Interrelationship of dietary Mg intake and electrolyte homeostasis in hamsters: I. Severe Mg deficiency, electrolyte homeostasis, and myocardial necrosis

Reuniting overnutrition and undernutrition, macronutrients, and micronutrients

Over-nutrition and its late consequences are a dominant theme in medicine today. In addition to the health hazards brought on by over-nutrition, the medical community has recently accumulated a roster of health benefits with obesity, grouped under "obesity paradox." Throughout the world and throughout history until the 20^th century, under-nutrition was a dominant evolutionary force. Under-nutrition brings with it a mix of benefits and detriments that are opposite to and continuous with those of over-nutrition. This continuum yields J-shaped or U-shaped curves relating body mass index to mortality. The overweight have an elevated risk of dying in middle age of degenerative diseases while the underweight are at increased risk of premature death from infectious conditions. Micronutrient deficiencies, major concerns of nutritional science in the 20^th century, are being neglected. This "hidden hunger" is now surprisingly prevalent in all weight groups, even among the overweight. Because micronutrient replacement is safe, inexpensive, and predictably effective, it is now an exceptionally attractive target for therapy across the spectrum of weight and age. Nutrition-related conditions worthy of special attention from caregivers include excess vitamin A, excess vitamin D, and deficiency of magnesium.

Toward a broader understanding of aldosterone in congestive heart failure

Discovered some 50 years ago, aldosterone (ALDO) has come to be recognised as a mineralocorticoid hormone with well-known endocrine properties in epithelial cells that contribute to the pathophysiology of congestive heart failure. This includes Na + resorption at the expense of K+ excretion in classic target tissues: kidneys, colon, sweat and salivary glands. Though less well known, Mg2+ excretion is likewise enhanced by ALDO, while adrenal ALDO secretion is regulated by extracellular Mg2+ ([Mg2+ ]o). An emerging body of information has and continues to identify other endocrine actions of ALDO receptor-ligand binding. They include: promoting an efflux of cytosolic free Mg2+, or [Mg2+]i, in exchange for Na+ in such non-epithelial cells as peripheral blood mononuclear cells; its influence on endothelial cell function; and its central actions that involve regulation of cerebrospinal fluid composition produced by epithelial cells of the choroid plexus, activity of the hypothalamic paraventricular nucleus involved in Na+ appetite, Na+ and H2O excretion and sympathetic nerve activity, and the regulation of TNF-α production from central and/or peripheral sources. Extra-adrenal steroidogenesi and auto/paracrine properties of ALDO generated de novo in the cardiovasculature are now under investigation and preliminary findings suggest they contribute to tissue repair. The past decade has witnessed a revival of interest in this steroid molecule. In years to come, an even broader understanding of ALDO's contribution to the pathophysiology of congestive heart failure will undoubtedly emerge.

Magnesium deficiency in critical illness

Magnesium (Mg) deficiency commonly occurs in critical illness and correlates with a higher mortality and worse clinical outcome in the intensive care unit (ICU). Magnesium has been directly implicated in hypokalemia, hypocalcemia, tetany, and dysrhythmia. Moreover, Mg may play a role in acute coronary syndromes, acute cerebral ischemia, and asthma. Magnesium regulates hundreds of enzyme systems. By regulating enzymes controlling intracellular calcium, Mg affects smooth muscle vasoconstriction, important to the underlying pathophysiology of several critical illnesses. The principle causes of Mg deficiency are gastrointestinal and renal losses; however, the diagnosis is difficult to make because of the limitations of serum Mg levels, the most common assessment of Mg status. Magnesium tolerance testing and ionized Mg2+ are alternative laboratory assessments; however, each has its own difficulties in the ICU setting. The use of Mg therapy is supported by clinical trials in the treatment of symptomatic hypomagnesemia and preeclampsia and is recommended for torsade de pointes. Magnesium therapy is not supported in the treatment of acute myocardial infarction and is presently undergoing evaluation for the treatment of severe asthma exacerbation, for the prevention of post-coronary bypass grafting dysrhythmias, and as a neuroprotective agent in acute cerebral ischemia.

