The biochemical function of Mg ²⁺ in insulin secretion, insulin signal transduction and insulin resistance

ARTICLE

Auteur(s) : Theodor Günther

Charité – Universitätsmedizin Berlin, Campus Benjamin Franklin, Institut für Molekularbiologie und Biochemie, Berlin, Germany

In the following review the direct and indirect functions of Mg²⁺ in the secretion and signal transduction of insulin are described and the possible role of Mg²⁺ in the development of insulin resistance is discussed.

Function of Mg²⁺ in insulin secretion

ATP binds Mg²⁺ and closes ATP-sensitive K⁺ channels, resulting in depolarization of the cell membrane. The depolarization opens voltage-sensitive Ca²⁺ channels and induces Ca²⁺ influx.

The Ca²⁺ channel seems to be unspecific. At increased glucose concentration, β-cells also take up ²⁸Mg²⁺ [1]. Extracellular Mg²⁺ inhibits Ca²⁺ influx competitively [2, 3]. Thus a reduction in the concentration of extracellular free Mg²⁺ ([Mg²⁺]_o) results in an increased Ca²⁺ influx and increased concentration of intracellular free Ca²⁺ ([Ca²⁺]_i). The increased [Ca²⁺]_i stimulates insulin secretion by β-cells, as was found in experiments with an insulinoma cell line [3]. This effect of extracellular Mg²⁺ on [Ca²⁺]_i and insulin secretion may occur at slightly reduced [Mg²⁺]_o within the normal range of [Mg²⁺]_o. It was found that, in healthy human subjects with 0.79 mM plasma Mg²⁺, fasting plasma insulin amounted to 23 μU/mL, while in healthy subjects with 0.87 or 1.00 mM plasma Mg²⁺, fasting plasma insulin amounted to 11 μU/mL [4].

However, there are inconsistent results concerning the effect of [Mg²⁺]_o on the insulin plasma concentration in experiments with Mg-deficient rats. An increased plasma insulin concentration was found in Mg-deficient rats [5]. In other experiments with Mg-deficient rats the plasma insulin concentration was not significantly changed [6-10]. The missing increased plasma insulin concentration in Mg-deficient rats may be explained by the reduction of protein and insulin biosynthesis, dependent on the degree and duration of Mg deficiency. Following i.p. injection of glucose into Mg-deficient rats, plasma insulin increased only to 32.9 μU/mL, and to 67.9 μU/mL in the control rats. A similar result was obtained in an i.v. glucose tolerance test with Mg-deficient rats [10]. Additional factors such as gut-derived peptide hormones and gastric emptying affect insulin secretion [11].

Function of Mg²⁺ in insulin binding and insulin signal transduction

The binding of insulin to its receptor results in receptor autophosphorylation and internalization of this complex. Internalized insulin receptors phosphorylate IRS 1-6 (insulin receptor substrate 1-6) and other kinases in the insulin signaling cascade [14, 15]. In these reactions, Mg²⁺ is operating together with ATP as a kinase substrate. Additionally, a second Mg²⁺ is bound to a regulatory site of the insulin receptor tyrosine kinase (IRTK). The apparent affinity of the IRTK for MgATP increased as the concentration of free Mg²⁺ increased, and the apparent affinity of the IRTK for free Mg²⁺ increased as the concentration of MgATP increased [16]. A second Mg²⁺ is an essential activator for receptor-type and soluble protein tyrosine kinases (PTKs) [17, 18]. The binding of the two Mg²⁺ to the IRTK and the crystal structure of this complex were determined [19].

In various cell types in diabetes, as well as in obesity, intracellular Ca²⁺ is increased [20, 21]. It has been discussed that the increased [Ca²⁺]_i may play a significant role in diabetes. For review see [21]. It may be possible that intracellular free Ca²⁺ interacts with intracellular free Mg²⁺ and competitively inhibits IRTK. However, it is uncertain whether Ca²⁺ inhibits the essential second Mg²⁺ in PTKs. In experiments with a soluble PTK, 10 μM Ca²⁺ in the presence of 0.2 mM ATP and 6 mM Mg²⁺ had no catalytic effect [22]. Unfortunately, the Mg²⁺ and Ca²⁺ concentration used did not correspond to the physiological [Mg²⁺]_i and [Ca²⁺]_i.

