Potassium depletion and salt-sensitive hypertension in Dahl rats: effect on calcium, magnesium, and phosphate excretions

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.

Nutraceutical support in heart failure: a position paper of the International Lipid Expert Panel (ILEP)

Heart failure (HF) is a complex clinical syndrome that represents a major cause of morbidity and mortality in Western countries. Several nutraceuticals have shown interesting clinical results in HF prevention as well as in the treatment of the early stages of the disease, alone or in combination with pharmacological therapy. The aim of the present expert opinion position paper is to summarise the available clinical evidence on the role of phytochemicals in HF prevention and/or treatment that might be considered in those patients not treated optimally as well as in those with low therapy adherence. The level of evidence and the strength of recommendation of particular HF treatment options were weighed up and graded according to predefined scales. A systematic search strategy was developed to identify trials in PubMed (January 1970 to June 2019). The terms 'nutraceuticals', 'dietary supplements', 'herbal drug' and 'heart failure' or 'left verntricular dysfunction' were used in the literature search. The experts discussed and agreed on the recommendation levels. Available clinical trials reported that the intake of some nutraceuticals (hawthorn, coenzyme Q10, l-carnitine, d-ribose, carnosine, vitamin D, probiotics, n-3 PUFA and beet nitrates) might be associated with improvements in self-perceived quality of life and/or functional parameters such as left ventricular ejection fraction, stroke volume and cardiac output in HF patients, with minimal or no side effects. Those benefits tended to be greater in earlier HF stages. Available clinical evidence supports the usefulness of supplementation with some nutraceuticals to improve HF management in addition to evidence-based pharmacological therapy.

The RING finger- and PDZ domain-containing protein PDZRN3 controls localization of the Mg2+ regulator claudin-16 in renal tube epithelial cells

Ion exchange in the renal tubules is fundamental to the maintenance of physiological ion levels. Claudin-16 (CLDN16) regulates the paracellular reabsorption of Mg²⁺ in the thick ascending limb of Henle's loop in the kidney, with dephosphorylation of CLDN16 increasing its intracellular distribution and decreasing paracellular Mg²⁺ permeability. CLDN16 is located in the tight junctions, but the mechanism regulating its localization is unclear. Using yeast two-hybrid systems, we found that CLDN16 binds to PDZRN3, a protein containing both RING-finger and PDZ domains. We also observed that the carboxyl terminus of the cytoplasmic CLDN16 region was required for PDZRN3 binding. PZDRN3 was mainly distributed in the cytosol of rat kidney cells and upon cell treatment with the protein kinase A inhibitor H-89, colocalized with CLDN16. H-89 also increased mono-ubiquitination and the association of CLDN16 with PDZRN3. Mono-ubiquitination levels of a K275A mutant were lower, and its association with PDZRN3 was reduced compared with wild-type (WT) CLDN16 and a K261A mutant, indicating that Lys-275 is the major ubiquitination site. An S217A mutant, a dephosphorylated form of CLDN16, localized to the cytosol along with PDZRN3 and the endosomal marker Rab7. PDZRN3 siRNA increased cell-surface localization of WT CLDN16 in H-89-treated cells or containing the S217A mutant and also suppressed CLDN16 endocytosis. Of note, H-89 decreased paracellular Mg²⁺ flux in WT CLDN16 cells, and PDZRN3 siRNA increased Mg²⁺ flux in the H-89-treated WT CLDN16 and S217A mutant cells. These results suggest that PDZRN3 mediates endocytosis of dephosphorylated CLDN16 and represents an important component of the CLDN16-trafficking machinery in the kidney.

Acid-base and electrolyte abnormalities in heart failure: pathophysiology and implications

Electrolyte and acid–base abnormalities are a frequent and potentially dangerous complication in subjects with congestive heart failure. This may be due either to the pathophysiological alterations present in the heart failure state leading to neurohumoral activation (stimulation of the renin–angiotensin–aldosterone system, sympathoadrenergic stimulation), or to the adverse events of therapy with diuretics, cardiac glycosides, and ACE inhibitors. Subjects with heart failure may show hyponatremia, magnesium, and potassium deficiencies; the latter two play a pivotal role in the development of cardiac arrhythmias. The early identification of these alterations and the knowledge of the pathophysiological mechanisms are very useful for the management of these patients.

