Cyclic AMP-induced Mg2+ release from rat liver hepatocytes, permeabilized hepatocytes, and isolated mitochondria

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.

Effects of cyclic nucleotide phosphodiesterases (PDEs) on mitochondrial skeletal muscle functions

Mitochondria play a critical role in skeletal muscle metabolism and function, notably at the level of tissue respiration, which conduct muscle strength as well as muscle survival. Pathological conditions induce mitochondria dysfunctions notably characterized by free oxygen radical production disturbing intracellular signaling. In that way, the second messengers, cyclic AMP and cyclic GMP, control intracellular signaling at the physiological and transcription levels by governing phosphorylation cascades. Both nucleotides are specifically and selectively hydrolyzed in their respective 5'-nucleotide by cyclic nucleotide phosphodiesterases (PDEs), which constitute a multi-genic family differently tissue distributed and subcellularly compartmentalized. These PDEs are presently recognized as therapeutic targets for cardiovascular, pulmonary, and neurologic diseases. However, very few data concerning cyclic nucleotides and PDEs in skeletal muscle, specifically in mitochondria, are reported in the literature. The knowledge of PDE implication in mitochondrial signaling would be helpful for resolving critical mitochondrial dysfunctions in skeletal muscle.

The Characterization of an Adrenergic Signalling System Involved in the Encystment of the Ocular Pathogen Acanthamoeba spp

The aim of this study was to identify and characterize the receptor system involved in controlling encystment in Acanthamoeba using specific agonists and antagonists and to examine whether endogenous stores of catecholamines are produced by the organism. Acanthamoeba trophozoites suspended in axenic growth medium were exposed to adrenoceptor agonists and antagonists to determine which compounds promoted or prevented encystment. Second, trophozoites were cultured in medium containing a catecholamine synthesis inhibitor to investigate the effect this had on natural encystment. Nonspecific adrenoceptor agonists including epinephrine, isoprotenerol, and the selective β1 adrenoceptor agonist dobutamine were found to cause > 90% encystment of Acanthamoeba trophozoites compared to < 30% with the controls. The selective β1 antagonist metoprolol was able to inhibit epinephrine mediated encystment by > 55%. Cultures of Acanthamoeba with the catecholamine synthesis inhibitor α-methyl-p-tyrosine significantly reduced the level of amoebic encystment compared to controls. In conclusion, Acanthamoeba appear to contain a functional adrenergic receptor system of unknown structure which is involved in initiating the encystment process that can be activated and blocked by β1 agonists and antagonists respectively. Furthermore, the presence of this receptor system in Acanthamoeba indicates that topical β adrenoceptor blockers may be effective adjunct therapy by reducing the transformation of trophozoites into the highly resistant cyst stage.

Magnesium physiology and clinical therapy in veterinary critical care

OBJECTIVE

To review magnesium physiology including absorption, excretion, and function within the body, causes of magnesium abnormalities, and the current applications of magnesium monitoring and therapy in people and animals.

ETIOLOGY

Magnesium plays a pivotal role in energy production and specific functions in every cell in the body. Disorders of magnesium can be correlated with severity of disease, length of hospital stay, and recovery of the septic patient. Hypermagnesemia is seen infrequently in people and animals with significant consequences reported. Hypomagnesemia is more common in critically ill people and animals, and can be associated with platelet, immune system, neurological, and cardiovascular dysfunction as well as alterations in insulin responsiveness and electrolyte imbalance.

DIAGNOSIS

Measurement of serum ionized magnesium in critically or chronically ill veterinary patients is practical and provides information necessary for stabilization and treatment. Tissue magnesium concentrations may be assessed using nuclear magnetic resonance spectroscopy as well as through the application of fluorescent dye techniques.

THERAPY

Magnesium infusions may play a therapeutic role in reperfusion injury, myocardial ischemia, cerebral infarcts, systemic inflammatory response syndromes, tetanus, digitalis toxicity, bronchospasms, hypercoagulable states, and as an adjunct to specific anesthetic or analgesic protocols. Further veterinary studies are needed to establish the frequency and importance of magnesium disorders in animals and the potential benefit of magnesium infusions as a therapeutic adjunct to specific diseases.

PROGNOSIS

The prognosis for most patients with magnesium disorders is variable and largely dependent on the underlying cause of the disorder.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

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.

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.

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.

A novel kinetic assay of mitochondrial ATP-ADP exchange rate mediated by the ANT

A novel method exploiting the differential affinity of ADP and ATP to Mg(2+) was developed to measure mitochondrial ADP-ATP exchange rate. The rate of ATP appearing in the medium after addition of ADP to energized mitochondria, is calculated from the measured rate of change in free extramitochondrial [Mg(2+)] reported by the membrane-impermeable 5K(+) salt of the Mg(2+)-sensitive fluorescent indicator, Magnesium Green, using standard binding equations. The assay is designed such that the adenine nucleotide translocase (ANT) is the sole mediator of changes in [Mg(2+)] in the extramitochondrial volume, as a result of ADP-ATP exchange. We also provide data on the dependence of ATP efflux rate within the 6.8-7.8 matrix pH range as a function of membrane potential. Finally, by comparing the ATP-ADP steady-state exchange rate to the amount of the ANT in rat brain synaptic, brain nonsynaptic, heart and liver mitochondria, we provide molecular turnover numbers for the known ANT isotypes.

