Magnesium-dependent calcium efflux in mammalian heart muscle

Hypomagnesemic down-regulation of L-type Ca(2+) channel in cardiomyocyte as an arrhythmogenic substrate in rats

The present study was designed to investigate the effect of magnesium (Mg) depletion on the expression of voltage-gated calcium (Ca(2+)) channels and Ca(2+) currents in the heart and thereby on hypomagnesemic arrhythmogenesis in adult male rats. Male Wistar rats were fed an Mg-free diet or a normal diet for up to 16 weeks. Serum Mg concentrations were significantly reduced at week 4 or later with an Mg-free diet, which experimentally represents hypomagnesemia. Myocardial Mg contents were also reduced at week 16 accompanied by myocardial hypertrophy. Telemetric ECG recordings revealed a long-term changes of ECG parameters in hypomagnesemic rats; RR shortening, QT prolongation and appreciable PR prolongation. At the same time, hypomagnesemic rats demonstrate various bradycardiac arrhythmias including ventricular premature beats, atrioventricular blocks and sinus arrest, which were never recoded in rats fed by a normal diet. Electrophysiological studies elucidated that the L-type Ca(2+) channel current was decreased in Mg-deficient cardiomyocytes, and these findings were consistent with down-regulation of CaV1.2-mRNA but not in levels of CaV1.3, CaV3.1 or CaV3.2. These findings provide novel insights into hypomagnesemic electrophysiological disorders in the heart, and should be considered when assessing the design of effective antiarrhythmic treatments in patients with hypomagnesemia.

Accelerated thymus involution in magnesium-deficient rats is related to enhanced apoptosis and sensitivity to oxidative stress

Experimental Mg deficiency leads to alterations in the immune response. Reduction of thymus weight and histological changes were previously observed in Mg-deficient rats after several weeks on a deficient diet, suggesting that functions of this immune organ may be affected by Mg deficiency. More recently, changes in the immune system during early Mg deficiency were shown. Thus, in the present study we examined modifications in the thymus during the early stages of Mg deficiency in weanling rats. From our results, it appears that Mg deficiency accelerates thymus involution. The assessment of apoptosis (enumeration of apoptotic cells on the basis of morphological criteria and intranucleosomal degradation of genomic DNA) showed greater values in thymuses from Mg-deficient rats as compared with controls. This was observed very early, since a significant difference was shown on the second day of deficiency, before reduced weight of thymus, which was recorded in the later period. These results indicate the relationship of accelerated thymus involution with an active process of cell death. Mg deficiency led to histological changes in the thymus. In the early stage of deficiency (second day) the presence of inflammatory cells was shown, suggesting that the inflammatory process was already occurring in the tissue studied. Later (eighth day) an increased proportion of epithelial reticular cells in the cortex was shown, indicating a remodelling process occurring in this period. Enhanced susceptibility to peroxidation also occurred very early during Mg deficiency. It may be hypothesized that disturbances in Mg status of short duration could have cellular effects with various deleterious consequences.

Manganese alters mitochodrial integrity in the hearts of swine marginally deficient in magnesium

It was previously reported that pigs marginally deficient in magnesium (Mg) and fed diets high in manganese (Mn) died suddenly with signs of sudden cardiac death. Manganese, which has properties similar to Mg, may exacerbate Mg-deficiency and be accumulated by mitochondria resulting in ultrastructural damage. The objective of this study was to determine whether deaths of the type previously observed were mediated by adverse interactions of Mn and Mg resulting in ultrastructural damage to the myocardium, alterations in electrocardiographic recordings and tissue retention of Mn, Mg and calcium (Ca). Forty-eight pigs were fed one of six diets in a 2 X 3 factorial arrangement of Mg (100 or 1000 mg Mg/kg) and Mn (5, 50 or 500 mg Mn/kg) for 8 weeks. Left ventricle muscle samples were collected for examination by transmission electron microscopy. No differences in heart muscle ultrastructure were observed between pigs fed low and adequate dietary Mg. However, marked myocardial necrosis and mitochondrial swelling were observed in pigs fed high dietary Mn when combined with low Mg. Feeding low dietary Mg elevated minimum (P < 0.01), maximum (P < 0.05) and average (P < 0.001) heart rates. Low dietary Mg resulted in a 55% probability of a ventricular beat being recorded (P = 0.05) and lower Mg (P < 0.02) and Ca (P < 0.04) contents in heart atria and ventricles. These results suggest that high Mn, when fed in combination with low Mg, disrupts mitochondrial ultrastructure and is associated with the sudden deaths previously reported.

Exacerbated immune stress response during experimental magnesium deficiency results from abnormal cell calcium homeostasis

The aim of this study was to assess the potential mechanism underlying the enhanced inflammatory processes during magnesium deficit. In this study, exacerbated response to live bacteria and platelet activating factors was shown in rats fed a magnesium-deficient diet. Peritoneal cells from these animals also showed enhanced superoxide anion production and calcium mobilising potency following in vitro stimulation. The latter effect occurred very early in the course of magnesium deficiency. These studies first showed that an abnormal calcium handling induced by extracellular magnesium depression in vivo may be at the origin of exacerbated inflammatory response.

Hypomagnesemia: characterization of a model of sudden cardiac death

OBJECTIVES

We sought to compare the incidence of sudden death in rats treated with magnesium-deficient and control diets and to address the electrophysiologic characteristics associated with these end points.

BACKGROUND

Although magnesium deficiency is associated with an increased incidence of sudden cardiac death in patients, there has been no clear cause and effect relation because of a number of covariables, including diuretic use, hypokalemia, digitalis use and left ventricular dysfunction.

METHODS

Hypomagnesemic rats and their paired control rats underwent in vivo electrophysiologic studies and measurements of the total calcium and magnesium content of their cardiac ventricles

RESULTS

Serum magnesium levels were 0.5 +/- 0.3 mEq/liter (mean +/- SD) in hypomagnesemic animals and 1.2 +/- 0.9 mEq/liter in control animals. A modest but significant prolongation of the repolarization time was seen at the apical epicardial site (83 +/- 8 ms in hypomagnesemic rats vs. 68 +/- 13 ms in control rats, p < 0.05), but not at the other sites studied. Bradyarrhythmias and tachyarrhythmias were observed in 82% of the hypomagnesemic rats during the in vivo electrophysiologic studies, compared with 0% in the control group. During these studies, sudden, unexpected asystolic deaths were observed in 4 of 11 hypomagnesemic rats and 0 of 8 control rats. Polymorphic nonsustained ventricular tachycardia was provoked by rapid pacing in 5 to 11 hypomagnesemic rats and 0 of 8 control rats. Three of six hypomagnesemic rats exposed to auditory stimuli developed seizures, followed immediately by sudden deaths-two due to asystole and one due to ventricular fibrillation-although no end points occurred in the control animals.

CONCLUSIONS

In this model, magnesium deficiency results in sudden cardiac death. The presence of startle induction of sudden death preceded by seizures suggests that sudden cardiac death results from a neurologic trigger.