Glucagon and arginine vasopressin stimulate Mg2+ uptake in mouse distal convoluted tubule cells

SGLT2 Inhibitors in Management of Severe Hypomagnesemia in Patients Without Diabetes: A Report of 4 Cases

Sodium/glucose cotransporter 2 (SGLT2) inhibitors have demonstrated a class effect in improving serum magnesium levels in patients with diabetes. Additionally, recent reports have shown their promising beneficial effects in the treatment of refractory hypomagnesemia in patients with diabetes. However, their role in treating hypomagnesemia in patients without diabetes remains unexplored. Here, we report 4 cases of severe and refractory hypomagnesemia that showed dramatic improvement after initiating SGLT2 inhibitors in patients without diabetes. Case 1 had calcineurin inhibitor-associated severe hypomagnesemia. Cases 2, 3, and 4 had refractory hypomagnesemia associated with platinum-based chemotherapy with or without gastrointestinal losses. Case 1 was able to withdraw from high-dose oral magnesium supplementation. Cases 2 and 3 achieved independence from intravenous magnesium supplementation, whereas case 4 had decreased intravenous magnesium requirements. All the cases demonstrated sustainably improved serum magnesium levels. Withdrawal of SGLT2 inhibitors in case 4 resulted in worsening serum magnesium levels and intravenous magnesium requirements. The extraglycemic benefit of this group of medications not only suggests the need for further studies to better understand the effect of SGLT2 inhibitors on magnesium homeostasis but also supports expanded use in a larger patient population.

Hypermagnesemia in critically ill patients with cancer: A case report

Hypermagnesemia is often an under reported finding in critically ill patients with cancer. Hypomagnesemia is a commonly encountered electrolyte abnormality in patients with cancer that is primarily caused by a reduced intake, secondary to chemotherapeutic drugs and malnutrition. Hypermagnesemia is rarely observed in patients with normal renal function, as excess intake can be compensated by renal excretion. However, in critically ill patients with reduced renal function, hypermagnesemia can add further to complications and increase mortality. Drugs such as lactulose, antacids, fentanyl and peptide hormones, including vasopressin, can further increase chances of hypermagnesemia, particularly when patients demonstrate decreased renal function and multiple organ failure. Prudence and caution must therefore be exercised while using these agents in critically ill patients with cancer to avoid increased complications and mortality. Herein, the current study reports three cases of critically ill patients with cancer admitted into intensive care who had refractory hypermagnesemia.

Modeling Distal Convoluted Tubule (Patho)Physiology: An Overview of Past Developments and an Outlook Toward the Future

The kidneys are essential for maintaining electrolyte homeostasis. Blood electrolyte composition is controlled by active reabsorption and secretion processes in dedicated segments of the kidney tubule. Specifically, the distal convoluted tubule (DCT) and connecting tubule are important for regulating the final excretion of sodium, magnesium, and calcium. Studies unravelling the specific function of these segments have greatly improved our understanding of DCT (patho)physiology. Over the years, experimental models used to study the DCT have changed and the field has advanced from early dissection studies with rats and rabbits to the use of various transgenic mouse models. Developments in dissection techniques and cell culture methods have resulted in immortalized mouse DCT cell lines and made it possible to specifically obtain DCT fragments for ex vivo studies. However, we still do not fully understand the complex (patho)physiology of this segment and there is need for advanced human DCT models. Recently, kidney organoids and tubuloids have emerged as new complex cell models that provide excellent opportunities for physiological studies, disease modeling, drug discovery, and even personalized medicine in the future. This review presents an overview of cell models used to study the DCT and provides an outlook on kidney organoids and tubuloids as model for DCT (patho)physiology. Impact statement This study provides a detailed overview of past and future developments on cell models used to study kidney (patho)physiology and specifically the distal convoluted tubule (DCT) segment. Hereby, we highlight the need for an advanced human cell model of this segment and summarize recent advances in the field of kidney organoids and tubuloids with a focus on DCT properties. The findings reported in this review are significant for future developments toward an advanced human model of the DCT that will help to increase our understanding of DCT (patho)physiology.

