Simultaneous Mg, Ca, P,K,Na and Cl analysis in rat tubular fluid. II. During acute Mg plasma loading

Magnesium Biology

Magnesium (Mg2+) is essential for energy metabolism, muscle contraction, and neurotransmission. As part of the Mg-ATP complex, it is involved in over 600 enzymatic reactions. Serum Mg2+ levels are tightly regulated between 0.7 mmol/L and 1.1 mmol/L by interplay of intestinal absorption and renal excretion. In the small intestine, Mg2+ is absorbed paracellularly via claudin-2, and -12. In the colon, transcellular absorption of Mg2+ is facilitated by TRPM6/7 and CNNM4. In the kidney, the proximal tubule reabsorbs only 20% of the filtered Mg2+. The majority of the filtered Mg2+ is reabsorbed in the thick ascending limb (TAL), where the lumen-positive transepithelial voltage drives paracellular transport via claudin-16/-19. Fine-tuning of Mg2+ reabsorption is achieved in the distal convoluted tubule (DCT). Here, TRPM6/7 tetramers facilitate apical Mg2+ uptake, which is hormonally regulated by insulin and EGF. Basolateral Mg2+ extrusion is Na+ dependent and achieved by CNNM2 and/or SLC41A3. Hypomagnesemia (serum Mg2+ < 0.7 mmol/L) develops when intestinal and/or renal Mg2+ (re)absorption is disturbed. Common causes include alcoholism, type 2 diabetes mellitus, and the use of pharmacological drugs, such as proton-pump inhibitors (PPIs), calcineurin inhibitors (CNIs) and thiazide diuretics. Over the last decade, research on rare genetic and acquired Mg2+ disorders have identified Mg2+ channel and transporter activity, DCT length, mitochondrial function, and autoimmunity as mechanisms explaining hypomagnesemia. Classically, treatment of hypomagnesemia depended on oral or intravenous Mg2+ supplementation. Recently, prebiotic dietary fibers and SGLT2 inhibitors have been proposed as promising new therapeutic pathways to treat hypomagnesemia.

Sodium/Glucose Cotransporter 2 Inhibitors and Magnesium Homeostasis: A Review

Magnesium (Mg2+), also known as "the forgotten ion," is the second most abundant intracellular cation and is essential in a broad range of intracellular physiological and biochemical reactions. Its deficiency, hypomagnesemia (Mg2+<1.8mg/dL), is a prevalent condition and routinely poses challenges in its management in clinical practice. Sodium/glucose cotransporter 2 (SGLT2) inhibitors have emerged as a new class of drugs with treating hypomagnesemia as their unique extraglycemic benefit. The beneficial effect of SGLT2 inhibitors on magnesium balance in patients with diabetes with or without hypomagnesemia has been noted as a class effect in recent meta-analysis data from randomized clinical trials. Some reports have demonstrated their role in treating refractory hypomagnesemia in patients with or without diabetes. Moreover, studies on animal models have attempted to illustrate the effect of SGLT2 inhibitors on Mg2+homeostasis. In this review, we discuss the current evidence and possible pathophysiological mechanisms, and we provide directions for further research. We conclude by suggesting the effect of SGLT2 inhibitors on Mg2+homeostasis is a class effect, with certain patients gaining significant benefits. Further studies are needed to examine whether SGLT2 inhibitors can become a desperately needed novel class of medicines in treating hypomagnesemia.

Magnesium Deficiency and Cardiometabolic Disease

Magnesium (Mg²⁺) has many physiological functions within the body. These include important roles in maintaining cardiovascular functioning, where it contributes to the regulation of cardiac excitation-contraction coupling, endothelial functioning and haemostasis. The haemostatic roles of Mg²⁺ impact upon both the protein and cellular arms of coagulation. In this review, we examine how Mg²⁺ homeostasis is maintained within the body and highlight the various molecular roles attributed to Mg²⁺ in the cardiovascular system. In addition, we describe how nutritional and/or disease-associated magnesium deficiency, seen in some metabolic conditions, has the potential to influence cardiac and vascular outcomes. Finally, we also examine the potential for magnesium supplements to be employed in the prevention and treatment of cardiovascular disorders and in the management of cardiometabolic health.

Mechanisms of paracellular transport of magnesium in intestinal and renal epithelia

Magnesium is the fourth most abundant cation in the body. It plays a critical role in many biological processes, including the process of energy release. Paracellular transport of magnesium is mandatory for magnesium homeostasis. In addition to intestinal absorption that occurs in part across the paracellular pathway, magnesium is reabsorbed by the kidney tubule. The bulk of magnesium is reabsorbed through the paracellular pathway in the proximal tubule and the thick ascending limb of the loop of Henle. The finding that rare genetic diseases due to pathogenic variants in genes encoding specific claudins (CLDNs), proteins located at the tight junction that determine the selectivity and the permeability of the paracellular pathway, led to an awareness of their importance in magnesium homeostasis. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is caused by a loss of function of CLDN16 or CLDN19. Pathogenic CLDN10 variants cause HELIX syndrome, which is associated with a severe renal loss of sodium chloride and hypermagnesemia. The present review summarizes the current knowledge of the mechanisms and factors involved in paracellular magnesium permeability. The review also highlights some of the unresolved questions that need to be addressed.

