Magnesium deficiency prolongs myocardial stunning in an open-chest swine model

Hypomagnesemia and inflammation: clinical and basic aspects

In recent years, increasing awareness of hypomagnesemia has resulted in clinical trials that associate this mineral deficiency with diabetes, metabolic syndrome, and drug therapies for cancer and cardiovascular diseases. However, diagnostic testing for tissue deficiency of magnesium still presents a challenge. Investigations of animal and cellular responses to magnesium deficiency have found evidence of complex proinflammatory pathways that may lead to greater understanding of mediators of the pathobiology in neuronal, cardiovascular, intestinal, renal, and hematological tissues. The roles of free radicals, cytokines, neuropeptides, endotoxin, endogenous antioxidants, and vascular permeability, and interventions to limit the inflammatory response associated with these parameters, are outlined in basic studies of magnesium deficiency. It is hoped that this limited review of inflammation associated with some diseases complicated by magnesium deficiency will prompt greater awareness by clinicians and other health providers and in turn increase efforts to prevent and treat this disorder.

The nerve-heart connection in the pro-oxidant response to Mg-deficiency

Magnesium is a micronutrient essential for the normal functioning of the cardiovascular system, and Mg deficiency (MgD) is frequently associated in the clinical setting with chronic pathologies such as CHF, diabetes, hypertension, and other pathologies. Animal models of MgD have demonstrated a systemic pro-inflammatory/pro-oxidant state, involving multiple tissues/organs including neuronal, hematopoietic, cardiovascular, and gastrointestinal systems; during later stages of MgD, a cardiomyopathy develops which may result from a cascade of inflammatory events. In rodent models of dietary MgD, a significant rise in circulating levels of proinflammatory neuropeptides such as substance P (SP) and calcitonin gene-related peptide among others, was observed within days (1-7) of initiating the Mg-restricted diet, and implicated a neurogenic trigger for the subsequent inflammatory events; this early "neurogenic inflammation" phase may be mediated in part, by the Mg-gated N: -methyl-D-aspartate (NMDA) receptor/channel complex. Deregulation of the NMDA receptor may trigger the abrupt release of neuronal SP from the sensory-motor C-fibers to promote the subsequent pro-inflammatory changes: elevations in circulating inflammatory cells, inflammatory cytokines, histamine, and PGE(2) levels, as well as formation of nitric oxide, reactive oxygen species, lipid peroxidation products, and depletion of key endogenous antioxidants. Concurrent elevations of tissue CD14, a high affinity receptor for lipopolyssacharide, suggest that intestinal permeability may be compromised leading to endotoxemia. If exposure to these early (1-3 weeks MgD) inflammatory/pro-oxidant events becomes prolonged, this might lead to impaired cardiac function, and when co-existing with other pathologies, may enhance the risk of developing chronic heart failure.

The role of magnesium in the endothelial dysfunction caused by global ischemia followed by reperfusion: in vitro study of canine coronary arteries

OBJECTIVE

To study the role of magnesium in the endothelial dysfunction of canine coronary arteries caused by cardiopulmonary bypass (CPB) global ischemia followed by reperfusion.

DESIGN

Segments of canine coronary arteries were suspended in organ chambers to measure isometric contraction by prostaglandin F (2alpha), and relaxed by acetylcholine (ACh), sodium fluoride (NaF), calcium ionophore (A23187) and sodium nitroprusside (SNP) in crescent concentrations. The investigation protocol had groups with six dogs: CONTROL group (without CPB), CPB group (105 min of CPB without aortic cross-clamping), ISCH group (45 min of CPB with aortic cross-clamping), ISCH/REP group (45 min of aortic cross-clamping followed by 60 min of reperfusion). The coronary relaxations were evaluated with (phase I), without (phase II) and restored magnesium (phase III) to the organ bath.

RESULTS

The presence of magnesium in the organ bath was associated with the greater relaxation in response to agonists of the nitric oxide production. The removal of magnesium from the organ bath was associated with the reduction in the intensity of vessel relaxation. The magnesium restoration to the organ bath was associated with the additional reduction in the intensity of relaxation with the exception of NaF that allowed re-acquisition of the relaxation observed in the presence of magnesium.

CONCLUSION

This in vitro study demonstrates that magnesium ion favorably influences the nitric oxide production by the coronary endothelium, attenuating the endothelial dysfunction caused by global ischemia followed by reperfusion.

Magnesium reduces free radical concentration and preserves left ventricular function after direct current shocks

OBJECTIVE

Our objective was to determine if magnesium reduces free radicals generated by direct current countershock and preserves left ventricular contractile function.

BACKGROUND

We have previously shown that magnesium reduces free radicals in a coronary occlusion-reperfusion model, and therefore also might reduce free radical generation by direct current shocks.

METHODS

In eight swine weighing 18-27 kg (mean: 22 kg), using electron paramagnetic resonance, we monitored continuously the coronary sinus concentration of ascorbate free radical, a measure of free radical generation (total oxidative flux). Epicardial shocks (30 J) using a truncated exponential biphasic waveform (5/5 ms) were administered. Each animal received two shocks, one without and one with magnesium, 80 mg/min IV, beginning 10 min before the shock and continuing to 15 min after the shock. Percent fractional area shortening of the left ventricular cavity was determined by 2-dimensional echocardiography.

