Abnormal magnesium status in patients with ... - Clinical Science

Clinical Science (2000) 98, 175–181 (Printed in Great Britain)

Abnormal magnesium status in patients with cardiovascular diseases Shota SASAKI*, Tetsuya OSHIMA†, Hideo MATSUURA*, Ryoji OZONO†, Yukihito HIGASHI*, Nobuo SASAKI*, Toshiyuki MATSUMOTO†, Yukiko NAKANO†, Atsushi UEDA*, Atsunori YOSHIMIZU*, Satoshi KURISU*, Masayuki KAMBE† and Goro KAJIYAMA* *First Department of Internal Medicine, Hiroshima University School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima, Japan 734-8551, and †Department of Clinical Laboratory Medicine, Hiroshima University School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima, Japan 734-8551

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To investigate magnesium status in patients with cardiovascular diseases and in those presenting high factors for these diseases, we measured the concentrations of serum total Mg, serum ionized Mg and intra-erythrocyte Mg. Mg is an important cofactor for many enzymes, especially those involved in phosphate transfer reactions. Mg deficiency has been shown to be associated with fatal cardiovascular diseases, as well as with risk factors for these diseases. Only measurement of the serum concentration of total Mg is routinely available, but ionized Mg is the physiologically active component. Furthermore, most of the body’s Mg is present in the intracellular space. Subjects included patients with ischaemic heart disease (n ¯ 80), cardiac arrhythmia (n ¯ 60), diabetes mellitus (n ¯ 36), essential hypertension (n ¯ 194) and hypercholesterolaemia (n ¯ 60). The same measurements were made in healthy controls (30 men and 26 women ; mean age 58³11 years). The serum ionized Mg concentration was measured with a selective ion electrode. The intra-erythrocyte Mg concentration was measured by atomic absorption. No gender difference was found for any Mg parameter, nor was age related to any Mg parameter. The serum albumin concentration was positively correlated only with the serum total Mg concentration. Although the serum total Mg concentration was similar in all groups, patients with diabetes mellitus and arrhythmia had lower serum levels of ionized Mg. Patients with essential hypertension exhibited higher intra-erythrocyte Mg concentrations than the healthy controls. Thus the measurement of serum total Mg concentration may obscure the presence of extracellular Mg deficiency in patients with arrhythmia and diabetes mellitus. Furthermore, the intracellular accumulation of Mg does not support the hypothesis of Mg deficiency in patients with essential hypertension.

INTRODUCTION Magnesium (Mg) is the fourth most abundant cation in the human body and the second most abundant intracellular cation, and is an important cofactor for many enzymes, especially those involved in phosphate transfer reactions [1,2]. Mg is therefore essential in the regulation of metabolism of other ions and cell functions. Recently, interest has been focused on the clinical importance of altered Mg status [3–5]. Mg deficiency has been shown to

be associated with fatal cardiovascular diseases, such as cardiac arrhythmia and coronary heart disease, as well as with risk factors for these diseases, such as hypertension, hypercholesterolaemia and diabetes mellitus [6–10]. Administration of Mg has been demonstrated to reduce the frequency of cardiac arrhythmia and sudden myocardial death [11,12]. Clinical evaluation of Mg status has been limited by the lack of suitable technology for measuring this cation. Indeed, only measurement of the serum concentration of

Key words : arrhythmia, cardiovascular disease, diabetes mellitus, hypertension, magnesium. Correspondence : Dr Shota Sasaki (e-mail sshota!mcai.med.hiroshima-u.ac.jp).

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S. Sasaki and others

total Mg is routinely available. In serum, Mg is present in three distinct fractions in equilibrium : ionized, complexed, and bound to serum protein. Because ionized Mg is physiologically active, the measurement of ionized Mg would be useful. Recently, a new electrode for the stable, reproducible measurement of ionized Mg has been made available [13,14]. Most of the body’s Mg is present in the intracellular space. Because the amount of Mg present in serum represents less than 1 % of total-body Mg, the serum Mg concentration may be a poor indicator of systemic Mg status. Measurement of intracellular Mg might better reflect a functional deficiency or excess of systemic Mg. Blood cells have frequently been used for the analysis of cellular cation metabolism. The intraerythrocyte concentration of Mg may reflect Mg status because of the relatively long life of this type of cell. The present study evaluated the serum concentrations of total Mg and of ionized Mg. We also measured the total intracellular Mg concentration in erythrocytes. Findings in patients with ischaemic heart disease, cardiac arrhythmia, diabetes mellitus, essential hypertension and hypercholesterolaemia were compared with Mg concentrations measured in healthy controls.