The action of intracellular Mg²⁺ and Ca²⁺ on PTKs cannot be quantified. The surface of the inner site of the cell membrane is negatively charged according to the zeta potential. Consequently, Mg²⁺ and Ca²⁺ are enriched at the inner surface of the cell membrane near to the IRTK. The concentration of MgATP^2- is reduced by the same mechanism. Hence the exact values of [Mg²⁺]_i, [Ca²⁺]_i and [MgATP^2-] at their binding sites are uncertain. Moreover, the Mg²⁺ kinetic (K_m values) of the various protein kinases and the Mg²⁺-dependent protein phosphatases in the phosphorylation/dephosphorylation processes in insulin signal transduction have not been studied. Therefore it is not known at which level of Mg²⁺ saturation these enzymes are operating in insulin-dependent cells.

There are controversial results on insulin signaling in Mg deficiency. In rats that had been fed an Mg-deficient diet for 4 days, insulin-stimulated autophosphorylation of the insulin receptor and IRTK activity of partially purified skeletal muscle insulin receptor were reduced, although autophosphorylation of the insulin receptor and IRTK activity were measured in the presence of 10 mM Mg²⁺ [10]. On the other hand, in skeletal muscle from rats that had been fed an Mg-deficient diet for 6 and 11 weeks, neither insulin-stimulated phosphorylation of the insulin receptor nor IRS-1 and IRS-1/PI (phosphatidyl inositol) 3-kinase association were changed. After 11 weeks of Mg deficiency, insulin stimulated phosphorylation of the insulin receptor, as well as IRS-1 and IRS-1/PI 3-kinase association in the liver, were increased [23]. There is no explanation for the controversial results or for the tissue-specific effect of Mg deficiency.

Mg²⁺ in obesity and insulin resistance

The most important risk factor in insulin resistance is obesity. It has been suggested that obesity-induced insulin resistance involves an alteration in Mg²⁺ metabolism plus other mechanisms (see below).

Total serum Mg²⁺ concentration in obese children was slightly lower than in lean children, amounting to 0.748 mM compared to 0.801 mM [25]. In patients with metabolic syndrome without diabetes, total serum Mg²⁺ was reduced from 1.0 mM to 0.74 mM. Total Mg²⁺ in mononuclear cells was reduced by 41%. There was an inverse correlation between serum Mg²⁺ and Mg²⁺ in mononuclear cells with body mass index [26]. Contrarily it was found that the Mg²⁺ concentration in plasma, erythrocytes and platelets was not changed in obese subjects [27]. Other studies have reported either a negative or no relationship between serum Mg²⁺ and body mass index [28]. For more literature on the relationship between Mg²⁺ and obesity see [146].

Obese Zucker fat rats were used as a model for the role of obesity in insulin resistance and diabetes type 2. Mg²⁺ supplementation of obese Zucker fat rats reduced blood glucose concentration resulting in a drop in the rate of diabetic rats from 8 of 8 to 1 of 8. [Mg²⁺]_o in the lean rats amounted to 0.38 mM, in the obese rats to 0.41 mM and in the Mg²⁺ supplemented obese rats to 0.59 mM [29]. In a similar experiment, total serum Mg²⁺ and [Mg²⁺]_o in obese Zucker fat rats was about 15% lower than in the lean controls. Glucosuria and plasma insulin concentration were reduced by feeding the high Mg²⁺ diet [30]. In another experiment with Zucker fat rats, total plasma Mg²⁺ concentration in the obese rats amounted to 0.49 mM and to 0.60 mM in the lean controls. A high fiber diet increased plasma Mg²⁺ to 0.67 mM in the lean rats and to 0.73 mM in the obese rats. The insulin concentration was halved in both groups by the high fiber diet [31]. Also, in epidemiological studies, the inverse association between Mg²⁺ intake and metabolic syndrome was greatly reduced or abolished for adjusting for other intakes, particularly fiber [28].

The probable effect of Mg²⁺ supplementation may be caused by the high Mg²⁺ concentration in the intestine, followed by increased fluid volume. Thus carbohydrates and enzymes are more diluted. Hydrolysis of carbohydrates within the intestinal lumen, glucose absorption and the increase in plasma glucose in the Mg²⁺-supplemented obese Zucker fat rats may be slower than in the controls. A fiber-rich diet may have a similar effect.

More important in obesity is the endocrinological function of adipocytes. Various effectors such as leptin, adiponectin, IL-1, IL-6, IL-8, IL-18, TNF-α, resistin, ghrelin, visfatin, orexin, adipsin and cortisol [24, 32-42] are produced by adipocytes or are related to obesity. Additionally, noradrenaline [27], adrenaline [43] and ROS [44-48] are involved in insulin resistance. Some of these effectors (IL-1, IL-6, IL-8, TNFα, noradrenaline, adrenaline and ROS) are increased in Mg deficiency and obesity [27, 33, 49-55, 147]. Elevated concentrations of TNF-α were related to low serum Mg²⁺ concentrations in obese subjects [37]. Via these effectors, Mg deficiency may enhance insulin resistance.