The role of phosphodiesterase 3 in endotoxin-induced acute kidney injury

Background

Acute kidney injury frequently accompanies sepsis. Endotoxin is known to reduce tissue levels of cAMP and low levels of cAMP have been associated with renal injury. We, therefore, hypothesized that endotoxin induced renal injury by activating phosphodiesterase 3 (PDE3) which metabolizes cAMP and that amrinone an inhibitor of PDE3 would prevent the renal injury.

Methods

Animals were divided into three groups (n = 7/group): 1) Control (0.9% NaCl infusion without LPS); 2) LPS (0.9% NaCl infusion with LPS); 3) Amrinone+LPS (Amrinone infusion with LPS). Either lipopolysaccharide (LPS) or vehicle was injected via the jugular vein and the rats followed for 3 hours. We explored the expression of PDE3 isoenzymes and the concentrations of cAMP in the tissue.

Results

The PDE3B gene but not PDE3A was upregulated in the kidney of LPS group. Immunohistochemistry also showed that PDE3B was expressed in the distal tubule in the controls and LPS caused PDE3B expression in the proximal as well. However, PDE3A was not expressed in the kidney either in the control or LPS treated groups. Tissue level of cAMP was decreased after LPS and was associated with an increase in blood urea nitrogen, creatinine, ultrastructural proximal tubular changes, and expression of inducible nitric oxide synthase (iNOS) in the endotoxemic kidney. In septic animals the phosphodiesterase 3 inhibitor, amrinone, preserved the tissue cAMP level, renal structural changes, and attenuated the increased blood urea nitrogen, creatinine, and iNOS expression in the kidney.

Conclusion

These findings suggest a significant role for PDE3B as an important mediator of LPS-induced acute kidney injury.

Cell (patho)physiology of magnesium

There is an unsettled debate about the role of magnesium as a ‘chronic regulator’ of biological functions, as opposed to the well-known role for calcium as an ‘acute regulator’. New and old findings appear to delineate an increasingly complex and important role for magnesium in many cellular functions. This review summarizes the available evidence for a link between the regulation of intracellular magnesium availability and the control of cell growth, energy metabolism and death, both in healthy and diseased conditions. A comprehensive view is precluded by technical difficulties in tracing magnesium within a multicompartment and dynamic environment like the cell; nevertheless, the last few years has witnessed encouraging progress towards a better characterization of magnesium transport and its storage or mobilization inside the cell. The latest findings pave the road towards a new and deeper appreciation of magnesium homoeostasis and its role in the regulation of essential cell functions.

Effect of Repeated Doses of Ethanol on Hepatic Mg2+Homeostasis and Mobilization

The acute administration of a first dose of ethanol (EtOH) to rat liver cells reduces the amount of Mg(2+) extruded by a second dose of EtOH or the subsequent addition of adrenergic agonists. In contrast, the Mg(2+) extrusion normally elicited by the alpha(1)-adrenergic or beta-adrenergic agonist does not impair the Mg(2+) mobilization induced by the subsequent addition of EtOH. Inhibition of EtOH metabolism by 4-methylpyrazole abolishes almost completely the Mg(2+) extrusion induced by the first dose of EtOH, and partially enlarges that elicited by the second dose of alcohol or the subsequent adrenergic stimulation. Ethanol-treated liver cells stimulated by the adrenergic agonist show a reduced level of membrane-bound Galphas as well as a reduced cellular cAMP content. Analysis of cellular Mg(2+) distribution indicates that EtOH administration decreases the Mg(2+) content of the cytoplasm, mitochondria, and endoplasmic reticulum to a comparable extent. These data indicate that acute EtOH administration directly impairs cellular Mg(2+) homeostasis and also prevents a further Mg(2+) mobilization by additional doses of alcohol or alpha(1)-adrenoceptor and beta-adrenoceptor agonist by decreasing cytosolic and intraorganelle Mg(2+) content and by affecting G-protein membrane distribution/signaling.

Functional characterization of two distinct Mg(2+) extrusion mechanisms in cardiac sarcolemmal vesicles

Cardiac ventricular myocytes extrude a sizeable amount of their total Mg(2+) content upon stimulation by beta-adrenergic agonists. This extrusion occurs within a few minutes from the application of the agonist, suggesting the operation of rapid and abundantly represented Mg(2+) transport mechanisms in the cardiac sarcolemma. The present study was aimed at characterizing the operation of these transport mechanisms under well defined conditions. Male Sprague-Dawley rats were used to purify a biochemical standardized preparation of sealed rat cardiac sarcolemmal vesicles. This experimental model has the advantage that trans-sarcolemmal cation transport can be studied under specific extra- and intra-vesicular ionic conditions, in the absence of intracellular organelles, and buffering or signaling components. Magnesium ion (Mg(2+)) transport was assessed by atomic absorbance spectrophotometry. The results reported here indicate that: (1) sarcolemma vesicles retained trapped intravesicular Mg(2+) in the absence of extravesicular counter-ions; (2) the addition of Na(+) or Ca(2+) induced a rapid and concentration-dependent Mg(2+) extrusion from the vesicles; (3) co-addition of maximal concentrations of Na(+) and Ca(2+) resulted in an additive Mg(2+) extrusion; (4) Mg(2+ )extrusion was blocked by addition of amiloride or imipramine; (5) pre-treatment of sarcolemma vesicles with alkaline phosphatase at the time of preparation completely abolished Na(+)- but not Ca(2+)-induced Mg(2+) extrusion; (6) Na(+)-dependent Mg(2+) transport could be restored by stimulating vesicles loaded with protein kinase A catalytic subunit and ATP with membrane-permeant cyclic-AMP analog; (7) extra-vesicular Mg(2+) could be accumulated in exchange for intravesicular Na(+) via a mechanism inhibited by amiloride or alkaline phosphatase treatment; (8) Mg(2+) accumulation could be restored via cAMP/protein kinase A protocol. Overall, these data provide compelling evidence for the operation of distinct Na(+)- and Ca(2+)-dependent Mg(2+) extrusion mechanisms in sarcolemma vesicles. The Na(+)-dependent mechanism appears to be specifically activated via protein kinase A/cAMP-dependent phosphorylation process, and can operate in either direction based upon the cation concentration gradient across the sarcolemma. The Ca(2+)-dependent mechanism, instead, only mediates Mg(2+) extrusion in a cAMP-independent manner.