Electrolyte disorders associated with the use of anticancer drugs

The use of anticancer drugs is beneficial for patients with malignancies but is frequently associated with the occurrence of electrolyte disorders, which can be hazardous and in many cases fatal. The review presents the electrolyte abnormalities that can occur with the use of anticancer drugs and provides the related mechanisms. Platinum-containing anticancer drugs induce hypomagnesemia, hypokalemia and hypocalcemia. Moreover, platinum-containing drugs are associated with hyponatremia, especially when combined with large volumes of hypotonic fluids aiming to prevent nephrotoxicity. Alkylating agents have been linked with the occurrence of hyponatremia [due to syndrome of inappropriate antidiuretic hormone secretion (SIADH)] and Fanconi's syndrome (hypophosphatemia, aminoaciduria, hypouricemia and/or glucosuria). Vinca alkaloids are associated with hyponatremia due to SIADH. Epidermal growth factor receptor monoclonal antibody inhibitors induce hypomagnesemia, hypokalemia and hypocalcemia. Other, monoclonal antibodies, such as cixutumumab, cause hyponatremia due to SIADH. Tyrosine kinase inhibitors are linked to hyponatremia and hypophosphatemia. Mammalian target of rapamycin inhibitors induce hyponatremia (due to aldosterone resistance), hypokalemia and hypophosphatemia. Other drugs such as immunomodulators or methotrexate have been also associated with hyponatremia. The administration of estrogens at high doses, streptozocin, azacitidine and suramin may induce hypophosphatemia. Finally, the drug-related tumor lysis syndrome is associated with hyperphosphatemia, hyperkalemia and hypocalcemia. The prevention of electrolyte derangements may lead to reduction of adverse events during the administration of anticancer drugs.

Medication-induced hypophosphatemia: a review

Hypophosphatemia (serum phosphorus concentration <2.5 mg/dl, 0.8 mmol/l), although rare in the general population, is commonly observed in hospitalized patients and may be associated with drug therapy. In fact, hypophosphatemia frequently develops in the course of treatment with drugs used in every-day clinical practice including diuretics and bisphosphonates. Proper diagnostic approach of patients with low serum phosphorus concentrations should involve a detailed medical history with special attention to the recent use of medications. The clinical manifestations of drug-induced hypophosphatemia are usually mild but might also be severe and potentially life-threatening. This review aims at a thorough understanding of the underlying pathophysiological mechanisms and risk factors of drug therapy-related hypophosphatemia thus allowing prevention and effective intervention strategies.

Prevalence of hypomagnesemia (HM) in a geriatric long-term care (LTC) setting

Electrolyte abnormalities are frequently observed in elderly long-term care (LTC) patients. Magnesium is a trace mineral, but is the second most abundant intracellular cation and the fourth most abundant cation in the body. This was a cross-sectional study to assess the prevalence of hypomagnesemia (HM) in non-selected elderly LTC patients. A total of 159 patients aged 65 years and older were included in the study. The attributes and variables related to the patients' hospital course were used to compare the two groups. We used univariate and multivariate analyses to correlate magnesium levels with demographic, clinical factors and laboratory data. HM was found in 36% of the patients, of whom 35% presented with moderate HM (0.8-0.9 microequiv./l) and 18% with severe HM (<or=0.7 microequiv./l). Patients with HM had a higher number of comorbid diseases per patient (p=0.038), low body mass index (BMI) (p=0.044) and more of them presented with laboratory markers of malnutrition, such as low total cholesterol (TC) and serum albumin (SA) levels. Coexistence with other electrolyte abnormalities was higher among patients with HM than without (p=0.006), predominantly hypocalcemia and hypokalemia (p=0.023 and 0.032, respectively). Using regression analysis, independent variables significantly associated with serum magnesium levels were serum albumin, calcium, potassium, urea levels, chronic renal failure (CRF), chronic heart failure (CHF), diabetes mellitus (DM) and diuretic drugs (R(2)=0.877). Both early (up to 30 days) and late rate of death were higher in patients with HM. The incidence of HM in LTC elderly patients is high and multifactorial. Understanding the causes of HM, correction of magnesium level, and definitive and effective treatment of the cause leading to HM is important to improve patient prognosis.

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.

Electrolyte disturbances in patients with hyponatremia

OBJECT

Electrolyte abnormalities are frequently observed in patients with hyponatremia. The aim of this study was to determine the incidence of various electrolyte abnormalities encountered in hyponatremic patients admitted to an internal medicine clinic, as well as to investigate the possible pathogenetic mechanisms responsible for these abnormalities.

PATIENTS AND METHODS

We prospectively studied 204 adult patients who either on admission to our clinic or during their hospitalization were found to have hyponatremia.

