Magnesium transport across cell membranes

Mg2+ limitation leads to a decrease in chlorophyll, resulting in an unbalanced photosynthetic apparatus in the cyanobacterium Synechocytis sp. PCC6803

Mg2+, the most abundant divalent cation in living cells, plays a pivotal role in numerous enzymatic reactions and is of particular importance for organisms performing oxygenic photosynthesis. Its significance extends beyond serving as the central ion of the chlorophyll molecule, as it also acts as a counterion during the light reaction to balance the proton gradient across the thylakoid membranes. In this study, we investigated the effects of Mg2+ limitation on the physiology of the well-known model microorganism Synechocystis sp. PCC6803. Our findings reveal that Mg2+ deficiency triggers both morphological and functional changes. As seen in other oxygenic photosynthetic organisms, Mg2+ deficiency led to a decrease in cellular chlorophyll concentration. Moreover, the PSI-to-PSII ratio decreased, impacting the photosynthetic efficiency of the cell. In line with this, Mg2+ deficiency led to a change in the proton gradient built up across the thylakoid membrane upon illumination.

Magnesium Supplementation Stimulates Autophagy to Reduce Lipid Accumulation in Hepatocytes via the AMPK/mTOR Pathway

Metabolic-associated fatty liver disease (MAFLD) (previously known as nonalcoholic fatty liver disease (NAFLD)) is a disease with high worldwide prevalence, but with limited available therapeutic interventions. Autophagy is a cell survival mechanism for clearing excess lipids in hepatocytes and affects the occurrence and development of MAFLD. In addition, some studies have shown that magnesium deficiency is common in patients with obesity and metabolic syndrome. Magnesium supplementation can effectively improve metabolism-related diseases such as obesity and fatty liver. Our study successfully constructed a cellular model of MAFLD by 1 mM free fatty acid (FFA) intervention in LO2 cells for 24 h, and there was an increase in lipid accumulation in hepatocytes after FFA intervention. Magnesium supplementation was shown to reduce lipid deposition in hepatocytes induced by FFA, and Western blotting (WB) analysis showed that magnesium supplementation could downregulate the expression of Fasn and SREBP1 and increase the expression of LPL, suggesting that magnesium can reduce lipid accumulation by reducing lipid synthesis and increasing lipid oxidation. Magnesium supplementation could affect cellular lipid metabolism by activating the AMPK/mTOR pathway to stimulate autophagy. Our results identified a relationship between magnesium and lipid accumulation in hepatocytes and showed that magnesium supplementation reduced lipid deposition in hepatocytes by activating autophagy by activating the AMPK-mTOR pathway.

Association between hypomagnesemia and severity of primary hyperparathyroidism: a retrospective study

BACKGROUND

The occurrence of hypomagnesemia in patients with primary hyperparathyroidism (PHPT) has been noted previously; however, the association of hypomagnesemia and severity of primary hyperparathyroidism remains unknown. The present study aimed to evaluate the association of hypomagnesemia with biochemical and clinical manifestations in patients with PHPT.

METHODS

This was a retrospective study conducted at a tertiary hospital. We obtained data from 307 patients with PHPT from January 2010 through August 2020. Data on demographics, history, laboratory findings, bone densitometry findings, and clinical presentation and complications were collected and were compared in normal magnesium group vs hypomagnesemia group.

RESULTS

Among the 307 patients with PHPT included in our study, 77 patients (33/102 [32.4%] males and 44/205 [21.5%] females) had hypomagnesemia. Mean hemoglobin levels in the hypomagnesemia group were significantly lower than those in the normal magnesium group in both males and females. In contrast, patients with hypomagnesemia had a higher mean serum calcium and parathyroid hormone than individuals with normal magnesium. The typical symptoms of PHPT, such as nephrolithiasis, bone pain/fractures, polyuria, or polydipsia, were more common in the hypomagnesemia group. In addition, patients with hypomagnesemia had a higher prevalence of osteoporosis, anemia, and hypercalcemic crisis. Even after adjusting for potential confounders, including age, sex, body mass index, estimated glomerular filtration rate, and parathyroid hormone levels, these associations remained essentially unchanged.

CONCLUSION

Biochemical and clinical evidence indicates that patients with PHPT with hypomagnesemia have more severe hyperparathyroidism than those without hypomagnesemia. In addition, PHPT patients with hypomagnesemia had a higher prevalence of osteoporosis, anemia, and hypercalcemic crisis.

Possibility of magnesium supplementation for supportive treatment in patients with COVID-19

Magnesium as an enzymatic activator is essential for various physiological functions such as cell cycle, metabolic regulation, muscle contraction, and vasomotor tone. A growing body of evidence supports that magnesium supplementation (mainly magnesium sulfate and magnesium oxide) prevents or treats various types of disorders or diseases related to respiratory system, reproductive system, nervous system, digestive system, and cardiovascular system as well as kidney injury, diabetes and cancer. The ongoing pandemic coronavirus disease 19 (COVID-19) characterized by respiratory tract symptoms with different degrees of important organ and tissue damages has attracted global attention. Particularly, effective drugs are still lacking in the COVID-19 therapy. In this review, we find and summarize the effectiveness of magnesium supplementation on the disorders or diseases, and provide a reference to the possibility of magnesium supplementation for supportive treatment in patients with COVID-19.

ArrhythmoGenoPharmacoTherapy

This review is focusing on the understanding of various factors and components governing and controlling the occurrence of ventricular arrhythmias including (i) the role of various ion channel-related changes in the action potential (AP), (ii) electrocardiograms (ECGs), (iii) some important arrhythmogenic mediators of reperfusion, and pharmacological approaches to their attenuation. The transmembrane potential in myocardial cells is depending on the cellular concentrations of several ions including sodium, calcium, and potassium on both sides of the cell membrane and active or inactive stages of ion channels. The movements of Na+, K+, and Ca2+ via cell membranes produce various currents that provoke AP, determining the cardiac cycle and heart function. A specific channel has its own type of gate, and it is opening and closing under specific transmembrane voltage, ionic, or metabolic conditions. APs of sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje cells determine the pacemaker activity (depolarization phase 4) of the heart, leading to the surface manifestation, registration, and evaluation of ECG waves in both animal models and humans. AP and ECG changes are key factors in arrhythmogenesis, and the analysis of these changes serve for the clarification of the mechanisms of antiarrhythmic drugs. The classification of antiarrhythmic drugs may be based on their electrophysiological properties emphasizing the connection between basic electrophysiological activities and antiarrhythmic properties. The review also summarizes some important mechanisms of ventricular arrhythmias in the ischemic/reperfused myocardium and permits an assessment of antiarrhythmic potential of drugs used for pharmacotherapy under experimental and clinical conditions.

Mg2+ homeostasis and transport in cyanobacteria - at the crossroads of bacterial and chloroplast Mg2+ import

Magnesium cation (Mg2+) is the most abundant divalent cation in living cells, where it is required for various intracellular functions. In chloroplasts and cyanobacteria, established photosynthetic model systems, Mg2+ is the central ion in chlorophylls, and Mg2+ flux across the thylakoid membrane is required for counterbalancing the light-induced generation of a ΔpH across the thylakoid membrane. Yet, not much is known about Mg2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg2+ transport across membranes has been studied in non-photosynthetic bacteria, and first observations and findings are reported for chloroplasts. Cyanobacterial cytoplasmic membranes appear to contain the well-characterized Mg2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg2+ flux down the electrochemical gradient. Both Mg2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a 'classical' bacterial membrane, essentially nothing is known about Mg2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the 'typical' bacterial Mg2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

[In vivo study of a novel micro-arc oxidation coated magnesium-zinc-calcium alloy scaffold/autologous bone particles repairing critical size bone defect in rabbit]

Objective

To evaluate the effect of a novel micro-arc oxidation (MAO) coated magnesium-zinc-calcium (Mg-Zn-Ca) alloy scaffold/autologous bone particles to repair critical size bone defect (CSD) in rabbit and explore the novel scaffold in vivo corrosion resistance and biocompatibility.