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.

Release of Ca2+ and Mg2+ from yeast mitochondria is stimulated by increased ionic strength

Background

Divalent cations are required for many essential functions of mitochondrial metabolism. Yet the transporters that mediate the flux of these molecules into and out of the mitochondrion remain largely unknown. Previous studies in yeast have led to the molecular identification of a component of the major mitochondrial electrophoretic Mg²⁺ uptake system in this organism as well as a functional mammalian homolog. Other yeast mitochondrial studies have led to the characterization of an equilibrative fatty acid-stimulated Ca²⁺ transport activity. To gain a deeper understanding of the regulation of mitochondrial divalent cation levels we further characterized the efflux of Ca²⁺ and Mg²⁺ from yeast mitochondria.

Results

When isolated mitochondria from the yeast Saccharomyces cerevisiae were suspended in a salt-based suspension medium, Ca²⁺ and Mg²⁺ were released from the matrix space. Release did not spontaneously occur in a non-ionic mannitol media. When energized mitochondria were suspended in a mannitol medium in the presence of Ca²⁺ they were able to accumulate Ca²⁺ by the addition of the electrogenic Ca²⁺ ionophore ETH-129. However, in a KCl or choline Cl medium under the same conditions, they were unable to retain the Ca²⁺ that was taken up due to the activation of the Ca²⁺ efflux pathway, although a substantial membrane potential driving Ca²⁺ uptake was maintained. This Ca²⁺ efflux was independent of fatty acids, which have previously been shown to activate Ca²⁺ transport. Endogenous mitochondrial Mg²⁺ was also released when mitochondria were suspended in an ionic medium, but was retained in mitochondria upon fatty acid addition. When suspended in a mannitol medium, metal chelators released mitochondrial Mg²⁺, supporting the existence of an external divalent cation-binding site regulating release. Matrix space Mg²⁺ was also slowly released from mitochondria by the addition of Ca²⁺, respiratory substrates, increasing pH, or the nucleotides ATP, ADP, GTP, and ATP-gamma-S.

Conclusion

In isolated yeast mitochondria Ca²⁺ and Mg²⁺ release was activated by increased ionic strength. Free nucleotides, metal ion chelators, and increased pH also stimulated release. In yeast cells this release is likely an important mechanism in the regulation of mitochondrial matrix space divalent cation concentrations.

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+.

Streptozotocin-induced diabetes impairs Mg2+ homeostasis and uptake in rat liver cells

Male Sprague-Dawley rats rendered diabetic by streptozotocin injection presented 10 and 20% decreases in total hepatic Mg2+ content at 4 and 8 wk, respectively, following diabetes onset. This decrease was associated with a parallel decrease in K+ and ATP content and an increase in Na+ level. In diabetic liver cells, the Mg2+ extrusion elicited by alpha1-adrenoceptor stimulation was markedly reduced compared with nondiabetic livers, whereas that induced by beta-adrenoceptor stimulation was unaffected. In addition, diabetic hepatocytes did not accumulate Mg2+ following stimulation of protein kinase C pathway by vasopressin, diacylglycerol analogs, or phorbol 12-myristate 13-acetate derivates despite the reduced basal content in cellular Mg2+. Experiments performed in purified plasma membrane from diabetic livers located the defect at the level of the bidirectional Na+/Mg2+ exchanger operating in the basolateral domain of the hepatocyte cell membrane, which could extrude but not accumulate Mg2+ in exchange for Na+. The impairment of Mg2+ uptake mechanism, in addition to the decrease in cellular ATP level, can contribute to explaining the decrease in liver Mg2+ content observed under diabetic conditions.

Chronic EtOH administration alters liver Mg2+ homeostasis

Ethanol (EtOH) administration to rats for 4 wk markedly decreased Mg(2+) content in several tissues, including liver. Total cellular Mg(2+) accounted for 26.8 +/- 2.4 vs. 36.0 +/- 1.4 nmol Mg(2+)/mg protein in hepatocytes from EtOH-fed and control rats, respectively, and paralleled a 13% decrease in cellular ATP content. Stimulation of alpha(1)- or beta-adrenergic receptor or acute EtOH administration did not elicit an extrusion of Mg(2+) from liver cells of EtOH-fed rats while releasing 5% of total tissue Mg(2+) content from hepatocytes of control rats. Despite the 25% decrease in Mg(2+) content, hepatocytes from EtOH-fed rats did not accumulate Mg(2+) following stimulation of protein kinase C signaling pathway, whereas control hepatocytes accumulated approximately 2 nmol Mg(2+). mg protein(-1). 4 min(-1). Together, these data indicate that Mg(2+) homeostasis and transport are markedly impaired in liver cells after prolonged exposure to alcohol. The inability of liver cells, and possibly other tissues, to accumulate Mg(2+) can help explain the reduction in tissue Mg(2+) content following chronic alcohol consumption.

