Intracellular and extracellular concentrations of Na+ modulate Mg2+ transport in rat ventricular myocytes

Quantitative 23Na magnetic resonance imaging in the abdomen at 3 T

OBJECTIVES

To assess the feasibility of sodium-23 MRI for performing quantitative and non-invasive measurements of total sodium concentration (TSC) and relaxation in a variety of abdominal organs.

MATERIALS AND METHODS

Proton and sodium imaging of the abdomen was performed in 19 healthy volunteers using a 3D cones sequence and a sodium-tuned 4-rung transmit/receive body coil on a clinical 3 T system. The effects of B1 non-uniformity on TSC measurements were corrected using the double-angle method. The long-component of 23Na T2* relaxation time was measured using a series of variable echo-times.

RESULTS

The mean and standard deviation of TSC and long-component 23Na T2* values were calculated across the healthy volunteer group in the kidneys, cerebrospinal fluid (CSF), liver, gallbladder, spleen, aorta, and inferior vena cava.

DISCUSSION

Mean TSC values in the kidneys, liver, and spleen were similar to those reported using 23Na-MRI previously in the literature. Measurements in the CSF and gallbladder were lower, potentially due to the reduced spatial resolution achievable in a clinically acceptable scan time. Mean long-component 23Na T2* values were consistent with previous reports from the kidneys and CSF. Intra-population standard error was larger in smaller, fluid-filled structures due to fluid motion and partial volume effects.

Plasma Membrane Channel TRPM4 Mediates Immunogenic Therapy-Induced Necrosis

Several emerging therapies kill cancer cells primarily by inducing necrosis. As necrosis activates immune cells, potentially, uncovering the molecular drivers of anticancer therapy-induced necrosis could reveal approaches for enhancing immunotherapy efficacy. To identify necrosis-associated genes, we performed a genome-wide CRISPR-Cas9 screen with negative selection against necrosis-inducing preclinical agents BHPI and conducted follow-on experiments with ErSO. The screen identified transient receptor potential melastatin member 4 (TRPM4), a calcium-activated, ATP-inhibited, sodium-selective plasma membrane channel. Cancer cells selected for resistance to BHPI and ErSO exhibited robust TRPM4 downregulation, and TRPM4 reexpression restored sensitivity to ErSO. Notably, TRPM4 knockout (TKO) abolished ErSO-induced regression of breast tumors in mice. Supporting a broad role for TRPM4 in necrosis, knockout of TRPM4 reversed cell death induced by four additional diverse necrosis-inducing cancer therapies. ErSO induced anticipatory unfolded protein response (a-UPR) hyperactivation, long-term necrotic cell death, and release of damage-associated molecular patterns that activated macrophages and increased monocyte migration, all of which was abolished by TKO. Furthermore, loss of TRPM4 suppressed the ErSO-induced increase in cell volume and depletion of ATP. These data suggest that ErSO triggers initial activation of the a-UPR but that it is TRPM4-mediated sodium influx and cell swelling, resulting in osmotic stress, which sustains and propagates lethal a-UPR hyperactivation. Thus, TRPM4 plays a pivotal role in sustaining lethal a-UPR hyperactivation that mediates the anticancer activity of diverse necrosis-inducing therapies.

SIGNIFICANCE

A genome-wide CRISPR screen reveals a pivotal role for TRPM4 in cell death and immune activation following treatment with diverse necrosis-inducing anticancer therapies, which could facilitate development of necrosis-based cancer immunotherapies.

Acute cytotoxicity of mineral fibres observed by time-lapse video microscopy

Inhalation of mineral fibres is associated with the onset of an inflammatory activity in the lungs and the pleura responsible for the development of fatal malignancies. It is known that cell damage is a necessary step for triggering the inflammatory response. However, the mechanisms by which mineral fibres exert cytotoxic activity are not fully understood. In this work, the kinetics of the early cytotoxicity mechanisms of three mineral fibres (i.e., chrysotile, crocidolite and fibrous erionite) classified as carcinogenic by the International Agency for Research on Cancer, was determined for the first time in a comparative manner using time-lapse video microscopy coupled with in vitro assays. All tests were performed using the THP-1 cell line, differentiated into M0 macrophages (M0-THP-1) and exposed for short times (8 h) to 25 μg/mL aliquots of chrysotile, crocidolite and fibrous erionite. The toxic action of fibrous erionite on M0-THP-1 cells is manifested since the early steps (2 h) of the experiment while the cytotoxicity of crocidolite and chrysotile gradually increases during the time span of the experiment. Chrysotile and crocidolite prompt cell death mainly via apoptosis, while erionite exposure is also probably associated to a necrotic-like effect. The potential mechanisms underlying these different toxicity behaviours are discussed in the light of the different morphological, and chemical-physical properties of the three fibres.