RESULTS

Magnesium shocks resulted in a significantly lower increase in the ascorbate free radical concentration (0.6+/-4.6%) than no-magnesium shocks (16+/-3.3%, P<0.05) at 12 min after the shock. Total radical flux was reduced 72% (P<0.05), and left ventricular fractional area shortening was preserved: baseline: 69+/-2.6%, no-magnesium shocks: 41+/-2.8% (P<0.05, versus baseline) and magnesium shocks 61+/-3.7%.

CONCLUSIONS

Magnesium pre-treatment reduced oxygen free radicals generated by direct current shocks; post-shock left ventricular contractile function was not impaired. Magnesium may be cardioprotective during epicardial ('surgical') defibrillation.

Does magnesium have a role in the treatment of patients with coronary artery disease?

Hypomagnesemia is common in hospitalized patients, especially in elderly patients with coronary artery disease (CAD) and/or those with chronic heart failure. Hypomagnesemia is associated with increased all cause mortality and mortality from CAD. Magnesium supplementation improves myocardial metabolism, inhibits calcium accumulation and myocardial cell death; it improves vascular tone, peripheral vascular resistance, afterload and cardiac output, reduces cardiac arrhythmias and improves lipid metabolism. Magnesium also reduces vulnerability to oxygen-derived free radicals, improves endothelial function and inhibits platelet function, including platelet aggregation and adhesion, which potentially confers upon magnesium physiologic and natural effects similar to adenosine-diphosphate inhibitors such as clopidogrel. However, data regarding the use of magnesium in patients with acute myocardial infarction (AMI) are conflicting. Although some previous relatively small randomized clinical trials demonstrated a remarkable reduction in mortality when intravenous magnesium was administered to relatively high risk AMI patients, two recently published large-scale randomized clinical trials (the Fourth International Study of Infarct Survival [ISIS 4] and Magnesium in Coronaries [MAGIC]) were unable to demonstrate any advantage of intravenous magnesium over placebo. Nevertheless, the theoretical benefits of magnesium supplementation as a cardio-protective agent in CAD patients, promising results from animal and human studies, its relatively low-cost and ease of handling requiring no special expertise, together with its excellent tolerability, gives magnesium a place in treating CAD patients, especially in those at high risk, such as CAD patients with heart failure, the elderly and hospitalized patients with hypomagnesemia. Furthermore, magnesium therapy is indicated in life-threatening ventricular arrhythmias such as torsades de pointes and intractable ventricular tachycardia.

Bioavailability and pharmacokinetics of magnesium after administration of magnesium salts to humans

Therapeutically, magnesium salts represent an important class of compounds and exhibit various pharmacologic actions. Examples of magnesium salts are ionic magnesium and magnesium citrate in nephrolithiasis, magnesium salicylate in rheumatoid arthritis, magnesium hydroxide as an antacid as well as a cathartic, and magnesium mandelate as urinary antiseptic. Various anions attached to the cation magnesium, such as oxide, chloride, gluconate, and lactate, affect the delivery of the amounts of elemental magnesium to the target site and thereby produce different pharmacodynamic effects. This review examines the bioavailability and pharmacokinetics of various magnesium salts and correlates pharmacodynamic action with the structure-activity relationship.

Effect of magnesium sulfate pretreatment and significance of matrix metalloproteinase-1 and interleukin-6 levels in coronary reperfusion therapy for patients with acute myocardial infarction

Magnesium (Mg) inhibits the influx of calcium in vascular smooth muscle cells. The purposes of this study were to test the hypothesis that an intravenous administration of magnesium might effect the complement response and to determine the effects of a magnesium pretreatment of patients with acute myocardial infarction (AMI) on the incidence of reperfusion injuries. Thirty-eight AMI patients were treated with coronary reperfusion therapy within 6 hours of onset. They were randomly divided into two groups: group pretreated with intravenous magnesium sulfate (0.27 mmol/kg) (magnesium group, n = 19), and nonpretreated controls (placebo group). The reperfusion injuries observed within 1 hour after the coronary reperfusion included arrhythmias, aggravated chest pain, and ST segment elevation in 12-lead electrocardiograms. Coronary recanalization was performed in 36 patients. The incidence of reperfusion arrhythmia was significantly lower in the magnesium group than in the placebo group (17% vs 78%, p<0.001). At the postreperfusion stage, there was a tendency for the degree of ST segment reelevation in the magnesium group lower than in the placebo group (2.5 +/- 2.3 mm vs 4.7 +/- 3.8 mm, p = 0.07). No marked difference was observed in the incidence of chest pain aggravation between the two groups (67% vs 73%, ns). The peak serum levels of interleukin-6 (IL-6) were significantly lower in the magnesium group than those in the placebo group (38.9 +/- 25.0 vs 92.3 +/- 76.5 pg/mL, p = 0.016). The peak serum levels of matrix metalloproteinase-1 (MMP-1) were lower than those in the placebo group (16.2 +/- 4.8 vs 19.7 +/- 9.0 ng/mL, p = 0.09), but the difference was not significant. A positive correlation was observed between the peak MMP-1 values and the peak IL-6 values (r = 0.57, p = 0.001) in all patients. Increased serum ionized Mg2+ may inhibit arrhythmic recurrence and the production of IL-6 and MMP-1 after reperfusion and prevent the increase of myocardial lesions caused by calcium overload on myocytes. The increased IL-6 production may induce MMP-1, leading to tissue organ injury. Pretreatment with magnesium sulfate may protect the myocardium of AMI patients from reperfusion injuries.