RESULTS

Ninety-two patients (45.5%) had at least one additional electrolyte abnormality. Hypophosphatemia was the most frequent electrolyte disorder observed (35 patients, 17%). Hypokalemia was seen in 32 patients (15.8%), hypomagnesemia in 31 patients (15.2%) and hyperkalemia in 12 patients (5.9%). Moreover, 5 patients (2.5%) had hyperphosphatemia, 4 patients (1.9%) exhibited hypermagnesemia, 3 patients (1.4%) had hypercalcemia, and 6 patients (2.9%) had true hypocalcemia. There were no statistically significant differences regarding the incidence of these electrolyte abnormalities (as a whole) between the main subgroups of hyponatremic patients (diuretic-induced, syndrome of inappropriate antidiuretic hormone, hypovolemia-induced and edematous state-related). However, hypokalemia and hypomagnesemia were more frequently observed in patients with diuretic-induced hyponatremia, while hyperkalemia was more frequently seen in edematous state-related hyponatremia.

CONCLUSIONS

Additional electrolyte abnormalities are frequently encountered in patients with hyponatremia of any origin admitted to an internal medicine clinic.

Dysfunction of paracellin-1 by dephosphorylation in Dahl salt-sensitive hypertensive rats

A high-salt diet reduced the levels of renal cAMP content and serine-phosphorylated paracellin-1 in Dahl salt-sensitive hypertensive rats. In MDCK cells expressing paracellin-1, protein kinase A inhibitor reduced the serine-phosphorylated paracellin-1 and transepithelial Mg(2+) transport, suggesting that a dephosphorylation of paracellin-1 induces the reduction of Mg(2+) reabsorption in salt-sensitive hypertension.

Hypomagnesemia and concurrent acid-base and electrolyte abnormalities in patients with congestive heart failure

BACKGROUND

Patients with severe decompensated congestive heart failure (CHF) commonly exhibit acid-base and electrolyte disturbances mainly due to the activation of several neurohumoral mechanisms as well as to drugs regularly used in this population. Magnesium deficit is not infrequently observed in CHF patients but its pathophysiology remains less well-studied as compared with other electrolyte alterations, such as hypokalemia. However, there is evidence that early detection and correction of magnesium abnormalities could obviate potentially deleterious arrhythmogenic effects.

AIM

To assess the incidence of magnesium level disorders and analyze the underlying pathophysiological mechanisms in patients with CHF.

METHODS

Eighty-six consecutive CHF patients (NYHA class III or IV) admitted to our hospital over a period of 5 years were studied. Patients with diabetes mellitus, liver or renal failure, and chronic obstructive lung disease were excluded. All patients received conventional treatment with digoxin, diuretic agents and an angiotensin converting enzyme inhibitor. On admission, blood and urine electrolytes and renal function parameters were determined. Arterial blood gases and serum anion gap determinations were also performed.

RESULTS

Hypomagnesemia was found in 15 [n=15 (17.4%)] CHF patients. The majority of these patients also exhibited other electrolyte abnormalities, such as hypokalemia, hypocalcemia and hypophosphatemia. Inappropriate magnesiuria (fractional excretion of magnesium >4%) was evident in eight hypomagnesemic patients. A variety of associated conditions, including poor dietary intake, also favored magnesium depletion.

CONCLUSION

Magnesium deficit is a common electrolyte disorder in CHF (NYHA class III/IV) patients and several interrelated mechanisms are implicated in its pathogenesis. Clinicians' awareness of the incidence of hypomagnesemia in this population as well as its related pathophysiology could be useful for the early detection and appropriate treatment to inhibit its arrhythmogenic potential.

Glycosphingolipids modulate renal phosphate transport in potassium deficiency

BACKGROUND

Potassium (K) deficiency (KD) and/or hypokalemia have been associated with disturbances of phosphate metabolism. The purpose of the present study was to determine the cellular mechanisms that mediate the impairment of renal proximal tubular Na/Pi cotransport in a model of K deficiency in the rat.

METHODS

K deficiency in the rat was achieved by feeding rats a K-deficient diet for seven days, which resulted in a marked decrease in serum and tissue K content.

