Characterization of Na(+)-dependent Mg2+ efflux from Mg2(+)-loaded rat erythrocytes

How the kidney regulates magnesium: a modelling study

The kidneys are crucial for maintaining Mg2+ homeostasis. Along the proximal tubule and thick ascending limb, Mg2+ is reabsorbed paracellularly, while along the distal convoluted tubule (DCT), Mg2+ is reabsorbed transcellularly via transient receptor potential melastatin 6 (TRPM6). TRPM6 and other renal transporter expressions are regulated by sex hormones. To investigate renal Mg2 handling, we have developed sex-specific computational models of electrolyte transport along rat superficial nephron. Model simulations indicated that along the proximal tubule and thick ascending limb, Mg2+ and Na+ transport occur parallelly, but they are dissociated along the DCT. In addition, our models predicted higher paracellular Mg2+ permeability in females to attain similar cortical thick ascending limb fractional Mg2+ reabsorption in both sexes. Furthermore, DCT fractional Mg2+ reabsorption is higher in females than in males, allowing females to better fine-tune Mg2+ excretion. We validated our models by simulating the administration of three classes of diuretics. The model predicted significantly increased, marginally increased and significantly decreased Mg2+ excretions for loop, thiazide and K-sparing diuretics, respectively, aligning with experimental findings. The models can be used to conduct in silico studies on kidney adaptations to Mg2+ homeostasis alterations during conditions such as pregnancy, diabetes and chronic kidney disease.

Magnesium efflux from Drosophila Kenyon cells is critical for normal and diet-enhanced long-term memory

Dietary magnesium (Mg²⁺) supplementation can enhance memory in young and aged rats. Memory-enhancing capacity was largely ascribed to increases in hippocampal synaptic density and elevated expression of the NR2B subunit of the NMDA-type glutamate receptor. Here we show that Mg²⁺ feeding also enhances long-term memory in Drosophila. Normal and Mg²⁺-enhanced fly memory appears independent of NMDA receptors in the mushroom body and instead requires expression of a conserved CNNM-type Mg²⁺-efflux transporter encoded by the unextended (uex) gene. UEX contains a putative cyclic nucleotide-binding homology domain and its mutation separates a vital role for uex from a function in memory. Moreover, UEX localization in mushroom body Kenyon cells (KCs) is altered in memory-defective flies harboring mutations in cAMP-related genes. Functional imaging suggests that UEX-dependent efflux is required for slow rhythmic maintenance of KC Mg²⁺. We propose that regulated neuronal Mg²⁺ efflux is critical for normal and Mg²⁺-enhanced memory.

The proverbial saying ‘you are what you eat’ perfectly summarizes the concept that our diet can influence both our mental and physical health. We know that foods that are good for the heart, such as nuts, oily fish and berries, are also good for the brain. We know too that vitamins and minerals are essential for overall good health. But is there any evidence that increasing your intake of specific vitamins or minerals could help boost your brain power?

While it might sound almost too good to be true, there is some evidence that this is the case for at least one mineral, magnesium. Studies in rodents have shown that adding magnesium supplements to food improves how well the animals perform on memory tasks. Both young and old animals benefit from additional magnesium. Even elderly rodents with a condition similar to Alzheimer’s disease show less memory loss when given magnesium supplements. But what about other species?

Wu et al. now show that magnesium supplements also boost memory performance in fruit flies. One group of flies was fed with standard cornmeal for several days, while the other group received cornmeal supplemented with magnesium. Both groups were then trained to associate an odor with a food reward. Flies that had received the extra magnesium showed better memory for the odor when tested 24 hours after training.

Wu et al. show that magnesium improves memory in the flies via a different mechanism to that reported previously for rodents. In rodents, magnesium increased levels of a receptor protein for a brain chemical called glutamate. In fruit flies, by contrast, the memory boost depended on a protein that transports magnesium out of neurons. Mutant flies that lacked this transporter showed memory impairments. Unlike normal flies, those without the transporter showed no memory improvement after eating magnesium-enriched food. The results suggest that the transporter may help adjust magnesium levels inside brain cells in response to neural activity.

Humans produce four variants of this magnesium transporter, each encoded by a different gene. One of these transporters has already been implicated in brain development. The findings of Wu et al. suggest that the transporters may also act in the adult brain to influence cognition. Further studies are needed to test whether targeting the magnesium transporter could ultimately hold promise for treating memory impairments.

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 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.

Erythrocyte magnesium fluxes in mice with nutritionally and genetically low magnesium status

Low intracellular magnesium (Mg) contents may be observed in case of severe Mg insufficient intake or because of genetic regulation. This work was conducted to investigate the influence of intracellular Mg content on erythrocyte Mg(2+) influx and efflux in mice with low nutritionally and genetically (MGL and MGH mice) Mg status. C57BL6 mice were fed for 2 wks a diet containing 1000 mg Mg/kg diet Mg (control group), 100 mg Mg/kg diet (Mg-marginal group) or 30 mg Mg/kg diet (Mg deficient group), while mice with low (MGL) and high (MGH) Mg levels were fed a control diet for 2 wks. The quantification of erythrocyte Mg(2+) influx and efflux was performed using a stable isotope of Mg. Our results showed that erythrocyte Mg(2+) influx and efflux were respectively increased and decreased in nutritional Mg deficiency; while in genetically determined Mg status Mg(2+) fluxes were lower in MGL mice compared to MGH mice. Moreover Mg(2+) efflux was significantly correlated to Mg level in erythrocytes in all the mice studied (p < 0.001). In conclusion, erythrocyte Mg(2+) influx and efflux are modulated by low Mg status, namely decreased Mg(2+) efflux compensate for nutritional Mg deficiency, while the genetic regulation of erythrocyte Mg(2+) content depends on modification of Mg(2+) influx.