Magnesium reabsorption in the kidney

Mg²⁺ is essential for many cellular and physiological processes, including muscle contraction, neuronal activity, and metabolism. Consequently, the blood Mg²⁺ concentration is tightly regulated by balanced intestinal Mg²⁺ absorption, renal Mg²⁺ excretion, and Mg²⁺ storage in bone and soft tissues. In recent years, the development of novel transgenic animal models and identification of Mendelian disorders has advanced our current insight in the molecular mechanisms of Mg²⁺ reabsorption in the kidney. In the proximal tubule, Mg²⁺ reabsorption is dependent on paracellular permeability by claudin-2/12. In the thick ascending limb of Henle's loop, the claudin-16/19 complex provides a cation-selective pore for paracellular Mg²⁺ reabsorption. The paracellular Mg²⁺ reabsorption in this segment is regulated by the Ca²⁺-sensing receptor, parathyroid hormone, and mechanistic target of rapamycin (mTOR) signaling. In the distal convoluted tubule, the fine tuning of Mg²⁺ reabsorption takes place by transcellular Mg²⁺ reabsorption via transient receptor potential melastatin-like types 6 and 7 (TRPM6/TRPM7) divalent cation channels. Activity of TRPM6/TRPM7 is dependent on hormonal regulation, metabolic activity, and interacting proteins. Basolateral Mg²⁺ extrusion is still poorly understood but is probably dependent on the Na⁺ gradient. Cyclin M2 and SLC41A3 are the main candidates to act as Na⁺/Mg²⁺ exchangers. Consequently, disturbances of basolateral Na⁺/K⁺ transport indirectly result in impaired renal Mg²⁺ reabsorption in the distal convoluted tubule. Altogether, this review aims to provide an overview of the molecular mechanisms of Mg²⁺ reabsorption in the kidney, specifically focusing on transgenic mouse models and human hereditary diseases.

Molecular mechanisms underlying paracellular calcium and magnesium reabsorption in the proximal tubule and thick ascending limb

Calcium and magnesium are the most abundant divalent cations in the body. The plasma level is controlled by coordinated interaction between intestinal absorption, reabsorption in the kidney, and, for calcium at least, bone storage and exchange. The kidney adjusts urinary excretion of these ions in response to alterations in their systemic concentration. Free ionized and anion-complexed calcium and magnesium are filtered at the glomerulus. The majority (i.e., >85%) of filtered divalent cations are reabsorbed via paracellular pathways from the proximal tubule and thick ascending limb (TAL) of the loop of Henle. Interestingly, the largest fraction of filtered calcium is reabsorbed from the proximal tubule (65%), while the largest fraction of filtered magnesium is reclaimed from the TAL (60%). The paracellular pathways mediating these fluxes are composed of tight junctional pores formed by claudins. In the proximal tubule, claudin-2 and claudin-12 confer calcium permeability, while the exact identity of the magnesium pore remains to be determined. Claudin-16 and claudin-19 contribute to the calcium and magnesium permeable pathway in the TAL. In this review, we discuss the data supporting these conclusions and speculate as to why there is greater fractional calcium reabsorption from the proximal tubule and greater fractional magnesium reabsorption from the TAL.

Pathophysiology of Drug-Induced Hypomagnesaemia

Magnesium (Mg²⁺) is the second most abundant intracellular and fourth extracellular cation found in the body and is involved in a wide range of functions in the human cell and human physiology. Its role in most of the enzyme processes (ATP-ases)-stabilisation of nucleic acids (DNA, RNA), regulation of calcium and potassium ion channels, proliferation, glucose metabolism and apoptosis-make it one of the most important cations in the cell. Three pathogenetic mechanisms are mainly implicated in the development of hypomagnesaemia: reduced food intake, decreased intestinal absorption and increased renal excretion of Mg²⁺. This review presents the function of Mg²⁺, how it is handled in the kidney and the drugs that cause hypomagnesaemia. The frequency and the number of drugs like diuretics and proton-pump inhibitors (PPIs) that are used daily in medical practice are discussed in order to prevent and treat adverse effects by providing an insight into Mg²⁺ homeostasis.

An overview of diagnosis and management of drug-induced hypomagnesemia

Magnesium (Mg) is commonly addressed as the "forgotten ion" in medicine. Nonetheless, hypomagnesemia should be suspected in clinical practice in patients with relevant symptomatology and also be considered a predisposing factor for the development of other electrolyte disturbances. Furthermore, chronic hypomagnesemia has been associated with diabetes mellitus and cardiovascular disease. Hypomagnesemia as a consequence of drug therapy is relatively common, with the list of drugs inducing low serum Mg levels expanding. Culprit medications linked to hypomagnesemia include antibiotics (e.g. aminoglycosides, amphotericin B), diuretics, antineoplastic drugs (cisplatin and cetuximab), calcineurin inhibitors, and proton pump inhibitors. In recent years, the mechanisms of drug-induced hypomagnesemia have been unraveled through the discovery of key Mg transporters in the gut and kidney. This narrative review of available literature focuses on the pathogenetic mechanisms underlying drug-induced hypomagnesemia in order to increase the insight of clinicians toward early diagnosis and effective management.