Acute myocardial infarction without thrombolytic therapy: beneficial effects of magnesium sulfate

Only one third of hospitalized patients with acute myocardial infarction (AMI) receive thrombolytic therapy despite its proven benefits on outcomes. Elderly patients, have a greater risk of death during myocardial infarction; however, thrombolytic therapy appears to be less used in these patients, as compared to the general AMI-patients. In order to evaluate the impact of magnesium supplementation in AMI-patients without thrombolytic therapy, 194 patients participated in a prospective, randomized and placebo-controlled study: 96 patients received a 48-hour intravenous magnesium sulfate and 98 isotonic glucose as placebo. Magnesium infusion reduced the incidence of arrhythmias, congestive heart failure and in-hospital-mortality compared with placebo (27 vs. 40%, p = 0.04; 18 vs. 23%, p = 0.27; 4 vs. 17%, p < 0.01, respectively); in the subgroup of elderly patients (> 70 years), the benefit was also obvious (42 vs. 50%; 18 vs. 25%; 9 vs. 23%, p = 0.09, respectively). These data suggest that intravenous magnesium supplementation might be justified in order to reduce myocardial damage and mortality rate in subsets of high-risk patients such the elderly and/or patients not suitable for thrombolysis. Additional trials appear to be indicated to evaluate the potential benefit of magnesium in well defined specific subsets of AMT-patients.

Is there a place for magnesium in the treatment of acute myocardial infarction?

Infusions of solutions of magnesium sulfate for patients with acute myocardial infarction were shown by a meta-analysis of seven small studies and a larger study of 2316 patients (LIMIT-2) to have clinical efficacy. However, the ISIS-4 study of 58,050 patients found no improvement in short-term mortality rates with magnesium therapy in patients with acute myocardial infarction. In this article we explore the following four differences between the ISIS-4 study and the earlier studies: (1) Time of initiation of magnesium treatment after acute myocardial infarction and thrombolytic therapy; (2) dosage of magnesium in the first 24 hours after acute myocardial infarction; (3) duration of magnesium infusion after acute myocardial infarction; and (4) differences in patient risks in control and treatment groups. These four differences may explain the different outcomes among these studies and indicate the type of additional studies that are needed to define the clinical utility of magnesium infusion in the treatment of patients with acute myocardial infarction.

Magnesium in acute myocardial infarction: overview of available evidence

Despite improvements in the outcome of patients with acute myocardial infarction (MI) during the past three decades, room for improvement exists in elderly patients and in patients who are not candidates for thrombolysis. Animal models suggest that magnesium supplementation before reperfusion reduces infarct size. Statistical analysis of the randomized trials of magnesium in MI reveals a gradient of response. When higher risk patients were enrolled, a greater benefit of magnesium was observed; progressively smaller benefits of magnesium occurred as the control group mortality approached 7%, at which point no benefit was detected. Although the ISIS-4 study enrolled more than 58,000 patients, no reduction in mortality was seen, probably as a result of a low control group mortality and relatively late administration of the magnesium. Because the potential benefit of magnesium in MI remains an open question, additional trials are needed before this inexpensive and easily administered therapy is prematurely cast aside.

The rationale of magnesium as alternative therapy for patients with acute myocardial infarction without thrombolytic therapy

Only one third of hospitalized patients with acute myocardial infarction receive thrombolytic therapy despite its proven benefits on outcomes. Elderly patients, for example, have a greater risk of death after myocardial infarction, but studies demonstrate that thrombolytic therapy is less likely to be used in older patients. Intravenous magnesium supplementation, both theoretically and experimentally, has been demonstrated to decrease myocardial damage and reduce the mortality rate in subsets of patients, including the elderly and/or patients not suitable for thrombolysis, if it is administered before reperfusion occurs. The aim of this study is to review the rationale for patients with acute myocardial infarction without thrombolytic therapy.

ISIS 4: clinical controversy regarding magnesium infusion, thrombolytic therapy, and acute myocardial infarction

Magnesium (Mg) infusions over 24 hours were given to patients with suspected acute myocardial infarction (AMI) at least 2 hours after thrombolysis. Patients showed no benefit and even some increased risk in contrast to reduction in mortality obtained by Mg therapy in smaller trials. Results of all of the studies were pooled and statistically analyzed, according to a fixed-effects model that is inappropriate for studies of different protocols. The panel concluded that further study of Mg in AMI is not needed. This conclusion has been questioned.