Rapid release of Mg(2+) from liver mitochondria by nonesterified long-chain fatty acids in alkaline media

Long-chain fatty acids induce a rapid release of Mg(2+) from both energized and nonenergized rat liver mitochondria suspended at pH 8 in isotonic saline but not sucrose media. The effect is observed only with fatty acids that possess protonophoric activity. The most active saturated fatty acids are myristic and palmitic, while the most active unsaturated acids are oleic, linolenic, and arachidonic. The rate of Mg(2+) release drastically decreases with decreasing medium pH to 7.2-7.6. However, at those pH values this rate is doubled by energization of mitochondria with respiratory substrates. Mg(2+) release is accompanied by cyclosporin A-insensitive large-amplitude swelling of mitochondria. This swelling is similar to that produced by the divalent metal ionophore A23187 and is interpreted as being due to activation of the inner membrane anion channel, the K(+) uniporter, and the K(+)/H(+) exchanger. In energized mitochondria, both swelling and Mg(2+) release are blocked by the exogenous K(+)/H(+) exchanger nigericin. It is proposed that fatty acids under conditions of alkaline mitochondrial matrix activate latent Mg(2+)-sensitive ion-conducting pathways in the inner mitochondrial membrane, which mediate swelling and Mg(2+) release. It is hypothesized that fatty acids activate an intrinsic Mg(2+)/H(+) exchanger that is related to, or identical with, the K(+)/H(+) exchanger.

Role of group II and group III metabotropic glutamate receptors in spinal cord injury

Spinal cord injury (SCI) produces an increase in extracellular excitatory amino acid (EAA) concentrations that results in glutamate receptor-mediated excitotoxic events. An important class of these receptors is the metabotropic glutamate receptors (mGluRs). mGluRs can activate a number of intracellular pathways that increase neuronal excitability and modulate neurotransmission. Group I mGluRs are known to modulate EAA release and the development of chronic central pain (CCP) following SCI; however, the role of group II and III mGluRs remains unclear. To begin evaluating group II and III mGluRs in SCI, we administered the specific agonists for group II, APDC, or group III, L-AP4, by interspinal injection immediately following SCI. Contusion injury was produced at spinal segment T10 with a New York University impactor (12.5-mm drop, 10-g rod 2 mm in diameter) in 30 adult male Sprague-Dawley rats (175-200 g). Evoked and spontaneous behavioral measures of CCP, locomotor recovery, changes in mGluR expression, and amount of spared tissue were examined. Neither APDC nor L-AP4 affected locomotor recovery or the development of thermal hyperalgesia; however, L-AP4 and APDC attenuated changes in mechanical thresholds and changes in exploratory behavior indicative of CCP. APDC- and L-AP4-treated groups had higher expression levels of mGluR2/3 at the epicenter of injury on post contusion day 28; however, there was no difference in the amount of spared tissue between treatment groups. These results demonstrate that treatment with agonists to group II and III mGluRs following SCI affects mechanical responses, exploratory behavior, and mGluR2/3 expression without affecting the amount of tissue spared, suggesting that the level of mGluR expression after SCI may modulate nociceptive responses.

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.

Relationship between total and free cellular Mg(2+) during metabolic stimulation of rat cardiac myocytes and perfused hearts

The changes in total Mg were compared with changes in cytosolic free Mg(2+) during metabolic stimulation of collagenase-dispersed rat cardiac myocytes or Langendorff-perfused rat hearts. In myocytes the addition of agents leading to cAMP increase or protein kinase C activation results in a loss or gain of more than 5% of total Mg content within 3 min (i.e., 3-4 nmol Mg/mg protein). Under the same conditions, changes in cytosolic free Mg(2+) measured with fluorescent indicator are small and result in changes of cytosolic free Mg(2+) equivalent to 90-140 microM. In perfused hearts, beta-adrenergic stimulation results in a loss of total Mg larger than 0.5 micromol per gram of heart corresponding to 9% loss of total Mg content of the heart (estimated to be 5.8 micromol). Under these conditions there is no change in cytosolic free Mg(2+) or the major buffer of cytosolic Mg(2+), ATP, as measured by (31)P NMR. These data suggest that a major redistribution of total Mg occurs in intracellular organelles or in cytosolic buffers in order to maintain cytosolic free Mg(2+) relatively unchanged during the observed cellular massive translocation of total Mg. Hence, Mg(2+) may regulate metabolic functions not within the cytosol but in locations where its concentration oscillates, such as extracellular fluid and intracellular compartments.

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.

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).

Mitochondrial transport of cations: channels, exchangers, and permeability transition

This review provides a selective history of how studies of mitochondrial cation transport (K+, Na+, Ca2+) developed in relation to the major themes of research in bioenergetics. It then covers in some detail specific transport pathways for these cations, and it introduces and discusses open problems about their nature and physiological function, particularly in relation to volume regulation and Ca2+ homeostasis. The review should provide the basic elements needed to understand both earlier mitochondrial literature and current problems associated with mitochondrial transport of cations and hopefully will foster new interest in the molecular definition of mitochondrial cation channels and exchangers as well as their roles in cell physiology.