Methods

Seventy-two New Zealand white rabbits were randomly divided into 3 groups ( n=24), group A was uncoated Mg-Zn-Ca alloy scaffold group, group B was 10 μm MAO coated Mg-Zn-Ca alloy scaffold group, and group C was control group with only autologous bone graft. The animals were operated to obtain bilateral ulnar CSD (15 mm in length) models. The bone fragment was removed and minced into small particles and were filled into the scaffolds of groups A and B. Then, the scaffolds or autologous bone particles were replanted into the defects. The animals were sacrificed at 2, 4, 8, and 12 weeks after surgery (6 rabbits each group). The local subcutaneous pneumatosis was observed and recorded. The ulna defect healing was evaluated by X-ray image and Van Gieson staining. The X-ray images were assessed and scored by Lane-Sandhu criteria. The percentage of the lost volume of the scaffold (ΔV) and corrosion rate (CR) were calculated by the Micro-CT. The Mg ²⁺ and Ca ²⁺ concentrations were monitored during experiment and the rabbit liver, brain, kidney, and spleen were obtained to process HE staining at 12 weeks after surgery.

Results

The local subcutaneous pneumatosis in group B was less than that in group A at 2, 4, and 8 weeks after surgery, showing significant differences between 2 groups at 2 and 4 weeks after surgery ( P<0.05); and the local subcutaneous pneumatosis was significantly higher in group B than that in group A at 12 weeks after surgery ( P<0.05). The X-ray result showed that the score of group C was significantly higher than those of groups A and B at 4 and 8 weeks after surgery ( P<0.05), and the score of group B was significantly higher than that of group A at 8 weeks ( P<0.05). At 12 weeks after surgery, the scores of groups B and C were significantly higher than that of group A ( P<0.05). Meanwhile, the renew bone moulding of group B was better than that in group A at 12 weeks after surgery. Micro-CT showed that ΔV and CR in group B were significantly lower than those in group A ( P<0.05). Van Gieson staining showed that group B had better biocompatibility and osteanagenesis than group A. The Mg ²⁺ and Ca ²⁺ concentrations in serum showed no significant difference between groups during experiments ( P>0.05). And there was no obvious pathological changes in the liver, brain, kidney, and spleen of the 3 groups with HE staining at 12 weeks.

Conclusion

The MAO coated Mg-Zn-Ca alloy scaffold/autologous bone particles could be used to repair CSD effectively. At the same time, 10 μm MAO coating can effectively improve the osteanagenesis, corrosion resistance, and biocompatibility of Mg-Zn-Ca alloy scaffold.

Magnesium-based bioceramics in orthopedic applications

Magnesium ions are directly involved in numerous biological mechanisms; for example, they play an important part in the regulation of ion channels, DNA stabilization, enzyme activation and stimulation of cell growth and proliferation. This alkaline earth metal has gained great popularity in orthopedic applications in recent years. Magnesium-based bioceramics include a large group of magnesium containing compounds such as oxides, phosphates and silicates, that are involved in orthopedic applications like bone cements, bone scaffolds or implant coatings. This article aims to give a comprehensive review on different magnesium-based bioceramics, e.g. magnesium phosphates (MgO-P₂O₅), calcium magnesium phosphates (CaO-MgO-P₂O₅), and magnesium glasses (SiO₂-MgO) with a strong focus on the chemistry and properties of magnesium phosphate containing cements as the main application form. In addition, the processing of magnesium phosphate minerals into macroporous scaffolds for tissue engineering applications by either using traditional porogens or by additive manufacturing approaches are reflected. Finally, the biological in vitro and in vivo properties of magnesium phosphates for bone regeneration are summarized, which show promising results regarding the application as bone replacement material, but still lack in terms of testing in large animal models, load-bearing application sites and clinical data.

STATEMENT OF SIGNIFICANCE

Though bone substitutes from calcium phosphates have been investigated for a long time, a new trend is visible in the biomaterials sector: magnesium based bioceramics from magnesium phosphates and silicates due to the special biological significance of magnesium ions in enzymatic activation, cell growth and proliferation, etc. In contrast to pure magnesium implants, such formulations do not release hydrogen during degradation. As with calcium based bioceramics, magnesium based bioceramics are used for the development of diverse applications such as cements, macroporous scaffolds and coatings. From this perspective, we present a systematic overview on diverse kinds of magnesium based bioceramics, their processing regimes for different clinical purposes and their behavior both in vitro and in vivo.

Magnesium Extravaganza: A Critical Compendium of Current Research into Cellular Mg2+ Transporters Other than TRPM6/7

Magnesium research has boomed within the last 20 years. The real breakthrough came at the start of the new millennium with the discovery of a plethora of possible Mg homeostatic factors that, in particular, included putative Mg²⁺ transporters. Until that point, Mg research was limited to biochemical and physiological work, as no target molecular entities were known that could be used to explore the molecular biology of Mg homeostasis at the level of the cell, tissue, organ, or organism and to translate such knowledge into the field of clinical medicine and pharmacology. Because of the aforementioned, Mg²⁺ and Mg homeostasis, both of which had been heavily marginalized within the biomedical field in the twentieth century, have become overnight a focal point of many studies ranging from primary biomedical research to translational medicine.The amount of literature concerning cellular Mg²⁺ transport and cellular Mg homeostasis is increasing, together with a certain amount of confusion, especially about the function(s) of the newly discovered and, in the majority of instances, still only putative Mg²⁺ transporters/Mg²⁺ homeostatic factors. Newcomers to the field of Mg research will thus find it particularly difficult to orient themselves.Here, we briefly but critically summarize the status quo of the current understanding of the molecular entities behind cellular Mg²⁺ homeostasis in mammalian/human cells other than TRPM6/7 chanzymes, which have been universally accepted as being unspecific cation channel kinases allowing the flux of Mg²⁺ while constituting the major gateway for Mg²⁺ to enter the cell.

Mg Isotope Fractionation during Uptake by a Rock-Inhabiting, Model Microcolonial Fungus Knufia petricola at Acidic and Neutral pH

The model rock-inhabiting microcolonial fungus Knufia petricola fractionates stable Mg isotopes in a time- and pH-dependent manner. During growth, the increase of ²⁶Mg/²⁴Mg in the fungal cells relative to the growth media amounted to 0.65 ± 0.14‰ at pH 6 and 1.11 ± 0.35‰ at pH 3. We suggest a constant equilibrium fractionation factor during incorporation of Mg into ribosomes and ATP as a cause of enrichment of ²⁶Mg in the cells. We suggest too that the proton gradient across the cell wall and cytoplasmic membrane controls Mg²⁺ transport into the fungal cell. As the strength of this gradient is a function of extracellular solution pH, the pH-dependence on Mg isotope fractionation is thus due to differences in fungal cell mass fluxes. Through a mass balance model we show that Mg uptake into the fungal cell is not associated with a unique Mg isotope fractionation factor. This Mg isotope fractionation dependence on pH might also be observed in any organism with cells that follow similar Mg uptake and metabolic pathways and serves to reveal Mg cycling in ecosystems.

A fully integrated new paradigm for lithium's mode of action - lithium utilizes latent cellular fail-safe mechanisms

It is proposed that lithium's therapeutic effects occur indirectly by augmenting a cascade of protective "fail-safe" pathways pre-configured to activate in response to a dangerous low cell [Mg⁺⁺] situation, eg, posttraumatic brain injury, alongside relative cell adenosine triphosphate depletion. Lithium activates cell protection, as it neatly mimics a lowered intracellular [Mg⁺⁺] level.