Computational Model for Membrane Transporters. Potential Implications for Cancer

To explain the increased transport of nutrients and metabolites and to control the movement of drug molecules through the transporters to the cancer cells, it is important to understand the exact mechanism of their structure and activity, as well as their biological and physical characteristics. We propose a computational model that reproduces the functionality of membrane transporters by quantifying the flow of substrates through the cell membrane. The model identifies the force induced by conformational changes of the transporter due to hydrolysis of ATP, in ABC transporters, or by an electrochemical gradient of ions, in secondary transporters. The transport rate is computed by averaging the velocity generated by the force along the paths followed by the substrates. The results obtained are in accordance with the experiments. The model provides an overall framework for analyzing the membrane transport proteins that regulate the flows of ions, nutrients and other molecules across the cell membranes, and their activities.

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.

TRPM7 channel activity in Jurkat T lymphocytes during magnesium depletion and loading: implications for divalent metal entry and cytotoxicity

TRPM7 is a cation channel-protein kinase highly expressed in T lymphocytes and other immune cells. It has been proposed to constitute a cellular entry pathway for Mg²⁺ and divalent metal cations such as Ca²⁺, Zn²⁺, Cd²⁺, Mn²⁺, and Ni²⁺. TRPM7 channels are inhibited by cytosolic Mg²⁺, rendering them largely inactive in intact cells. The dependence of channel activity on extracellular Mg²⁺ is less well studied. Here, we measured native TRPM7 channel activity in Jurkat T cells maintained in external Mg²⁺ concentrations varying between 400 nM and 1.4 mM for 1-3 days, obtaining an IC₅₀ value of 54 μM. Maintaining the cells in 400 nM or 8 μM [Mg²⁺]_o resulted in almost complete activation of TRPM7 in intact cells, due to cytosolic Mg²⁺ depletion. A total of 1.4 mM [Mg²⁺]_o was sufficient to fully eliminate the basal current. Submillimolar concentrations of amiloride prevented cellular Mg²⁺ depletion but not loading. We investigated whether the cytotoxicity of TRPM7 permeant metal ions Ni²⁺, Zn²⁺, Cd²⁺, Co²⁺, Mn²⁺, Sr²⁺, and Ba²⁺ requires TRPM7 channel activity. Mg²⁺ loading modestly reduced cytotoxicity of Zn²⁺, Co²⁺, Ni²⁺, and Mn²⁺ but not of Cd²⁺. Channel blocker NS8593 reduced Co²⁺ and Mn²⁺ but not Cd²⁺ or Zn²⁺ cytotoxicity and interfered with Mg²⁺ loading as evaluated by TRPM7 channel basal activity. Ba²⁺ and Sr²⁺ were neither detectably toxic nor permeant through the plasma membrane. These results indicate that in Jurkat T cells, entry of toxic divalent metal cations primarily occurs through pathways distinct from TRPM7. By contrast, we found evidence that Mg²⁺ entry requires TRPM7 channels.

Modulation of Mg2+ influx and cytoplasmic free Mg2+ concentration in rat ventricular myocytes

To examine whether TRPM7, a member of the melastatin family of transient receptor potential channels, is a physiological pathway for Mg2+ entry in mammalian cells, we studied the effect of TRPM7 regulators on cytoplasmic free Mg2+ concentration ([Mg2+]i) of rat ventricular myocytes. Acutely isolated single cells were AM-loaded with the fluorescent indicator furaptra, and [Mg2+]i was estimated at 25 °C. After [Mg2+]i was lowered by soaking the cells with a high-K+ and Mg2+-Ca2+-free solution, [Mg2+]i was recovered by extracellular perfusion of Ca2+-free Tyrode's solution that contained 1 mM Mg2+. The initial rate of increase in [Mg2+]i was analyzed as the Mg2+ influx rate. The Mg2+ influx rate was increased by the TRPM7 activator, naltriben (2-50 μM), in a concentration-dependent manner with a half maximal effective concentration (EC50) of 24 μM. This EC50 value is similar to that reported for the activation of recombinant TRPM7 overexpressed in HEK293 cells. Naltriben (50 μM) caused little change in basal [Mg2+]i (~ 0.9 mM) in Ca2+-free Tyrode's solution, but significantly raised [Mg2+]i to 1.31 ± 0.03 mM in 94 min after the removal of extracellular Na+. Re-introduction of extracellular Na+ lowered [Mg2+]i back to the basal level even in the presence of naltriben. Application of 10 μM NS8593, an inhibitor of TRPM7, significantly lowered [Mg2+]i to 0.72 ± 0.03 mM in 50-60 min independent of extracellular Na+. The results suggest that Mg2+ entry through TRPM7 significantly contributes to physiological Mg2+ homeostasis in mammalian heart cells.