Sodium-dependent recovery of ionised magnesium concentration following magnesium load in rat heart myocytes

Our objectives were to investigate regulation of intracellular ionised Mg2+ concentration ([fMg2+]i) in cardiac muscle and cardiac Na+/Mg2+ antiport stoichiometry. [fMg2+]i was measured at 37 degrees C in isolated rat ventricular myocytes with mag-fura-2. Superfusion of myocytes with Na+ and Ca2+ free solutions containing 30 mM Mg2+ for 15 min more than doubled [fMg2+]i from its basal level (0.75 mM). Re-addition of Na+ caused [fMg2+]i to fall exponentially with time to basal level, the rate increasing linearly with [Na+]. Log(recovery rate) increased linearly with log([Na+]), the slope of 1.06 (95% confidence limits, 0.94-1.17) suggesting one Na+ ion is exchanged for each Mg2+. [fMg2+]i recovery was complete even if the membrane potential was depolarised to 0 mV or if superfusate [Mg2+] was increased to 3 mM. Recovery was rapid in normal Tyrode (0.3 min(-1)) with a Q10 of 2.2. It was completely inhibited by 200 microM imipramine but was unaffected by 20 microM KB-R7943 or 1 microM SEA0400, suggesting the Na+ /Ca2+ antiporter is not involved. Membrane depolarisation by increasing superfusate [K+] to 70 mM, or voltage clamp to 0 mV, increased recovery rate in Na+ containing solutions more than threefold. We conclude [fMg2+]i recovery is by Mg2+ efflux on a 1 Na+:1 Mg2+ antiport.

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

Apparent free cytoplasmic concentrations of Mg2+ ([Mg2+]i) and Na+ ([Na+]i) were estimated in rat ventricular myocytes using fluorescent indicators, furaptra (mag-fura-2) for Mg2+ and sodium-binding benzofuran isophthalate for Na+, at 25 degrees C in Ca2+-free conditions. Analysis included corrections for the influence of Na+ on furaptra fluorescence found in vitro and in vivo. The myocytes were loaded with Mg2+ in a solution containing 24 mM Mg2+ either in the presence of 106 mM Na+ plus 1 mM ouabain (Na+ loading) or in the presence of only 1.6 mM Na+ to deplete the cells of Na+ (Na+ depletion). The initial rate of decrease in [Mg2+]i from the Mg2+-loaded cells was estimated in the presence of 140 mM Na+ and 1 mM Mg2+ as an index of the rate of extracellular Na+-dependent Mg2+ efflux. Average [Na+]i, when estimated from sodium-binding benzofuran isophthalate fluorescence in separate experiments, increased from 12 to 31 mM and 47 mM after Na+ loading for 1 and 3 h, respectively, and decreased to approximately 0 mM after 3 h of Na+ depletion. The intracellular Na+ loading significantly reduced the initial rate of decrease in [Mg2+]i, on average, by 40% at 1 h and by 64% at 3 h, suggesting that the Mg2+ efflux was inhibited by intracellular Na+ with 50% inhibition at approximately 40 mM. A reduction of the rate of Mg2+ efflux was also observed when Na+ was introduced into the cells through the amphotericin B-perforated cell membrane (perforated patch-clamp technique) via a patch pipette that contained 130 mM Na+. When the cells were heavily loaded with Na+ with ouabain in combination with intracellular perfusion from the patch pipette containing 130 mM Na+, removal of extracellular Na+ caused an increase in [Mg2+]i, albeit at a very limited rate, which could be interpreted as reversal of the Mg2+ transport, i.e., Mg2+ influx driven by reversed Na+ gradient. Extracellular Na+ dependence of the rate of Mg2+ efflux revealed that the Mg2+ efflux was activated by extracellular Na+ with half-maximal activation at 55 mM. These results contribute to a quantitative characterization of the Na+-Mg2+ exchange in cardiac myocytes.

Regulation of Na+/Mg2+ antiport in rat erythrocytes

In rat erythrocytes, the regulation of Na+/Mg2+ antiport by protein kinases (PKs), protein phosphatases (PPs), intracellular Mg2+, ATP and Cl- was investigated. In untreated erythrocytes, Na+/Mg2+ antiport was slightly inhibited by the PK inhibitor staurosporine, slightly stimulated by the PP inhibitor calyculin A and strongly stimulated by vanadate. PMA stimulated Na+/Mg2+ antiport. This effect was completely inhibited by staurosporine and partially inhibited by the PKC inhibitors Ro-31-8425 and BIM I. Participation of other PKs such as PKA, the MAPK cascade, PTK, CK I, CK II, CAM II-K, PI 3-K, and MLCK was excluded by use of inhibitors. Na+/Mg2+ antiport in rat erythrocytes can thus be stimulated by PKCalpha. In non-Mg2+ -loaded erythrocytes, ATP depletion reduced Mg2+ efflux and PMA stimulation in NaCl medium. A drastic activation of Na+/Mg2+ antiport was induced by Mg2+ loading which was not further stimulated by PMA. Staurosporine, Ro-31-8425, BIM I and calyculin A did not inhibit Na+/Mg2+ antiport of Mg2+ -loaded cells. Obviously, at high [Mg2+]i Na+/Mg2+ antiport is maximally stimulated. PKCalpha or PPs are not involved in stimulation by intracellular Mg2+. ATP depletion of Mg2+ -loaded erythrocytes reduced Mg2+ efflux and the affinity of Mg2+ binding sites of the Na+/Mg2+ antiporter to Mg2+. In non-Mg2+ -loaded erythrocytes Na+/Mg2+ antiport essentially depends on Cl-. Mg2+ -loaded erythrocytes were less sensitive to the activation of Na+/Mg2+ antiport by [Cl-]i.