Lack of an effect of nephron-specific deletion of adenylyl cyclase 3 on renal sodium and water excretion or arterial pressure

Adenylyl cyclase (AC)-stimulated cAMP plays a key role in modulating transport and channel activity along the nephron. However, the role of individual adenylyl cyclase isoforms in such regulation is largely unknown. Since adenylyl cyclase 3 (AC3) is expressed throughout nephron, we investigated its role in the physiologic regulation of renal Na⁺ and water transport. Mice were generated with inducible nephron knockout of AC3 (AC3 KO) by breeding mice with loxP-flanked critical exons in the Adcy3 gene with mice expressing Pax8-rtTA and LC-1 transgenes. After doxycycline treatment at 1 month of age, nephron AC3 KO mice had 100% Adcy3 gene recombination in all renal tubule segments, but not in glomeruli. Sodium intake, urinary Na⁺ excretion, glomerular filtration rate, and blood pressure were similar between nephron KO mice and the controls during normal, high, and low Na⁺ diets. Plasma renin concentration was not different between the two groups during varied Na⁺ intake. Moreover, there were no differences in urine volume and urine osmolality between the two genotypes during normal or restricted water intake. In conclusion, these data suggested that AC3 is not involved in the physiological regulation of nephron Na⁺ and water handling.

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.

In vitro characterization of aldosterone and cAMP effects in mouse distal convoluted tubule cells

The distal nephron plays a capital role in the fine regulation of sodium reabsorption. Compared with the cortical collecting duct, much less information is available on the hormonal regulation of sodium transporter genes in the distal convoluted tubule (DCT), where the thiazide-sensitive Na+-Cl-cotransporter (NCC) is the major entry pathway for Na+. The purpose of this study was to characterize the in vitro effects of aldosterone (Aldo; 1 μM) and cAMP (8-BrcAMP; 0.5 mM) on mouse DCT (mDCT) by using an immortalized mDCT cell line. Western blot analysis and semiquantitative RT-PCR were performed to analyze the expression of genes involved in sodium transport. The mDCTcell line expressed the 11β-hydroxysteroid dehydrogenase type 2 gene and both the mineralocorticoid and glucocorticoid receptor genes, suggesting Aldo responsiveness. In this sense, we found that mDCT cells expressed the amiloride-sensitive Na+channel (ENaC) and responded to Aldo by upregulating the α-subunit protein. Similarly, α1Na+-K+-ATPase protein was upregulated by Aldo and 8-BrcAMP. In addition, the Aldo intermediate gene sgk1 mRNA was increased in response to both Aldo and 8-BrcAMP, and the transcription factor HNF–3α mRNA was induced by 8-BrcAMP. With respect to NCC regulation, although Aldo induced NCC protein levels in mice in vivo, neither Aldo nor 8-BrcAMP significantly induced the NCC mRNA or protein levels in mDCT cells. These results suggest that in mDCT, Aldo and cAMP modulate some downstream mediators and effectors in vitro but do not influence the expression of NCC in this cell model.

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.

Adenosine modulates Mg(2+) uptake in distal convoluted tubule cells via A(1) and A(2) purinoceptors

