Bella T. Altura

Intracellular and extracellular magnesium depletion in Type 2 (non-insulin-dependent) diabetes mellitus

SummaryTo investigate alterations of magnesium metabolism in Type 2 (non-insulin-dependent) diabetes mellitus, we utilized a new magnesium-specific selective ion electrode apparatus to measure serum ionized magnesium (Mg-io) in fasting subjects with and without Type 2 diabetes, and compared these values to levels of serum total magnesium, and of intracellular free magnesium (Mgi) analysed by 31P-NMR spectroscopy. Both Mg-io (0.630±0.008 vs 0.552± 0.008 mmol/l, p<0.001) and Mgi (223.3±8.3 vs 184± 13.7 mmol/l,p<0.001), but not serum total magnesium, were significantly reduced in Type 2 diabetes compared with nondiabetic control subjects. Furthermore, a close relationship was observed between serum Mg-io and Mgi (r=0.728, p<0.001). We suggest that magnesium deficiency, both extracellular and intracellular, is a characteristic of chronic stable mild Type 2 diabetes, and as such, may predispose to the excess cardiovascular morbidity of the diabetic state. Furthermore, by more adequately reflecting cellular magnesium metabolism than total serum magnesium levels, Mg-io measurements may provide a more readily available tool than has heretofore been available to analyse magnesium metabolism in a variety of diseases.

Withdrawal of magnesium causes vasospasm while elevated magnesium produces relaxation of tone in cerebral arteries

Abstract Excised middle cerebral and basilar arteries from dogs were incubated in Krebs-Ringer bicarbonate solution and exposed to normal, high and low concentrations of magnesium [Mg] in the medium. Sudden withdrawal of extracellular Mg 2+ ([Mg 2+ ] o ) from the medium produced contraction of arterial smooth muscle, whereas elevated concentrations of [Mg 2+ ] o decreased the basal tensions in a concentration-dependent manner.

Mechanisms of hydrogen peroxide-induced contraction of rat aorta

It has been suggested that reactive oxygen species may be involved in the regulation of vascular tone. However, the underlying mechanisms remain to be elucidated. The present studies were designed to investigate the contractile effects of hydrogen peroxide (H2O2), one of the reactive oxygen species, on isolated ring segments of rat aorta with and without endothelium. H2O2 induced an endothelium-independent contraction in isolated rat aorta ring segments in a concentration-dependent manner at concentrations from 5 x 10(-6) to 5 x 10(-3) M. H2O2-induced contractions of denuded rat aorta rings were stronger than those on intact rat aorta segments. The contractile effects of H2O2 were inhibited completely by 1200 u/ml catalase. The presence of 1.0 microM Fe2+ or 10 microM proadifen, a cytochrome P450 monooxygenase inhibitor, potentiated the contractile effect of H2O2 on isolated rat aorta segments. 1 mM deferoxamine (a Fe2+ chelator) or 100 microM dimethyl sulfoxide (a hydroxyl radical scavenger) significantly attenuated the vessel contractions induced by hydrogen peroxide plus Fe2+ or hydrogen peroxide itself. Removal of extracellular Ca2+ ([Ca2+]0), addition of 5 microM verapamil, administration of a protein kinase C inhibitor (staurosporine), treatment with an inhibitor of protein tyrosine phosphorylation (genistein) or employment of 5.0 microM indomethacin resulted in a significant attenuation of the contractile responses of the vessels to H2O2. Pharmacological antagonists (e.g. a muscarinic acetylcholine receptor antagonist (atropine), an antagonist of histamine H1 receptors (diphenhydramine), an antagonist of histamine H2 receptors (cimetidine), an alpha-adrenoceptor antagonist (phentolamine), a beta-adrenoceptor antagonist (propranolol) and an antagonist of serotonin receptor (methysergide)) did not inhibit or attenuate the contractions induced by H2O2. Exposure of primary aortic smooth muscle cells to H2O2 (5 x 10(-6) to 5 x 10(-3) M) produced significant rises of intracellular Ca2+ ([Ca2+]i) within 20 s. Employment of 1.0 microM Fe2+ markedly enhanced the increment in [Ca2+]i in the smooth muscle cells. 10 microM proadifen treatment failed to alter the hydrogen peroxide-induced increment in [Ca2+]i of the smooth muscle cells. However, the presence of 5 microM indomethacin significantly attenuated the rise in [Ca2+]i in smooth muscle cells. The present results suggest that H2O2 can induce contractions of rat aorta segments, at pathophysiological concentrations, which are Ca2+-dependent. Hydroxyl radicals (.OH), cyclooxygenase products, protein kinase C and products of protein tyrosine phosphorylation appear to play some role in hydrogen peroxide-induced contractions. Metabolites catalyzed by cytochrome P450-dependent enzymes (upon treatment with hydrogen peroxide) appear to exert a vasodilator effect on rat aorta segments. Lastly, some unidentified mediators, produced by a cytochrome P450 inhibitor (proadifen), during hydrogen peroxide treatment, appear to play some role in contraction of vascular smooth muscle of rat aorta segments in vitro.