Intracellular Mg2+ surge follows Ca2+ increase during depolarization in cultured neurons

The intracellular magnesium and calcium concentrations in cultured dorsal root ganglion neurons were measured using a fluorescent Mg2+ indicator, Mag-Fura-2 and a Ca2+ indicator, Fura-2, respectively. The magnesium concentration in the cytoplasm was higher than that in the nuclei at rest; 0.68+/-0.10 mM (mean+/-S.E.M., n=7) in the cytoplasm and 0.11+/-0.05 mM in the nucleus. When depolarized by a 60 mM KCl solution, the magnesium concentration increased remarkably in the cytoplasm; 1.52+/-0.26 mM (n=7) in the cytoplasm and 0.25+/-0. 12 mM in the nucleus. This is in contrast to a Ca2+ increase due to depolarization in which the increase was remarkable also in the nucleus. The Mg2+ response displayed a rapid spontaneous recovery even in the presence of the high K+ solution. The Ca2+ response, on the other hand, accompanied a slow recovery 'plateau'. Simultaneous measurements of Mg2+ and Ca2+ by a double-labeling experiment revealed that the Ca2+ concentration started to rise 0.46+/-0.05 s (n=32) earlier, and it reached its peak 1.38+/-0.12 s (n=32) earlier than Mg2+. These results support the scheme of 'calcium induced magnesium release', that the depolarization-induced elevation of the Ca2+ concentration causes an increase in the Mg2+ concentration in the cytoplasm.

Acute effect of EtOH on Mg2+ homeostasis in liver cells: evidence for the activation of an Na+/Mg2+ exchanger

The acute administration of ethanol mobilizes a considerable amount of Mg2+ from perfused rat livers and isolated hepatocytes in a dose-dependent fashion in the absence of release of cellular K+ or lactate dehydrogenase (LDH) in the extracellular medium. Mg2+ extrusion becomes detectable within 2 min and reaches the maximum within 8 min after ethanol addition, declining toward the basal value thereafter irrespective of the persistence of alcohol in the perfusion system and the dose of ethanol administered. The effect is the result of a specific impairment of Mg2+ transport and/or regulatory mechanisms. In fact, Mg2+ extrusion does not occur under conditions in which 1) ethanol is replaced by an equivalent dose of DMSO, 2) amiloride or imipramine are used as inhibitors of the Na+/Mg2+ exchanger, 3) extracellular Na+ is replaced by an equimolar concentration of choline chloride, and 4) 4-methylpyrazole is used to specifically inhibit alcohol dehydrogenase and cytochrome P-4502E1. Finally, the observation that the cellular level of ATP is markedly reduced after acute ethanol administration would suggest that Mg2+ extrusion results from a decreased buffering capacity of cytosolic Mg-ATP complex.

Characterization of two Mg2+ transporters in sealed plasma membrane vesicles from rat liver

The plasma membrane of mammalian cells possesses rapid Mg2+ transport mechanisms. The identity of Mg2+ transporters is unknown, and so are their properties. In this study, Mg2+ transporters were characterized using a biochemically and morphologically standardized preparation of sealed rat liver plasma membranes (LPM) whose intravesicular content could be set and controlled. The system has the advantages that it is not regulated by intracellular signaling machinery and that the intravesicular ion milieu can be designed. The results indicate that 1) LPM retain trapped intravesicular total Mg2+ with negligible leak; 2) the addition of Na+ or Ca2+ induces a concentration- and temperature-dependent efflux corresponding to 30-50% of the intravesicular Mg2+; 3) the rate of flux is very rapid (137.6 and 86.8 nmol total Mg2+ . micrometer -2 . min-1 after Na+ and Ca2+ addition, respectively); 4) coaddition of maximal concentrations of Na+ and Ca2+ induces an additive Mg2+ efflux; 5) both Na+- and Ca2+-stimulated Mg2+ effluxes are inhibited by amiloride, imipramine, or quinidine but not by vanadate or Ca2+ channel blockers; 6) extracellular Na+ or Ca2+ can stimulate Mg2+ efflux in the absence of Mg2+ gradients; and 7) Mg2+ uptake occurs in LPM loaded with Na+ but not with Ca2+, thus indicating that Na+/Mg2+ but not Ca2+/Mg2+ exchange is reversible. These data are consistent with the operation of two distinct Mg2+ transport mechanisms and provide new information on rates of Mg2+ transport, specificity of the cotransported ions, and reversibility of the transport.

