magnesium homeostasis. Both proteins share the unique feature of an ion channel fused to a kinase domain with homology to the family of atypical alpha kinases. The aim of this review is to summarize the data emerging from clinical and molecular genetic studies as well as from electrophysiologic and biochemical studies on these fascinating two new proteins and their role in human magnesium metabolism.

Cotransporters, WNKs and hypertension: important leads from the study of monogenetic disorders of blood pressure regulation

Major advances are being made in identifying the structure and behaviour of regulatory cascades that control the activity of cation-Cl(-) cotransporters and certain Na(+), K(+) and Cl(-) channels. These transporters play key roles in regulating arterial blood pressure as they are not only responsible for NaCl reabsorption in the thick ascending limb and distal tubule of the kidney, but are also involved in regulating smooth muscle Ca(2+) levels. It is now apparent that defects in these transporters, and particularly in the regulatory cascades, cause some monogenetic forms of hypertension and may contribute to essential hypertension and problems with K(+) homoeostasis. Two families of kinases are prominent in these processes: the Ste-20-related kinases [OSR1 (oxidative stress-responsive kinase 1) and SPAK (Ste20/SPS1-related proline/alanine-rich kinase)] and the WNKs [with no lysine kinases]. These kinases affect the behaviour of their targets through both phosphorylation and by acting as scaffolding proteins, bringing together regulatory complexes. This review analyses how these kinases affect transport by activating or inhibiting individual transporters at the cell surface, or by changing the surface density of transporters by altering the rate of insertion or removal of transporters from the cell surface, and perhaps through controlling the rate of transporter degradation. This new knowledge should not only help us target antihypertensive therapy more appropriately, but could also provide the basis for developing new therapeutic approaches to essential hypertension.

Hypomagnesemia in patients with type 2 diabetes

Hypomagnesemia has been reported to occur at an increased frequency among patients with type 2 diabetes compared with their counterparts without diabetes. Despite numerous reports linking hypomagnesemia to chronic diabetic complications, attention to this issue is poor among clinicians. This article reviews the literature on the metabolism of magnesium, incidence of hypomagnesemia in patients with type 2 diabetes, implicated contributing factors, and associated complications. Hypomagnesemia occurs at an incidence of 13.5 to 47.7% among patients with type 2 diabetes. Poor dietary intake, autonomic dysfunction, altered insulin metabolism, glomerular hyperfiltration, osmotic diuresis, recurrent metabolic acidosis, hypophosphatemia, and hypokalemia may be contributory. Hypomagnesemia has been linked to poor glycemic control, coronary artery diseases, hypertension, diabetic retinopathy, nephropathy, neuropathy, and foot ulcerations. The increased incidence of hypomagnesemia among patients with type 2 diabetes presumably is multifactorial. Because current data suggest adverse outcomes in association with hypomagnesemia, it is prudent to monitor magnesium routinely in this patient population and treat the condition whenever possible.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

NIPA1(SPG6), the basis for autosomal dominant form of hereditary spastic paraplegia, encodes a functional Mg2+ transporter

Mutations in the NIPA1(SPG6) gene, named for "nonimprinted in Prader-Willi/Angelman" has been implicated in one form of autosomal dominant hereditary spastic paraplegia (HSP), a neurodegenerative disorder characterized by progressive lower limb spasticity and weakness. However, the function of NIPA1 is unknown. Here, we show that reduced magnesium concentration enhances expression of NIPA1 suggesting a role in cellular magnesium metabolism. Indeed NIPA1 mediates Mg2+ uptake that is electrogenic, voltage-dependent, and saturable with a Michaelis constant of 0.69+/-0.21 mM when expressed in Xenopus oocytes. Subcellular localization with immunofluorescence showed that endogenous NIPA1 protein associates with early endosomes and the cell surface in a variety of neuronal and epithelial cells. As expected of a magnesium-responsive gene, we find that altered magnesium concentration leads to a redistribution between the endosomal compartment and the plasma membrane; high magnesium results in diminished cell surface NIPA1 whereas low magnesium leads to accumulation in early endosomes and recruitment to the plasma membrane. The mouse NIPA1 mutants, T39R and G100R, corresponding to the respective human mutants showed a loss-of-function when expressed in oocytes and altered trafficking in transfected COS7 cells. We conclude that NIPA1 normally encodes a Mg2+ transporter and the loss-of function of NIPA1(SPG6) due to abnormal trafficking of the mutated protein provides the basis of the HSP phenotype.