Aldosteronism: an immunostimulatory state precedes proinflammatory/fibrogenic cardiac phenotype

Chronic inappropriate (relative to dietary Na+ intake) elevations in circulating aldosterone (ALDO), termed aldosteronism, are associated with remodeling of intramural arteries of the right and left heart. Lesions appear at week 4 of treatment with ALDO and 1% dietary NaCl in uninephrectomized rats (ALDOST) and include invading monocytes, macrophages and lymphocytes with intracellular evidence of oxidative and nitrosative stress, myofibroblasts, and perivascular fibrosis. In this study, we tested the hypothesis that an immunostimulatory state with activated circulating peripheral blood mononuclear cells (PBMCs) precedes this proinflammatory and profibrogenic cardiac phenotype and is initiated by reduction in the cytosolic free Mg2+ concentration ([Mg2+]i). At 1 and 4 wk of ALDOST (preclinical and clinical stages, respectively), we monitored serum Mg2+, PBMC [Mg2+]i and cytosolic free [Ca2+] (via fluorimetry), and expressed genes (via microchip array) as well as markers of oxidative and nitrosative stress in plasma [alpha1-antiproteinase activity (alpha1-AP)] and cardiac tissue (immunohistochemical detection of gp91phox subunit of NADPH oxidase and 3-nitrotyrosine). Age- and gender-matched unoperated and untreated (UO) rats and uninephrectomized salt-treated (UN) rats served as controls. Serum [Mg2+] was unchanged by ALDOST. In contrast with UO and UN, [Mg2+]i and plasma alpha1-AP were each reduced (P < 0.05) at weeks 1 and 4. The decline in PBMC [Mg2+]i was accompanied by Ca2+ loading. Differential (twofold and higher) expression (up- and downregulation) in PBMC transcriptomes was present at week 1 and progressed at week 4. Involved were genes for the alpha1-isoform of Na+-K+-ATPase, the ATP-dependent Ca2+ pump, antioxidant reserves, inducible nitric oxide synthase, and PBMC activation with autoimmune responses. Expression of 3-nitrotyrosine and activation of gp91phox were seen in inflammatory cells that invaded intramural arteries. Thus early in aldosteronism (preclinical stage), an immunostimulatory state featuring activated circulating PBMCs with reduced ionized [Mg2+]i and oxidative and nitrosative stress precedes and may even predispose to coronary vascular lesions that first appear at week 4.

Aldosteronism revisited: perspectives on less well-recognized actions of aldosterone

Aldosterone is a mineralocorticoid with protean actions in both epithelial and nonepithelial cells. These include endocrine properties of circulating aldosterone that promote Na(+) resorption at the expense of well-recognized K(+) excretion and less well-recognized Mg(2+) excretion in classic target tissues: kidneys, colon, and sweat and salivary glands. The regulation of adrenal aldosterone secretion by [Mg(2+)](o) is also less well appreciated. More recently recognized endocrine actions of aldosterone include induction of Mg(2+) efflux in exchange for Na(+) in such nonepithelial cells as peripheral-blood mononuclear cells and influence on epithelial cells of the choroid plexus, where aldosterone alters the composition of cerebrospinal fluid that contributes to blood-pressure regulation. An association between primary aldosteronism and idiopathic intracranial hypertension has recently been reported. Extraadrenal steroidogenesis with de novo aldosterone production by the cardiovasculature, where its auto-/paracrine properties may contribute to tissue repair at sites of injury, has been observed. These less well-recognized actions of aldosterone have led to a revival of interest in how this steroid molecule contributes to the pathophysiology of various clinical disorders.