Magnesium Handling in the Kidney

Magnesium is a divalent cation that fills essential roles as regulator and cofactor in a variety of biological pathways, and maintenance of magnesium balance is vital to human health. The kidney, in concert with the intestine, has an important role in maintaining magnesium homeostasis. Although micropuncture and microperfusion studies in the mammalian nephron have shone a light on magnesium handling in the various nephron segments, much of what we know about the protein mediators of magnesium handling in the kidney have come from more recent genetic studies. In the proximal tubule and thick ascending limb, magnesium reabsorption is believed to occur primarily through the paracellular shunt pathway, which ultimately depends on the electrochemical gradient setup by active sodium reabsorption. In the distal convoluted tubule, magnesium transport is transcellular, although magnesium reabsorption also appears to be related to active sodium reabsorption in this segment. In addition, evidence suggests that magnesium transport is highly regulated, although a specific hormonal regulator of extracellular magnesium has yet to be identified.

Regulation of calcium reabsorption along the rat nephron: a modeling study

We expanded a mathematical model of transepithelial transport along the rat nephron to include the transport of Ca2+ and probe the impact of calcium-sensing mechanisms on Ca2+ reabsorption. The model nephron extends from the medullary thick ascending limb (mTAL) to the inner medullary collecting duct (IMCD). Our model reproduces several experimental findings, such as measurements of luminal Ca2+ concentrations in cortical tubules, and the effects of furosemide or deletion of the transient receptor potential channel vanilloid subtype 5 (TRPV5) on urinary Ca2+ excretion. In vitro microperfusion of rat TAL has demonstrated that activation of the calcium-sensing receptor CaSR lowers the TAL permeability to Ca2+, PCaTAL (Loupy A, Ramakrishnan SK, Wootla B, Chambrey R, de la Faille R, Bourgeois S, Bruneval P, Mandet C, Christensen EI, Faure H, Cheval L, Laghmani K, Collet C, Eladari D, Dodd RH, Ruat M, Houillier P. J Clin Invest 122: 3355, 2012). Our results suggest that this regulatory mechanism significantly impacts renal Ca2+ handling: when plasma Ca2+ concentration ([Ca2+]) is raised by 10%, the CaSR-mediated reduction in PCaTAL per se is predicted to enhance urinary Ca2+ excretion by ∼30%. If high [Ca2+] also induces renal outer medullary potassium (ROMK) inhibition, urinary Ca2+ excretion is further raised. In vitro, increases in luminal [Ca2+] have been shown to activate H+-ATPase pumps in the outer medullary CD and to lower the water permeability of IMCD. Our model suggests that if these responses exhibit the sigmoidal dependence on luminal [Ca2+] that is characteristic of CaSR, then the impact of elevated Ca2+ levels in the CD on urinary volume and pH remains limited. Finally, our model suggests that CaSR inhibitors could significantly reduce urinary Ca2+ excretion in hypoparathyroidism, thereby reducing the risk of calcium stone formation.

Segmental transport of Ca²⁺ and Mg²⁺ along the gastrointestinal tract

Calcium (Ca(2+)) and magnesium (Mg(2+)) ions are involved in many vital physiological functions. Since dietary intake is the only source of minerals for the body, intestinal absorption is essential for normal homeostatic levels. The aim of this study was to characterize the absorption of Ca(2+) as well as Mg(2+) along the gastrointestinal tract at a molecular and functional level. In both humans and mice the Ca(2+) channel transient receptor potential vanilloid subtype 6 (TRPV6) is expressed in the proximal intestinal segments, whereas Mg(2+) channel transient receptor potential melastatin subtype 6 (TRPM6) is expressed in the distal parts of the intestine. A method was established to measure the rate of Mg(2+) absorption from the intestine in a time-dependent manner by use of (25)Mg(2+). In addition, local absorption of Ca(2+) and Mg(2+) in different segments of the intestine of mice was determined by using surgically implanted intestinal cannulas. By these methods, it was demonstrated that intestinal absorption of Mg(2+) is regulated by dietary needs in a vitamin D-independent manner. Also, it was shown that at low luminal concentrations, favoring transcellular absorption, Ca(2+) transport mainly takes place in the proximal segments of the intestine, whereas Mg(2+) absorption predominantly occurs in the distal part of the gastrointestinal tract. Vitamin D treatment of mice increased serum Mg(2+) levels and 24-h urinary Mg(2+) excretion, but not intestinal absorption of (25)Mg(2+). Segmental cannulation of the intestine and time-dependent absorption studies using (25)Mg(2+) provide new ways to study intestinal Mg(2+) absorption.