Characterization of a New Ion Selective Electrode for Ionized Magnesium in Whole Blood, Plasma, Serum, and Aqueous Samples

Results from a novel ion selective electrode (ISE) for ionized magnesium (Mg2+) correlate well with atomic absorption spectroscopy on aqueous solutions containing from 0.1-3.0 mmol MgCl2/L. Day to day precision (coefficient of variation) of the electrode on protein-based controls is < 4%; aqueous-based controls < 6%. The new ISE is selective for Mg2+ with a selectivity constant for Ca2+ (KMgCa) of 8 x 10(-2). Adding pathophysiologic concentrations of Cd2+, Ca2+, Cu2+, Fe3+, K+, Na+, or Zn2+ to serum and aqueous solutions gave negligible to minimal changes in measured Mg2+. Ligand binding studies in aqueous solution indicate that pathophysiologic concentrations of different anions (e.g. heparin, lactate, bicarbonate, phosphate, acetate and sulfate) bind to Mg2+, effectively reducing its concentration in solution. Likewise, silicon (as either found in Vacuutainer tubes or as chlorosilane) failed to exert any significant effect on measured Mg2+. Addition of Intralipid (up to 500 mg/dL) gave negligible to minimal changes in Mg2+. Mg2+ measurements on whole blood, plasma, and serum for a given human subjects samples are virtually identical, at least within the reference range for Mg2+. Typically, Mg2+ is 71% of TMg, but varies from subject to subject; i.e. Mg2+ cannot be predicted from TMg. Clinical studies revealed that the Mg2+/TMg ratio could be remarkably consistent in sequential samples (e.g., throughout the course of coronary bypass surgery) taken from one patient, but that this ratio could differ dramatically from the ratio in sequential samples taken from another. Mg2+ is held within a narrow range (0.53-0.67 mmol/L) in normal, healthy subjects when compared to TMg (0.70-0.96 mmol/L).