Inherited Renal Tubulopathies Associated With Metabolic Alkalosis: Effects on Blood Pressure

Inherited tubular disorders associated with metabolic alkalosis are caused by several gene mutations encoding different tubular transporters responsible for NaCl renal handling. Body volume and renin-angiotensin-aldosterone system status are determined by NaCl reabsorption in the distal nephron. Two common hallmarks in affected individuals: hypokalemia and normal / high blood pressure, support the differential diagnosis. Bartter's syndrome, characterized by hypokalemia and normal blood pressure, is a heterogenic disease caused by the loss of function of SLC12A1 (type 1), KCNJ1 (type 2), CLCNKB (type 3), or BSND genes (type 4). As a result, patients present with renal salt wasting and hypercalciuria. Gitelman's syndrome is caused by the loss of funcion of the SLC12A3 gene and may resemble Bartter's syndrome, though is associated with the very low urinary calcium. Liddle's syndrome, also with similar phenotype but with hypertension, is produced by the gain of function of the SNCC1B or SNCC1G genes, and must be distinguished from other entities of inherited hypertension such as Apparently Mineralocorticoid Excess, of glucocorticoid remediable hypertension.

Increased renal calcium and magnesium transporter abundance in streptozotocin-induced diabetes mellitus

Diabetes is associated with renal calcium and magnesium wasting, but the molecular mechanisms of these defects are unknown. We measured renal calcium and magnesium handling and investigated the effects of diabetes on calcium and magnesium transporters in the thick ascending limb and distal convoluted tubule in streptozotocin (STZ)-induced diabetic rats. Rats were killed 2 weeks after inducing diabetes, gene expression of calcium and magnesium transporters in the kidney was determined by real-time polymerase chain reaction, and the abundance of protein was assessed by immunoblotting. Our results showed that diabetic rats had significant increase in the fractional excretion for calcium and magnesium (both P < 0.01), but not for sodium. Reverse transcriptase-polymerase chain reaction revealed significant increases in messenger RNA abundance of transient potential receptor (TRP) V5 (223 +/- 10%), TRPV6 (177 +/- 9%), calbindin-D28k (231 +/- 8%), and TRPM6 (165 +/- 8%) in diabetic rats. Sodium chloride cotransporter was also increased (207 +/- 10%). No change was found in paracellin-1 (cortex: 108 +/- 8%; medulla: 110 +/- 10%). Immunofluorescent studies of renal sections showed significant increase in calbindin-D28k (238 +/- 10%) and TRPV5 (211 +/- 10%), but no changes in paracellin-1 in Western blotting (cortex: 110 +/- 7%; medulla: 99 +/- 7%). Insulin administration completely corrected the hyperglycemia-associated hypercalciuria and hypermagnesiuria, and reversed the increase of calcium and magnesium transporter abundance. In conclusion, our results demonstrated increased renal calcium and magnesium transporter abundance in STZ-induced diabetic rats, which may represent a compensatory adaptation for the increased load of calcium and magnesium to the distal tubule.

Organic potassium salts or fibers effects on mineral balance and digestive fermentations in rats adapted to an acidogenic diet

BACKGROUND

Fibers and potassium (K) organic salts in plant foods are liable to affect Ca and Mg balance at digestive and renal levels, respectively. K organic salts could counteract the acidifying effects of western diets and consequences of excess NaCl.

AIM OF THE STUDY

To study this question, male rats were adapted to a basal acidifying low-K (LK) diet, or to diets supplemented with a fiber mix (LK/F), or K citrate (HK) or both (HK/F).

RESULTS

HK and HK/F diets displayed a marked alkalinizing effect in urine and promoted citraturia, but this effect was not modulated by fibers. The effect of fibers on Ca digestive absorption was more potent than K citrate effect on Ca renal excretion. In contrast, K citrate effect on kidney Mg excretion was more effective than that of fibers on Mg digestive absorption, a maximal effect on Mg balance was observed in rats fed the HK/F diet. Digestive fermentations in rats fed the LK/F diet were characterized by high-propionic acid fermentations and succinate accumulation. In rats adapted to the HK/F diet, K citrate supplementation depressed succinate and increased butyrate concentrations.