Intermolecular G-Quadruplex Induces Hyaluronic Acid-DNA Superpolymers Causing Cancer Cell Swelling, Blebbing, and Death

Over the past decade, nanomedicine has gained considerable attraction through its relevance, for example, in "smart" delivery, thus creating platforms for novel treatments. Here, we report a natural polymer-DNA conjugate that undergoes self-assembly in a K⁺-dependent fashion to form a G-quadruplex (GQ) and generate superpolymeric structures. We derivatized a thiolated conjugate of the naturally occurring glycosaminoglycan polymer hyaluronic acid (HASH) with short G-rich DNA (HASH-DNA) that can form an intermolecular noncanonical GQ structure. Gel mobility shift assay and circular dichroism measurements confirmed HASH conjugation to DNA and K⁺-dependent GQ formation, respectively. Transmission electron microscopy and scanning electron microscopy results indicated that the addition of K⁺ to the HASH-DNA conjugate led to the formation of micron-range structures, whereas control samples remained unordered and as a nebulous globular form. Confocal microscopy of a fluorescently labeled form of the superpolymer verified increased cellular uptake. The HASH-DNA conjugates showed toxicity in HeLa cells, whereas a scrambled DNA (Mut) conjugate HASH-Mut showed no cytotoxicity, presumably because of nonformation of the superpolymeric structure. To understand the mechanism of cell death and if the superpolymeric structure is responsible for it, we monitored the cell size and observed an average of 23% increase in size compared to 4.5% in control cells at 4.5 h. We believe that cellular stress is generated presumably by the intracellular assembly of this large superpolymeric nanostructure causing cell blebbing with no exit option. This approach provides a new strategy of cellular delivery of a targeted naturally occurring polymer and a novel way to induce superpolymeric structure formation that acts as a therapeutic.

Physiological pathway of magnesium influx in rat ventricular myocytes

Cytoplasmic free Mg(2+) concentration ([Mg(2+)]i) was measured in rat ventricular myocytes with a fluorescent indicator furaptra (mag-fura-2) introduced by AM-loading. By incubation of the cells in a high-K(+) (Ca(2+)- and Mg(2+)-free) solution, [Mg(2+)]i decreased from ? 0.9 mM to 0.2 to 0.5 mM. The lowered [Mg(2+)]i was recovered by perfusion with Ca(2+)-free Tyrode's solution containing 1 mM Mg(2+). The time course of the [Mg(2+)]i recovery was fitted by a single exponential function, and the first derivative at time 0 was analyzed as being proportional to the initial Mg(2+) influx rate. The Mg(2+) influx rate was inversely related to [Mg(2+)]i, being higher at low [Mg(2+)]i. The Mg(2+) influx rate was augmented by the high extracellular Mg(2+) concentration (5 mM), whereas it was greatly reduced by cell membrane depolarization caused by high K(+). Known inhibitors of TRPM7 channels, 2-aminoethoxydiphenyl borate (2-APB), NS8593, and spermine reduced the Mg(2+) influx rate with half inhibitory concentrations (IC50) of, respectively, 17 ?M, 2.0 ?M, and 22 ?M. We also studied Ni(2+) influx by fluorescence quenching of intracellular furaptra by Ni(2+). The Ni(2+) influx was activated by lowering intra- and extracellular Mg(2+) concentrations, and it was inhibited by 2-APB and NS8593 with IC50 values comparable with those for the Mg(2+) influx. Intracellular alkalization (caused by pulse application of NH4Cl) enhanced, whereas intracellular acidification (induced after the removal of NH4Cl) slowed the Mg(2+) influx. Under the whole-cell patch-clamp configuration, the removal of intracellular and extracellular divalent cations induced large inward and outward currents, MIC (Mg-inhibited cation) currents or IMIC, carried by monovalent cations likely via TRPM7 channels. IMIC measured at -120 mV was diminished to ? 50% by 100 ?M 2-APB or 10 ?M NS8593. These results suggest that TRPM7/MIC channels serve as a major physiological pathway of Mg(2+) influx in rat ventricular myocytes.