Intravenous Magnesium Sulfate Rapidly Alleviates Headaches of Various Types

Background.—Circumstantial evidence points to the possible role of magnesium deficiency in the pathogenesis of headaches and has raised questions about the clinical utility of magnesium as a therapeutic regimen in some headaches. Methods.—We evaluated the efficacy of intravenous infusion of I gram of magnesium sulfate (MgSO4) for the treatment of patients with headaches and attempted to correlate clinical responses to the basal serum ionized magnesium (IMg2+) level. We also determined if patients with certain headache types exhibit low serum IMg2+ as opposed to total serum magnesium. Using a case‐control comparison at an outpatient headache clinic, a consecutive sample of patients presenting with a moderate or severe headache of any type were included in the study. Of the 40 patients in the study (mean age 38.2 ± 9.4 years; range 14 to 55; 11 men [39.2 ± 7.3 years] and 29 women [37.8 ± 10.2 years]), 16 patients had migraines without aura, 9 patients had cluster headaches, 4 patients had chronic tension‐type headaches, and 11 had chronic migrainous headaches. Total serum magnesium was measured with atomic absorption spectroscopy and a Kodak Ektachem DT‐60. Sensitive ion selective electrodes were utilized to measure serum IMg2+ and ionized calcium (ICa2+); ICa2+/IMg2+ ratios were calculated. Results.—Complete elimination of pain was observed in 80% of the patients within 15 minutes of infusion of MgSO4. No recurrence or worsening of pain was observed within 24 hours in 56% of the patients. Patients treated with MgSO4 observed complete elimination of migraine‐associated symptoms such as photophobia and phonophobia as well as nausea Correlation was noted between immediate and 24‐hour responses with the serum IMg2+ levels. Immediate pain relief was observed in 32 (80%) of 40 patients (P <0.001). In 18 of the 32 patients, pain relief persisted for at least 24 hours (P<0.005) Of these 18 patients, 16 (89%) had a low serum IMg2+ level Total magnesium levels in contrast in all subjects were within normal range (0.70‐0.99 mmol/L). No side effects were observed, except for a brief flushed feeling. Of the 8 patients with no relief, only 37.5% had a low IMg2+ level. Patients demonstrating no return of headache or associated symptoms within 24 hours of intravenous MgSO4 exhibited the lowess initial basal levels of IMg2+. Non‐responders exhibited significantly elevated total magnesium levels compared to responders. Although most subcategories of headache types investigated (ie, migraine, cluster, chronic migrainous) exhibited low serum IMg2+ during headache and prior to intravenous MgSO4 the patients with cluster headaches exhibited the lowest basal levels of IMg2+ (P<0.01). All headache subjects except for the chronic tension group exhibited rather high serum ICa2+/IMg2+ ratios (P<0.01, compared to controls). Conclusions.—Intravenous infusion of 1 gram of MgSO4 results in rapid relief of headache pain in patients with low serum IMg2+ levels. Measurement of serum IMg2+ levels may have a practical application in many types of headache patients. Low serum and brain tissue ionized magnesium levels may precipitate headache symptoms in susceptible patients.

Magnesium and Vascular Tone and Reactivity

It has been reported that the concentration of extracellular magnesium ions ([Mg++]o) can affect blood flow, blood pressure and vascular reactivity in intact mammals. The present

Endothelium-dependent relaxation in coronary arteries requires magnesium ions

A great deal of interest has recently focused upon the mechanism(s) associated with the generation and action of endothelium‐derived relaxant factors (EDRFs) in blood vessels. Since we have shown that extracellular magnesium ions ([Mg2+]o) are important in control of coronary vascular tone and reactivity, we wondered whether these divalent cations play any role in the generation or action of EDRF in coronary arterial smooth muscle. Using isolated canine coronary arterial rings, we have now found that removal of [Mg2+]o inhibits the ability of these vascular preparations to relax when challenged with acetylcholine; in the absence of [Mg2+]o, the relaxation concentration‐response curves for acetylcholine are shifted markedly to higher concentrations with small maxima. It, thus, appears that [Mg2+]o is an important co‐factor for acetylcholine‐induced endothelium‐dependent relaxation in canine coronary arteries. These findings support our previous hypothesis that dietary deficiency of Mg may be an important factor in aetiology of coronary vasopasm.

Low levels of serum ionized magnesium are found in patients early after stroke which result in rapid elevation in cytosolic free calcium and spasm in cerebral vascular muscle cells

Ninety-eight patients admitted to the emergency rooms of three urban hospitals with a diagnosis of either ischemic stroke or hemorrhagic stroke exhibited early and significant deficits in serum ionized Mg2+ (IMg2+), but not total Mg, as measured with a unique Mg2+-sensitive ion-selective electrode. Twenty-five percent of these stroke patients exhibited >65% reductions in the mean serum IMg2+ found in normal healthy human volunteers or patients admitted for minor bruises, cuts or deep lacerations. The stroke patients also demonstrated significant elevation in the serum ionized Ca2+ (ICa2+)/IMg2+ ratio, a sign of increased vascular tone and cerebrovasospasm. Exposure of primary cultured canine cerebral vascular smooth muscle cells to the low concentrations of IMg2+ found in the stroke patients, e.g. 0.30-0.48 mM, resulted in rapid and marked elevations in cytosolic free calcium ions ([Ca2+]i) as measured with the fluorescent probe, fura-2, and digital image analysis. Coincident with the rise in [Ca2+]i, many of the cerebral vascular cells went into spasm. Reintroduction of normal extracellular Mg2+ ion concentrations failed to either lower the [Ca2+]i overload or reverse the rounding-up of the cerebral vascular cells. These results suggest that changes in Mg2+ metabolism play important roles in stroke syndromes and in the etiology of cerebrovasospasm associated with cerebral hemorrhage.