tk;1Adenosine plays a role in the control of water and electrolyte reabsorption in the distal tubule. As the distal convoluted tubule is important in the regulation of renal Mg(2+) balance, we determined the effects of adenosine on cellular Mg(2+) uptake in this segment. The effect of adenosine was studied on immortalized mouse distal convoluted tubule (MDCT) cells, a model of the intact distal convoluted tubule. The rate of Mg(2+) uptake was measured with fluorescence techniques using mag-fura 2. To assess Mg(2+) uptake, MDCT cells were first Mg(2+) depleted to 0.22 +/- 0.01 mM by being cultured in Mg(2+)-free media for 16 h and then placed in 1.5 mM MgCl(2); next, changes in intracellular Mg(2+) concentration ([Mg(2+)](i)) were determined. [Mg(2+)](i) returned to basal levels, 0.53 +/- 0.02 mM, with a mean refill rate, d([Mg(2+)](i))/dt, of 137 +/- 16 nM/s. Adenosine stimulates basal Mg(2+) uptake by 41 +/- 10%. The selective A(1) purinoceptor agonist N(6)-cyclopentyladenosine (CPA) increased intracellular Ca(2+) and decreased parathyroid hormone (PTH)-stimulated cAMP formation and PTH-mediated Mg(2+) uptake. On the other hand, the selective A(2) receptor agonist 2-[p-(2-carbonyl-ethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS) stimulated Mg(2+) entry in a concentration-dependent fashion. CGS increased cAMP formation and the protein kinase A inhibitor RpcAMPS inhibited CGS-stimulated Mg(2+) uptake. Selective inhibition of phospholipase C, protein kinase C, or mitogen-activated protein kinase enzyme cascades with U-73122, Ro-31-8220, and PD-98059, respectively, diminished A(2) agonist-mediated Mg(2+) entry. Aldosterone potentiated CGS-mediated Mg(2+) entry, and elevation of extracellular Ca(2+) diminished CGS-responsive cAMP formation and Mg(2+) uptake. Accordingly, MDCT cells possess both A(1) and A(2) purinoceptor subtypes with intracellular signaling typical of these respective receptors. We conclude that adenosine has dual effects on Mg(2+) uptake in MDCT cells through separate A(1) and A(2) purinoceptor pathways.

ATP inhibits Mg(2+) uptake in MDCT cells via P2X purinoceptors

Nucleotides have diverse effects on water and electrolyte reabsorption within the distal tubule of the nephron. As the distal tubule is important in control of renal Mg(2+) balance, we determined the effects of ATP on cellular Mg(2+) uptake in this segment. The effects of ATP on immortalized mouse distal convoluted tubule (MDCT) cells were studied by measuring Mg(2+) uptake with fluorescence techniques. The mean basal Mg(2+) uptake rate was 165 +/- 6 nM/s. ATP inhibited basal Mg(2+) uptake and hormone-stimulated Mg(2+) entry by 40%. Both P2X (P2X1-P2X5 subtypes) and P2Y2 receptor subtypes were identified in MDCT cells using differential RT-PCR. Activation of both receptor subtypes with selective agonists increased intracellular Ca(2+) concentration, P2X purinoceptors by ionotropic-gated channels, and P2Y receptors via G protein-mediated intracellular Ca(2+) release. The more relatively selective P2X agonists [beta,gamma-methylene ATP (beta,gamma-Me-ATP) and 2'- and -3'-O-(4-benzoyl-benzoyl)-ATP] inhibited arginine vasopressin (AVP)- and parathyroid hormone (PTH)-mediated Mg(2+) uptake whereas agonists more selective for P2Y purinoceptors (UTP, ADP, and 2-methylthio-ATP) were without effect. Removal of extracellular Ca(2+) diminished beta,gamma-Me-ATP-mediated increase in intracellular Ca(2+) and inhibition of AVP-stimulated Mg(2+) entry. We conclude from this information that ATP inhibited Mg(2+) uptake in MDCT cells through P2X purinoceptors expressed in this distal convoluted tubule cell line.

1,25(OH)(2)D(3) stimulates Mg2+ uptake into MDCT cells: modulation by extracellular Ca2+ and Mg2+