Magnesium metabolism in mice selected for high and low erythrocyte magnesium levels

A genetic control of blood magnesium (Mg) levels has been suggested. To investigate the mechanisms and the biologic significance of this genetic regulation, a mouse model, ie, mice selected for low magnesium level (MGL) and high magnesium level (MGH), was developed. The purpose of this study was to explore the Mg status and Mg metabolism in female MGL and MGH mice. We observed that MGL mice had reduced total and ionized plasma Mg, lower erythrocyte Mg, lower tibia, and kidney Mg levels. In contrast, total urinary Mg and (25)Mg levels were significantly higher in MGL mice. MGL mice had smaller total Mg exchangeable pool masses compared with MGH, and fractional transport rates of Mg (exchange constant) were different. In vitro (25)Mg enrichments in erythrocytes from MGL mice were significantly lower. Moreover, Mg efflux from erythrocytes was significantly higher in MGL. In conclusion, this work demonstrates that MGL mice present lower body stores of Mg than MGH mice and lower body Mg retention. This is confirmed at a cellular level by a lower enrichment of (25)Mg in erythrocytes. The lower retention of Mg by MGL erythrocyte in comparison to MGH appears to be partly due to a higher Mg efflux in MGL erythrocyte. It can be hypothesized that a genetic factor that modulates Na(+)/Mg(2+) exchanger activity may be important in the regulation of Mg metabolism. Further investigations on the mechanisms responsible for differences in Mg retention between MGL and MGH mice could contribute to a better understanding of the genetic regulation of cellular Mg.

Hormone-stimulated Mg(2+) accumulation into rat hepatocytes: a pathway for rapid Mg(2+) and Ca(2+) redistribution

Many diseases such as cardiac arrhythmia, diabetes, and chronic alcoholism are associated with a marked decrease of plasma and parenchymal Mg(2+), and Mg(2+) administration is routinely used therapeutically. This study uses isolated rat hepatocytes to ascertain if and under which conditions increases in extracellular Mg(2+) result in an increase in intracellular Mg(2+). In the absence of stimulation, changing extracellular Mg(2+) had no effect on total cellular Mg(2+) content. By contrast, carbachol or vasopressin administration promoted an accumulation of Mg(2+) that increased cellular Mg(2+) content by 13.2 and 11.8%, respectively, and stimulated Mg(2+) uptake was unaffected by the absence of extracellular Ca(2+). Mg(2+) efflux resulting from stimulation of alpha- or beta-adrenergic receptors operated with a Mg(2+):Ca(2+) exchange ratio of 1. These data indicate that cellular Mg(2+) uptake can occur rapidly and in large amounts, through a process distinct from Mg(2+) release, but operating only upon specific hormonal stimulation.

Angiotensin II type I receptor modulates intracellular free Mg2+ in renally derived cells via Na+-dependent Ca2+-independent mechanisms

Treatment of Madin-Darby canine kidney (MDCK) cells with the peptide hormone angiotensin II (Ang II) results in an increase in the concentrations of cytosolic free calcium ([Ca(2+)](i)) and sodium ([Na(+)](i)) with a concomitant decrease in cytosolic free Mg(2+) concentration ([Mg(2+)](i)). In the present study we demonstrate that this hormone-induced decrease in [Mg(2+)](i) is independent of [Ca(2+)](i) but dependent on extracellular Na(+). [Mg(2+)](i), [Ca(2+)](i), and [Na(+)](i) were measured in Ang II-stimulated MDCK cells by fluorescence digital imaging using the selective fluoroprobes mag-fura-2AM, fura-2AM, and sodium-binding benzofuran isophthalate (acetoxymethyl ester), respectively. Ang II decreased [Mg(2+)](i) and increased [Na(+)](i) in a dose-dependent manner. These effects were inhibited by irbesartan (selective AT(1) receptor blocker) but not by PD123319 (selective AT(2) receptor blocker). Imipramine and quinidine (putative inhibitors of the Na(+)/Mg(2+) exchanger) and removal of extracellular Na(+) abrogated Ang II-mediated [Mg(2+)](i) effects. In cells pretreated with thapsigargin (reticular Ca(2+)-ATPase inhibitor), Ang II-stimulated [Ca(2+)](i) transients were attenuated (p < 0.01), whereas agonist-induced [Mg(2+)](i) responses were unchanged. Clamping the [Ca(2+)](i) near 50 nmol/liter with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) inhibited Ang II-induced [Ca(2+)](i) increases but failed to alter Ang II-induced [Mg(2+)](i) responses. Benzamil, a selective blocker of the Na(+)/Ca(2+) exchanger, inhibited [Na(+)](i) but not [Mg(2+)](i) responses. Our data demonstrate that in MDCK cells, AT(1) receptors modulate [Mg(2+)](i) via a Na(+)-dependent Mg(2+) transporter that is not directly related to [Ca(2+)](i). These data support the notion that rapid modulation of [Mg(2+)](i) is not simply a result of Mg(2+) redistribution from intracellular buffering sites by Ca(2+) and provide evidence for the existence of a Na(+)-dependent, hormonally regulated transporter for Mg(2+) in renally derived cells.

Intralymphocyte free magnesium in patients with primary aldosteronism: aldosterone and lymphocyte magnesium homeostasis

It is known that hyperaldosteronism has been associated with magnesium deficiency, yet there are no data on the intracellular concentration of ionized magnesium ([Mg(2+)(i)]) in subjects with primary aldosteronism (PA). We measured intralymphocyte free magnesium ([Mg(2+)(i)]) and intralymphocyte free calcium ([Ca(2+)(i)]) in 16 patients with PA and 26 normotensive control subjects (NCs). [Mg(2+)(i)] and [Ca(2+)(i)] were also measured in blood lymphocytes incubated in vitro with aldosterone, according to a fluorimetric method. In subjects with PA, [Mg(2+)(i)] was significantly lower than that in NCs (mean+/-SD; PA 203+/-56 micromol/L, NCs 291+/-43 micromol/L, 95% confidence interval 57 to 119, P=0.001). In the patients, [Ca(2+)(i)] did not prove to be statistically different from that of NCs (mean+/-SD; PA 47.2+/-10.6 nmol/L, NCs 53.2+/-11 nmol/L). The lymphocytes exposed to the action of aldosterone showed a significant reduction in [Mg(2+)(i)] (n=15, NCs 271+/-28 micromol/L, aldosterone treatment 188+/-39 micromol/L, P=0.001, 95% confidence interval 57 to 108). The dose-effect curve of aldosterone on [Mg(2+)(i)] showed an EC(50) value of approximately 0.5 to 1 nmol/L aldosterone. The reduction in [Mg(2+)(i)] mediated by aldosterone is antagonized by the receptor inhibitor of aldosterone; it is inhibited by inhibitors of protein synthesis and is not measurable when the lymphocytes are incubated in an Na(+)-free medium. The data are consistent with the hypothesis that aldosterone affects the cellular homeostasis of magnesium, probably through modification of the activity of the Na(+)-Mg(2+) antiporter.