Magnesium and the role of MgtC in growth of Salmonella typhimurium

Salmonella typhimurium has three distinct transport systems for Mg2+: CorA, MgtA, and MgtB. The mgtCB operon encodes two proteins, MgtC, a hydrophobic protein with a predicted molecular mass of 22.5 kDa, and MgtB, a 102-kDa P-type ATPase Mg2+ transport protein. The mgtCB locus has been identified as part of a new Salmonella pathogenicity island, SPI-3. Transcription of mgtCB is regulated by extracellular Mg2+ via the two-component PhoPQ regulatory system important for virulence. To elucidate MgtC's role in a low-Mg2+ environment, we looked at growth and transport in strains lacking the CorA and MgtA Mg2+ transporters but expressing MgtB, MgtC, or both. mgtC mgtB+ and mgtC+ mgtB+ strains exhibited growth in N minimal medium without added Mg2+ with a 1- to 2-h lag phase. An mgtC+ mgtB strain was also able to grow in N minimal medium without added Mg2+ but only after a 24-h lag phase. In N minimal medium containing 10 mM Mg2+, all strains grew after a short lag phase; the mgtC+ mgtB strain grew to a higher optical density at 600 nm than an mgtC+ mgtB+ strain and was comparable to wild type. The lengthy lag phase before growth in an mgtC+ mgtB strain was not due to lack of expression of MgtC. Western blot analysis indicated that substantial MgtC protein is present by 2 h after suspension in N minimal medium. Surprisingly, in an mgtC+ mgtB+ strain, MgtC was undetectable during Mg2+ starvation, although large amounts of MgtB were observed. The lack of expression of MgtC is not dependent on functional MgtB, since a strain carrying a nonfunctional MgtB with a mutation (D379A) also did not make MgtC. Since, during invasion of eukaryotic cells, S. typhimurium appears to be exposed to a low-pH as well as a low-Mg2+ environment, the growth of an mgtC+ mgtB strain was tested at low pH with and without added Mg2+. While significant quantities of MgtC could be detected after suspension at pH 5.2, the mgtC+ mgtB strain was unable to grow at pH 5.2 whether or not Mg2+ was present. Finally, using 63Ni2+ and 57Co2+ as alternative substrates for the unavailable 28Mg2+, cation uptake could not be detected in an mgtC+ mgtB strain after Mg2+ starvation. We conclude that MgtC is not a Mg2+ transporter and that it does not have a primary role in the survival of S. typhimurium at low pH.

Modulation of oxidative phosphorylation by Mg2+ in rat heart mitochondria

The effect of varying the Mg2+ concentration on the 2-oxoglutarate dehydrogenase (2-OGDH) activity and the rate of oxidative phosphorylation of rat heart mitochondria was studied. The ionophore A23187 was used to modify the mitochondrial free Mg2+ concentration. Half-maximal stimulation (K0.5) of ATP synthesis by Mg2+ was obtained with 0.13 +/- 0.02 mM (n = 7) with succinate (+rotenone) and 0.48 +/- 0.13 mM (n = 6) with 2-oxoglutarate (2-OG) as substrates. Similar K0.5 values were found for NAD(P)H formation, generation of membrane potential, and state 4 respiration with 2-OG. In the presence of ADP, an increase in Pi concentration promoted a decrease in the K0.5 values of ATP synthesis, membrane potential formation and state 4 respiration for Mg2+ with 2-OG, but not with succinate. These results indicate that 2-OGDH is the main step of oxidative phosphorylation modulated by Mg2+ when 2-OG is the oxidizable substrate; with succinate, the ATP synthase is the Mg2+-sensitive step. Replacement of Pi by acetate, which promotes changes on intramitochondrial pH abolished Mg2+ activation of 2-OGDH. Thus, the modulation of the 2-OGDH activity by Mg2+ has an essential requirement for Pi (and ADP) in intact mitochondria which is not associated to variations in matrix pH.

Direct inhibition of mitochondrial respiratory chain complex III by cell-permeable ceramide

Ceramide is a lipid second messenger that mediates the effects of tumor necrosis factor alpha and other agents on cell growth and differentiation. Ceramide is believed to act via activation of protein phosphatase, proline-directed protein kinase, or protein kinase C. Tumor necrosis factor alpha-induced common pathway of apoptosis is associated with an early impairment of mitochondria. Herein, we demonstrate that ceramide can directly inhibit mitochondrial respiratory chain function. In isolated mitochondria, a rapid decline of mitochondrial oxidative phosphorylation occurs in the presence of N-acetylsphingosine (C2-ceramide), a synthetic cell-permeable ceramide analog. An investigation of the site of ceramide action revealed that the activity of respiratory chain complex III is reduced by C2-ceramide with half-maximum effect at 5-7 microM. In contrast, N-acetylsphinganine (C2-dihydroceramide), which lacks a functionally critical double bond and is ineffective in cells, did not alter mitochondrial respiration or complex III activity. We suggest that these in vitro observations may set the stage for identifying a novel mechanism of regulation of mitochondrial function in vivo.

Regulation of magnesium efflux from rat spleen lymphocytes

Rat spleen lymphocytes (RSL) incubated at 37 degrees C in Mg-free medium (O-trans conditions) exibited Mg2+ efflux with apparent velocity of 0.2 nmol/mg protein/min. After 30 min, this process accounted for the mobilization of about 15% of cell total Mg2+. Half of the Mg2+ efflux depended on extracellular Na+ and was stimulated by cAMP. IFN-alpha significantly enhanced Mg2+ efflux under O-trans conditions as well as in the presence of physiological extracellular Mg2+. Pretreatment of RSL with indomethacin completely abolished IFN-alpha-induced Mg2+ efflux, suggesting a crucial role for cyclooxygenase-dependent arachidonate metabolism. On the other hand, pretreatment of RSL with the PKA inhibitor (Rp)8-Br-cAMPS prevented IFN-alpha stimulation of Mg2+ efflux, indicating the involvement of cAMP. Consistently, both IFN-alpha and exogenous PGE1 increased cAMP from 50 to 125 pmol/mg protein. Altogether these results show that IFN-alpha stimulates Mg2+ efflux by activating arachidonate metabolism and synthesis of prostaglandins. By influencing adenylcyclase activity, PGEs can eventually promote cAMP-dependent Mg2+ efflux, possibly through the activity of a Na-Mg antiport. In RSL, therefore, magnesium movements can be under the control of IFN-alpha and, perhaps, of other cytokines, suggesting the involvement of Mg2+ in cell response to receptor-mediated stimuli.