CONCLUSION

Organic anions arising from digestive fermentations seem to be not directly involved in the alkalinizing effects of plant foods. Fibers and organic K salts exert distinct effects on Ca and Mg metabolism, but with interesting interactions as to Mg balance, digestive fermentations and urine pH.

Prevention of thiazide-induced hypokalemia without magnesium depletion by potassium-magnesium-citrate

Thiazide can cause magnesium depletion, which may exaggerate renal potassium wasting and hypokalemia. The purpose of this double-blind, randomized trial was to compare the metabolic effects of potassium-magnesium-citrate (K-Mg-citrate) and potassium chloride (KCl) during long-term treatment with thiazide. Twenty-two normal volunteers received hydrochlorothiazide 50 mg/d. Ten subjects concurrently took K-Mg-citrate (42 mEq K/d and 21 mEq Mg/d), and 12 subjects were given KCl 42 mEq/d. Serum potassium concentration remained unchanged during K-Mg-citrate supplementation, with a change from baseline of 21.7% over 6 months, compared with 26.4% with KCl supplementation. Serum electrolytes were normal and not significantly different between K-Mg-citrate and KCl. During K-Mg-citrate treatment, serum magnesium increased significantly by about 10%, associated with an adequate increase in urinary magnesium and a nonsignificant increase in monocyte and free muscle magnesium. Serum magnesium was unchanged, and monocyte and free muscle magnesium showed a nonsignificant decline during KCl supplementation. K-Mg-citrate provided an alkali load, increasing urinary pH, and reducing urinary undissociated uric acid. It also increased urinary citrate and tended to lower the saturation of calcium oxalate. KCl supplementation lacked these actions. K-Mg-citrate prevents thiazide-induced hypokalemia without provoking metabolic alkalosis. It seems to prevent magnesium depletion. By providing an alkali load, it retards the propensity for the crystallization of uric acid and probably of calcium oxalate. Though not conclusive, KCl supplementation may be less effective than K-Mg-citrate in maintaining normokalemia because of a subtle magnesium wasting. Moreover, KCl is devoid of protective action toward crystallization of stone-forming salts.

Phosphorylation of paracellin-1 at Ser217 by protein kinase A is essential for localization in tight junctions

Although paracellin-1 (PCLN-1) is known to have a crucial role in the control of Mg2+ reabsorption in the kidney, the molecular pathways involved in the regulation of PCLN-1 have not been clarified. We used FLAG-tagged PCLN-1 to investigate these pathways further, and found that PCLN-1 is phosphorylated at Ser217 by protein kinase A (PKA) under physiological conditions in Madin-Darby canine kidney (MDCK) cells. PCLN-1 expression decreased Na+ permeability, resulting in a decrease in the transepithelial electrical resistance (TER). By contrast, PCLN-1 enhanced transepithelial Mg2+ transport. PKA inhibitors, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89) and myristoylated protein kinase A inhibitor 14-22 amide PKI, and an adenylate cyclase inhibitor, 2′,5′-dideoxy adenosine (DDA), reduced the phosphoserine level of PCLN-1. The inhibitory effect of DDA was rescued by 8-bromoadenosine-3′,5′-cyclic monophosphate (8-Br-cAMP). PKA and adenylate cyclase inhibitors decreased transepithelial Mg2+ transport and TER. Dephosphorylated PCLN-1 moved from detergent-insoluble to soluble fractions and was dissociated from ZO-1. A fusion protein of PCLN-1 with glutathione-S-transferase revealed that Ser217 was phosphorylated by PKA. Phosphorylated PCLN-1 was localized in the tight junction (TJ) along with ZO-1, whereas dephosphorylated PCLN-1 and the S217A mutant were translocated into the lysosome. The degradation of dephosphorylated PCLN-1 and S217A mutant was inhibited by chloroquine, a specific lysosome inhibitor. Thus, the PKA-dependent phosphorylation of Ser217 in PCLN-1 is essential for its localization in the TJ and transepithelial Mg2+ transport.