An in vivo study on the metabolism and osteogenic activity of bioabsorbable Mg-1Sr alloy

Previous studies indicated that local delivery of strontium effectively increased bone quality and formation around osseointegrating implants. Therefore, implant materials with long-lasting and controllable strontium release are avidly pursued. The central objective of the present study was to investigate the in vivo biocompatibility, metabolism and osteogenic activity of the bioabsorbable Mg-1Sr (wt.%, nominal composition) alloy for bone regeneration. The general corrosion rate of the alloy implant as a femoral fracture fixation device was 0.55±0.03mm·y(-1) (mean value±standard deviation) in New Zealand White rabbits which meet the bone implantation requirements and can be adjusted by material processing methods. All rabbits survived and the histological evaluation showed no abnormal physiology or diseases 16 weeks post-implantation. The degradation process of the alloy did not significantly alter 16 primary indexes of hematology, cardiac damage, inflammation, hepatic functions and metabolic process. Significant increases in peri-implant bone volume and direct bone-to-implant contact (48.3%±15.3% and 15.9%±5.6%, respectively) as well as the expressions of four osteogenesis related genes (runt-related transcription factor 2, alkaline phosphatase, osteocalcin, and collagen, type I, alpha 1) were observed after 16 weeks implantation for the Mg-1Sr group when compared to the pure Mg group. The sound osteogenic properties of the Mg-1Sr alloy by long-lasting and controllable Sr release suggesting a very attractive clinical potential.

STATEMENT OF SIGNIFICANCE

Sr (strontium) has exhibited pronounced effects to reduce the bone fracture risk in osteoporotic patients. Nonetheless, long-lasting local Sr release is hardly achieved by traditional methods like surface treatment. Therefore, a more efficient Sr local delivery platform is in high clinical demand. The stable and adjustable degradation process of Mg alloy makes it an ideal Sr delivery platform. We combine the well-known osteogenic properties of strontium with magnesium to manufacture bioabsorbable Mg-1Sr alloy with stable Sr release based on our previous studies. The in vitro and in vivo results both showed the alloy's suitable degradation rate and biocompatibility, and the sound osteogenic properties and stimulation effect on bone formation suggest its very attractive clinical potential.

Salinity effects on plasma ion levels, cortisol, and osmolality in Chinook salmon following lethal sampling

Studies on hydromineral balance in fishes frequently employ measurements of electrolytes following euthanasia. We tested the effects of fresh- or salt-water euthanasia baths of tricaine mesylate (MS-222) on plasma magnesium (Mg(2+)) and sodium (Na(+)) ions, cortisol and osmolality in fish exposed to saltwater challenges, and the ion and steroid hormone fluctuations over time following euthanasia in juvenile spring Chinook salmon (Oncorhynchus tshawytscha). Salinity of the euthanasia bath affected plasma Mg(2+) and Na(+) concentrations as well as osmolality, with higher concentrations in fish euthanized in saltwater. Time spent in the bath positively affected plasma Mg(2+) and osmolality, negatively affected cortisol, and had no effect on Na(+) concentrations. The difference of temporal trends in plasma Mg(2+) and Na(+) suggests that Mg(2+) may be more sensitive to physiological changes and responds more rapidly than Na(+). When electrolytes and cortisol are measured as endpoints after euthanasia, care needs to be taken relative to time after death and the salinity of the euthanasia bath.

Hypocalcemia secondary to hypomagnesemia in a patient with Crohn's disease

A 42-year-old female with Crohn's disease who had previously undergone multiple surgical interventions developed marked hypocalcemia, which could not be resolved with calcium administration. Markedly reduced serum magnesium levels were also observed. After intravenous magnesium administration, serum calcium levels rapidly normalized. In addition, the plasma levels of intact parathyroid hormone increased immediately after magnesium administration. These data strongly suggest that hypocalcemia resulted from disturbance of appropriate parathyroid hormone secretion caused by hypomagnesemia. After introduction of infliximab therapy, her abdominal symptoms and endoscopic findings improved, and serum calcium and magnesium levels stabilized within the normal range without magnesium administration.

Magnesium in health and disease

Mammalian cells tightly regulate cellular Mg(2+) content through a variety of transport and buffering mechanisms under the control of various hormones and cellular second messengers. The effect of these hormones and agents results in dynamic changes in the total content of Mg(2+) being transported across the cell membrane and redistributed within cellular compartments. The importance of maintaining proper cellular Mg(2+) content optimal for the activity of various cellular enzymes and metabolic cycles is underscored by the evidence that several diseases are characterized by a loss of Mg(2+) within specific tissues as a result of defective transport, hormonal stimulation, or metabolic impairment. This chapter will review the key mechanisms regulating cellular Mg(2+) homeostasis and their impairments under the most common diseases associated with Mg(2+) loss or deficiency.

Cellular magnesium homeostasis

Magnesium, the second most abundant cellular cation after potassium, is essential to regulate numerous cellular functions and enzymes, including ion channels, metabolic cycles, and signaling pathways, as attested by more than 1000 entries in the literature. Despite significant recent progress, however, our understanding of how cells regulate Mg(2+) homeostasis and transport still remains incomplete. For example, the occurrence of major fluxes of Mg(2+) in either direction across the plasma membrane of mammalian cells following metabolic or hormonal stimuli has been extensively documented. Yet, the mechanisms ultimately responsible for magnesium extrusion across the cell membrane have not been cloned. Even less is known about the regulation in cellular organelles. The present review is aimed at providing the reader with a comprehensive and up-to-date understanding of the mechanisms enacted by eukaryotic cells to regulate cellular Mg(2+) homeostasis and how these mechanisms are altered under specific pathological conditions.

Regulation of Alr1 Mg transporter activity by intracellular magnesium

Mg homeostasis is critical to eukaryotic cells, but the contribution of Mg transporter activity to homeostasis is not fully understood. In yeast, Mg uptake is primarily mediated by the Alr1 transporter, which also allows low affinity uptake of other divalent cations such as Ni(2+), Mn(2+), Zn(2+) and Co(2+). Using Ni(2+) uptake to assay Alr1 activity, we observed approximately nine-fold more activity under Mg-deficient conditions. The mnr2 mutation, which is thought to block release of vacuolar Mg stores, was associated with increased Alr1 activity, suggesting Alr1 was regulated by intracellular Mg supply. Consistent with a previous report of the regulation of Alr1 expression by Mg supply, Mg deficiency and the mnr2 mutation both increased the accumulation of a carboxy-terminal epitope-tagged version of the Alr1 protein (Alr1-HA). However, Mg supply had little effect on ALR1 promoter activity or mRNA levels. In addition, while Mg deficiency caused a seven-fold increase in Alr1-HA accumulation, the N-terminally tagged and untagged Alr1 proteins increased less than two-fold. These observations argue that the Mg-dependent accumulation of the C-terminal epitope-tagged protein was primarily an artifact of its modification. Plasma membrane localization of YFP-tagged Alr1 was also unaffected by Mg supply, indicating that a change in Alr1 location did not explain the increased activity we observed. We conclude that variation in Alr1 protein accumulation or location does not make a substantial contribution to its regulation by Mg supply, suggesting Alr1 activity is directly regulated via as yet unknown mechanisms.

Adenosine triphosphate depletion by cyanide results in a Na(+)-dependent Mg(2+) extrusion from liver cells

Addition of NaCN to isolated hepatocytes results in a marked and rapid decrease in cellular adenosine triphosphate (ATP) content, and in the extrusion of a sizable amount of cellular Mg(2+). This extrusion starts after a 10-minute lag phase and reaches a maximum of 35 to 40 nmol Mg(2+) per milligram protein within 60 minutes from the addition of CN(-). A quantitatively similar Mg(2+) extrusion is also observed after the addition of the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone but not that of the glycolysis inhibitor iodoacetate. The Mg(2+) extrusion is completely inhibited by the removal of extracellular Na(+) or the addition of imipramine, quinidine, or glibenclamide, whereas it persists after the removal of extracellular Ca(2+) or K(+), or the addition of amiloride. An acidic extracellular pH or the removal of extracellular HCO₃⁻ inhibits the cyanide-induced Mg(2+) extrusion by at least 80%. Taken together, these data suggest that the decrease in cellular adenosine triphosphate content removes a major Mg(2+) complexing agent from the hepatocyte and results in an extrusion of hepatic Mg(2+) exclusively through a Na(+)-dependent exchange mechanism modulated by acidic changes in extracellular pH.