RESULTS

K deficiency resulted in a marked increase in urinary Pi excretion and a decrease in the V(max) of brush-border membrane (BBM) Na/Pi cotransport activity (1943 +/- 95 in control vs. 1184 +/- 99 pmol/5 sec/mg BBM protein in K deficiency, P < 0.02). Surprisingly, the decrease in Na/Pi cotransport activity was associated with increases in the abundance of type I (NaPi-1), and type II (NaPi-2) and type III (Glvr-1) Na/Pi protein. The decrease in Na/Pi transport was associated with significant alterations in BBM lipid composition, including increases in sphingomyelin, glucosylceramide, and ganglioside GM3 content and a decrease in BBM lipid fluidity. Inhibition of glucosylceramide synthesis resulted in increases in BBM Na/Pi cotransport activity in control and K-deficient rats. The resultant Na/Pi cotransport activity in K-deficient rats was the same as in control rats (1148 +/- 52 in control + PDMP vs. 1152 +/- 61 pmol/5 sec/mg BBM protein in K deficiency + PDMP). These changes in transport activity occurred independent of further changes in BBM NaPi-2 protein or renal cortical NaPi-2 mRNA abundance.

CONCLUSION

K deficiency in the rat causes inhibition of renal Na/Pi cotransport activity by post-translational mechanisms that are mediated in part through alterations in glucosylceramide content and membrane lipid dynamics.

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.

Renal vascular morphology and haemodynamics in Dahl salt-sensitive rats on high salt-low potassium diet: neural and genetic influences

OBJECTIVE

A dietary combination of high salt and low potassium (HS-LK) exacerbates hypertension in Dahl salt-sensitive (DS) rats and renders Dahl salt-resistant (DR) rats hypertensive. In both strains, the hypertension is accompanied by remodelling of the renal resistance vasculature, and is attenuated by peripheral chemical sympathectomy. In the current study, we sought to determine whether the sympathetic nervous system is causally involved in mediating the renal vascular and haemodynamic alterations associated with HS-LK feeding in Dahl rats.

DESIGN

Two groups each of DS and DR rats were maintained on HS-LK diet (8% NaCl, 0.2% KCl) for 8 weeks. One group of DS (n = 9) and DR (n = 8) were treated with 6-hydroxydopamine (6-OHDA) in 0.001 N HCl vehicle to chemically ablate peripheral sympathetic nerve terminals. The two remaining groups (n = 8 each) received equivalent injections of vehicle.

METHODS

At the end of the dietary regimen, arterial blood pressure (ABP), glomerular filtration rate (GFR) and renal blood flow (RBF) were measured, and the structure of intra-renal resistance vessels was examined by planar morphometric analysis of coronal sections prepared from perfusion-fixed kidneys.

RESULTS

Both 6-OHDA-treated and untreated DS rats presented a greater degree of intra-renal vessel remodelling characterized by reduced lumen diameter in the absence (eutrophic) or presence (hypertrophic) of cross-sectional area expansion, higher renal vascular resistance (RVR) and lower GFR and RBF than DR rats. Chemical sympathectomy increased lumen diameters and reduced vascular wall expansion, resulting in a decrease in RVR and a concomitant increase in RBF and GFR in both strains; however, the effect was more prominent in the DS rats.

CONCLUSIONS

We conclude that HS-LK-induced changes in intra-renal vessel structure and renal haemodynamic function in Dahl rats are, at least in part, dependent on the activity of the sympathetic nervous system.

Regulation of sodium, calcium and vitamin D metabolism in Dahl rats on a high-salt/low-potassium diet: genetic and neural influences

1. A dietary combination of high salt and low potassium (HSLK) exacerbates hypertension in Dahl salt-sensitive (DS) rats and renders previously normotensive Dahl salt-resistant (DR) rats hypertensive. In both strains, the severity of hypertension correlates with urinary calcium loss. However, the magnitude of excretory calcium losses is significantly greater in DS rats and is potentiated by chemical sympathectomy in both strains. 2. We hypothesized that a defect in vitamin D metabolism may underlie the observed strain-dependent differences in calcium balance. 3. Arterial blood pressure (ABP), water and mineral balance and serum concentrations of 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3) and 25-hydroxyvitamin D3 (25(OH)D3) were measured in intact and chemically sympathectomized (6-hydroxydopamine; 6-OHDA) DS and DR rats after 8 weeks on a HSLK diet. 4. Chronic ingestion of this diet resulted in marked and moderate levels of hypertension in DS and DR rats, respectively. The hypertension was abated and eliminated by 6-OHDA in the DS and DR strains, respectively. Independent of treatment, DS rats had significantly higher urinary excretion of calcium and reduced intestinal absorption of the ion compared with DR rats. The DS rats had significantly higher serum levels of 1,25(OH)2 D3 and markedly lower serum levels of 25(OH)D3 than DR rats. Chemical sympathectomy tended to increase 1,25(OH)2 D3 and to decrease 25(OH)D3 levels in both strains. 5. These data indicate a genetic difference in vitamin D metabolism between DS and DR rats. The abnormally elevated levels of 1,25(OH)2 D3 in DS rats may be an appropriate compensatory response to excessive excretory calcium loss and reduced target organ sensitivity to the hormone and may, maladaptively, directly contribute to hypertension, by stimulating vascular smooth muscle contractility.