Captopril protects against myocardial injury induced by magnesium deficiency

We have previously reported that antioxidant drug intervention protects against magnesium deficiency-induced myocardial lesions. In the present study, Golden Syrian male hamsters were fed either a magnesium-deficient diet or a magnesium-supplemented diet. Animals from each group received sulfhydryl-containing angiotensin converting enzyme inhibitors: captopril, epi-captopril (a stereoisomer of captopril), and zofenopril* (arginine blend of zofenopril containing a free SH group); another group of animals received the non-sulfhydryl-containing angiotensin converting enzyme inhibitor enalaprilat. The animals were killed after 14 days, and their hearts were isolated for morphological and morphometric analyses. Hematoxylin and eosin-stained sections were examined by a computer image analysis system for a morphometric determination of the severity of myocardial injury. Captopril reduced both the density of lesions, from 0.32 to 0.08 lesions/(mm2) (p less than 0.01), and the area fraction of lesions, from 7.42 x 10(-4) to 2.03 x 10(-4) lesion area/(mm2) (p less than 0.01), as well as the degree of inflammatory infiltration around the blood vessels. Epi-captopril and zofenopril* were virtually equipotent to captopril, but enalaprilat afforded only slight (nonsignificant) protection. These results indicate that a significant component of the protective effect of captopril in this model was attributable to its sulfhydryl moiety, rather than solely due to the inhibition of the angiotensin converting enzyme. These data further support our previous findings of possible free radical participation in cardiomyopathy due to magnesium deficiency.

Myocardial magnesium: relation to laboratory and clinical variables in patients undergoing cardiac surgery

Magnesium concentration was measured in the right atrial appendage of 100 patients undergoing cardiac surgery and associations with serum and mononuclear blood cell magnesium, other laboratory values and patient clinical variables were studied. In addition, magnesium was measured in the right atrial appendage and left ventricular free wall in 23 autopsy subjects to determine whether there was a proportional relation between right atrial appendage and left ventricular free wall magnesium. The mean left ventricular free wall/right atrial appendage magnesium ratio was 2.13 +/- 0.39 (r = 0.67, p = 0.0009). In the group with cardiac surgery, the right atrial appendage magnesium concentration correlated inversely with age (r = -0.54, p = 0.001). The mean right atrial appendage magnesium concentration (micrograms/g wet weight tissue) was lower in patients with postoperative cardiac arrhythmia than in those without arrhythmia (103 +/- 13 versus 111 +/- 10, p = 0.009) and in diabetic than in nondiabetic patients (103 +/- 13 versus 109 +/- 12, p = 0.02). The right atrial appendage magnesium concentration also tended to be lower in patients receiving potassium/magnesium-losing diuretics, although this difference did not achieve statistical significance (105 +/- 14 versus 109 +/- 11, p = 0.16). Right atrial appendage magnesium concentration correlated positively with serum creatinine concentration (r = 0.31, p = 0.002) and negatively with serum calcium concentration (r = -0.29, p = 0.013). Serum magnesium did not correlate with right atrial appendage or mononuclear blood cell magnesium concentration or clinical variables. There was a statistically significant correlation between mononuclear blood cell and right atrial appendage magnesium concentrations in some subgroups of patients.

Magnesium deficiency-induced cardiomyopathy: protection by vitamin E

Syrian male hamsters weighing 80-100g were placed on either a magnesium deficient diet (MgD) or an identical diet supplemented with 100 mmols/Kg MgCl. Animals from each group received vitamin E 10, 15, and 25mg three-week slow release pellets, as subcutaneous implants. The animals were sacrificed after 14 days and their hearts isolated for morphological analysis. H&E stained sections were examined by a computer image analysis system for a morphometric determination of the severity of myocardial injury. Vitamin E significantly (p less than 0.01) reduced both the numerical density and the area fraction of MgD lesions. These data indicate possible free radical participation in the mechanism of injury.

Geographic variation in the incidence of myocardial calcification associated with acute myocardial infarction

There is reason to believe that calcium influx into heart muscle during acute myocardial infarction (AMI) can aggravate myocyte injury. Furthermore, the degree of such influx might correlate with the occurrence of microscopic myocyte calcification observed at autopsy. We have searched for evidence of myocyte calcification in hearts of patients found to have AMI at autopsy at the Veterans Administration Medical Center in Salt Lake City (SLCVA), a region with a low myocardial infection death rate, and at the George Washington University Medical Center in Washington, DC (GWUMC), a region with a high myocardial infection death rate. Of 23 consecutive cases examined under "blind" conditions at the GWUMC in which AMI was found, there were 15 instances of cardiac myocyte calcification observed in von Kossa-stained sections. Not a single example of myocyte calcification was found in 23 comparable cases at the SLCVA. The basis of this difference in myocyte calcification is unknown, but may be related to the fact that the Salt Lake City drinking water contains a higher level of magnesium, which is known to protect against soft tissue calcification, than does that of Washington, DC. This may be the basis for the apparent protection that dietary magnesium exerts against myocardial infarction death.