Magnesium, Electrolyte Transport and Coronary Vascular Tone

SummaryCoronary heart diseases (CHD) have high indices of mortality and morbidity. A number of CHD and myocardial ischaemic syndromes such as unstable angina pectoris, sudden death ischaemic heart disease, acute myocardial infarction and ventricular arrhythmias have been associated with losses of myocardial magnesium and potassium.Mg++ ions are essential for regulation of Na+ and K+ transport across cell membranes, including those found in cardiac and vascular smooth muscle cells. Mg++ activates an Na+-K+-ATPase pump which in turn plays a major role in regulating Na+-K+ transport. Loss of cellular Mg++ results in loss of critically important phosphagens: MgATP and creatine phosphate. Thus, under conditions where cellular Mg++ is depleted (e.g. hypoxia, ischaemia, anoxia), the Na+-K+ pump and phosphagen stores will be compromised, leading to alterations in resting membrane potentials. Cellular Mg++ depletion has been found to result in concomitant depletion of K+ in a number of cells, including cardiac and vascular muscles. The consequences of these events are often production of cardiac arrhythmias. Myocardial and vascular injury lead to disturbances in electrolyte transport across cell membranes, whereby Na+ and Ca++ uptakes are enhanced and, just prior or concomitantly, Mg++ and K+ are lost. Such electrolyte disturbances often lead to necrotic foci.Considerable evidence has accumulated to indicate that the extracellular concentracion of Mg++ is important in control of arterial tone and blood pressure via regulation of vascular membrane Mg++-Ca++ exchange sites. A reduction in the extracellular Mg++ concentracion can produce hypertension, coronary vasospasm and potentiation of vasoconstrictor agents by allowing excess entry of Ca++; concomitantly, the potency of vasodilator agents is reduced. Alterations in vascular membrane Mg++ result in arterial and nrteriolar membranes which are ‘leaky’, thus contributing to the cellular reduction in K+ and gain of Na+ and Ca+. Alterations in extracellular K+ or Na+ concentracions over physiological ranges, in the face of a Mg++ deficit, can exacerbate the coronary vasospasm noted with reduction in only extracellular Mg++. Since free Mg++ ions are necessary for maintaining Ca+ ions (both plasma membrane-bound and sarcoplasmic reticulum membrane-bound via Ca++ ATPases), intracellular free Mg++ would rise in conditions which result in cellular loss of Mg++, thereby exacerbating and contributing to elevation of blood pressure and coronary vasospasm. Data are also accumulating to suggest that Mg++ may control a Na+-Ca++ pump which is essential for maintenance of normal coronary vascular tone.Overall, the available data suggest that Mg++ plays critical roles in maintenance of normal cardiac excitability and coronary vascular tone. Based on these and other findings reviewed herein, magnesium prophylaxis and intervention may be extremely useful in the prevention and treatment of CHD, sudden death ischaemic heart disease, hypertension and acute myocardial infarction.RésuméL’insuffisance coronarienne est grevée d’indices élevés de mortalité et de morbidité. Un certain nombre de formes cliniques d’insuffisance coronarienne et d’ischémie myocardique ont été mises en rapport avec un déficit en potassium et en magnésium. C’est le cas de l’angine de poitrine instable, de la mort subite par ischémie myocardique, de l’infarctus aigu du myocarde et des arythmies ventriculaires.Les ions Mg++ sont essentiels au transport normal du Na+ et du K+ à travers la membrane cellulaire y compris celle du muscle cardiaque et celle du muscle vasculaire lisse. Le Mg++ active la Na+ -K+ -ATPase de la pompe membranaire laquelle régularise le transport Na+ -K+. Une perte de Mg + + cellulaire équivaut à une perte de phosphagènes dont le rôle est primordial à savoir le MgA TP et le phosphate de créatinine. Ainsi, dans les circonstances où existe une déplétion en Mg + + (i.e. l’hypoxie, l’ischémie, l’anoxie), le fonctionnement de la pompe Na+ -K+ et les réserves en phosphagènes sont compromises et il en résulte des altérations dans le potentiel membranaire de repos. La perte de Mg+ + entraîne une perte de K+ dans de nombreuses cellules y compris celles du coeur et du muscle vasculaire. La conséquence en est souvent le développement d’arythmies. Les altérations du myocarde et des vaisseaux coronaires conduit à des modifications du transport électrolytique à travers la membrane cellulaire où l’entrée de Na+ et de Ca+ sont facilités parallèlement à une perte de Mg+ + et K+. Ceci favorise la formation de foyers de nécrose.On possède des preuves solides de l’importance de la concentration en Mg+ + extracellulaire dans le contrôle du tonus artériel et de la pression sanguine par le biais de sites d’échanges Mg + + -Ca + + au niveau de la membrane vasculaire. Une diminution du Mg+ + extracellulaire peut être à l’origine d’une réaction hypertensive, d’un spasme coronaire et d’une