Characterization of Mg(2+) efflux from rat erythrocytes non-loaded with Mg(2+)

Non-Mg(2+)-loaded rat erythrocytes with a physiological level of Mg(2+)(i) exhibited Mg(2+) efflux when incubated in nominally Mg(2+)-free media. Two types of Mg(2+) efflux were shown: (1) An Na(+)-dependent Mg(2+) efflux in NaCl and Na gluconate medium, which was inhibited by amiloride and quinidine, as was Na(2+)/Mg(2+) antiport in Mg(2+)-loaded rat erythrocytes; and (2) an Na(+)-independent Mg(2+) efflux in sucrose medium and choline Cl medium, which may be differentiated into SITS-sensitive Mg(2+) efflux at low Cl(-)(o) (in sucrose) and into SITS-insensitive Mg(2+) efflux at high Cl(-)(o) (in 150 mmol/l choline Cl).

Extracellular Mg2+ inhibits both histamine-stimulated Ca(2+)-signaling and exocytosis in human tracheal secretory gland cells

The effects of extracellular Mg2+ concentration have been investigated on the histamine-stimulated exocytotic process of human tracheal secretory gland (HTG) cells. The exocytosis of secretory granules (SG) was observed concomitantly with dynamic changes of intracellular Ca2+ ([Ca2+]i) and Mg2+ concentrations ([Mg2+]i). The rate of SG exocytosis was appraised by the decrease of quinacrine fluorescence emission. Dynamic changes of [Mg2+]i and [Ca2+]i in HTG cells were determined by the combined use of UV-microspectrofluorometry with Mag-Indo-1 and Indo-1 probes, respectively. High Mg2+ medium significantly inhibited the histamine-stimulated secretion. The influence of the extracellular and intracellular Mg2+ concentrations on [Ca2+]i was analyzed. Basal [Mg2+]i increased from 0.8 mM in a Mg(2+)-free medium to 1.7 mM in 10 mM Mg2+ medium. Histamine induced a [Mg2+]i increase which is dependent on extracellular Mg2+ concentration. The histamine stimulated [Ca2+]i rise was reduced in the presence of elevated Mg2+ extracellular medium and inhibitory effects of extracellular Mg2+ were concomitant with changes in [Mg2+]i. Our data suggest that the inhibition by extracellular Mg2+ of stimulated exocytosis is dependent on both the increase of [Mg2+]i and the inhibition of cytosolic Ca2+ influx.

Na+-dependent release of Mg2+ from an intracellular pool in rat sublingual mucous acini

Muscarinic stimulation induces release of Mg2+ from an intracellular pool in rat sublingual mucous acini (Zhang, G. H., and Melvin, J. E. (1992) J. Biol. Chem. 267, 20721-20727). In the present study we examined the interdependence of Mg2+ mobilization on intracellular Na+ and Ca2+ by monitoring the intracellular free concentrations of Na+ ([Na+]i), Mg2+ ([Mg2+]i), and Ca2+ ([Ca2+]i) using ion-sensitive fluorescent indicators. Gramicidin increased the intracellular concentrations of all three ions. Comparable to agonist-stimulated mobilization of Mg2+, the gramicidin-induced [Mg2+]i increase was independent of extracellular Mg2+ indicating release of Mg2+ from an intracellular pool. Clamping the [Ca2+]i near 30 nM with the Ca2+-selective chelator BAPTA failed to alter the [Na+]i or [Mg2+]i increases generated by gramicidin. In contrast, depletion of intracellular Na+ markedly suppressed the muscarinic-stimulated [Mg2+]i increase, whereas the [Ca2+]i increase was similar to that seen in physiological extracellular Na+. These results revealed that intracellular Mg2+ mobilization did not directly relate to the [Ca2+]i, but required an increase in [Na+]i. Consistent with this hypothesis, increasing [Na+]i by activating Na+ influx via the Na+/H+ exchanger also increased the [Mg2+]i. The Na+/Mg2+ exchange inhibitor quinidine suppressed both the gramicidin- and muscarinic-induced discharge of internal Mg2+. These results suggest that release of Mg2+ from an intracellular pool is mediated by a Na+-dependent Mg2+ transport mechanism in salivary acinar cells.

Magnesium transport in magnesium-loaded ferret red blood cells

Mg efflux from ferret red blood cells is stimulated when cells are Mg loaded, but the properties of efflux depend on the loading method. When cell Mg content is altered using A23187, which is subsequently washed away, Mg efflux is minimal until intracellular ionized [Mg] ([Mg2+]i) is greater than 0.9 mM, it then increases substantially with [Mg2+]i. Efflux from loaded cells falls as external [Na] ([Na]o) is reduced, and net Mg influx (against an electrochemical gradient) is seen when [Na]o is sufficiently low. Both influx and efflux are amiloride sensitive. Mg influx from media containing a normal or low [Na] is not affected by reducing [Mg2+]i to very low levels. When cells are Mg loaded by incubating them in media containing 5 mM Na and Mg, Mg efflux is again minimal until [Mg2+]i is greater than 0.9 mM and then it increases with [Mg2+]i, but at a rate approximately 4 times faster than in cells loaded using A23187. This efflux is little affected by 1 mM amiloride. Thus Mg-loading using A23187 reveals the [Mg2+]i dependence of a transporter which is amiloride sensitive, reversible and can operate against an electrochemical gradient, consistent with Na-Mg antiport. Loading by incubation in low-[Na] media activates a high-capacity Mg transporter which obscures the antiporter.