Regulation of Mg2+ homeostasis by insulin in perfused rat livers and isolated hepatocytes

Several recent studies demonstrate that adrenergic receptor stimulation evokes marked changes in Mg2+ homeostasis. As insulin counter-regulates many of the metabolic consequences of adrenergic receptor stimulation, we evaluated the potential influence of insulin on Mg2+ movements in response to adrenergic stimulation. The data demonstrate that insulin is able to block the Mg2+ efflux from perfused rat livers stimulated by isoproterenol or 8-Br-cAMP, but has little or no effect on epinephrine or phenylephrine induced Mg2+ efflux. Thus, evidence is provided demonstrating that there are redundant adrenergic pathways regulating Mg2+ efflux from liver tissue. One of these pathways, the beta-adrenergic component, is selectively blocked by insulin. Furthermore, these findings may provide a cellular explanation for hypomagnesemia associated with diabetes.

Mg2+ control of respiration in isolated rat liver mitochondria

The role of endogenous mitochondrial Mg2+ as a potential regulator of mitochondrial dehydrogenase activity, and therefore of cellular respiration, was measured in isolated mitochondria containing matrix Ca2+ and Mg2+ levels resembling those occurring in vivo. Ca2+ and Mg2+ depletion was carried out using the cation ionophore A23187 in the presence or absence of the Ca2+ uniporter inhibitor ruthenium red (RR). Divalent cation depletion inhibits the oxidation of alpha-ketoglutarate or pyruvate in states 4 and 3, slows uncoupled respiration and results in decreased membrane potential. Since the addition of Mg2+ could not restore respiration, these dehydrogenases appear not to be regulated by Mg2+. In contrast, similar cation depletion stimulates succinate dehydrogenase (or glutamate dehydrogenase) in state 4 without decreasing membrane potential. The addition of RR caused authentic uncoupling, accompanied by a decrease in membrane potential and an increase in membrane permeability. These effects could be completely reversed by Mg2+. These and other data, showing that Mg2+ depletion results in a change of respiration depending on the substrate oxidized and the metabolic state, indicate that Mg2+ removal may have direct and indirect effects on mitochondrial respiration. A clear direct effect is the stimulation of succinate or glutamate dehydrogenase by decreasing matrix Mg2+. Hence, changes in matrix Mg2+ (in addition to those of Ca2+) could be of great consequence, not only for the control of respiration but also for metabolic pathways affected by changes in concentrations of matrix substrates.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

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.

Calcium ion-dependent signalling and mitochondrial dysfunction: mitochondrial calcium uptake during hormonal stimulation in intact liver cells and its implication for the mitochondrial permeability transition

Hormones that elevate cytosolic Ca2+ concentrations ([Ca2+]cyt) often use Ca2+ as a messenger to activate intramitochondrial metabolic processes. However, the mitochondrial Ca2+ level also regulates the activation of the mitochondrial permeability transition (MPT), a process that involves the assembly of a high conductance proteinaceous pore across the inner and outer membrane. Studies on intact liver cells indicate that the MPT is a critical step in the cell killing induced by anoxia or respiratory inhibitors. In this study, we used freshly isolated hepatocytes to investigate to what extent the elevation of [Ca2+]cyt by vasopressin or other agonists causes Ca2+ accumulation in the mitochondria and how this treatment affects the mitochondrial susceptibility to undergo the MPT. Hepatocytes were incubated with vasopressin, glucagon, or with thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ pump) prior to permeabilization with digitonin. Mitochondrial Ca2+ accumulation was determined by following the ionomycin-induced Ca2+ release in permeabilized cells and mitochondrial swelling was studied by following cyclosporin A-sensitive light scattering changes induced by phenyl-arsenoxide and rotenone. The results indicate that agents that elevate [Ca2+]cyt cause a significant Ca2+ accumulation in the mitochondria. Excessive Ca2+ accumulation (> 10-fold increase over basal levels) was obtained with the combination of vasopressin and glucagon or with incubations containing thapsigargin. These conditions were also associated with a marked increase in rotenone-induced mitochondrial swelling. However, the more modest increase in mitochondrial Ca2+ content after treating cells with vasopressin alone did not enhance the swelling response; instead, vasopressin suppressed mitochondrial swelling compared to control incubations. Vasopressin also partly suppressed the swelling associated with thapsigargin treatment, although it did not significantly affect the Ca2+ accumulation under these conditions. This effect of vasopressin was mimicked by phorbol ester, suggesting a role for protein kinase C. The data indicate that mitochondrial Ca2+ accumulation following elevation of elevation of [Ca2+]cyt enhances the susceptibility for activation of the MPT, a response that may increase cell injury during anoxia or in response to other challenges. However, hormones also activate protective responses in the cell that suppress the MPT.