Leptin deficiency or leptin resistance induces obesity and insulin resistance via central and peripheral effects [24]. The production of leptin is affected by nutrients (glucose, amino acids, fatty acids), hormones and cytokines (insulin, glucocorticoids, catecholamines, TNF-α, IL-6). Their relative importance and mechanisms of action remain unclear [148].

The effects of adiponectin are mediated through the binding of adiponectin to one of two adiponectin receptors and subsequent activation of AMP-activated protein kinase and downstream signaling molecules. Adiponectin augments insulin-mediated tyrosine phosphorylation of the insulin receptor and IRS-1 [56]. Finally, adiponectin decreases plasma glucose by increasing glucose uptake and by suppressing glucose production in the liver, and decreases FFA in serum by increased oxidation of FFA in muscle [57]. Adiponectin levels are decreased in obesity [58-60]. A 21% reduction of body mass index increased adiponectin by 46% [59]. Adiponectin synthesis and adiponectin concentration in plasma have been positively correlated to the intake of fiber and Mg²⁺ [61, 62]. The synthesis of adiponectin is induced by PPARγ (peroxisome proliferator-activated receptor γ) [58, 63]. The higher oligomeric forms of adiponectin are responsible for its actions [63]. The synthesis of adiponectin is inhibited by insulin, catecholamines, glucocorticoids, IL-1β, IL-6, TNF-α and ROS [33, 54, 64, 65].

The increased plasma insulin concentration in insulin resistance can induce an additional secretion of catecholamines [66].

Catecholamines stimulate lipolysis in adipose tissue [67], resulting in an increase in saturated FFA. FFA are an important link between obesity and insulin resistance. FFA are elevated in most obese subjects. FFA inhibit insulin-stimulated peripheral glucose uptake, decrease muscle glycogen synthesis and stimulate insulin secretion [68]. FFA, particularly palmitate, reacts with Toll-like receptors, which associate with MyD 88 (myeloid differentiation factor 88) and activates JNK (c-Jun N-terminal kinase), IKK (IκB kinase) and PKC (protein kinase C). These protein kinases phosphorylate serine residues of IRS-1 and IRS-2, resulting in an inhibition of IRS-1 and IRS-2 and thus blocking the insulin signal [69-71]. Adrenaline infusion in rats reduced insulin-stimulated tyrosine phosphorylation of the insulin receptor and IRS-1 in muscle. IRS-1 contains over 30 potential serine/threonine phosphorylation sites in motifs recognized by various protein kinases [43]. Thus catecholamines induce serine phosphorylation via cAMP and PKA. Moreover, cAMP increases the activity of endogenous protein tyrosine phosphatase [43]. There are about 150 protein serine/threonine phosphatases and protein tyrosine phosphatases. Some of them are regulated by phosphorylation/dephosphorylation and some are activated by Mg²⁺ [72]. By these mechanisms, catecholamines contribute to insulin resistance.

Additionally, palmitate, which is increased in obesity, decreases the expression of PPARγ coactivator-1α and -1β which are regulators of mitochondrial biosynthesis and function in skeletal muscle. Thus, the expression of many mitochondrial tricarboxylic acid cycle and oxidative phosphorylation genes is reduced [73]. PPARγ coactivator-1α overexpression has caused hepatic insulin resistance and improved muscle insulin sensitivity [74].

TNF-α binds to two distinct cell surface receptors followed by a signaling cascade and stimulation of genes for IL-1α, IL-1β, IL-6, IL-8 and IL-18 [75]. TNF-α also induces serine phosphorylation of IRS-1 via activation of JNK and IKK, thus blocking insulin signaling [53].

IκB, the inhibitor protein of NF-κB (nuclear factor-κB), is phosphorylated by IKK-β which is phosphorylated and activated by serine kinases [46]. Thereafter, IκB is degraded. NF-κB is transferred to the nucleus, is bound to DNA and regulates the transcription of a large number of genes, including genes of the proinflammatory effectors TNF-α and IL-6 [69, 70]. The action of NF-κB can be suppressed by activation of PPARγ [24, 53].

The JNK and NF-κB pathways can be activated by ROS [46-48, 53] which are elevated in Mg deficiency [51].

TNF-α and IL-6 reduce the expression of adiponectin [58] and impair insulin signaling [33]. IL-6 secretion is stimulated by β-adrenergics [33]. Finally, IL-6 impairs insulin-induced glycogenesis and induces gluconeogenesis, leading to hyperglycemia and to compensatory hyperinsulinemia [33].