METHODS

tension was excluded by appropriate clinical examination. Patients receiving anti-hypertensive treatment were also included in the group. The 60 patients with hypercholesterolaemia had a serum concentration of total cholesterol of " 5.7 mmol}l or were receiving antihyperlipidaemic treatment. Patients were excluded from the study if they had renal failure, congestive heart failure or severe metabolic disturbances. In a preliminary study, we examined the effects of 3 months of treatment with anti-hypertensive drugs on Mg metabolism. Calcium antagonists, β-blockers or angiotensin-converting enzyme inhibitors did not change Mg status, but diuretics reduced serum total and ionized Mg concentrations. Therefore patients receiving diuretics were excluded from the present study. The healthy controls exhibited normal findings on physical and laboratory examinations (systolic}diastolic blood pressure ! 140}90 mmHg ; fasting blood sugar ! 6.7 mmol}l ; haemoglobin A1c ! 6.0 % ; total cholesterol ! 5.4 mmol}l). Chest radiographs and ECGs were also normal. Control subjects were recruited from healthy members of the medical staff and people who were undergoing annual medical check-ups. The study was approved by the Ethical Committee of the First Department of Internal Medicine, Hiroshima University School of Medicine. Informed consent for participation was obtained from each subject.

Subjects The 430 patients (246 Japanese men and 184 Japanese women ; mean age 61³12 years) attended the outpatient clinic at the First Department of Internal Medicine, Hiroshima University School of Medicine during the course of 1 month ( June 1998). Healthy controls (30 men and 26 women ; mean age 58³11 years) were also studied. The patients were grouped according to the main diagnosis as follows. Eighty patients suffered from ischaemic heart disease (including angina pectoris and previous myocardial infarction), which was diagnosed according to conventional criteria and confirmed by coronary angiography. Sixty patients had cardiac arrhythmia, defined as more than 1000 symptomatic supraventricular extra systoles daily or ventricular extra systoles (more than Lown grade 3) diagnosed by 24-h Holter ECG. Structural heart disease, such as cardiomyopathy, valvular disease and congenital heart disease, was excluded by echocardiography, and ischaemic heart disease was excluded by coronary angiography. The 36 patients with diabetes mellitus were diagnosed according to World Health Organization criteria [15], or were receiving anti-hyperglycaemic treatment other than insulin. The largest group comprised 194 patients with essential hypertension. Hypertension was defined as a systolic blood pressure of " 160 mmHg and}or a diastolic blood pressure of " 90 mmHg in the sitting position on at least three separate occasions when measured in the outpatient clinic. Secondary hyper# 2000 The Biochemical Society and the Medical Research Society

Procedures Venous blood was drawn in the morning under fasting conditions. Routine chemical methods were used to determine serum concentrations of total cholesterol, haemoglobin A1c and albumin. The serum total Mg concentration was measured with an auto-analyzer (enzymic method with glucokinase). The serum ionized Mg concentration was measured with a selective ion electrode (NOVA 8 ; NOVA Biomedical, Waltham, MA, U.S.A.). For calculation of intra-erythrocyte total Mg concentration, erythrocytes were washed three times with ice-cold isotonic choline chloride solution. Atomic absorption in haemolysates prepared from a known volume of cells previously packed in a microcapillary tube was measured by centrifugation at 15 000 g for 5 min. Intra-assay coefficients of variation for serum total, serum ionized and intra-erythrocyte Mg concentrations were 1.9 %, 3.6 % and 4.2 % respectively. Interassay coefficients of variation were 3.6 %, 3.9 % and 5.6 % respectively.

Statistics Data are presented as means³S.D. Significant differences between groups were evaluated by analysis of variance. This was followed by post-testing with Fisher’s protected least significant difference. Simple linear regression analysis was performed to assess the relationship between each Mg parameter and the subject’s age and

Mg status and cardiovascular diseases

serum concentration of total cholesterol, haemoglobin A1c and albumin. A level of P ! 0.05 was considered statistically significant.