Induction of the tetracycline repressor: characterization by molecular-dynamics simulations

Extensive molecular-dynamics simulations show that the distance between the centers of gravity of the two equivalent helices 3 in the DNA-binding heads of the dimer of the tetracycline-repressor protein (TetR) can be used as a reliable diagnostic of induction. This is not, however, true for X-ray structures, but only for molecular-dynamics simulations. This is suggested to be because TetR is inherently flexible along the coordinate of the allosteric change (as is always likely to be the case for allosteric proteins), so that crystal-packing forces can determine the conformation of the protein. However, the time scale of the allosteric rearrangement in the absence of DNA-complexation is found to be of the order of tens of nanoseconds, so that rearrangements can be observed reproducibly in 100 ns simulations. Metastable (pre-equilibrium) conformations of TetR have been observed for up to 60 ns. The likely equilibrium processes and key features of the TetR system are discussed.

Molecular determinants of magnesium homeostasis: insights from human disease

The past decade has witnessed multiple advances in our understanding of magnesium (Mg(2+)) homeostasis. The discovery that mutations in claudin-16/paracellin-1 or claudin-19 are responsible for familial hypomagnesemia with hypercalciuria and nephrocalcinosis provided insight into the molecular mechanisms governing paracellular transport of Mg(2+). Our understanding of the transcellular movement of Mg(2+) was similarly enhanced by the realization that defects in transient receptor potential melastatin 6 (TRPM6) cause hypomagnesemia with secondary hypocalcemia. This channel regulates the apical entry of Mg(2+) into epithelia. In so doing, TRPM6 alters whole-body Mg(2+) homeostasis by controlling urinary excretion. Consequently, investigation into the regulation of TRPM6 has increased. Acid-base status, 17beta estradiol, and the immunosuppressive agents FK506 and cyclosporine affect plasma Mg(2+) levels by altering TRPM6 expression. A mutation in epithelial growth factor is responsible for isolated autosomal recessive hypomagnesemia, and epithelial growth factor activates TRPM6. A defect in the gamma-subunit of the Na,K-ATPase causes isolated dominant hypomagnesemia by altering TRPM6 activity through a decrease in the driving force for apical Mg(2+) influx. We anticipate that the next decade will provide further detail into the control of the gatekeeper TRPM6 and, therefore, overall whole-body Mg(2+) balance.

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

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

Magnesium–calcium signalling in rat parotid acinar cells: effects of acetylcholine

This study investigated the effects of extracellular Mg(2+) ([Mg(2+)](o)) on basal and acetylcholine (ACh)-evoked amylase secretion and intracellular free Ca(2+) ([Ca(2+)](i)) in rat parotid acinar cells. In a medium containing 1.1 mM [Mg(2+)](o), ACh evoked significant increases in amylase secretion and [Ca(2+)](i). Either low (0 mM) or elevated (5 and 10 mM) [Mg(2+)](o) attenuated ACh-evoked responses. In a nominally Ca(2+) free medium, elevated [Mg(2+)](o) attenuated basal and ACh-evoked amylase secretion and [Ca(2+)](i). In parotid acinar cells incubated with either 0, 1.1, 5 or 10 mM [Mg(2+)](o), ACh evoked a gradual decrease in [Mg(2+)](i). These results indicate that the ACh-evoked Mg(2+) efflux is an active process since Mg(2+) has to move against its gradient. Either lidocaine, amiloride, N-methyl-D: -glucamine, quinidine, dinitrophenol or bumetanide can elevate [Mg(2+)](i) above basal level. In the presence of these membrane transport inhibitors, ACh still evoked a decrease in [Mg(2+)](i) but the response was less pronounced with either [Na(+)](o) removal or in the presence of either amiloride or quinidine. These results indicate marked interactions between Ca(2+) and Mg(2+) signalling in parotid acinar cells and that ACh-evoked Mg(2+) transport was not dependent upon [Na(+)](o).

Physiology of epithelial Ca2+ and Mg2+ transport

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

Mechanism of Exocrine Pancreatic Insufficiency in Streptozotocin-Induced Type 1 Diabetes Mellitus

Diabetes mellitus (DM) is a major health problem at present affecting about 180 million people worldwide. DM is associated with many metabolic abnormalities in the body including the indigestion of carbohydrates leading to malnutrition and weight loss. In this article we investigate the cellular and molecular mechanisms of exocrine pancreatic insufficiency in streptozotocin (STZ, 60 mg kg(-1), i.p.)-induced DM in male rats compared to healthy age-matched controls. Either electrical field stimulation (EFS) or cholecystokinin octapeptide (CCK-8, 10(-8) M) can elicit large and significant (P < 0.05) increases in amylase output from pancreatic segments compared to basal secretion. Insulin (10(-6) M) alone has no significant effect on amylase output compared to basal but it enhanced the secretory responses to either EFS or CCK-8. When rats were rendered diabetic with STZ, either EFS or CCK-8-evoked amylase output was significantly (P < 0.01) decreased compared to the responses obtained with either EFS or CCK-8 alone in healthy age-matched control pancreas. In addition, CCK-8 can elicit large dose-dependent release of amylase in age-matched control and diabetic acinar cells with significantly (P < 0.05) reduced responses in diabetic acinar cells. CCK-8 evoked a large rapid increase in peak cytosolic free calcium concentration ([Ca2+]c) followed by a decrease to a plateau phase in age-matched control fura-2-loaded pancreatic acinar cells. These responses were significantly (P < 0.05) decreased in STZ-induced diabetic acinar cells. In the presence of 10(-6) M insulin, CCK-8 evoked a much larger increase in the Ca2+ transient compared to the response obtained with CCK-8 alone. These effects were significantly (P < 0.01) inhibited in STZ-induced diabetic acinar cells. Similarly, in zero extracellular Ca2+ [Ca2+]ĉ, the CCK-8-evoked [Ca2+]c was significantly (P < 0.05) reduced in both diabetic and age-matched control acinar cells, but with more pronounced reduction in diabetic acinar cells. CCK(A) receptor mRNA levels remained unchanged in diabetic rat acinar cells compared to age-matched healthy control. In contrast, amylase mRNA was significantly (P < 0.05) reduced in diabetic acinar cells compared to control. The results indicate that reduced amylase secretion in response to either EFS or CCK-8 in the diabetic pancreas may be due to reduced [Ca2+]c and gene expression for amylase and not to the gene expression of CCK(A) receptor in pancreatic acinar cells.

Protein kinase A dependent phosphorylation activates Mg2+ efflux in the basolateral region of the liver

Isolated hepatocytes in physiological [Na(+)]( 0 ) tightly maintain [Mg(2+)]( i ). Upon beta-adrenergic stimulation or in the presence of permeable cAMP, hepatocytes release 5-10% (1-3 mM Mg(2+)) of their total Mg(2+) content. However, isolated basolateral liver plasma membranes (bLPM), release Mg(2+) in the presence of [Na(+)]( o ) even in the absence of catecholamine stimulation. The data indicate that a physiological brake for Mg(2+) efflux is present in the hepatocyte and is removed upon cellular signaling. In contrast, this regulation "brake" is absent in purified bLPM thus rendering them fully active. The present study was carried out to reconstruct the missing regulatory component. Activation of Mg(2+) extrusion in intact cells is consistent with cAMP dependent phosphorylation of the transporter or a regulatory protein. Treatment of bLPM with a non-specific phosphatase such as alkaline phosphatase (AP), decreased Mg(2+) efflux by 70% compared to untreated bLPM. When AP-treated bLPM were loaded with protein kinase A (PKA), and stimulated with permeable cAMP, Mg(2+) transport fully recovered. These data suggest that phosphorylation of the Na(+)/Mg(2+) exchanger or a nearby protein activates the Mg(2+) transport mechanism in hepatocytes.