Mechanisms and regulation of renal magnesium transport

Magnesium's most important role is in the release of chemical energy. Although most magnesium is stored outside of the extracellular fluid compartment, the regulated value is blood magnesium concentration. Cellular magnesium and bone magnesium do not play a major role in the defense of blood magnesium concentration; the major role is played by the kidney, where the renal tubule matches the urinary magnesium excretion and the net entry of magnesium into the extracellular fluid. In the kidney, magnesium is reabsorbed in the proximal tubule, the thick ascending limb of the loop of Henle, and the distal convoluted tubule. Magnesium absorption is mainly paracellular in the proximal tubule and in the thick ascending limb of the loop of Henle, whereas it is transcellular in the distal convoluted tubule. Several hormones and extracellular magnesium itself alter the distal tubular handling of magnesium, but the hormone(s) regulating extracellular magnesium concentration remains unknown.

A model of calcium transport along the rat nephron

We developed a mathematical model of calcium (Ca2+) transport along the rat nephron to investigate the factors that promote hypercalciuria. The model is an extension of the flat medullary model of Hervy and Thomas ( Am J Physiol Renal Physiol 284: F65–F81, 2003). It explicitly represents all the nephron segments beyond the proximal tubules and distinguishes between superficial and deep nephrons. It solves dynamic conservation equations to determine NaCl, urea, and Ca2+ concentration profiles in tubules, vasa recta, and the interstitium. Calcium is known to be reabsorbed passively in the thick ascending limbs and actively in the distal convoluted (DCT) and connecting (CNT) tubules. Our model predicts that the passive diffusion of Ca2+ from the vasa recta and loops of Henle generates a significant axial Ca2+ concentration gradient in the medullary interstitium. In the base case, the urinary Ca2+ concentration and fractional excretion are predicted as 2.7 mM and 0.32%, respectively. Urinary Ca2+ excretion is found to be strongly modulated by water and NaCl reabsorption along the nephron. Our simulations also suggest that Ca2+ molar flow and concentration profiles differ significantly between superficial and deep nephrons, such that the latter deliver less Ca2+ to the collecting duct. Finally, our results suggest that the DCT and CNT can act to counteract upstream variations in Ca2+ transport but not always sufficiently to prevent hypercalciuria.

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.

Mechanism of urinary calcium regulation by urinary magnesium and pH

Urinary magnesium and pH are known to modulate urinary calcium excretion, but the mechanisms underlying these relationships are unknown. In this study, the data from 17 clinical trials in which urinary magnesium and pH were pharmacologically manipulated were analyzed, and it was found that the change in urinary calcium excretion is directly proportional to the change in magnesium excretion and inversely proportional to the change in urine pH; a regression equation was generated to relate these variables (R(2) = 0.58). For further exploration of these relationships, intravenous calcium chloride, magnesium chloride, or vehicle was administered to rats. Magnesium infusion significantly increased urinary calcium excretion (normalized to urinary creatinine), but calcium infusion did not affect magnesium excretion. Parathyroidectomy did not prevent this magnesium-induced hypercalciuria. The effect of magnesium loading on calciuria was still observed after treatment with furosemide, which disrupts calcium and magnesium absorption in the thick ascending limb, suggesting that the effect may be mediated by the distal nephron. The calcium channel TRPV5, normally present in the distal tubule, was expressed in Xenopus oocytes. Calcium uptake by TRPV5 was directly inhibited by magnesium and low pH. In summary, these data are compatible with the hypothesis that urinary magnesium directly inhibits renal calcium absorption, which can be negated by high luminal pH, and that this regulation likely takes place in the distal tubule.

The human paracellin-1 gene (hPCLN-1): renal epithelial cell-specific expression and regulation

Tubular reabsorption of Mg2+ is mediated by the tight junction protein paracellin-1, which is encoded by the gene PCLN-1 (CLDN16) and exclusively expressed in the kidney. Tubular Mg2+ reclamation is modulated by many hormones and factors. The aim of this study was to define regulatory elements essential for renal tubular cell-specific expression of human PCLN-1 (hPCLN-1) and to explore the effect of Mg2+ transport modulators on the paracellin-1 gene promoter. Endogenous paracellin-1 mRNA and protein were detected in renal cell lines opossom kidney (OK), HEK293, and MDCT, but not in the fibroblast cell line NIH3T3. A 7.5-kb hPCLN-1 5'-flanking DNA sequence along with seven 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the renal and nonrenal cells. The highest levels of luciferase activity resulted from transfection of a 5'-flanking 2.5-kb fragment (pJ2M). This activity was maximal in OK cells, was orientation dependent, and was absent in NIH3T3 cells. Mg2+ deprivation significantly increased pJ2M-driven activity in transfected OK cells, whereas Mg2+ load decreased it compared with conditions of normal Mg2+. Deletion analysis along with electrophoretic mobility-shift assay demonstrated that OK cells contain nuclear proteins, which bind a 70-bp region between -1633 and -1703 of major functional significance. Deleting this 70-bp segment, which contains a single peroxisome proliferator-response element (PPRE), or mutating the PPRE, caused a 60% reduction in luciferase activity. Stimulating the 70-bp sequence with 1,25(OH)2 vitamin D decreased luciferase activity by 52%. This effect of 1,25(OH)2 vitamin D was abolished in the absence of PPRE or in the presence of mutated PPRE. We conclude that the PPRE within this 70-bp DNA region may play a key role in the cell-specific and regulatory activity of the hPCLN-1 promoter. Ambient Mg2+ concentration and 1,25(OH)2 vitamin D may modulate paracellular, paracellin-1-mediated, Mg2+ transport at the transcriptional level. 1,25(OH)2 vitamin D exerts its activity on the hPCLN-1 promoter likely via the PPRE site.