The distal convoluted tubule plays a significant role in renal magnesium conservation. Although the cells of the distal convoluted tubule possess the vitamin D receptor, little is known about the effects of 1alpha,25-dihydroxyvitamin D [1,25(OH)(2)D(3)] on magnesium transport. In this study, we examined the effect of 1,25(OH)(2)D(3) on distal cellular magnesium uptake and the modulation of this response by extracellular Ca2+ and Mg2+ in an immortalized mouse distal convoluted tubule (MDCT) cell line. MDCT cells possess the divalent cation-sensing receptor (CaSR) that responds to elevation of extracellular Ca2+ and Mg2+ concentrations to diminish peptide hormone-stimulated Mg2+ uptake. Mg2+ uptake rates were determined by microfluorescence in Mg2+ -depleted MDCT cells. Treatment of MDCT cells with 1,25(OH)(2)D(3) for 16-24 h stimulated basal Mg2+ uptake in a concentration-dependent manner from basal levels of 164 +/- 5 to 210 +/- 11 nM/s, representing a 28 +/- 3% change. Pretreatment with actinomycin D or cycloheximide abolished 1,25(OH)(2)D(3)-stimulated(.)Mg2+ uptake (154 +/- 18 nM/s), suggesting that 1,25(OH)(2)D(3) stimulates Mg2+ uptake through gene activation and protein synthesis. Elevation of extracellular Ca2+ inhibited 1,25(OH)(2)D(3)-stimulated Mg2+ uptake (143 +/- 5 nM/s). Preincubation of the cells with an antibody to the CaSR prevented the inhibition by elevated extracellular Ca2+ of 1,25(OH)(2)D(3)-stimulated Mg2+ uptake (202 +/- 8 nM/s). Treatment with an antisense CaSR mRNA oligodeoxynucleotide also abolished the effects of extracellular Ca2+ on 1,25(OH)(2)D(3)-responsive Mg2+ entry. This showed that elevated extracellular calcium modulates 1,25(OH)(2)D-mediated responses through the CaSR. In summary, 1,25(OH)(2)D(3) stimulated Mg2+ uptake in MDCT cells, and this is dependent on de novo protein synthesis. Elevation of extracellular Ca2+, acting via the CaSR, inhibited 1,25(OH)(2)D(3)-stimulated Mg2+ entry. These data indicate that 1,25(OH)(2)D(3) has important effects on the control of magnesium entry in MDCT cells and these responses can be modulated by extracellular divalent cations.

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.

beta-Adrenergic agonists stimulate Mg(2+) uptake in mouse distal convoluted tubule cells

beta-Adrenergic agonists influence electrolyte reabsorption in the proximal tubule, loop of Henle, and distal tubule. Although isoproterenol enhances magnesium absorption in the thick ascending limb, it is unclear what effect, if any, beta-adrenergic agonists have on tubular magnesium handling. The effects of isoproterenol were studied in immortalized mouse distal convoluted tubule (MDCT) cells by measuring cellular cAMP formation with radioimmunoassays and Mg(2+) uptake with fluorescence techniques. Intracellular free Mg(2+) concentration ([Mg(2+)](i)) was measured in single MDCT cells by using microfluorescence with mag-fura-2. To assess Mg(2+) uptake, MDCT cells were first Mg(2+) depleted to 0.22 +/- 0.01 mM by culturing in Mg(2+)-free media for 16 h and then placed in 1.5 mM MgCl(2), and the changes in [Mg(2+)](i) were determined. [Mg(2+)](i) returned to basal levels, 0.53 +/- 0.02 mM, with a mean refill rate, d([Mg(2+)](i))/dt, of 168 +/- 11 nM/s. Isoproterenol stimulated Mg(2+) entry in a concentration-dependent manner, with a maximal response of 252 +/- 11 nM/s, at a concentration of 10(-7) M, that represented a 50 +/- 7% increase in uptake rate above control values. This was associated with a sixfold increase in intracellular cAMP generation. Isoproterenol-stimulated Mg(2+) uptake was completely inhibited with RpcAMPS, a protein kinase A inhibitor, and U-73122, a phospholipase C inhibitor, and partially blocked by RO 31-822, a protein kinase C inhibitor. Accordingly, isoproterenol-mediated Mg(2+) entry rates involve multiple intracellular signaling pathways. Aldosterone potentiated isoproterenol-stimulated Mg(2+) uptake (326 +/- 31 nM/s), whereas elevation of extracellular Ca(2+) inhibited isoproterenol-mediated cAMP accumulation and Mg(2+) uptake, 117 +/- 37 nM/s. These studies demonstrate that isoproterenol stimulates Mg(2+) uptake in a cell line of mouse distal convoluted tubules that is modulated by hormonal and extracellular influences.