Hypertension development in Dahl S and R rats on high salt-low potassium diet: calcium, magnesium and sympathetic nervous system

Dietary combination of high salt with low potassium (HSLK) exacerbates hypertension development in Dahl salt-sensitive (S) rats, and produces a mild degree of hypertension in otherwise salt-resistant (R) rats. Increased blood pressure in both strains is associated with increased urinary excretion of calcium and magnesium. The objective of this study was to determine the effect of blood pressure on body balance of these ions in Dahl rats on HSLK diet. Two groups of S and two groups of R weanlings were all placed on HSLK diet (NaCl=8%, K=0.2%) for eight weeks. One group of each strain was subjected to chemical sympathectomy with 6-hydroxydopamine (6-OHDA) to counteract hypertension development. Urinary norepinephrine was used to determine efficacy of 6-OHDA treatment. Systolic blood pressures of conscious animals were measured daily throughout the study. The last three days on the diet were used to determine total dietary intake and urinary as well as fecal excretion of sodium, calcium and magnesium. At the end of the study, extracellular fluid volume, serum aldosterone and parathyroid hormone were analyzed. Final systolic blood pressures in the 4 groups were as follows: S=235+/-9 mmHg (n=9); R=155+/-3 mmHg (n=8); 6-OHDA S=151+/-6 mm Hg (n=8); 6-OHDA R=117+/-6 mm Hg. Chemical sympathectomy decreased blood pressure in both S and R rats. There was no indication of sodium accumulation in S rats. Associated with reduced parathyroid hormone levels the S strain had significantly less positive balance for calcium than the R strain, primarily due to increased urinary excretion. A less positive balance for magnesium was also observed, due mainly to relatively reduced intestinal absorption of the ion. We conclude that the HSLK diet is associated with inappropriate activation of the sympathetic nervous system and increased arterial pressure in both strains. In addition, since divalent cations may influence blood pressure, we suggest that the observed abnormalities in calcium and magnesium metabolism might independently promote hypertension development in the S strain.

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

Cellular mechanisms of chlorothiazide and cellular potassium depletion on Mg2+ uptake in mouse distal convoluted tubule cells

The use of the distally-acting diuretic, chlorothiazide, has been reported to have important effects on renal magnesium handling. The cellular mechanisms of chlorothiazide action on Mg2+ uptake was investigated in immortalized mouse distal convoluted tubule (MDCT) cells. Intracellular free Mg2+ concentration was determined by microfluorescence. Mg2+ transport was measured as a function of change in intracellular Mg2+ concentration with time following placement of Mg2+-depleted cells into a buffer containing 1.5 mM magnesium. The uptake rate of Mg2+ into Mg2+-depleted cells was 179 +/- 28 nM/second. Mg2+ uptake was dependent on the membrane voltage as membrane hyperpolarization enhanced uptake whereas depolarization diminished transport. Chlorothiazide increased Mg2+ uptake by 58%, from 179 +/- 28 to 283 +/- 23 nM/second. The ability of chlorothiazide to stimulate Mg2+ uptake in MDCT cells was concentration-dependent and related to the diuretic-induced hyperpolarization of the plasma membrane. These studies support the notion that acute chlorothiazide administration enhances renal magnesium conservation through its effects on Mg2+ transport within the distal convoluted tubule. Since chronic chlorothiazide administration may result in hypokalemia as well as hypomagnesemia, Mg2+ uptake was determined in potassium-depleted MDCT cells. Mg2+ uptake was diminished, 80 +/- 24 nM/second, in potassium depleted cells. Hyperpolarization of the plasma membrane with the cell permanent anion, SCN-, corrected Mg2+ uptake in potassium depleted cells suggesting that the basis for diminished uptake may, in part, be due to depolarization of the membrane voltage. In summary, acute chlorothiazide stimulates Mg2+ transport in MDCT cells. We postulate that chronic chlorothiazide use may lead to hypokalemia that in turn diminishes Mg2+ transport in the distal tubule resulting in urinary magnesium-wasting.