Magnesium and the parathyroid

The serum levels of parathyroid hormone and magnesium depend on each other in a complex manner. The secretion of parathyroid hormone by the parathyroid is physiologically controlled by the serum calcium level, but magnesium can exert similar effects. While low levels of magnesium stimulate parathyroid hormone secretion, very low serum concentrations induce a paradoxical block. This block leads to clinically relevant hypocalcemia in severely hypomagnesiemic patients. The mechanism of this effect has recently been traced to an activation of the alpha-subunits of heterotrimeric G-proteins. This activation mimicks activation of the calcium sensing receptor and thus causes inhibition of parathyroid hormone secretion. In addition to the effects of magnesium on parathyroid hormone secretion, parathyroid hormone in turn regulates magnesium homeostasis by modulating renal magnesium reabsorption. The distal convoluted tubule is of crucial importance for parathyroid hormone-regulated magnesium homeostasis.

Magnesium transport in the renal distal convoluted tubule

The distal tubule reabsorbs approximately 10% of the filtered Mg(2+), but this is 70-80% of that delivered from the loop of Henle. Because there is little Mg(2+) reabsorption beyond the distal tubule, this segment plays an important role in determining the final urinary excretion. The distal convoluted segment (DCT) is characterized by a negative luminal voltage and high intercellular resistance so that Mg(2+) reabsorption is transcellular and active. This review discusses recent evidence for selective and sensitive control of Mg(2+) transport in the DCT and emphasizes the importance of this control in normal and abnormal renal Mg(2+) conservation. Normally, Mg(2+) absorption is load dependent in the distal tubule, whether delivery is altered by increasing luminal Mg(2+) concentration or increasing the flow rate into the DCT. With the use of microfluorescent studies with an established mouse distal convoluted tubule (MDCT) cell line, it was shown that Mg(2+) uptake was concentration and voltage dependent. Peptide hormones such as parathyroid hormone, calcitonin, glucagon, and arginine vasopressin enhance Mg(2+) absorption in the distal tubule and stimulate Mg(2+) uptake into MDCT cells. Prostaglandin E(2) and isoproterenol increase Mg(2+) entry into MDCT cells. The current evidence indicates that cAMP-dependent protein kinase A, phospholipase C, and protein kinase C signaling pathways are involved in these responses. Steroid hormones have significant effects on distal Mg(2+) transport. Aldosterone does not alter basal Mg(2+) uptake but potentiates hormone-stimulated Mg(2+) entry in MDCT cells by increasing hormone-mediated cAMP formation. 1,25-Dihydroxyvitamin D(3), on the other hand, stimulates basal Mg(2+) uptake. Elevation of plasma Mg(2+) or Ca(2+) inhibits hormone-stimulated cAMP accumulation and Mg(2+) uptake in MDCT cells through activation of extracellular Ca(2+)/Mg(2+)-sensing mechanisms. Mg(2+) restriction selectively increases Mg(2+) uptake with no effect on Ca(2+) absorption. This intrinsic cellular adaptation provides the sensitive and selective control of distal Mg(2+) transport. The distally acting diuretics amiloride and chlorothiazide stimulate Mg(2+) uptake in MDCT cells acting through changes in membrane voltage. A number of familial and acquired disorders have been described that emphasize the diversity of cellular controls affecting renal Mg(2+) balance. Although it is clear that many influences affect Mg(2+) transport within the DCT, the transport processes have not been identified.

Divalent cation transport by the distal nephron: insights from Bartter's and Gitelman's syndromes

Elucidation of the gene defects responsible for many disorders of renal fluid and electrolyte homeostasis has provided new insights into normal and abnormal physiology. Identifying the causes of Gitelman's and Bartter's syndromes has greatly enhanced our understanding of ion transport by thick ascending limb and distal convoluted tubule cells. Despite this information, several phenotypic features of these diseases remain confusing, even in the face of molecular insight. Paramount among these are disorders of divalent cation homeostasis. Bartter's syndrome is caused by dysfunction of thick ascending limb cells. It is associated with calcium wasting, but magnesium wasting is usually mild. Loop diuretics, which inhibit ion transport by thick ascending limb cells, markedly increase urinary excretion of both calcium and magnesium. In contrast, Gitelman's syndrome is caused by dysfunction of the distal convoluted tubule. Hypocalciuria and hypomagnesemia are universal parts of this disorder. Yet although thiazide diuretics, which inhibit ion transport by distal convoluted tubule cells, reduce urinary calcium excretion, they have minimal effects on urinary magnesium excretion, when given acutely. This review proposes mechanisms that may account for the differences between the effects of diuretic drugs and the phenotypic features of Gitelman's and Bartter's syndromes. These mechanisms are based on recent insights from another inherited disease of ion transport, inherited magnesium wasting, and from a review of the chronic effects of diuretic drugs in animals and people.