Regulation of magnesium homeostasis and transport in mammalian cells

Magnesium is the second most abundant cation within the cell after potassium and plays an important role in numerous biological functions. Several pieces of experimental evidence indicate that mammalian cells tightly regulate Mg(2+) content by precise control mechanisms operating at the level of Mg(2+) entry and efflux across the cell membrane, as well as at the level of intracellular Mg(2+) buffering and organelle compartmentation under resting conditions and following hormonal stimuli. This review will attempt to elucidate the mechanisms involved in hormonal-mediated Mg(2+) extrusion and accumulation, as well as the physiological implications of changes in cellular Mg(2+) content following hormonal stimuli.

Mg2+ release coupled to Ca2+ uptake: a novel Ca 2+ accumulation mechanism in rat liver

Isolated hepatocytes release 2-3 nmol Mg2+/mg protein or approximately 10% of the total cellular Mg2+ content within 2 minutes from the addition of agonists that increase cellular cAMP, for example, isoproterenol (ISO). During Mg2+ release, a quantitatively similar amount of Ca2+ enters the hepatocyte, thus suggesting a stoichiometric exchange ratio of 1 Mg2+:1Ca2+. Calcium induced Mg2+ extrusion is also observed in apical liver plasma membranes (aLPM), in which the process presents the same 1 Mg2+:1Ca2+ exchange ratio. The uptake of Ca2+ for the release of Mg2+ occurs in the absence of significant changes in Deltapsi as evidenced by electroneutral exchange measurements with a tetraphenylphosphonium (TPP+) electrode or 3H-TPP+. Collapsing the Deltapsi by high concentrations of TPP+ or protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) does not inhibit the Ca2+-induced Mg2+ extrusion in cells or aLPM. Further, the process is strictly unidirectional, serving only in Ca2+ uptake and Mg2+ release. These data demonstrate the operation of an electroneutral Ca2+/Mg2+ exchanger which represents a novel pathway for Ca2+ accumulation in liver cells following adrenergic receptor stimulation.

The epithelial Mg2+ channel transient receptor potential melastatin 6 is regulated by dietary Mg2+ content and estrogens

The kidney is the principal organ responsible for the regulation of the body Mg(2+) balance. Identification of the gene defect in hypomagnesemia with secondary hypocalcemia recently elucidated transient receptor potential melastatin 6 (TRPM6) as the gatekeeper in transepithelial Mg(2+) transport, whereas its homolog, TRPM7, is implicated in cellular Mg(2+) homeostasis. The aim of this study was to determine the tissue distribution in mouse and regulation of TRPM6 and TRPM7 by dietary Mg(2+) and hormones. This study demonstrates that TRPM6 is expressed predominantly in kidney, lung, cecum, and colon, whereas TRPM7 is distributed ubiquitously. Dietary Mg(2+) restriction in mice resulted in hypomagnesemia and renal Mg(2+) and Ca(2+) conservation, whereas a Mg(2+)-enriched diet led to increased urinary Mg(2+) and Ca(2+) excretion. Conversely, Mg(2+) restriction significantly upregulated renal TRPM6 mRNA levels, whereas a Mg(2+) enriched diet increased TRPM6 mRNA expression in colon. Dietary Mg(2+) did not alter TRPM7 mRNA expression in mouse kidney and colon. In addition, it was demonstrated that 17beta-estradiol but not 1,25-dihydroxyvitamin D(3) or parathyroid hormone regulates TRPM6 renal mRNA levels. Renal TRPM7 mRNA abundance remained unaltered under these conditions. The renal TRPM6 mRNA level in ovariectomized rats was significantly reduced, whereas 17beta-estradiol treatment normalized TRPM6 mRNA levels. In conclusion, kidney, lung, cecum, and colon likely constitute the main sites of active Mg(2+) (re)absorption in the mouse. In addition, Mg(2+) restriction and 17beta-estradiol upregulated renal TRPM6 mRNA levels, whereas a Mg(2+)-enriched diet stimulated TRPM6 mRNA expression in colon, supporting the gatekeeper function of TRPM6 in transepithelial Mg(2+) transport.

Effects of multiple metal binding sites on calcium and magnesium-dependent activation of BK channels

BK channels are activated by physiological concentrations of intracellular Ca²⁺ and Mg²⁺ in a variety of cells. Previous studies have identified two sites important for high-affinity Ca²⁺ sensing between [Ca²⁺]_i of 0.1–100 μM and a site important for Mg²⁺ sensing between [Mg²⁺]_i of 0.1–10 mM. BK channels can be also activated by Ca²⁺ and Mg²⁺ at concentrations >10 mM so that the steady-state conductance and voltage (G-V) relation continuously shifts to more negative voltage ranges when [Mg²⁺]_i increases from 0.1–100 mM. We demonstrate that a novel site is responsible for metal sensing at concentrations ≥10 mM, and all four sites affect channel activation independently. As a result, the contributions of these sites to channel activation are complex, depending on the combination of Ca²⁺ and Mg²⁺ concentrations. Here we examined the effects of each of these sites on Ca²⁺ and Mg²⁺-dependent activation and the data are consistent with the suggestion that these sites are responsible for metal binding. We provide an allosteric model for quantitative estimation of the contributions that each of these putative binding sites makes to channel activation at any [Ca²⁺]_i and [Mg²⁺]_i.

Increased renal tubular reabsorption of calcium and magnesium by the offspring of diabetic rat pregnancy

Diabetic pregnancy has a marked influence on offspring calcium and magnesium homeostasis. Urinary excretion of calcium and magnesium is reduced, yet offspring of diabetic pregnancy exhibit hypomagnesemia and hypocalcemia. The aim of this study was to measure renal hemodynamic and tubular function in the offspring of diabetic (OD) and control, nondiabetic (OC) rats at 4 and 8 wk of age to determine the glomerular and tubular mechanisms through which renal calcium and magnesium handling are programmed in utero. The fraction of filtered calcium that was excreted was significantly lower in OD at both 4 and 8 wk of age [8 wk: OC (n = 6), 11.8 +/- 2.9 versus OD (n = 5), 4.3 +/- 0.6%; p < 0.05] and that of magnesium was lower at 8 wk of age [OC (n = 6), 42.4 +/- 7.5 versus OD (n = 5), 13.0 +/- 1.7%; p < 0.01]. This increased reabsorption occurred despite an elevated GFR in OD. These findings clearly indicate that tubular reabsorptive mechanisms for calcium and magnesium are increased markedly in OD. Serum PTH concentration was reduced in 8-wk-old OD [OC (n = 7), 539.4 +/- 142.1 versus OD (n = 9), 174.3 +/- 69.4 pg/ml; p < 0.05], consistent with previous reports in human infants. Taken together, these observations suggest that the basis for the altered renal magnesium and calcium handling in OD involves increased tubular transport activity and possibly increased sensitivity of these mechanisms to PTH.

Identification and characterization of a novel mammalian Mg2+ transporter with channel-like properties

BACKGROUND

Intracellular magnesium is abundant, highly regulated and plays an important role in biochemical functions. Despite the extensive evidence for unique mammalian Mg2+ transporters, few proteins have been biochemically identified to date that fulfill this role. We have shown that epithelial magnesium conservation is controlled, in part, by differential gene expression leading to regulation of Mg2+ transport. We used this knowledge to identify a novel gene that is regulated by magnesium.