IL-1 is another cytokine that is increased in Mg deficiency [49] and obesity [54]. After i.p. injection of IL-1β, the concentration of glucose in blood and glycogen in liver were decreased, probably by increased insulin-independent glucose transport into adipocytes or muscle cells [76].

IL-1β inhibited insulin-induced phosphorylation of the insulin receptor, IRS-1 and other kinases of the insulin-signaling cascade. Treatment of adipocytes with IL-1β decreased the production of adiponectin and increased IL-6 secretion and led to the down-regulation of PPARγ [54].

IL-1 receptor antagonist is increased in the adipose tissue and serum of obese humans. The IL-1 receptor antagonist binds to IL-1 receptors, antagonizing the effects of IL-1α and IL-1β. The functional interaction between IL-1 and IL-1 receptor antagonists in obesity remains speculative [77]. The IL-1 receptor antagonist reduces insulin sensitivity through a muscle-specific decrease in glucose uptake [78].

Adipose tissue can produce steroid hormones including cortisol [24]. Glucocorticoid binds to its receptor, the receptor translocates to the nucleus where it binds to glucocorticoid response elements on DNA and regulates the transcription of specific genes. One of the consequences is increased expression of the p85α subunit of the PI 3-kinase in excess to the p110 catalytic subunit. Excess free p85α competes with PI 3-kinase for binding to IRS-1 and reduces the insulin-induced activation of PI 3-kinase, thus contributing to insulin resistance [79].

Inflammatory processes in obesity and Mg deficiency

Extracellular Mg²⁺ in diabetes mellitus type 2

The frequency of hypomagnesemia in diabetes type 2 was reported to vary from 25% to 39% [81] or from 25% to 48% [82]. It was argued that [Mg²⁺]_o would be a better indicator than total serum Mg²⁺. [Mg²⁺]_o in diabetes type 2 patients was reduced from 0.52 mM to 0.49 mM without a significant alteration in total serum Mg²⁺ [83]. In another study, [Mg²⁺]_o in control subjects amounted to 0.630 mM and to 0.552 mM in type 2 diabetic subjects. Total serum Mg²⁺, amounting to 0.86 mM in the controls and to 0.81 mM in the diabetics, was not significantly different [84]. In non-diabetic subjects, [Mg²⁺]_o amounted to 0.49 mM and in diabetic obese subjects to 0.45 mM [85].

On the other hand, increased total serum Mg²⁺ (controls: 0.89 mM, non-insulin treated diabetics: 0.95 mM) [86] and increased [Mg²⁺]_o (controls: 0.51 mM, gestational diabetics: 0.57 mM) [87] have been reported. The values for [Mg²⁺]_o in diabetics [83-85] are within the reference interval for normal individuals [88].

The reduction of extracellular Mg²⁺ in some diabetic patients may result from Mg²⁺ loss through osmotic diuresis and insufficient Mg²⁺ supplied by food, particularly when the resorption of Mg²⁺ in the intestine and kidney is reduced, for example by mutation of TRPM-6 [89]. Thus hypomagnesemia and the reduction of [Mg²⁺]_o in some diabetic type 2 patients is a secondary effect.

It has been discussed that diabetic complications may be enhanced by hypomagnesemia. There are correlations between the reductions of total and free Mg²⁺ in serum and the increase in HbA_1c and diabetic complications [90].

The diabetic complications are caused by hyperglycemia. Hyperglycemia leads to increased non-enzymatic glycation of proteins and via various steps to advanced glycation end products (AGEs) [91] and ROS [45-48]. As mentioned above, the formation of ROS is enhanced by Mg deficiency [51]. AGEs react with AGE receptors on various cell types [92] and induce signal transduction via ROS and activation of NF-κB and the production of cytokines, growth factors and matrix proteins [45-48].

Hyperglycemia increases the concentration of dihydroxyacetone phosphate, glycerol phosphate, de-novo sythesis of diacylglycerol and activation of PKCβ and PKCδ and leads finally to microvascular complications in the retina, kidney and nerves. For details see [44]. These mechanisms indicate that hyperglycemia is a primary cause and hypomagnesemia and diabetic complications are secondary to hyperglycemia.

Alteration of [Mg²⁺]_i by insulin, catecholamines and hyperglycemia

Insulin

The Mg²⁺ efflux via the Na⁺/Mg²⁺ antiport of liver plasma membrane vesicles (LPMV) from streptozotocin-diabetic rats was 3 times the Mg²⁺ efflux from controls. The Na⁺/Mg²⁺ antiport from diabetic rats lost reversibility, only operating as a Mg²⁺ efflux [95].