SLC41 transporters--molecular identification and functional role

The solute carrier family 41 (SLC41) encompasses three members A1, A2, and A3. Based on their distant homology to the bacterial Mg²⁺ channel MgtE, all have been linked to Mg²⁺ transport. There is only very limited knowledge on the molecular biology and exact functions of SLC41A2 and SLC41A3. SLC41A1 is ubiquitously expressed and data on its functional and molecular properties, regulation, complex-forming ability, and spectrum of binding partners are available. SLC41A1 was recently identified as being the Na⁺/Mg²⁺ exchanger (NME)-a predominant Mg²⁺ efflux system. Mg²⁺-dependent and hormonal regulation of NME activity is now known to depend on the intracellular N terminus of SLC41A1 that is involved in Mg²⁺ sensing and contains phosphorylation sites for protein kinase (PK) A and PKC. Data showing a link between SLC41A1 and human disorders such as Parkinson's disease, nephronophthisis (induced by the null mutation c.698G>T in renal SLC41A1), and preeclampsia make the protein a candidate therapeutic target.

Solute Carrier Family SLC41, what do we really know about it?

The 41^st family of solute carriers (SLC41) comprises three members A1, A2 and A3, which are distantly homologous to bacterial Mg²⁺ channel MgtE. SLC41A1 was recently characterized as being an Na⁺/Mg²⁺ exchanger (NME; a predominant cellular Mg²⁺ efflux system). Little is known about the exact function of SLC41A2 and SLC41A3, although, these proteins have also been linked to Mg²⁺ transport in human (animal) cells. The molecular biology (including membrane topology, cellular localization, transcriptomics and proteomics) of SLC41A2 and SLC41A3 compared with SLC41A1 has only been poorly explored. Significantly more data with regard to function, functional regulation, involvement in cellular signalling, complex-forming ability, spectrum of binding partners and involvement in the pathophysiology of human diseases are available for SLC41A1. Three recent observations namely the identification of the null mutation, c.698G>T, in SLC41A1 underlying the nephronophthisis-like phenotype, the recognition of a putative link between SLC41A1 and Parkinson's disease, and the observation that nearly 55% of preeclamptic placental samples overexpress SLC41A1, marks the protein as a possible therapeutic target of these diseases. A potential role of the SLC41 family of Mg²⁺ transporters in the pathophysiology of human diseases is further substantiated by the finding that SLC41A3 knockout mice develop abnormal locomotor coordination.

Magnesium homeostasis in cardiac myocytes of Mg-deficient rats

To study possible modulation of Mg²⁺ transport in low Mg²⁺ conditions, we fed either a Mg-deficient diet or a Mg-containing diet (control) to Wistar rats for 1–6 weeks. Total Mg concentrations in serum and cardiac ventricular tissues were measured by atomic absorption spectroscopy. Intracellular free Mg²⁺ concentration ([Mg²⁺]_i) of ventricular myocytes was measured with the fluorescent indicator furaptra. Mg²⁺ transport rates, rates of Mg²⁺ influx and Mg²⁺ efflux, were estimated from the rates of change in [Mg²⁺]_i during Mg loading/depletion and recovery procedures. In Mg-deficient rats, the serum total Mg concentration (0.29±0.026 mM) was significantly lower than in control rats (0.86±0.072 mM) after 4–6 weeks of Mg deficiency. However, neither total Mg concentration in ventricular tissues nor [Mg²⁺]_i of ventricular myocytes was significantly different between Mg-deficient rats and control rats. The rates of Mg²⁺ influx and efflux were not significantly different in both groups. In addition, quantitative RT-PCR revealed that Mg deficiency did not substantially change mRNA expression levels of known Mg²⁺ channels/transporters (TRPM6, TRPM7, MagT1, SLC41A1 and ACDP2) in heart and kidney tissues. These results suggest that [Mg²⁺]_i as well as the total Mg content of cardiac myocytes, was well maintained even under chronic hypomagnesemia without persistent modulation in function and expression of major Mg²⁺ channels/transporters in the heart.