RESULTS

Oligonucleotide microarray analysis was used to identify a novel human gene that encodes a protein involved with Mg2+-evoked transport. We have designated this magnesium transporter (MagT1) protein. MagT1 is a novel protein with no amino acid sequence identity to other known transporters. The corresponding cDNA comprises an open reading frame of 1005 base pairs encoding a protein of 335 amino acids. It possesses five putative transmembrane (TM) regions with a cleavage site, a N-glycosylation site, and a number of phosphorylation sites. Based on Northern analysis of mouse tissues, a 2.4 kilobase transcript is present in many tissues. When expressed in Xenopus laevis oocytes, MagT1 mediates saturable Mg2+ uptake with a Michaelis constant of 0.23 mM. Transport of Mg2+ by MagT1 is rheogenic, voltage-dependent, does not display any time-dependent inactivation. Transport is very specific to Mg2+ as other divalent cations did not evoke currents. Large external concentrations of some cations inhibited Mg2+ transport (Ni2+, Zn2+, Mn2+) in MagT1-expressing oocytes. Ca2+and Fe2+ were without effect. Real-time reverse transcription polymerase chain reaction and Western blot analysis using a specific antibody demonstrated that MagT1 mRNA and protein is increased by about 2.1-fold and 32%, respectively, in kidney epithelial cells cultured in low magnesium media relative to normal media and in kidney cortex of mice maintained on low magnesium diets compared to those animals consuming normal diets. Accordingly, it is apparent that an increase in mRNA levels is translated into higher protein expression.

CONCLUSION

These studies suggest that MagT1 may provide a selective and regulated pathway for Mg2+ transport in epithelial cells.

Regulation by Magnesium of Potato Tuber Mitochondrial Respiratory Activities

Dehydrogenase activities of potato tuber mitochondria and corresponding phosphorylation rates were measured for the dependence on external and mitochondrial matrix Mg2+. Magnesium stimulated state 3 and state 4 respiration, with significantly different concentrations of matrix Mg2+ required for optimal activities of the several substrates. Maximal stimulation of respiration with all substrates was obtained at 2-mM external Mg2+. However, respiration of malate, citrate, and alpha-ketoglutarate requires at least 4-mM Mg2+ inside mitochondria for maximization of dehydrogenase activities. The phosphorylation system, requires a low level of internal Mg2+ (0.25 mM) to reach high activity, as judged by succinate-dependent respiration. However, mitochondria respiring on citrate or alpha-ketoglutarate only sustain high levels of phosphorylation with at least 4-mM matrix Mg2+. Respiration of succinate is active without external and matrix Mg2+, although stimulated by the cation. Respiration of alpha-ketoglutarate was strictly dependent on external Mg2+ required for substrate transport into mitochondria, and internal Mg2+ is required for dehydrogenase activity. Respiration of citrate and malate also depend on internal Mg2+ but, unlike alpha-ketoglutarate, some activity still remains without external Mg2+. All the substrates revealed insensitive to external and internal mitochondrial Ca2+, except the exogenous NADH dehydrogenase, which requires either external Ca2+ or Mg2+ for detectable activity. Calcium is more efficient than Mg2+, both having cumulative stimulation. Unlike Ca2+, Mn2+ could substitute for Mg2+, before and after addition of A23, showing its ability to regulate phosphorylation and succinate dehydrogenase activities, with almost the same efficiency as Mg2+.

A Multi-Model Approach to Nucleic Acid-Based Drug Development

With the advent of functional genomics and the shift of interest towards sequence-based therapeutics, the past decades have witnessed intense research efforts on nucleic acid-mediated gene regulation technologies. Today, RNA interference is emerging as a groundbreaking discovery, holding promise for development of genetic modulators of unprecedented potency. Twenty-five years after the discovery of antisense RNA and ribozymes, gene control therapeutics are still facing developmental difficulties, with only one US FDA-approved antisense drug currently available in the clinic. Limited predictability of target site selection models is recognized as one major stumbling block that is shared by all of the so-called complementary technologies, slowing the progress towards a commercial product. Currently employed in vitro systems for target site selection include RNAse H-based mapping, antisense oligonucleotide microarrays, and functional screening approaches using libraries of catalysts with randomized target-binding arms to identify optimal ribozyme/DNAzyme cleavage sites. Individually, each strategy has its drawbacks from a drug development perspective. Utilization of message-modulating sequences as therapeutic agents requires that their action on a given target transcript meets criteria of potency and selectivity in the natural physiological environment. In addition to sequence-dependent characteristics, other factors will influence annealing reactions and duplex stability, as well as nucleic acid-mediated catalysis. Parallel consideration of physiological selection systems thus appears essential for screening for nucleic acid compounds proposed for therapeutic applications. Cellular message-targeting studies face issues relating to efficient nucleic acid delivery and appropriate analysis of response. For reliability and simplicity, prokaryotic systems can provide a rapid and cost-effective means of studying message targeting under pseudo-cellular conditions, but such approaches also have limitations. To streamline nucleic acid drug discovery, we propose a multi-model strategy integrating high-throughput-adapted bacterial screening, followed by reporter-based and/or natural cellular models and potentially also in vitro assays for characterization of the most promising candidate sequences, before final in vivo testing.

Magnesium and experimental vasospasm

Object. Interest has developed in the use of magnesium (Mg++) as a neuroprotectant and antivasospastic agent. Magnesium may increase cerebral blood flow (CBF) and reduce the contraction of cerebral arteries caused by various stimuli. In this study the authors tested the hypothesis that a continuous intravenous infusion of Mg++ reduces cerebral vasospasm after experimental subarachnoid hemorrhage (SAH).

Methods. A dose-finding study was conducted in five monkeys (Macaca fascicularis) to determine what doses of intravenous MgSO4 elevate the cerebrospinal fluid (CSF) concentrations of Mg++ to vasoactive levels and to determine what effects these doses have on the diameters of cerebral arteries, as shown angiographically. After a standard dose of MgSO4 had been selected it was then administered in a randomized, controlled, blinded study to 10 monkeys (five animals/group) with SAH, beginning on Day 0 and continuing for 7 days, at which time angiography was repeated. A 0.086-g/kg bolus of MgSO4 followed by an infusion of 0.028 g/kg/day MgSO4 significantly elevated serum and CSF levels of Mg++ (five monkeys). Magnesium sulfate significantly elevated the serum level of total Mg++ from a control value of 0.83 ± 0.04 mmol/L to 2.42 ± 1.01 mmol/L on Day 7 and raised the CSF level from 1.3 ± 0.04 mmol/L to 1.76 ± 0.14 mmol/L. There was no angiographic evidence of any effect of MgSO4 on normal cerebral arteries. After SAH, the vasospasm in the middle cerebral artery was not significantly reduced (46 ± 8% in the MgSO4-treated group compared with 35 ± 6% in the placebo [vehicle]-treated group, p > 0.05, unpaired t-test).

Conclusions. Magnesium sulfate did not significantly reduce cerebral vasospasm after SAH in the doses tested. An investigation of SAH is warranted mainly to test whether a benefit can be achieved by neuroprotection or by augmentation of CBF by dilation of small vessels and/or collateral pathways.