Effects of magnesium, ouabain and bumetanide on 86rubidium uptake in a human atrial cell line

1. The effects of extracellular magnesium concentrations (0, 0.6, 1.2 mM) on 86Rb (used as an analogue of potassium) uptake were investigated in the Girardi human atrial cell line in the presence and absence of drugs. 2. Increasing extracellular magnesium resulted in significantly higher 86Rb uptake. Compared to uptake in 0.6 mM (the physiological extracellular magnesium concentration), uptake of 86Rb was significantly higher in the 1.2 mM magnesium medium and significantly lower in the magnesium-free medium. 3. Ouabain (10(-3)M) and bumetanide (10(-4)M) were added to inhibit, respectively, the Na-K-ATPase and the Na-K-Cl co-transport system in the media containing the three magnesium concentrations. The ouabain-sensitive, bumetanide-sensitive and residual transport were found to be 58%, 29% and 13% of the total uptake in the medium containing 0.6 mM magnesium. 4. The ouabain-sensitive 86Rb uptake was inhibited significantly by reducing the magnesium concentrations to zero whereas the bumetanide-sensitive and residual uptake were not significantly affected by different magnesium concentrations. 5. At three different ouabain concentrations (10(-7) M, 10(-5) M, 10(-3) M) studied there was significantly greater uptake of 86Rb in 1.2 mM magnesium compared to uptake in 0 mM magnesium. 6. The present findings indicate that extracellular magnesium is important for 86Rb (potassium) transport in cardiac cells, and suggest that the main effect is on the Na-K-ATPase component of transport.

Intracellular magnesium of mononuclear cells from venous blood of clinically healthy subjects

Measurement of intracellular magnesium (Mg2+) may have advantages over serum Mg2+ measurements in the assessment of Mg2+ homeostasis in patients. An accurate and reproducible method of measuring the Mg2+ content of mononuclear cells (lymphocytes and monocytes) from the venous blood of human subjects utilizing commonly available reagents and equipment is described. A well-defined clinical population of 88 subjects (48 males and 40 females) was used as the normal population. The mean mononuclear cell Mg2+ content was 67.8 +/- 13.8 (SD) fg/cell giving a 95% confidence interval normal range of 40-95 fg/cell which compared favorably to the empirical 95% limits of 44-94 fg/cell. The serum Mg2+ measured at the same time was 0.90 +/- 0.06 (SD) mmol/l. No significant correlation was found when comparing serum and mononuclear cell Mg2+ concentrations. Additionally, no significant differences were found when serum and intracellular Mg2+ were analyzed for sex and age with the exception that intracellular Mg2+ tended to be slightly higher in the younger age groups. The routine performance of this assay may require as little as 7 ml of venous blood and was done with a coefficient of variation of 3.0-3.6%.

Acquired magnesium deficiency and myocardial tolerance to ischemia

Isolated hearts from rats subjected to high and low dietary Mg for 4 weeks were perfused with buffers containing 0.8 and 0.3 mM Mg, these values reflecting the plasma Mg concentrations on the two intakes. After a 20 min period of subtotal, global ischemia the recovery values of cardiac function were significantly lower in hearts from Mg-depleted rats perfused with 0.3 mM Mg, whereas the control values of cardiac function of the two groups were nearly identical before the ischemic episode. An additional group of hearts from Mg depleted rats perfused with 0.8 mM Mg did not present higher values of post-ischemic cardiac function compared to those perfused with 0.3 mM Mg. Rats fed a low dietary Mg had lower concentrations of Mg in plasma, bone and skeletal muscle, but not in the heart. The only myocardial electrolyte difference was a higher Na in the low dietary Mg group. It is concluded that an acquired extracellular Mg deficiency may reduce the myocardial tolerance to ischemia. The rapid restoration to high extracellular Mg in a prolonged Mg deficiency state did not improve the myocardial tolerance to an immediate ischemic episode.