Human geneSLC41A1encodes for the Na+/Mg2+exchanger

Magnesium (Mg2+), the second most abundant divalent intracellular cation, is involved in the vast majority of intracellular processes, including the synthesis of nucleic acids, proteins, and energy metabolism. The concentration of intracellular free Mg2+([Mg2+]i) in mammalian cells is therefore tightly regulated to its optimum, mainly by an exchange of intracellular Mg2+for extracellular Na+. Despite the importance of this process for cellular Mg2+homeostasis, the gene(s) encoding for the functional Na+/Mg2+exchanger is (are) still unknown. Here, using the fluorescent probe mag-fura 2 to measure [Mg2+]ichanges, we examine Mg2+extrusion from hSLC41A1-overexpressing human embryonic kidney (HEK)-293 cells. A three- to fourfold elevation of [Mg2+]iwas accompanied by a five- to ninefold increase of Mg2+efflux. The latter was strictly dependent on extracellular Na+and reduced by 91% after complete replacement of Na+with N-methyl-d-glucamine. Imipramine and quinidine, known unspecific Na+/Mg2+exchanger inhibitors, led to a strong 88% to 100% inhibition of hSLC41A1-related Mg2+extrusion. In addition, our data show regulation of the transport activity via phosphorylation by cAMP-dependent protein kinase A. As these are the typical characteristics of a Na+/Mg2+exchanger, we conclude that the human SLC41A1 gene encodes for the Na+/Mg2+exchanger, the predominant Mg2+efflux system. Based on this finding, the analysis of Na+/Mg2+exchanger regulation and its involvement in the pathogenesis of diseases such as Parkinson's disease and hypertension at the molecular level should now be possible.

KB-R7943 inhibits Na+-dependent Mg2+ efflux in rat ventricular myocytes

Na(+)-dependent Mg(2+) efflux activity was studied with the fluorescent Mg(2+) indicator furaptra in the presence of various potential antagonists known to inhibit other transporters and channels. Among the compounds tested, KB-R7943, an inhibitor of Na(+)/Ca(2+) exchange, most potently inhibited the Na(+)/Mg(2+) exchange with half inhibitory concentrations (IC(50)) of 21 μM: (25°C) and 16 μM: (35°C). These IC(50) values were a factor of three to four lower than those of imipramine, a widely used inhibitor of Na(+)/Mg(2+) exchange. Apart from the inhibitory effect on Na(+)/Mg(2+) exchange, relatively high concentrations of KB-R7943 (100 μM: at 25°C and ≥20 μM: at 35°C), in combination with prolonged UV-illumination, caused cell shortening, probably because of the phototoxicity of the compound and the formation of rigor crossbridges. We conclude that KB-R7943 may be a useful tool to study cellular Mg(2+) homeostasis if care is taken to minimize its phototoxicity.

Metabolic inhibition strongly inhibits Na+-dependent Mg2+ efflux in rat ventricular myocytes

We measured intracellular Mg2+ concentration ([Mg2+]i) in rat ventricular myocytes using the fluorescent indicator furaptra (25 degrees C). In normally energized cells loaded with Mg2+, the introduction of extracellular Na+ induced a rapid decrease in [Mg2+]i: the initial rate of decrease in [Mg2+]i (initial Delta[Mg2+]i/Deltat) is thought to represent the rate of Na+-dependent Mg2+ efflux (putative Na+/Mg2+ exchange). To determine whether Mg2+ efflux depends directly on energy derived from cellular metabolism, in addition to the transmembrane Na+ gradient, we estimated the initial Delta[Mg2+]i/Deltat after metabolic inhibition. In the absence of extracellular Na+ and Ca2+, treatment of the cells with 1 microM carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, an uncoupler of mitochondria, caused a large increase in [Mg2+]i from approximately 0.9 mM to approximately 2.5 mM in a period of 5-8 min (probably because of breakdown of MgATP and release of Mg2+) and cell shortening to approximately 50% of the initial length (probably because of formation of rigor cross-bridges). Similar increases in [Mg2+]i and cell shortening were observed after application of 5 mM potassium cyanide (KCN) (an inhibitor of respiration) for > or = 90 min. The initial Delta[Mg2+]i/Deltat was diminished, on average, by 90% in carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone-treated cells and 92% in KCN-treated cells. When the cells were treated with 5 mM KCN for shorter times (59-85 min), a significant decrease in the initial Delta[Mg2+]i/Deltat (on average by 59%) was observed with only a slight shortening of the cell length. Intracellular Na+ concentration ([Na+]i) estimated with a Na+ indicator sodium-binding benzofuran isophthalate was, on average, 5.0-10.5 mM during the time required for the initial Delta[Mg2+]i/Deltat measurements, which is well below the [Na+]i level for half inhibition of the Mg2+ efflux (approximately 40 mM). Normalization of intracellular pH using 10 microM nigericin, a H+ ionophore, did not reverse the inhibition of the Mg2+ efflux. From these results, it seems likely that a decrease in ATP below the threshold of rigor cross-bridge formation (approximately 0.4 mM estimated indirectly in the this study), rather than elevation of [Na+]i or intracellular acidosis, inhibits the Mg2+ efflux, suggesting the absolute necessity of ATP for the Na+/Mg2+ exchange.