Mg2+/Ca2+ sensing inhibits hormone-stimulated Mg2+ uptake in mouse distal convoluted tubule cells

The distal convoluted tubule plays a significant role in renal magnesium conservation. An immortalized mouse distal convoluted tubule (MDCT) cell line has been extensively used to study the cellular mechanisms of magnesium transport in this nephron segment. MDCT cells possess an extracellular polyvalent cation-sensing mechanism responsive to Mg2+, Ca2+, and neomycin. The present studies determined the effect of Mg2+/Ca2+ sensing on hormone-mediated cAMP formation and Mg2+ uptake in MDCT cells. MDCT cells were Mg2+ depleted by culturing in Mg2+-free media for 16 h, and Mg2+ uptake was measured by microfluorescence after placing the depleted cells in 1.5 mM MgCl2. The mean rate of Mg2+ uptake was 164 +/- 5 nM/s in control MDCT cells. Activation of Mg2+/Ca2+ sensing with neomycin did not affect basal Mg2+ uptake (155 +/- 5 nM/s). We have previously reported that treatment of MDCT cells with either glucagon or arginine vasopressin (AVP) stimulated Mg2+ entry. In the present studies, the addition of extracellular Mg2+ or Ca2+ inhibited glucagon- and AVP-stimulated cAMP formation and Mg2+ uptake in concentration-dependent manner with half-maximal concentrations of approximately 1.5 and 3.0 mM, respectively. Exogenous cAMP or forskolin stimulated Mg2+ uptake in the presence of Mg2+/Ca2+ sensing activation. We infer from these studies that Mg2+/Ca2+-sensing mechanisms located in the distal convoluted tubule may play a role in control of distal magnesium absorption.

Renal magnesium handling: new insights in understanding old problems

Recent research has provided new concepts in our understanding of renal magnesium handling. Although the majority of the filtered magnesium is reabsorbed within the loop of Henle, it is now recognized that the distal tubule also plays an important role in magnesium conservation. Magnesium absorption within the cTAL segment of the loop is passive and dependent on the transepithelial voltage. Magnesium transport in the DCT is active and transcellular in nature. Many of the hormonal (PTH, calcitonin, glucagon, AVP) and nonhormonal (magnesium-restriction, acid-base changes, potassium-depletion) influences that affect magnesium transport within the cTAL similarly alter magnesium absorption within the DCT. However, the cellular mechanisms are different. Actions within the loop affect either the transepithelial voltage or the paracellular permeability. Influences acting in the DCT involve changes in active transcellular transport either Mg2+ entry across the apical membrane or Mg2+ exit from the basolateral side. These transport processes are fruitful areas for future research. An additional regulatory control has recently been recognized that involves an extracellular Ca2+/Mg(2+)-sensing receptor. This receptor is present in the basolateral membrane of the TAL and DCT and modulates magnesium and calcium conservation with elevation in plasma divalent cation concentration. Further studies are warranted to determine the physiological role of the Ca2+/Mg(2+)-sensing receptor, but activating and inactivating mutations have been described that result in renal magnesium-wasting and hypermagnesemia, respectively. All of these receptor-mediated controls change calcium absorption in addition to magnesium transport. Selective magnesium control is through intrinsic control of Mg2+ entry into distal tubule cells. The cellular mechanisms that intrinsically regulate magnesium transport have yet to be described. Familial diseases associated with renal magnesium-wasting provide a unique opportunity to study these intrinsic controls. Loop diuretics such as furosemide increase magnesium excretion by virtue of its effects on the transepithelial voltage thereby inhibiting passive magnesium absorption. Distally acting diuretics, like amiloride and chlorothiazide, enhance Mg2+ entry into DCT cells. Amiloride may be used as a magnesium-conserving diuretic whereas chlorothiazide may lead to potassium-depletion that compromises renal magnesium absorption. Patients with Bartter's and Gitelman's syndromes, diseases of salt transport in the loop and distal tubule, respectively, are associated with disturbances in renal magnesium handling. These may provide useful lessons in understanding segmental control of magnesium reabsorption. Metabolic acidosis diminishes magnesium absorption in MDCT cells by protonation of the Mg2+ entry pathway. Metabolic alkalosis increases magnesium permeability across the cTAL paracellular pathway and stimulates Mg2+ entry into DCT cells. Again, these changes are likely due to protonation of charges along the paracellular pathway of the cTAL and the putative Mg2+ channel of the DCT. Cellular potassium-depletion diminishes the voltage-dependent magnesium absorption in the TAL and Mg2+ entry into MDCT cells. However, the relationship between potassium and magnesium balance is far from clear. For instance, magnesium-wasting is more commonly found in patients with Gitelman's disease than Bartter's but both have hypokalemia. Further studies are needed to sort out these discrepancies. Phosphate deficiency also decreases Mg2+ uptake in distal cells but it apparently does so by mechanisms other than those observed in potassium depletion. Accordingly, potassium depletion, phosphate deficiency, and metabolic acidosis may be additive. The means by which cellular potassium and phosphate alter magnesium handling are unclear. Research in the nineties has increased our understanding of renal magnesium transport and regulation, but there are many in

Flow-correlated influx of K, Ca, P, and Mg during continuous microperfusion of the loop of Henle in the rat