synergie d’effet avec des substances vasoconstrictrices en favorisant une entrée massive de Ca+ +. Parallèlement, l’effet des vasodilatateurs est diminué. Des modifications du Mg+ + de la membrane vasculaire rendent ‘perméables’ les artères et artérioles ce qui contribue à l’appauvrissement cellulaire en K+ et à son enrichissement en Na+ et Ca+ +. Une élévation du K+ ou du Na+ extracellulaires au-dessus des concentrations physiologiques accompagnée d’un déficit en Mg+ + peut renforcer le spasme coronaire observé avec la seule diminution de Mg + + extracellulaire. Puisque le Mg + + libre est indispensable au maintien d’un Ca+ + normal (Ca+ + lié à la membrane cytoplasmique et au réticulum endoplasmique via les Ca+ + ATPases) le Mg+ + libre intracellulaire s’élèverait dans les situations où existe une perte cellulaire de Mg + + contribuant de ce fait à une élévation de la pression sanguine et à un renforcement du spasme coronarien. Il existe aussi des données qui tendent à prouver que le Mg + + peut exercer un contrôle sur la pompe Na + -Ca + + qui est essentielle au maintien d’une tonicité normale des vaisseaux coronaires.Au total, les données disponibles suggèrent que le Mg+ + joue un rôle fondamental dans le maintien d’un niveau normal d’excitabilité myocardique et de tonicité coronaire. Elles conduisent à penser que l’utilisation prophylactique et curative du magnésium est d’une grande utilité pour la prévention et le traitement de l’insuffisance coronarienne, la mort subite par ischémie myocardique, l’hypertension, l’infarctus aigu du myocarde.ZusammenfassungKoronare Herzkrankheiten (KHK) besitzen eine hohe Mortalitüt und Morbiditüt. Eine A nzahl von KHK und ischümischen Myokardsyndromen wie instabile Angina pectoris, plötzlicher Herztod bei ischümischer Herzkrankheit, akuter Herzinfarkt und ventrikulüre Arrhythmien wurden mit Verlusten an myokardialem Magnesium und Kalium in Verbindung gebracht.Mg++ -Ionen sind für die Regulation des Na+ - und K+ -Transports durch die Zellmembran einschlieβlich der von kardialen und vaskulüren glatten Muskelzellen erforderlich. Mg+ + aktiviert die Na+ -K+ -ATPase die ihrerseits eine bedeutende Rolle für die Regulation des Na+ -K+ -Transports spielt. Ein Verlust an zellulürem Mg+ + führt zu Verlusten der kritisch wichtigen Phosphagene MgATP und Kreatinphosphat. Unter den Bedingungen, die zu einer zellulären Mg+ + -Erschöpfung führen (z.B. Hypoxie, Ischümie, Anoxie) wird die Funktion der Na+ -K+ -Pumpe und der Phosphagenspeicher beeinträchtigt was zu Veränderungen der Ruhemembranpotentiale führt. Es wurde nachgewiesen, daβ eine zellulüre Mg+ + -Verarmung in einer Anzahl von Zellen, einschlieβlich der Herz- und Gefäβmuskeln, eine gleichzeitige Verarmung an K+ ergibt. Die Konsequenzen dieser Ereignisse ist häufig das Auftreten von Herzarrhythmien. Myokardiale und vaskulüre Schüdigungen führen zu Störungen im Elektrolyttransport durch die Zellmembran, wodurch die Aufnahme von Na+ undCa+ + verstärkt und unmittelbar vorher oder gleichzeitig Mg+ + undK+ verloren gehen. Solche Elektrolytstörungen führen oft zu nekrotischen Bezirken.Die vorliegenden Befunde zeigen daβ die extrazelluläre Konzentration von Mg+ + für die Kontrolle des arteriellen Tonus und Blutdrucks via Regulation der vaskulären Mg+ + -Ca+ + -Membranaustauschstellen wichtig ist. Eine Senkung der extrazellulären Mg+ + -Konzentration kann eine Hypertonie, koronare Vasospasmen und eine Potenzierung der Wirkung von Vasokonstriktoren durch einen gesteigerten Einstrom von Ca + + hervorrufen; gleichzeitig wird die Wirksamkeit von Vasodilatatoren vermindert. Veränderungen des Mg + + in der Gefäβmembran führen zu ‘durchlässigen’ und arteriolären Membranen, und begünstigen dadurch die Abnahme des intrazellulären K+ und Zunahme von Na+ und Ca+ +. Liegen neben dem Mg + + -Mangel Veränderungen der extrazellulären K+ - und Na+ -Konzentrationen vor, die über den physiologischen Bereich hinausgehen, kann dies zu einer weiteren Verschlimmerung der koronaren Vasospasmen führen. Da freie Mg++ -Ionen zum Erhalt von Ca++ -Ionen notwendig sind (sowohl über die Plasmamembran-gebundenen als auch am sarkoplasmatischen Retikulum Membran-gebundenen Ca+ + -ATPasen), steigt das intrazellulüre freie Mg+ + unter Bedingungen, die zu einem Verlust von zellulärem Mg + + und dadurch zu Exazerbationen und Beteiligungen an der Erhöhung des Blutdrucks und koronarer Vasospasmen führen. Es hüufen sich auch Hin weise, die vermuten lassen, daβ Mg + + eine Na+ -Ca++ -Pumpe kontrolliert, die zurAufrechterhaltung eines normalen Koronargefäβtonus erforderlich ist.Insgesamt lassen die verfügbaren Daten vermuten, daβ Mg + + eine kritische Rolle für die Aufrechterhaltung der normalen kardialen Erregbarkeit und des koronaren Gefüβtonus spielt. Aufgrund dieser und anderer hier besprochener Befunde kann die Magnesium-Prophylaxe und Intervention für die Verhinderung und Behandlung von KHK, plötzlichem Tod bei ischämischer Herzkrankheit, Hypertonie und akutem Myokardinfarkt, auβerordentlich nützlich s