Iron transport into erythroid cells by the Na+/Mg2+ antiport

Rabbit erythroid cells can take up non-transferrin-bound iron by a high-affinity and a low-affinity transport mechanism (Hodgson et al. (1995) J. Cell. Physiol. 162, 181-190). The latter process, which is present in mature erythrocytes as well as reticulocytes, was investigated in this study using rabbit reticulocytes and erythrocytes. Iron uptake was optimal in isotonic KCI (pH 7.0), was shown to be much greater for Fe(II) than Fe(III), to be saturable with a Km value of approx. 15 microM Fe(II), temperature-dependent and inhibited by inhibitors of cell energy metabolism, by Na+ and many divalent cations and by several known inhibitors of membrane cation transport mechanisms. Uptake was more rapid with rabbit than with rat or human erythrocytes. The Fe(II) transport process was much more sensitive to inhibition by Mg2+ than by Ca2+ and the inhibition by both Mg2+ and Na+ was of competitive type. Cells depleted of intracellular Mg2+ by the use of the ionophore A23187 had low rates of Fe(II) uptake. High rates of uptake could be achieved by replenishment of intracellular Mg2+, and the Mg(2+)-dependent uptake of Fe(II) was inhibited by the same reagents which reduced the uptake by untreated cells. Many features of the Fe(II) transport process are very similar to those of the previously described Na+/Mg2+ antiport. These features, plus the stimulation of Fe(II) uptake by intracellular Mg2+ and inhibition by extracellular Mg2+ or Na+, strongly suggest that the iron is transported into the cells by the antiport.

Sodium-magnesium antiport in Retzius neurones of the leech Hirudo medicinalis

1. Intracellular free magnesium ([Mg2+]i) and sodium ([Na+]i) concentrations were measured in Retzius neurones of the leech Hirudo medicinalis using ion-sensitive microelectrodes. 2. The mean steady-state values for [Mg2+]i and [Na+]i were 0.46 mM (pMg, 3.34 +/- 0.23; range, 0.1-1.2 mM; n = 32) and 8.95 mM (pNa, 2.05 +/- 0.15; range, 5.1-15.5 mM, n = 21), respectively, at a mean membrane potential (Em) of -35.6 +/- 6.1 mV (n = 32). Thus, [Mg2+]i is far below the value calculated for a passive distribution (16.9 mM) but close to the equilibrium value calculated for a hypothetical 1 Na(+)-1 Mg2+ antiport (0.41 mM). 3. Simultaneous measurements of [Mg2+]i, [Na+]i and Em in Retzius neurones showed that an increase in the extracellular Mg2+ concentration ([Mg2+]o) resulted in an increase in [Mg2+]i, a parallel decrease in [Na+]i and a membrane depolarization, while a decrease in [Mg2+]o had opposite effects. These results are compatible with calculations based on a 1 Na(+)-1 Mg2+ antiport. 4. Na+ efflux at high [Mg2+]o still occurred when the Na(+)-K+ pump was inhibited by the application of ouabain or in K(+)-free solutions. This efflux was blocked by amiloride. 5. In the absence of extracellular Na+ ([Na+]o), no Mg2+ influx occurred. Mg2+ influx at high [Mg2+]o was even lower than in the presence of [Na+]o. Mg2+ efflux was blocked in the absence of [Na+]o. 6. The rate of Mg2+ extrusion was reduced by lowering [Na+]o, even if the Na+ gradient across the membrane remained almost unchanged. 7. Mg2+ efflux was blocked by amiloride (half-maximal effect at 0.25 mM amiloride; Hill coefficient, 1.3) but not by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA). 8. No changes in intracellular Ca2+ and pH (pHi) could be detected when [Mg2+]o was varied between 1 and 30 mM. 9. Changing pHi by up to 0.4 pH units had no effect on [Mg2+]i. 10. The results suggest the presence of an electrogenic 1 Na(+)-1 Mg2+ antiport in leech Retzius neurones. This antiport can be reversed and is inhibited by low extracellular and/or intracellular Na+ and by amiloride.

The role of magnesium in regulating CCK-8-evoked secretory responses in the exocrine rat pancreas

This study investigates the effect of magnesium (Mg2+) on the secretory responses and the mobilization of calcium (Ca2+) and Mg2+ evoked by cholecystokinin-octapeptide (CCK-8) in the exocrine rat pancreas. In the isolated intact perfused pancreas CCK-8 (10(-10) M) produced marked increases in juice flow and total protein output in zero and normal (1.1 mM) extracellular Mg2+ [Mg2+]o compared to a much reduced secretory response in elevated (5 mM and 10 mM) [Mg2+]o. Similar effects of perturbation of [Mg2+]o on amylase secretion and 45 Ca2+ uptake (influx) were obtained in isolated pancreatic segments. In pancreatic acinar cells loaded with the fluorescent bioprobe fura-2 acetomethylester (AM), CCK-8 evoked marked increases in cytosolic free Ca2+ concentration [Ca2+]i in zero and normal [Mg2+]o compared to a much reduced response in elevated [Mg2+]o. Pretreatment of acinar cells with either dibutyryl cyclic AMP (DB2 cAMP) or forskolin had no effect on the CCK-8 induced changes in [Ca2+]i. In magfura-2-loaded acinar cells CCK-8 (10(-8) M) stimulated an initial transient rise in intracellular free Mg2+ concentration [Mg2+]i followed by a more prolonged and sustained decrease. This response was abolished when sodium (Na+) was replaced with N-methyl-D-glucamine (NMDG). Incubation of acinar cells with 10 mM Mg2+ resulted in an elevation in [Mg2+]i. Upon stimulation with CCK-8, [Mg2+]i decreased only slightly compared with the response obtained in normal [Mg2+]o. CCK-8 caused a net efflux of Mg2+ in pancreatic segments; this effect was abolished when extracellular sodium [Na+]o was replaced with either NMDG or choline. The results indicate that Mg2+ can regulate CCK-8-evoked secretory responses in the exocrine pancreas possibly via Ca2+ mobilization. Moreover, the movement of Mg2+ in pancreatic acinar cells is dependent upon extracellular Na+.