RESULTS No gender difference was found for any Mg parameter in healthy controls or in any of the patient groups (Table 1). Age was not related to any Mg parameter in healthy controls or any of the patient groups (results not shown). The clinical characteristics of each group are shown in Table 2. Patients with hypercholesterolaemia exhibited significantly higher total cholesterol than the healthy controls. Patients with diabetes mellitus exhibited significantly higher haemoglobin A1c than the healthy controls. However, the serum concentrations of total cholesterol (results not shown) or haemoglobin A1c (Figure 1) were not related to any Mg parameter in healthy controls. In each of the patient groups, these two parameters were also not related to any Mg parameter (results not shown). The total population studied (n ¯ 486) exhibited a positive correlation between serum total Mg concentration and serum ionized Mg concentration (r ¯ 0.587, P ! 0.001 ; Figure 2). The intra-erythrocyte Mg concentration was not correlated with either of the other two Mg parameters. In addition, the serum albumin concentration was positively, but weakly, correlated with only one Mg parameter, i.e. the serum total Mg concentration (r ¯ 0.211, P ! 0.001 ; Figure 3). The three Mg parameters for each group are shown in Table 3. The serum total Mg concentration was similar in

Table 1

all groups. Patients with arrhythmia and those with diabetes mellitus exhibited significantly lower serum concentrations of ionized Mg than the healthy controls. The serum ionized Mg concentrations did not differ in the other groups. Patients with hypertension showed a significantly higher intra-erythrocyte Mg concentration. The other patients did not differ significantly from the healthy controls, although the patients with arrhythmia tended to have reduced intra-erythrocyte Mg concentrations (P ¯ 0.086 compared with healthy controls). There were no differences in the serum concentrations of ionized sodium, calcium or potassium among the study groups.

DISCUSSION In the present study, the serum total Mg concentration, which was positively correlated with the serum albumin concentration, was similar in all groups. A positive correlation between the serum concentrations of albumin and of total Mg may be expected, because the serum total Mg concentration includes complexed or protein-bound fractions. We could detect a significant decrease in the serum ionized Mg concentration, but not in the serum total Mg concentration, in diabetes mellitus and cardiac arrhythmia. The measurement of serum ionized Mg may be an earlier indicator of Mg deficiency in these patients than measurement of serum total Mg. Several investigators have reported a high incidence of systemic Mg deficiency in patients with diabetes mellitus [9,10]. However, it is not known whether the Mg

Mg parameters by gender in healthy control subjects

All data are expressed as means³S.D.

Male (n ¯ 30) Female (n ¯ 26)

Table 2

Serum total Mg (mmol/l)

Serum ionized Mg (mmol/l)

Intra-erythrocyte Mg (mmol/l of cells)

0.913³0.027 0.960³0.061

0.532³0.054 0.545³0.027

1.558³0.523 1.468³0.514

Demographic clinical characteristics of each study group

All data are expressed as means³S.D. Significance of differences : *P ! 0.05 compared with healthy controls. Group

Age (years)

Sex (M/F)

Albumin (g/dl)

Total cholesterol (mmol/l)

Haemoglobin A1c (%)

Healthy controls Patients Ischaemic heart disease Arrhythmia Diabetes mellitus Hypertension Hypercholesterolaemia

58³11

30/26

42.4³3.2

5.09³0.65

5.2³0.7

61³7 59³14 60³9 59³11 58³11

56/24 36/24 21/13 95/99 37/23

43.1³3.1 43.0³3.4 43.7³3.6 43.4³4.2 44.3³4.1

5.24³0.80 5.06³1.00 5.30³0.74 5.33³0.88 6.10³0.73*

5.9³1.2 5.5³1.2 7.9³1.2* 5.7³1.1 5.6³0.9


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Figure 1 Scatterplots showing the relationship between haemoglobin A1c concentration and the three Mg parameters in healthy controls

The concentrations of serum total Mg (A), serum ionized Mg (B) and intra-erythrocyte Mg (C) were not related to the haemoglobin A1c level (N.S., not significant).

Figure 2 Scatterplot showing the relationship between serum total and serum ionized Mg concentrations

The total population studied (n ¯ 486) exhibited a positive correlation between serum total Mg concentration and serum ionized Mg concentration. deficiency is a cause or a result of the diabetes mellitus. The effect of low Mg status on glucose disposition has been evaluated. A lower insulin response and an impairment of glucose tolerance was observed in Mg# 2000 The Biochemical Society and the Medical Research Society

Figure 3 Scatterplot showing the relationship between serum albumin concentration and serum total Mg concentration

The serum albumin concentration was positively correlated with the serum total Mg concentration. deficient rats [16]. Paolisso et al. [17] reported that Mg supplementation improves insulin sensitivity and glucose oxidation in the course of a euglycaemic}


Table 3

Mg parameters of each study group

All data are expressed as means³S.D. Significance of differences : *P ! 0.05 compared with healthy controls. Group Healthy controls Patients Ischaemic heart disease Arrhythmia Diabetes mellitus Hypertension Hypercholesterolaemia

Serum total Mg (mmol/l)

Serum ionized Mg (mmol/l)

Intra-erythrocyte Mg (mmol/l of cells)