Iron and magnesium exchange via the low affinity iron transporter in rabbit erythroid cells—exchange rates and the action of valinomycin, diethylstilbestrol and protein kinase inhibitors

Evidence was presented previously that rabbit erythroid cells possess a low-affinity Fe2+ transport system which operates via the Na+/Mg2+antiport [Biochim. Biophys. Acta 1282 (1996) 163]. This was investigated further by measurements of Mg2+ efflux as well as Fe2+ uptake by the cells and by examining the inhibitory effects of valinomycin, diethylstilbestrol (DES) and protein kinase inhibitors. Mg2+ efflux and Fe2+ uptake were measured using rabbit reticulocytes and mature erythrocytes incubated in isotonic KCl or NaCl solutions. Both processes were slower in mature cells than reticulocytes. Mg2+ efflux into KCl solution was much lower than into NaCl solution but was stimulated by addition of Fe2+ to the solution. The rate of Fe2+-stimulated Mg2+ efflux closely followed that of Fe2+ uptake in a one-to-one molar ratio. Valinomycin, DES and the protein kinase inhibitors all inhibited Fe2+ uptake from KCl solution. Valinomycin also inhibited Fe2+-stimulated Mg2+ efflux into KCl solution but markedly stimulated the efflux into NaCl. Maximal inhibition of Fe2+ uptake from KCl solution required the presence of K+, Rb+ or Cs+ ions with which valinomycin forms strong complexes. The results could not be explained on the basis of changes in cell membrane potential or cell volume. By contrast, the increase in Mg2+ efflux into NaCl solution produced by valinomycin was accompanied by cell shrinkage and production of a more negative membrane potential, either of which may be responsible for the effect. The inhibition produced by the protein kinase inhibitors indicate that phosphorylation of the transporter or an associated protein by protein tyrosine kinase is probably required to activate the transporter.

Slowing effects of Mg2+ on contractile kinetics of skinned preparations of rat hearts depending on myosin heavy chain isoform content

The effects of changes in Mg2+ concentration on the kinetics of stretch activation were investigated on skinned rat heart preparations under maximal Ca2+ activation. Muscle strips of hyper- and hypothyroid rat hearts were investigated at 0.5 and 1 mM free Mg2+; the total ATP concentration was 8 mM which resulted in saturating MgATP2- concentrations above 5 mM. Preparations containing exclusively the cardiac alpha-myosin heavy chain (hyper- and hypothyroid atria, hyperthyroid ventricles) showed an acceleration of the kinetics of stretch activation by a factor of about 1.5 (P<0.01, paired t-test) when free Mg2+ was decreased from 1 to 0.5 mM. Conversely, preparations containing exclusively the beta-myosin heavy chain isoform showed only a small acceleration by a factor of 1.05 (P<0.05, paired t-test) under the same conditions. The fact that the Mg2+ sensitivity was dependent on the myosin heavy chain isoform excludes the possibility that Mg2+ exhibits only unspecific effects on contractile proteins. Several hypotheses for explaining the observed Mg2+ effects are discussed. The conditions used in our experiments might be close to the physiological situation and, thus, changes of Mg2+ concentration must be considered as possible factors modulating the contractile kinetics especially of atrial muscle tissue.

Glucose-induced alterations of intracellular ionized magnesium in human lymphocytes

The intracellular ionic content of human erythrocytes may be altered by hyperglycaemia. Despite this, very little is known about the cellular mechanisms linking glucose and cellular magnesium homeostasis. We measured intracellular ionized magnesium in human lymphocytes, by means of a fluorimetric technique, total intracellular magnesium by means of atomic absorption spectrophotometry and intracellular ATP by means of HPLC. The incubation of lymphocytes with D-glucose in the absence of insulin was followed by a significant decrease in intracellular ionized magnesium; this effect did not occur when the cells were incubated with L-glucose. The effect of glucose on intracellular ionized magnesium was blocked by amphotericin B and the EC(50) of the effect of glucose on intracellular ionized magnesium was about 5 mmol/l of glucose. The increase of intracellular ionized magnesium in cells incubated in the absence of glucose was followed by a decrease in intracellular ATP. In a Na(+)-free medium the decrease of intracellular ionized magnesium in the presence of glucose was still present and the incubation of lymphocytes with glucose did not modify total intralymphocyte magnesium. By selective permeabilization of cell membranes, we established that glucose could not increase compartmentalized intracellular ionized magnesium. Our data supports the hypothesis that glucose per se induces a substantial decrease in intracellular ionized magnesium, which is probably due to an augmented binding of intracellular ionized magnesium to cellular ATP.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Intracellular Mg(2+) enhances the function of BK-type Ca(2+)-activated K(+) channels

BK channels modulate neurotransmitter release due to their activation by voltage and Ca(2+). Intracellular Mg(2+) also modulates BK channels in multiple ways with opposite effects on channel function. Previous single-channel studies have shown that Mg(2+) blocks the pore of BK channels in a voltage-dependent manner. We have confirmed this result by studying macroscopic currents of the mslo1 channel. We find that Mg(2+) activates mslo1 BK channels independently of Ca(2+) and voltage by preferentially binding to their open conformation. The mslo3 channel, which lacks Ca(2+) binding sites in the tail, is not activated by Mg(2+). However, coexpression of the mslo1 core and mslo3 tail produces channels with Mg(2+) sensitivity similar to mslo1 channels, indicating that Mg(2+) sites differ from Ca(2+) sites. We discovered that Mg(2+) also binds to Ca(2+) sites and competitively inhibits Ca(2+)-dependent activation. Quantitative computation of these effects reveals that the overall effect of Mg(2+) under physiological conditions is to enhance BK channel function.

Mitochondrial Ca(2+) uptake depends on the spatial and temporal profile of cytosolic Ca(2+) signals

Using confocal imaging of Rhod-2-loaded HeLa cells, we examined the ability of mitochondria to sequester Ca(2+) signals arising from different sources. Mitochondrial Ca(2+) (Ca(2+)mit) uptake was stimulated by inositol 1,4,5-trisphosphate (InsP(3))-evoked Ca(2+) release, capacitative Ca(2+) entry, and Ca(2+) leaking from the endoplasmic reticulum. For each Ca(2+) source, the relationship between cytosolic Ca(2+) (Ca(2+)cyt) concentration and Ca(2+)mit was complex. With Ca(2+)cyt < 300 nm, a slow and persistent Ca(2+)mit uptake was observed. If Ca(2+)cyt increased above approximately 400 nm, Ca(2+)mit uptake accelerated sharply. For equivalent Ca(2+)cyt increases, the rate of Ca(2+)mit rise was greater with InsP(3)-evoked Ca(2+) signals than any other source. Spatial variation of the Ca(2+)mit response was observed within individual cells. Both the fraction of responsive mitochondria and the amplitude of the Ca(2+)mit response were graded in direct proportion to stimulus concentration. Trains of repetitive Ca(2+) oscillations did not maintain elevated Ca(2+)mit levels. Only low frequency Ca(2+) transients (<1/15 min) evoked repetitive Ca(2+)mit signals. Our data indicate that there is a lag between Ca(2+)cyt and Ca(2+)mit increases but that mitochondria will accumulate calcium when it is elevated over basal levels regardless of its source. Furthermore, in addition to the characteristics of Ca(2+) signals, Ca(2+) uniporter desensitization and proximity of mitochondria to InsP(3) receptors modulate mitochondrial Ca(2+) responses.

Calcium-magnesium interactions in pancreatic acinar cells

Although the role of calcium (Ca2+) in the signal transduction and pathobiology of the exocrine pancreas is firmly established, the role of magnesium (Mg2+) remains unclear. We have characterized the intracellular distribution of Mg2+ in response to hormone stimulation in isolated mouse pancreatic acinar cells and studied the role of Mg2+ in modulating Ca2+ signaling using microspectrofluorometry and digital imaging of Ca2+- or Mg2+-sensitive fluorescent dyes as well as Mg2+-sensitive intracellular microelectrodes. Our results indicate that an increase in intracellular Mg2+ concentrations reduced the cholecystokinin (CCK) -induced Ca2+ oscillations by inhibiting the capacitive Ca2+ influx. An intracellular Ca2+ mobilization, on the other hand, was paralleled by a decrease in [Mg2+]i, which was reversible upon hormone withdrawal independent of the electrochemical gradients for Mg2+, Ca2+, Na+, and K+, and not caused by Mg2+ efflux from acinar cells. In an attempt to characterize possible Mg2+ stores that would explain the reversible, hormone-induced intracellular Mg2+ movements, we ruled out mitochondria or ATP as potential Mg2+ buffers and found that the CCK-induced [Mg2+]i decrease was initiated at the basolateral part of the acinar cells, where most of the endoplasmic reticulum (ER) is located, and progressed from there toward the apical pole of the acinar cells in an antiparallel fashion to Ca2+ waves. These experiments represent the first characterization of intracellular Mg2+ movements in the exocrine pancreas, provide evidence for possible Mg2+ stores in the ER, and indicate that the spatial and temporal distribution of intracellular Mg concentrations profoundly affects acinar cell Ca2+ signaling.