Differential regulation of circulating Mg2+ in the rat by beta 1- and beta 2-adrenergic receptor stimulation

Extracellular Mg2+ homeostasis was studied in vivo in the anesthetized rat. Animals were infused with isoproterenol (ISO) for 10 minutes, and serum Mg2+ was measured after the infusion and then 10 and 20 minutes later. A dose-dependent increase in circulating Mg2+ was observed in animals infused with ISO at a rate of 0.1 microgram.kg-1.min-1 or higher. The time course of the effect demonstrated that circulating Mg2+ continued to increase 20 minutes after the end of the ISO infusion. A predicted maximal increase in serum Mg2+ concentration of 19.3% was derived with a predicted EC50 of 0.08 microgram.kg-1.min-1. The maximal percent increase corresponded to a net increase of 6.7 mumol/300 g body wt. Because infusion of ISO resulted in changes in hemodynamic parameters, most notably a drop in blood pressure, a group of animals was infused with nitroprusside to mimic the hypotensive response via a nonadrenergic mechanism. Under these conditions, there was a transient increase in circulating Mg2+ that was largely inhibited by propranolol, indicating that hypotension per se was not responsible for the mobilization of Mg2+. Infusion of salbutamol, but not prenalterol, also induced an increase in circulating Mg2+. Pretreatment with butoxamine, ICI-118551, or propranolol prevented the ISO-induced increase in serum Mg2+. Pretreatment with atenolol minimally affected the ISO-induced changes in circulating Mg2+. Pretreatment with CGP-20271A actually enhanced the ISO-induced increase in circulating Mg2+. This evidence demonstrates the existence of a pool of Mg2+ that is mobilized into the circulation in response to selective beta 2-adrenergic stimulation.

Regulation by extracellular Na+ of cytosolic Mg2+ concentration in Mg(2+)-loaded rat sublingual acini

The regulation of cytosolic free Mg2+ concentration ([Mg2+]i) in Mg(2+)-loaded rat sublingual mucous acini was examined using the Mg(2+)-sensitive fluorescent indicator mag-fura-2. Loading sublingual acini with 5 mM Mg2+ elevated the [Mg2+]i from 0.35 +/- 0.01 mM to 0.66 +/- 0.01 mM. Removal of extracellular Mg2+ resulted in a significantly faster [Mg2+]i decrease in Mg(2+)-loaded acini than in unloaded acini. Membrane depolarization with high extracellular [K+] and inhibition of P-type ATPases by vanadate did not alter the [Mg2+]i decrease, indicating that the Mg2+ efflux mechanism is not electrogenic. Na(+)-free medium inhibited 80% of the [Mg2+]i decrease suggesting that a Na(+)-dependent Mg2+ efflux pathway mediates the [Mg2+]i decrease. Accordingly, the Na(+)-dependent antiport inhibitor quinidine reduced > 80% of the [Mg2+]i decrease, suggesting that the Na(+)-dependent Mg2+ efflux is mediated by the Na+/Mg2+ antiport system. Mg2+ efflux was also partly driven by K+. The [Mg2+]i decreased was significantly inhibited by carbachol, a muscarinic agonist, but not by cAMP. These results indicate that in sublingual acinar cells a Na(+)-dependent pathway mediates Mg2+ efflux and that muscarinic stimulation may regulate Mg2+ extrusion.

Second messenger role of magnesium in pancreatic acinar cells of the rat

Application of either acetylcholine (ACh, 10(-5) M) or cholecystokinin-octapeptide (CCK-8, 10(-8) M) to the isolated rat pancreas elicited large increases in amylase secretion, radiolabelled 45Ca2+ influx and cytosolic free calcium [Ca2+]i levels in zero and normal (1.1 mM) extracellular magnesium [Mg2+]o. Elevated [Mg2+]o significantly (p < 0.001) reduced the secretagogue-evoked secretory responses and Ca2+ mobilisation. Stimulation of pancreatic segments with either ACh (10(-5) and 10(-6) M) or CCK-8 (10(-8) and 10(-10) M) resulted in marked elevation in Mg2+ concentration in effluent samples (net efflux). On removal of either ACh or CCK-8, Mg2+ concentration returned to resting level. In pancreatic acinar cells loaded the fluorescent dye magfura, ACh and CCK-8 evoked marked reduction in cytosolic free Mg2+ concentration [Mg2+]i compared to the resting value of 0.82 +/- 0.03 mM (n = 50) in normal medium in the absence of secretagogues. In elevated [Mg2+]o (10 mM) medium, [Mg2+]i rises to 0.98 +/- 0.04 mM (n = 6). Addition of CCK-8 led to only a small reduction in [Mg2+]i in elevated [Mg2+]o. In Mg2+ loaded pancreatic acinar cells, Mg2+ is released in a time dependent manner and this efflux of Mg2+ was sensitive to sodium, extracellular amiloride (1 mM), dinitrophenol (10 mM) and lidocaine (1 mM). The results indicate that Mg2+ is acting as an intracellular messenger to regulate the mobilisation of Ca2+ which in turn mediates enzyme secretion.

Reversibility of Na+/Mg2+ antiport in rat erythrocytes

Rat erythrocytes loaded with Mg2+ plus Na+ performed Mg2+ uptake under an intracellular/extracellular Na+ gradient. Mg2+ uptake was coupled to Na+ release at a stoichiometric ratio of 1 Mg2+/2 Na+.Mg2+ uptake was inhibited by amiloride, imipramine and quinidine. Mn2+ was taken up by the same transporter as Mg2+. Similar results had been found for net Mg2+ efflux via Na+/Mg2+ antiport in such rat erythrocytes. Hence, it can be concluded that Na+/Mg2+ antiport in Mg(2+)-loaded rat erythrocytes operates reversibly according to the direction of the Na+ gradient which is a contributing driving force. Net Mg2+ influx was dependent on ATP which increased the affinity of intracellular Mg2+ by activating Na+/Mg2+ antiport. Mg2+ uptake was increased by phorbol ester and inhibited by staurosporine, indicating that ATP may function via protein phosphorylation by protein kinase C.