0.935³0.105

0.538³0.042

1.517³0.516

0.949³0.082 0.923³0.085 0.913³0.084 0.934³0.077 0.944³0.084

0.542³0.042 0.520³0.048* 0.517³0.049* 0.533³0.042 0.530³0.052

1.513³0.437 1.420³0.447 1.617³0.549 1.695³0.503* 1.580³0.439

hyperinsulinaemic glucose clamp in patients with type II diabetes. Therefore a lowered serum Mg concentration may attenuate the secretion of insulin and the responsiveness of peripheral tissues to insulin. In addition, the development of diabetic complications may be associated with a lowered serum Mg concentration. Hatwal et al. [18] reported that patients with retinopathy had lower serum total Mg concentrations compared with diabetic patients without retinopathy. On the other hand, the effects of hyperinsulinaemia on Mg metabolism have also been evaluated. Several studies have suggested a shift of Mg from the serum to the intracellular space [19,20], or the urinary loss of Mg [21], under conditions of hyperinsulinemia. In the present study, only the serum ionized Mg concentration was decreased in diabetic patients. These findings are consistent with a previous study suggesting Mg deficiency in diabetes mellitus. However, the patients defined as suffering from diabetes mellitus represented a small group with relatively non-severe forms of the illness. Therefore the relationship between Mg deficiency and diabetes mellitus should be investigated in large populations in future studies. It has been suggested that Mg deficiency is associated with tachyarrhythmias, especially as a complication of acute myocardial infarction [22,23]. Mg infusion has been used clinically for treating patients with ventricular arrhythmia [24,25]. Furthermore, Zehender et al. [11] reported a decrease in ventricular extra systoles in patients administered oral Mg supplements. On the other hand, another study showed no effect of Mg infusion in acute myocardial infarction [26]. In an experimental study, Mg prolonged the PR interval (conduction time from the sino-atrial node to the left ventricle) as well as sino-atrial conduction, and increased the atrioventricular nodal refractory period [6]. Mg deficiency leads to a negative resting membrane potential, which may allow the cells to be depolarized more easily [27]. We showed a decrease in the serum ionized Mg concentration and a tendency for a reduced intra-erythrocyte Mg concentration in patients with cardiac arrhythmia. These findings may reflect the presence of a systemic deficiency

of Mg in tachyarrhythmia, and may neglect compartment maldistribution. As we have not performed a systemic Mg balance study, it is unclear whether the low Mg status results from decreased intake or increased loss. The role of Mg in blood pressure regulation remains unclear. It is controversial whether the oral administration of Mg lowered the blood pressure in intervention studies [28–30]. In essential hypertension the serum Mg concentration has been variously reported [31,32], and the intracellular Mg concentration has been reported to be higher [33], lower [8] or unchanged [34]. These inconsistent findings may be attributed to differences in methods of measurement or to racial differences. In the present study, in Japanese individuals, the concentrations of serum total and ionized Mg were unchanged, and the intra-erythrocyte Mg concentration was increased, in essential hypertension. This finding is consistent with our previous report of an increase in the intraplatelet concentration of free Mg in patients with essential hypertension [35], and with findings in spontaneously hypertensive rats [36]. In these human subjects and in the rats with genetic hypertension, the intracellular Ca#+ level was increased in the resting state and in stimulated conditions [37,38]. Mg is known to be a natural calcium antagonist [39]. The higher level of intracellular Mg observed in patients with essential hypertension may thus represent a compensatory mechanism. There are some limitations to our study. Our subjects were recruited from an outpatient clinic for cardiovascular diseases during 1 month. All participants in this period were included. The sample number was not manipulated. Thus there is a possibility that the sample number in the subgroups may be too small to detect small differences in Mg concentrations. In addition, some of the patients suffered from two or more diseases. In conclusion, the present study evaluated three Mg parameters to assess a possible role of Mg deficiency in patients with various cardiovascular diseases and in those presenting risk factors for such diseases. Although there were no differences in the serum total Mg concentration, a decrease in the serum ionized Mg concentration was # 2000 The Biochemical Society and the Medical Research Society

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observed in patients with diabetes mellitus and arrhythmia. In patients with essential hypertension, the concentrations of serum total and ionized Mg were normal, but the intra-erythrocyte Mg concentration was increased, which does not support the hypothesis of a Mg deficiency in essential hypertension. Measurements of the serum total Mg concentration alone may obscure the diagnosis of an abnormality in Mg metabolism in patients with these diseases.

ACKNOWLEDGMENTS We thank Yuko Omura for secretarial assistance.

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Received 30 March 1999/19 August 1999; accepted 9 November 1999


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