Alcohol, the cerebral circulation and strokes ☆

Inasmuch as ethanol is thought to exert its major effects on the CNS, it is important to determine whether this abused substance can exert any direct action on cerebral blood vessels. Since chronic ingestion of alcohol: (1) can produce a loss (and degeneration) of neurons and glial cells in the brain, and (2) is associated, often, with hallucinations in human subjects particularly those undergoing withdrawal, it is possible that ethanol could produce hypoxia in select regions of the brain. The available indirect evidence in man and animals, albeit equivocal, does indicate that ethanol in certain concentrations might produce deficits in cerebral blood flow in select regions of the brain. Direct in-situ observations on the rat brain, using high-resolution, quantitative TV image-intensification microscopy, indicates that administration of ethanol, irrespective of the route of administration (e.g., perivascularly, intraarterially or systemically), produces graded concentration-dependent spasms of arterioles and venules. Concentrations of ethanol approximately greater than 250 mg/dl produce intense spasms resulting in rupture of these vessels. Recent in-situ studies in conscious dogs, using radiolabelled microspheres, also indicate that ethanol can produce deficits in regional brain blood flow. Studies with isolated canine middle cerebral and basilar arteries clearly demonstrate that low concentrations of ethanol (e.g., (less than 10 mM) can produce concentration-dependent spasms by a direct vascular action. Collectively, these new findings could be used to support the concept that heavy use of alcohol or binge-drinking can produce stroke-like effects. Specific calcium antagonists prevented or reversed the alcohol-induced cerebrovasospasms in rats and may prove valuable in treating the hypertension and strokes observed in heavy users of alcohol.