Basic fibroblast growth factor increases intracellular magnesium concentration through the specific signaling pathways

Basic fibroblast growth factor (bFGF) plays an important role in angiogenesis. However, the underlying mechanisms are not clear. Mg(2+) is the most abundant intracellular divalent cation in the body and plays critical roles in many cell functions. We investigated the effect of bFGF on the intracellular Mg(2+) concentration ([Mg(2+)](i)) in human umbilical vein endothelial cells (HUVECs). bFGF increased [Mg(2+)](i) in a dose-dependent manner, independent of extracellular Mg(2+). This bFGF-induced [Mg(2+)](i) increase was blocked by tyrosine kinase inhibitors (tyrphostin A-23 and genistein), phosphatidylinositol 3-kinase (PI3K) inhibitors (wortmannin and LY294002) and a phospholipase Cgamma (PLCgamma) inhibitor (U73122). In contrast, mitogen-activated protein kinase inhibitors (SB202190 and PD98059) did not affect the bFGF-induced [Mg(2+)](i) increase. These results suggest that bFGF increases the [Mg(2+)](i) from the intracellular Mg(2+) stores through the tyrosine kinase/PI3K/PLCgamma-dependent signaling pathways.

Effects of anoxia, aglycemia, and acidosis on cytosolic Mg2+, ATP, and pH in rat sensory neurons

Sensory neurons can detect ischemia and transmit pain from various organs. Whereas the primary stimulus in ischemia is assumed to be acidosis, little is known about how the inevitable metabolic challenge influences neuron function. In this study we have investigated the effects of anoxia, aglycemia, and acidosis upon intracellular Mg(2+) concentration [Mg(2+)](i) and intracellular pH (pH(i)) in isolated sensory neurons. Anoxia, anoxic aglycemia, and acidosis all caused a rise in [Mg(2+)](i) and a fall in pH(i). The rise in [Mg(2+)](i) in response to acidosis appears to be due to H(+) competing for intracellular Mg(2+) binding sites. The effects of anoxia and aglycemia were mimicked by metabolic inhibition and, in a dorsal root ganglia (DRG)-derived cell line, the rise in [Mg(2+)](i) during metabolic blockade was closely correlated with fall in intracellular ATP concentration ([ATP](i)). Increase in [Mg(2+)](i) during anoxia and aglycemia were therefore assumed to be due to MgATP hydrolysis. Even brief periods of anoxia (<3 min) resulted in rapid internal acidosis and a rise in [Mg(2+)](i) equivalent to a decline in MgATP levels of 15-20%. With more prolonged anoxia (20 min) MgATP depletion is estimated to be around 40%. With anoxic aglycemia, the [Mg(2+)](i) rise occurs in two phases: the first beginning almost immediately and the second after an 8- to 10-min delay. Within 20 min of anoxic aglycemia [Mg(2+)](i) was comparable to that observed following complete metabolic inhibition (dinitrophenol + 2-deoxyglucose, DNP + 2-DOG) indicating a near total loss of MgATP. The consequences of these events therefore need to be considered in the context of sensory neuron function in ischemia.