Overexpression of a novel member of the mitochondrial carrier family rescues defects in both DNA and RNA metabolism in yeast mitochondria

The PIF1 and MRS2 gene products have previously been shown to be essential for mitochondrial DNA maintenance at elevated temperatures and mitochondrial group II intron splicing, respectively, in the yeast Saccharomyces cerevisiae. A multicopy suppressor capable of rescuing the respiratory deficient phenotype associated with null alleles of either gene has been isolated. This suppressor is a nuclear gene that was called RIM2/MRS12. The RIM2/MRS12 gene encodes a predicted protein of 377 amino acids that is essential for mitochondrial DNA metabolism and proper cell growth. Inactivation of this gene causes the total loss of mitochondrial DNA and, compared to wild-type rhoo controls, a slow-growth phenotype on media containing glucose. Analysis of the RIM2/MRS12 protein sequence suggests that RIM2/MRS12 encodes a novel member of the mitochondrial carrier family. In particular, a typical triplicate structure, where each repeat consists of two putative transmembrane segments separated by a hydrophilic loop, can be deduced from amino acid sequence comparisons and the hydropathy profile of RIM2/MRS12. Antibodies directed against the aminoterminus of RIM2/MRS12 detect this protein in mitochondria. The function of the RIM2/MRS12 protein and the substrates it might transport are discussed.

Magnesium transport by mitochondria

The pathways for the uptake and extrusion of Mg2+ by mitochondria are now well defined, the present evidence suggests that uptake occurs by nonspecific diffusive pathways in response to elevated membrane potential. There is disagreement as to some of the properties of Mg2+ efflux from mitochondria, but the reaction resembles K+ efflux in many ways and may occur in exchange for H+. Matrix free magnesium ion concentration, [Mg2+], can be measured using fluorescent probes and is set very close to cytosol [Mg2+] by a balance between influx and efflux and by the availability of ligands, such as Pi. There are indications that matrix [Mg2+] may be under hormonal control and that it contributes to the regulation of mitochondrial metabolism and transport reactions.

cAMP-induced changes of intracellular free Mg2+ levels in human erythrocytes

To examine the role of cAMP in the regulation of intracellular free magnesium concentration ([Mg2+]i), we measured [Mg2+]i in human erythrocytes by 31P-NMR spectroscopy. (-)-Isoproterenol, forskolin, Bt2cAMP and 8-bromo-cAMP decreased [Mg2+]i in human erythrocytes. Bt2cAMP did not increase the efflux rate of Mg2+ from erythrocytes. HA1004, a potent inhibitor of cAMP-dependent kinases, markedly increased the [Mg2+]i in a Mg(2+)-free buffer solution. Addition of 8-bromo-cGMP or 12-O-tetradecanoylphorbol 13-acetate (TPA) did not affect the [Mg2+]i. These results suggest that beta-adrenergic stimulation and cAMP play an important role in the regulation of [Mg2+]i in human erythrocytes.

Regulation of magnesium uptake and release in the heart and in isolated ventricular myocytes

Perfused rat hearts release or accumulate approximately 10% of total Mg2+ content when stimulated with norepinephrine (NE) or carbachol, respectively. Collagenase-dispersed rat ventricular myocytes increase or decrease total cell Mg2+ by 1 mM within 5 minutes when stimulated with these same transmitters. Measurements of Mg2+ transport using 28Mg or atomic absorbance spectrophotometry indicate that the rate and the extent of both stimulated Mg2+ efflux and influx are independent of the concentration of extracellular Mg2+ (0 to 1.2 mM). Mg2+ release induced by NE is rapidly reversed by the addition of carbachol, and Mg2+ uptake induced by carbachol is reversed by NE. Decreasing extracellular Na+ or Ca2+ decreases or abolishes Mg2+ efflux from myocytes. Cd2+ or other Ca2+ channel blockers also inhibit Mg2+ efflux in the presence of a physiological concentration of extracellular Ca2+. Replacement of extracellular Ca2+ with Sr2+ or with Mn2+ decreases or abolishes both stimulated efflux and influx of Mg2+. Redistribution of 85Sr in myocytes and in the supernatant indicates that under those conditions Sr2+ is released or accumulated by NE or carbachol in a manner similar to that of Mg2+. Hence, at least in the case of Sr2+, the inhibition of Mg2+ fluxes can be explained by the transport of Sr2+ rather than Mg2+ through the transport(s) systems. By contrast, replacement of extracellular Ca2+ with Ba2+ inhibits stimulated Mg2+ uptake but not Mg2+ release. These results indicate that cardiac myocytes have a major pool of Mg2+ that can be rapidly mobilized upon hormonal stimulation. The net uptake and release of Mg2+ are quantitatively similar and appear to be independent of the extracellular Mg2+ concentrations but are affected, to various degrees, by the presence of other cellular or extracellular cations.

Cytosolic free magnesium concentration in human platelets

Although the magnesium ion (Mg2+) is considered to play an important role in cell activation, information is limited by the lack of suitable methods for measuring cytosolic free Mg2+ concentration ([Mg2+]i). We measured [Mg2+]i in resting and activated human platelets using a new fluorescent Mg(2+)-indicator, mag-fura-2. [Mg2+]i was 0.54 +/- 0.14 mM in resting platelets from 15 healthy volunteers. [Mg2+]i was elevated to 1.33 +/- 0.44 mM and 0.92 +/- 0.37 mM in platelets stimulated with thrombin and collagen, respectively. Increased Mg2+ was considered to be derived chiefly from intracellular Mg2+ mobilization. These results suggest that platelet activation is associated with the increase in [Mg2+]i. The estimation of [Mg2+]i using this method is useful for the investigation of mechanism for various cell activation.