Investigations on hydropenic rats were undertaken enabling us to determine the relative loop influx rates for potassium, calcium, magnesium, and phosphorus during end proximal continuous microperfusions at 15 or 35 nl . min-1 with saline free of these ions. Replicate quantitative collections were taken at collected flow rates of 9 or 26 nl . min-1. Element concentrations were determined by electron probe analysis. Results showed a significant influx of all four ions in the fluid collected at the early distal site, with magnesium showing the smallest entry rate. As expected, calcium and potassium concentrations at the early distal site increased with the perfusion flow rate. In contrast, phosphorus concentration decreased with flow. Phosphorus concentration at low flow was 0.43 +/- 0.06 mmole . liter-1 vs. 0.25 +/- 0.03 at high flow (P less than 0.01). However, increasing influx was seen with increasing flow for phosphorus (r = 0.58, P less than 0.001). Magnesium concentrations did not significantly change with flow although they were clearly different from zero: 0.12 +/- 0.02 vs. 0.10 +/- 0.02 mmole . liter-1. The rate of magnesium influx as a function of flow was also highly significant (r = 0.48, P less than 0.01). Thus, there is an increase in loop influx in all four ions with high flow rates despite disparate flow influences on early distal concentrations. In conclusion, these experiments demonstrate that (1) magnesium can enter the tubular lumen of the loop when this structure is perfused with a magnesium-free solution and (2) under our experimental conditions, phosphate can also enter the lumen suggesting that the concentration of phosphate at the early distal site is limited by the rate of entry.

Volume and ion regulating renal functions in the big gerbil (Rhombomys opimus L.) and the water vole (Arvicola terrestris L.)

1. After iso-osmotic salt loading (1% NaCl, 1.25% KCl, 0.75% MgCl2 solutions, each load making up 5% body weight) the water voles excreted 66.2% sodium, 84.4% potassium, 18.8% magnesium over a 4 hr period. The big gerbil excreted 20%, 58.9% and 7.1% respectively over the same period. The volume of the water excreted was greater in the case of the water vole. 2. There were no considerable changes in plasma ion concentration in rodents of the species studied after salt loading. 3. The gerbils and water voles had no significant changes in the renal cortex electrolyte concentrations as a result of isotonic salt loads. The highest sodium cortico-papillar gradient was found in the gerbils when experimenting with the isotonic NaCl loading. It was somewhat lower with the KCl load, and significantly lower with water and MgCl2 loads. 4. Under the same experimental conditions, no major changes in the papilla sodium concentration were found in the water voles. 5. The concentrations of potassium, calcium and magnesium were practically alike in all zones of the renal tissue of both rodent species, ion loads producing no effect. 6. The comparison of the renal volume and ion regulating function in rodents with different urine osmotic concentration systems proves the independent existence of renal functions. The greater rate of renal fluid and ion excretion in the water voles is coupled with less specific ion regulation.

Renal function and concentrating ability in a desert rodent: the gundi (Ctenodactylus vali)

Clearance and cortical micropuncture experiments were carried out on non diuretic gundis. In this species, the kidney has a long and well developed papilla but, unlike other desert rodents, the vascular organization of the outer medulla is very simple. After withdrawal of water supply for either 24 h or 3 days before the experiments, the urine osmolality was only 1,361 +/- 57, n = 9, before and 1,136 +/- 89 mosmol . kg-1 during anesthesia. The GFR per 100 g B.W.(0.450 ml . min-1) is lower than in the rat studied under similar conditions. With regard to electrolytes the tubular handling of Na, Ca, K and Mg is similar to that observed for another desert rodent, psammomys obesus. For P, massive reabsorption (more than 30% of the filtered load) takes place along the distal convoluted tubule. The relatively poor concentrating ability of the gundi's kidney is not due to a lack of medullary recycling of urea since a net addition of urea to short loops of Henle is observed in this species. Physiological and morphological observations concerning the gundi and other desert rodent species suggest that the vascular bundle development in the outer medulla might affect the renal response to water deprivation.

Renal magnesium transport and the effects of hypermagnesemia, hypercalcemia, body magnesium stores and parathyroid hormone

Magnesium reabsorption occurs throughout the proximal, loop and distal segments of the nephron. The proximal tubule is less permeable to magnesium than calcium and sodium and most of the filtered load is reclaimed in the thick ascending limb of the loop of Henle. Thus one would expect that factors which regulate magnesium reabsorption should act within this important segment. No single hormone or agent appears to regulate magnesium reabsorption sufficiently to account for urinary changes; rather it appears to be a number of intracellular and extracellular factors acting in concert to effect day to day magnesium homeostasis.