Ionized magnesium in the homeostasis of cells: intracellular threshold for Mg(2+) in human platelets

The role of extracellular magnesium ions in the homeostasis of intracellular ionized magnesium ([Mg(2+)](i)) in human platelets was studied. For media containing 0.00 to 0.60 mmol/l of extracellular ionized magnesium ([Mg(2+)](o)), the mean [Mg(2+)](i) fluctuated between 533 and 760 micromol/l. As the [Mg(2+)](o) was increased to 1.5 mmol/l, the [Mg(2+)](i) increased proportionately and peaked at 1470.1 micromol/l. Additional increase in the [Mg(2+)](o) from 1.50 to 6.00 mmol/l resulted in decreased [Mg(2+)](i) until it equilibrated between 739 and 776 micromol/l. The influx of Mg(2+) at [Mg(2+)](o) of 0.60 and 1.50 mmol/l was studied using verapamil, a calcium channel inhibitor, and ouabain, an inhibitor of the Na/K pump, respectively. The verapamil (25 mmol/l) blocking experiments resulted in a 92.4% inhibition of the Mg(2+) influx into the platelet at a [Mg(2+)](o) of 1.50 mmol/l. Ouabain (0.5 and 2.5 mmol/l) showed an enhancement effect on the influx of Mg(2+) at [Mg(2+)](o) of 0.60 mmol/l and no effect at 1.50 mmol/l. The effect of verapamil indicates that ion channels that are homologous to calcium ion channels may be involved in the influx of Mg(2+) into the platelets. The inhibition of Mg(2+) influx for [Mg(2+)](o) greater than 1.50 mmol/l may illustrate a protective mechanism that attempts to maintain the viability of platelets at abnormally high [Mg(2+)](o). These results suggest that there is an intracellular Mg(2+) threshold of 1500 micromol/l, above which an active mechanism prevents further influx of Mg(2+).

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.

Activation of Na(+)- and Ca(2+)-dependent Mg(2+) extrusion by alpha(1)- and beta-adrenergic agonists in rat liver cells

The administration of selective alpha(1) (phenylephrine)-, beta (isoproterenol)-, or mixed (epinephrine) adrenergic agonists induces a marked Mg(2+) extrusion from perfused rat livers. In the absence of extracellular Ca(2+), phenylephrine does not induce a detectable Mg(2+) extrusion, isoproterenol-induced Mg(2+) mobilization is unaffected, and epinephrine induces a net Mg(2+) extrusion that is lower than in the presence of extracellular Ca(2+) and quantitatively similar to that elicited by isoproterenol. In the absence of extracellular Na(+), no Mg(2+) is extruded from the liver irrespective of the agonist used. Similar results are observed in perfused livers stimulated by glucagon or 8-chloroadenosine 3', 5'-cyclic monophosphate. In the absence of extracellular Na(+) or Ca(2+), adrenergic-induced glucose extrusion from the liver is also markedly decreased. Together, these results indicate that liver cells extrude Mg(2+) primarily via a Na(+)-dependent mechanism. This extrusion pathway can be activated by the increase in cellular cAMP that follows the stimulation by glucagon or a specific beta-adrenergic receptor agonist or, alternatively, by the changes in cellular Ca(2+) induced by the stimulation of the alpha(1)-adrenoceptor. In addition, the stimulation of the alpha(1)-adrenoceptor appears to activate an auxiliary Ca(2+)-dependent Mg(2+) extrusion pathway. Finally, our data suggest that experimental conditions that affect Mg(2+) mobilization also interfere with glucose extrusion from liver cells.

Determination of epithelial magnesium transport with stable isotopes

The inability to adequately determine Mg(2+) flux rates with radiotracer studies has stymied our efforts to understand how magnesium is transported by epithelial cells. To evaluate epithelial Mg(2+) transport, a stable 25Mg isotope was used to measure magnesium uptake into normal and Mg(2+)-depleted Madin-Darby canine kidney (MDCK) cells. 25Mg entry rates were significantly increased in Mg(2+)-depleted cells relative to those cultured in normal magnesium media, 0.5 mM. 25Mg uptake was inhibited by external La(3+) but not Ca(2+) in both normal and Mg(2+)-depleted cells suggesting a specific entry pathway. These results with 25Mg were the same as with microfluorescence determinations using mag-fura-2. We have shown that Mg(2+) entry into epithelial cells reflects transepithelial transport; accordingly, increased Mg(2+) uptake in Mg(2+)-depleted cells provides an important intrinsic control of renal magnesium absorption. Furthermore, these studies indicate that cellular Mg(2+) transport may be quantitated with the use of stable isotopes that may be successfully applied to cells other than epithelia.

Differential localization and operation of distinct Mg(2+) transporters in apical and basolateral sides of rat liver plasma membrane

Upon activation of specific cell signaling, hepatocytes rapidly accumulate or release an amount of Mg(2+) equivalent to 10% of their total Mg(2+) content. Although it is widely accepted that Mg(2+) efflux is Na(+)-dependent, little is known about transporter identity and the overall regulation. Even less is known about the mechanism of cellular Mg(2+) uptake. Using sealed and right-sided rat liver plasma membrane vesicles representing either the basolateral (bLPM) or apical (aLPM) domain, it was possible to dissect three different Mg(2+) transport mechanisms based upon specific inhibition, localization within the plasma membrane, and directionality. The bLPM possesses only one Mg(2+) transporter, which is strictly Na(+)-dependent, bi-directional, and not inhibited by amiloride. The aLPM possesses two separate Mg(2+) transporters. One, similar to that in the bLPM because it strictly depends on Na(+) transport, and it can be differentiated from that of the bLPM because it is unidirectional and fully inhibited by amiloride. The second is a novel Ca(2+)/Mg(2+) exchanger that is unidirectional and inhibited by amiloride and imipramine. Hence, the bLPM transporter may be responsible for the exchange of Mg(2+) between hepatocytes and plasma, and vice versa, shown in livers upon specific metabolic stimulation, whereas the aLPM transporters can only extrude Mg(2+) into the biliary tract. The dissection of these three distinct pathways and, therefore, the opportunity to study each individually will greatly facilitate further characterization of these transporters and a better understanding of Mg(2+) homeostasis.

A novel DIDS-sensitive, anion-dependent Mg(2+) efflux pathway in rat ventricular myocytes

The aim of this study was to identify the existence of anion-dependent Mg transport systems in cardiac muscle. DIDS-sensitive and anion-dependent (either Cl(-)(o) or NO(-)(3o)) increases in [Mg(2+)](i) occurred during Mg(2+) loading conditions. Much larger elevations of [Mg(2+)](i) occurred under Cl(-)(o)-free conditions with 0.1 mmol l(-1) DIDS, compared to Cl(-)(o) replacement alone. All these effects were abolished in Mg(2+)(o)-free medium. These data suggest a novel Mg(2+)-anion symport for Mg(2+) efflux against the electrochemical gradient that is fueled mostly by the efflux of an endogenous anion (HCO(-)(3)?), but with a small contribution from intracellular Cl(-) probably supplied via the Cl(-)-HCO(-)(3) exchanger.