Effect of oxygen free radicals on Mg2+ efflux from erythrocytes

Hydrogen peroxide destroyed the Na+/Mg2+ antiport in Mg(2+)-loaded human and rat erythrocytes and increased the leakage of intracellular Mg2+ and K+. These effects are opposite to the increase of Na+/Mg2+ antiport and unchanged Na(+)-independent Mg2+ efflux from erythrocytes of patients with cystic fibrosis score 3. Thus, the increase of Na+/Mg2+ antiport in these patients is not caused by increased formation of free radicals.

Na(+)- and anion-dependent Mg2+ influx in isolated hepatocytes

Hepatocytes, which were Mg(2+)-depleted during isolation, took up Mg2+ during reincubation. Mg2+ uptake was dependent on the concentration of extracellular Mg2+, Na+, Cl-, bicarbonate and phosphate. Li+ and choline+ did not substitute for extracellular Na+ in Mg2+ influx. Mg2+ influx was maximal when all three anion species were present, and did not occur when these anions were replaced by gluconate. Bicarbonate, phosphate and Cl- could substitute for each other. Mg2+ uptake in hepatocytes was inhibited by p-chloromercuribenzene sulfonate, ouabain, gramicidin D, amiloride and verapamil. The results were explained by the assumption that net Mg2+ influx in hepatocytes is operating via electroneutral Na+, Mg2+/anion cotransport driven by the Na+ gradient. However, electrogenic Mg2+ uptake gated by extracellular Na+ and anions could not be excluded.

Asymmetry of the magnesium sodium exchange across the human red cell membrane

(1) Net Mg2+ inflow into human red cells through the Mg(2+)-Na+ exchange system is slower, for a given driving force defined by the ionic gradients and Em, than outflow for a similar and opposite force. This is not incompatible with an asymmetric, equilibrating exchange carrier. (2) However, the finding that near zero force the rate of outflow does not tend towards zero implies an active component, i.e., direct input of metabolic energy in addition to the energy provided via the Na(+)-concentration gradient by the Na+/K(+)-pump.

Cold activation of Na influx through the Na-H exchange pathway in guinea pig red cells

Previous work showed that amiloride partially inhibits the net gain of Na in cold-stored red cells of guinea pig and that the proportion of unidirectional Na influx sensitive to amiloride increases dramatically with cooling. This study shows that at 37 degrees C amiloride-sensitive (AS) Na influx in guinea pig red blood cells is activated by cytoplasmic H+, hypertonic incubation, phorbol ester in the presence of extracellular Ca2+ and is correlated with cation-dependent H+ loss from acidified cells. Cytoplasmic acidification increases AS Na efflux into Na-free medium. These properties are consistent with the presence of a Na-H exchanger with a H+ regulatory site. Elevation of cytoplasmic free Mg2+ above 3 mM greatly increases AS Na influx: this correlates with a Na-dependent loss of Mg2+, indicating the presence of a Na-Mg exchanger. At 20 degrees C activators of Na-H exchange have little or no further stimulatory effect on the already elevated AS Na influx. AS Na influx is much larger than either Na-dependent H+ loss or AS Na efflux at 20 degrees C. The affinity of the AS Na influx for cytoplasmic H+ is greater at 20 degrees C than at 37 degrees C. Depletion of cytoplasmic Mg2+ does not abolish the high AS Na influx at 20 degrees C. Thus, elevation of AS Na influx with cooling appears to be due to increased activity of a Na-H exchanger (operating in a "slippage" mode) caused by greater sensitivity to H+ at a regulatory site.

Cellular mechanisms of vasopressin and endothelin to mobilize [Mg2+]i in vascular smooth muscle cells

The present study was undertaken to examine the effects of arginine vasopressin (AVP) and endothelin-1 (ET-1) on cytosolic free Mg2+ ([Mg2+]i) in cultured rat vascular smooth muscle cells (VSMC). [Mg2+]i was measured using the fluorescence indicator dye mag-fura-2. AVP and ET-1 at a concentration of 1 x 10(-9) M or higher induced the mobilization of [Mg2+]i and cytosolic free Ca2+ ([Ca2+]i) in a dose-dependent manner in rat VSMC. Atrial natriuretic peptide and sodium nitroprusside producing cellular guanosine 3',5'-cyclic monophosphate did not affect [Mg2+]i and [Ca2+]i. A diterpene activator of adenylate cyclase, forskolin, also did not alter [Mg2+]i and [Ca2+]i. The removal of extracellular Mg2+ enhanced the AVP-mobilized [Ca2+]i and did not change the AVP-mobilized [Mg2+]i. The Ca(2+)-free and nominally Mg2+/Ca(2+)-free states decreased the AVP-mobilized [Mg2+]i and [Ca2+]i. The Na(+)-free state enhanced the sustained, but not peak, level of the AVP-mobilized [Mg2+]i. These results indicate that AVP and ET-1 mobilize [Mg2+]i mediated through their intracellular second messenger [Ca2+]i and independent of extracellular Mg2+. Also, an increase in [Mg2+]i is indicated to stimulate the Na(+)-Mg2+ exchange to increase cellular Mg2+ efflux.

Isoproterenol-induced Mg2+ uptake in liver

Isoproterenol increased the Mg2+ content of hepatocytes after injection into rats or after addition to collagenase-dispersed hepatocytes. cAMP also the increased cellular Mg2+ content of isolated hepatocytes. This effect was prevented by staurosporine. Phorbol ester had no effect on the Mg2+ content of isolated hepatocytes, and after injection of isoproterenol into rats, protein kinase C of liver was not affected. It was concluded that isoproterenol induced long-term Mg2+ influx via the activation of protein kinase A which can be inhibited by staurosporine.