Effect of magnesium deficiency on renal magnesium and calcium transport in the rat

Recollection of micropuncture experiments were performed on acutely thyroparathyroidectomized rats rendered magnesium deficient by dietary deprivation. Urinary magnesium excretion fell from a control of 15 to 3% of the filtered load after magnesium restriction. The loop of Henle, presumably the thick ascending limb, was the major modulator for renal magnesium homeostasis. The transport capacity for magnesium, however, was less in deficient rats than control animals. Absolute magnesium reabsorption increased with acute infusions of magnesium chloride but was always less in magnesium-deficient rats than control rats for any given filtered load, which suggests either a defect of a resetting of the reabsorption mechanism. Recollection micropuncture demonstrated that this was a characteristic of the loop of Henle. Proximal magnesium reabsorption remained unchanged at 15% of the filtered load and was unaffected by magnesium deficiency or acute magnesium repletion. Distal tubular magnesium reabsorption was limited during depletion and increased to a similar extent in control and deficient rats with enhanced magnesium delivery. Calcium reabsorption was not altered in magnesium deficiency; however, elevations of extracellular magnesium resulted in a specific inhibition of calcium reabsorption within the loop of Henle. These data suggest that overall control of renal magnesium reabsorption occurs within the loop of Henle and that the proximal tubule reabsorbs a constant fraction of the filtered load despite variations in body magnesium status.

Magnesium handling in the dog kidney: a micropuncture study

Micropuncture and clearance experiments were done on normal dogs to investigate magnesium handling by proximal and distal nephron segments. Tubular fluid electrolytes were analyzed with the electron microprobe. Tubular fluid to ultrafilterable magnesium ratio (TF/UF magnesium) was observed to rise above unity but less than the TF/P insulin ratio generated along the proximal tubule. This is in contrast to the other major cations, the ratios of which remain close to unity as water is abstracted. Tubular fluid obtained from the distal tubule contained less magnesium than the glomerular filtrate (mean TF/UF magnesium of 0.6) indicating the loop of Henle is the major nephron segment reclaiming a significant portion of the filtered load. The faction of filtered load remaining at the distal sampling site was similar to the fraction appearing in the urine (8% vs 7%) indicating very little reabsorption beyond the distal tubule in these normal states.

Magnesium handling by the papilla of the young rat

In a recent study it was found that in Mg loaded rats, the fraction of filtered Mg (% E Mg) recovered in the bend of the loop of Henle of papilla was greater than the filtered load. However, the site of this Mg addition was unspecified and could be either the juxtamedullary proximal tubule, the pars recta, or in the papilla, the descending limb of the loop of Henle. In order to investigate the movement of Mg in the various structures of the papilla, we have studied: 1. The transport of this electrolyte along the collecting duct. 2. Its relative concentration in the loop of Henle and in the adjacent vasa recta. The experiments have been performed in hydropenic and Mg loaded rats. In the collecting duct, the inulin and Mg concentrations increase proportionally, indicating an absence of any transport of Mg along this part of this nephron. In the vasa recta of the accessible papilla, the capillary over peripheral plasma Mg ratio (C/UF Mg) in hydropenia and after Mg loading [1.88 +/- 0.15 (ES) and 2.89 +/- 0.25] were significantly lower than the corresponding TF/UF Mg in the adjacent loops of Henle (2.90 +/- 0.17 and 4.04 +/- 0.37). This finding reduces the possibilities of a Mg passive diffusion from the capillaries to the tubular lumen, unless the electrical potential of the descending limb is more negative than -5 mV. The hypothesis of an active secretion, or a passive diffusion of Mg in the deep proximal tubule, in the pars recta, or in the early non accessible descending limb constitutes the other alternative.

Renal handling of magnesium

In summary, magnesium reabsorption occurs throughout the proximal and distal segments of the nephron. The proximal tubule is less permeable to magnesium than calcium and sodium with most of the filtered load being reclaimed in the ascending loop of Henle. In contrast to calcium and sodium a tubular reabsorptive maximum has been demonstrated for magnesium and under certain circumstances secretion has been demonstrated in the terminal nephron segments. Although many factors are known to affect magnesium reabsorption the mechanism of the renal homeostasis remains to be determined.

Influence of extracellular fluid volume expansion on magnesium, calcium and phosphate handling along the rat nephron

Renal tubular handling of P, Ca, Mg and Na was studied in the rat both before and during mild hypertonic NaCl loading (ECVE), using micropuncture and clearance techniques and electron microprobe analysis. Micropuncture was performed at the late proximal and early distal tubule sites. ECVE significantly increased the urinary output of all four elements. In the case of Mg, the increase was relatively small and depended on slight but statistically unsignificant inhibition of reabsorption all along the entire length of the nephron. For Ca, it depended on the inhibition of proximal reabsorption, partially compensated by increased reabsorption along the loop. For P, it depended on proximal inhibition, no important net phosphate movement occurring in the loop during both periods. Ca reabsorption was highly correlated to that of sodium along the proximal tubule and Henel's loop, Ca and Mg reabsorption were closely related to the load delivered at the beginning of the structure. These observations are compatible with the view that tubular reabsorption of Ca and Mg is concentration rather than Tm limited, and that reabsorption of Ca, unlike that of Mg, is linked to the movements of sodium. Following ECVE, the difference between early distal and urinary deliveries increased significantly for Ca and P, but not for Mg. For phosphate, this difference accounted for by 45% of the delivery at the early distal tubule site, at variance with microinjection data obtained in the rat under similar salt loading conditions, which indicated that 17% only of the phosphate distal delivery were reabsorbed along the terminal segments. This discrepancy is discussed in terms of nephron functional heterogeneity.