Increase in serum magnesium level in haemodialysis patients receiving sevelamer hydrochloride

BACKGROUND

Clinical studies have shown that sevelamer hydrochloride improves lipid profiles and attenuates the progression of the cardiovascular calcifications in haemodialysis patients. It is known that both of these properties are associated with increased magnesium levels. The effect of sevelamer on serum magnesium level is not well documented. The aim of this study was to determine the effects of sevelamer treatment on serum magnesium in haemodialysis patients and to assess the association of magnesium levels with lipid profiles and intact parathyroid hormone (iPTH).

METHODS

Phosphate binders were discontinued during a two week washout period. Forty-seven patients, whose serum phosphate was greater than 6.0 mg/dl at the end of washout, received sevelamer hydrochloride for eight weeks. The patients were then washed off sevelamer for another two weeks.

RESULTS

Mean serum phosphorus concentration declined from 7.5 +/- 1.3 to 6.4 +/- 1.2 mg/dl (P < 0.001), mean serum magnesium levels increased from 2.75 +/- 0.35 to 2.90 +/- 0.41 mg/dl (P < 0.001) and median serum iPTH levels decreased from 297 to 213 pg/ml (P=0.001) during the eight weeks of sevelamer treatment. After the two week post-treatment washout phosphorus levels increased to 7.3 +/- 1.3 mg/dl (P < 0.001), magnesium levels were reduced to 2.77 +/- 0.39 mg/dl (P < 0.001) and iPTH levels increased to 240 pg/ml (P=0.012). No change was observed in serum calcium levels during the sevelamer treatment period and the subsequent washout period. The mean decline in total and low density lipoprotein (LDL) cholesterol during sevelamer treatment was 16.3 and 28.3 (P < 0.001), respectively. The mean increase in high density lipoprotein (HDL) cholesterol and in apolipoprotein A1 was 2.9 +/- 5.8 mg/dl (P=0.004) and 6.8 +/- 11.1 mg/dl (P=0.001), respectively. Multivariate analysis showed that the rise in serum magnesium concentration significantly correlated with reductions in iPTH levels (r=-0.40, P=0.016), but did not have any significant correlation with the changes in lipid profiles.

CONCLUSIONS

Our findings indicate that patients on haemodialysis receiving sevelamer have a significant increase in serum magnesium concentrations. This increase in serum magnesium is associated with reduction in iPTH levels. The changes in lipid profiles of these patients however are not related to changes in serum magnesium levels.

Hydrochlorothiazide in CLDN16 mutation

BACKGROUND

Hydrochlorothiazide (HCT) is applied in the therapy of familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) caused by claudin-16 (CLDN16) mutation. However, the short-term efficacy of HCT to reduce hypercalciuria in FHHNC has not yet been demonstrated in a clinical trial.

METHODS

Four male and four female patients with FHHNC and CLDN16 mutation, under long-standing HCT therapy (0.4-1.2 mg/kg, median 0.9 mg/kg, dose according to calciuria), aged 0.7-22.4 years, were included in a clinical study to investigate the effect of HCT on calciuria. The study design consisted of three periods: continued therapy for 4 weeks, HCT withdrawal for 6 weeks and restart of therapy at the same dose for 4 weeks. Calciuria and magnesiuria were assessed weekly as Ca/creat and Mg/creat ratio, every 2 weeks in 24 h urine, and serum Mg, K and kaliuria (s-Mg, s-K and K/creat) at weeks 0, 6, 10 and 14. The data of each study period were averaged and analysed by Friedman and Wilcoxon test.

RESULTS

Ca/creat was significantly reduced by HCT (median before/at/after withdrawal 0.76/1.24/0.77 mol/mol creat; n = 8, P<0.05). The reduction of Ca/24 h by HCT was not statistically significant (0.13/0.19/0.13 mmol/kg x 24 h; n = 5). Serum Mg (0.51/0.64/0.56 mmol/l; n = 8, P<0.05) and Serum K (3.65/4.35/3.65 mmol/l; n = 8, P<0.05) were significantly higher during withdrawal. However, Mg/creat (0.98/0.90/0.90 mol/mol creat; n = 8), Mg/24 h (0.14/0.12/0.18 mmol/kg x 24h; n = 5) and K/creat (6.3/8.4/6.2 mol/mol creat; n = 8) remained statistically unchanged during withdrawal.

CONCLUSIONS

We demonstrated that HCT is effective in reducing hypercalciuria due to CLDN16 mutation on a short-term basis. However, the efficacy of HCT to attenuate disease progression remains to be elucidated.