Diversities of transport of sodium in rodent red cells

1. Na-K pumps of rodent red cells reveal variations among species in terms of kinetic properties such as ouabain sensitivity, Na/K coupling and temperature sensitivity and variations within an individual organism related to such physiological challenges as K deficiency, calorie deficiency and seasonal changes in temperature. 2. Passive Na entry among rodents collectively occurs through the same routes as in red cells of other mammals, but red cells of hamsters, rats and thirteen-lined ground squirrels lack or are deficient in an amiloride-sensitive, shrinkage-activated Na-H exchange. 3. In guinea-pig this pathway appears to be both activated and uncoupled by cooling from 37 to 20 degrees C. 4. Red cells of rodents in general and hamsters in particular are rich in a Na-Mg exchange pathway. In hamsters, this appears to be the only amiloride-sensitive pathway in simple media. 5. In hamster cells, Na entry through the amiloride-sensitive Mg-activated pathway exhibits the same kinetics as previously shown for Na activation of Mg extrusion.

Mineral metabolism in erythrocytes from patients with cystic fibrosis

In patients with cystic fibrosis plasma concentrations of Zn and Mg were unchanged, plasma Ca concentrations were somewhat decreased, but plasma Fe concentrations were drastically reduced; the ratio Cu/Fe in plasma was increased. The Mg and Zn contents of erythrocytes from patients were unchanged. Therefore, the Mg and Zn content of erythrocytes cannot serve for the detection of patients with cystic fibrosis and their heterozygotes, as has been suggested. Cl(-)-dependent Mg2+ efflux from Mg(2+)-loaded erythrocytes was not affected in cystic fibrosis. Na(+)-dependent Mg2+ efflux was increased only in erythrocytes from patients with the most severe clinical symptoms.

Regulation of Mg2+ uptake in isolated rat myocytes and hepatocytes by protein kinase C

A large Mg2+ cell uptake against concentration gradients is stimulated in collagenase-dispersed rat myocytes by carbachol and in hepatocytes by carbachol or vasopressin. The signalling pathway(s) responsible for this stimulation of Mg2+ uptake was investigated by using various activators or inhibitors of protein kinase C (PKC) and by correlating Mg2+ uptake with cell PKC activity and cAMP content. In both cell preparations, the direct stimulation of PKC by diacylglycerol analogs or phorbol esters reproduce the same pattern of Mg2+ uptake as that induced by carbachol or vasopressin. These data indicate that the activation of PKC is responsible for a stimulation of Mg2+ uptake by myocytes or hepatocytes, whereas increase in cAMP in these cells stimulates Mg2+ release.

Intracellular Mg2+ and magnesium depletion in isolated renal thick ascending limb cells

Magnesium reabsorption and regulation within the kidney occur principally within the cortical thick ascending limb (cTAL) cells of the loop of Henle. Fluorometry with the dye, mag-fura-2, was used to characterize intracellular Mg2+ concentration ([Mg2+]i) in single cTAL cells. Primary cell cultures were prepared from porcine kidneys using a double antibody technique (goat anti-human Tamm-Horsfall and rabbit anti-goat IgG antibodies). Basal [Mg2+]i was 0.52 +/- 0.02 mM, which was approximately 2% of the total cellular Mg. Cells cultured (16 h) in high magnesium media (5 mM) maintained basal [Mg2+]i, 0.48 +/- 0.02, in the normal range. However, cells cultured in nominally magnesium-free media possessed [Mg2+]i, 0.27 +/- 0.01 mM, which was associated with a significant increase in net Mg transport, (control, 0.19 +/- 0.03 and low Mg, 0.35 +/- 0.01 nmol.mg-1 protein.min-1) as assessed by 28Mg uptake. Mg(2+)-depleted cells were subsequently placed in high Mg solution (5 mM) and the Mg2+ refill rate was assessed by fluorescence. [Mg2+]i returned to normal basal levels, 0.53 +/- 0.03 mM, with a refill rate of 257 +/- 37 nM/s. Mg2+ entry was not changed by 5.0 mM Ca2+ or 2 mM Sr2+, Cd2+, Co2+, nor Ba2+ but was inhibited by Mn2+ approximately La3+ approximately Gd3+ approximately Zn2+ approximately Be2+ at 2 mM. Intracellular Ca2+ and 45Ca uptake was not altered by Mg depletion or Mg2+ refill, indicating that the entry is relatively specific to Mg2+. Mg2+ uptake was inhibited by nifedipine (117 +/- 20 nM/s), verapamil (165 +/- 34 nM/s), and diltiazem (194 +/- 19 nM/s) but enhanced by the dihydropyridine analogue, Bay K 8644 (366 +/- 71 nM/s). These antagonists and agonists were reversible with removal and [Mg2+]i subsequently returned to normal basal levels. Mg2+ entry rate was concentration and voltage dependent and maximally stimulated after 4 h in magnesium-free media. Cellular magnesium depletion results in increases in a Mg2+ refill rate which is dependent, in part, on de novo protein synthesis. These data provide evidence for novel Mg2+ entry pathways in cTAL cells which are specific for Mg2+ and highly regulated. These entry pathways are likely involved with renal Mg2+ homeostasis.

Characterization of Na(+)-independent Mg2+ efflux from erythrocytes

Na(+)-independent Mg2+ efflux from Mg2(+)-loaded human, rat and chicken erythrocytes was reduced by extracellular Cl-. Na(+)-independent Mg2+ efflux at low extracellular Cl- concentration (sucrose medium) was inhibited by SITS and was nearly insensitive to SITS in 150 mM choline Cl medium. The inhibition of Mg2+ efflux by extracellular Cl- and DIDS could be overcome by the lipophilic permeant tetraphenylphosphonium cation. Na(+)-independent Mg2+ efflux from human and rat erythrocytes in sucrose and choline Cl medium was inhibited by cAMP and by amiloride and amiloride analogues. The results indicate that Na(+)-independent Mg2+ efflux in high Cl- medium is performed by a similar or the same Mg2+ efflux system, operating in sucrose medium in which the efflux of Mg2+ is accompanied by the efflux of Cl- for charge compensation.