Quantification of Mg2+ extrusion and cytosolic Mg2+-buffering in Xenopus oocytes

Intracellular Mg(2+) buffering and Mg(2+) extrusion were investigated in Xenopus laevis oocytes. Mg(2+) or EDTA were pressure injected and the resulting changes in the intracellular Mg(2+) concentration were measured simultaneously with Mg(2+)-selective microelectrodes. In the presence of extracellular Na(+), injected Mg(2+) was extruded from the oocytes with an estimated v(max) and K(M) of 74 pmol cm(-2)s(-1) and 1.28 mM, respectively. To investigate genuine cytosolic Mg(2+) buffering, measurements were carried out in the nominal absence of extracellular Na(+) to block Mg(2+) extrusion, and during the application of CCCP (inhibiting mitochondrial uptake). Under these conditions, Mg(2+) buffering calculated after both MgCl(2) and EDTA injections could be described by a buffer equivalent with a concentration of 9.8mM and an apparent dissociation constant, K(d-app), of 0.6mM together with an [ATP](i) of 0.9 mM with a K(d-app) 0.12 mM. Xenopus oocytes thus possess highly efficient mechanisms to maintain their intracellular Mg(2+) concentration.

Loading rat heart myocytes with Mg2+ using low-[Na+] solutions

The objective of our study was to investigate how Mg2+ enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg2+ entry, and now provide a preliminary account of its properties. Changes in Mg2+ influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg2+ concentration ([fMg2+]i) measured from the fluorescence of mag-fura-2 loaded into isolated cells. Superfusion of myocytes at 37 degrees C with Ca2+-free solutions with both reduced [Na+] and raised [Mg2+] caused myocytes to load with Mg2+. Uptake was seen with solutions containing 5 mm Mg2+ and 95 mm Na+, and increased linearly with increasing extracellular [Mg2+] or decreasing extracellular [Na+]. It was very sensitive to temperature (Q(10) > 9, 25--37 degrees C), was observed even in myocytes with very low Na+ contents, and stopped abruptly when external [Na+] was returned to normal. Uptake was greatly reduced by imipramine or KB-R7943 if these were added when [fMg2+]i was close to the physiological level, but was unaffected if they were applied when [fMg2+]i was above 2 mm. Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K+] or voltage clamp to 0 mV. We suggest that initial Mg2+ uptake may involve several transporters, including reversed Na+-Mg2+ antiport and, depending on the exact conditions, reversed Na+-Ca2+ antiport. The ensuing rise of [fMg2+]i, in conjunction with reduced [Na+], may then activate a new Mg2+ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB-R7943, but is inactivated by depolarization.

Effects of intracellular and extracellular concentrations of Ca2+, K+, and Cl- on the Na+-dependent Mg2+ efflux in rat ventricular myocytes

Intracellular Mg2+ concentration ([Mg2+]i) was measured in rat ventricular myocytes with the fluorescent indicator furaptra (25 degrees C). After the myocytes were loaded with Mg2+, the initial rate of decrease in [Mg2+]i (initial Delta[Mg2+]i/Deltat) was estimated upon introduction of extracellular Na+, as an index of the rate of Na+-dependent Mg2+ efflux. The initial Delta[Mg2+]i/Deltat values with 140 mM [Na+]o were essentially unchanged by the addition of extracellular Ca2+ up to 1 mM (107.3+/-8.7% of the control value measured at 0 mM [Ca2+]o in the presence of 0.1 mM EGTA, n=5). Intracellular loading of a Ca2+ chelator, either BAPTA or dimethyl BAPTA, by incubation with its acetoxymethyl ester form (5 microM for 3.5 h) did not significantly change the initial Delta[Mg2+]i/Deltat: 115.2+/-7.5% (seven BAPTA-loaded cells) and 109.5+/-10.9% (four dimethyl BAPTA loaded cells) of the control values measured in the absence of an intracellular chelator. Extracellular and/or intracellular concentrations of K+ and Cl- were modified under constant [Na+]o (70 mM), [Ca2+]o (0 mM with 0.1 mM EGTA), and membrane potential (-13 mV with the amphotericin-B-perforated patch-clamp technique). None of the following conditions significantly changed the initial Delta[Mg2+]i/Deltat: 1), changes in [K+]o between 0 mM and 75 mM (65.6+/-5.0% (n=11) and 79.0+/-6.0% (n=8), respectively, of the control values measured at 140 mM [Na+]o without any modification of extracellular and intracellular K+ and Cl-); 2), intracellular perfusion with K+-free (Cs+-substituted) solution from the patch pipette in combination with removal of extracellular K+ (77.7+/-8.2%, n=8); and 3), extracellular and intracellular perfusion with K+-free and Cl--free solutions (71.6+/-5.1%, n=5). These results suggest that Mg2+ is transported in exchange with Na+, but not with Ca2+, K+, or Cl-, in cardiac myocytes.