In human platelets, the addition of 100 or 200 μU/mL insulin increased [Mg²⁺]_i from 0.614 to 1.270 mM [96], in other experiments from 0.276 to 0.355 mM [97]. Again, [Ca²⁺]_i was not changed by insulin [97].

The insulin-induced increase in [Mg²⁺]_i may be caused by Mg²⁺ uptake. Incubation of rat uteri with 0.1 U/mL insulin increased the total Mg²⁺ content by 15% or 7%, dependent on [Mg²⁺]_o [98].

Perfusion of isolated hearts with 10 mU/mL insulin induced Mg²⁺ uptake [99] and increased [Mg²⁺]_i as measured by ³¹P NMR [100]. Insulin-induced Mg²⁺ uptake in heart muscle cells is coupled to the activity of insulin-sensitive GLUT 4 [99].

On the other hand, perfusion of liver with insulin did not induce Mg²⁺ uptake [101] and did not change [Mg²⁺]_i as measured by ¹³C NMR [102].

A lack of insulin, as in streptozotocin- or alloxan-treated animals, did not significantly change total Mg²⁺ content in skeletal muscle or liver [103, 104].

Contrarily to lymphocytes and platelets, 10 ng/mL insulin increased [Ca²⁺]_i in isolated normal adipocytes from 146 to 391 nM. Adipocytes isolated from obese subjects expressed a higher [Ca²⁺]_i (203 nM). Insulin increased [Ca²⁺]_i in adipocytes from obese subjects only to 230 nM, indicating insulin resistance [105]. Insulin may have reduced the activity of Ca ATPase in adipocytes.

The Mg²⁺ efflux via the Na⁺/Mg²⁺ antiport from Mg²⁺-loaded erythrocytes was enhanced by insulin, due to an increased affinity for extracellular Na⁺. The effect of insulin was mediated through the activation of PI 3-kinase [106]. Also, protein phosphatases may be involved in insulin-stimulated Mg²⁺ efflux [106]. On the other hand, incubation of normal human erythrocytes with 200 μU/mL insulin increased [Mg²⁺]_i from 0.208 mM to 0.264 mM and [Ca²⁺]_i from 19.8 to 63.3 μM [107].

These results indicate that insulin affects [Mg²⁺]_i and [Ca²⁺]_i differently, dependent on the tissue and experimental conditions. The reason is not known.

Catecholamines

The Mg²⁺ efflux via the Na⁺/Mg²⁺ antiport in various cell types, such as lymphocytes, HL-60 cells, Ehrlich ascites tumor cells, liver cells and heart muscle cells, is increased by β-adrenergics via cAMP and PKA. For a review see [108]. Phosphorylation of the Na⁺/Mg²⁺ antiporter may increase its affinity for intracellular Mg²⁺, resulting in an increased Mg²⁺ efflux. If the Mg²⁺ efflux exceeds a certain degree, for example in heart muscle cells [100] or lymphocytes [109], [Mg²⁺]_i is reduced [108]. A noradrenaline-induced decrease in [Mg²⁺]_i in lymphocytes occurred at unchanged ATP content [109].

The β-adrenergic- or 8BrcAMP-induced Mg²⁺ efflux from perfused liver was inhibited by 10 mU/mL insulin. Insulin may activate a calmodulin-activated phosphodiesterase that degrades cAMP to AMP [101]. The insulin-induced increase in [Mg²⁺]_i in heart muscle cells was abolished by isoproterenol [101]. Insulin may act in heart muscle cells by the involvement of PKC [101].

Contrarily to heart muscle cells, isoproterenol increased [Mg²⁺]_i in platelets of healthy individuals dose-dependently from 0.234 mM to 0.681 mM and was ineffective in the platelets of obese subjects [110]. The different effects of β-adrenergics on [Mg²⁺]_i in various cell types and their missing effect in obesity is not elucidated. It must be considered that high doses of insulin and catecholamine were applied.

Hyperglycemia

These alterations in [Mg²⁺]_i, [Ca²⁺]_i and pH_i were obtained in experiments within a short period of time, up to 3 hours. Cells are able to normalize altered intracellular ion concentrations by regulated transport systems. Reduction of [Mg²⁺]_i can be normalized by Mg²⁺ influx via TRPM 7, which is stopped when the physiological [Mg²⁺]_i is reached. An elevated [Mg²⁺]_i can be normalized by Mg²⁺ efflux via the Na⁺/Mg²⁺ antiport which is activated at increased [Mg²⁺]_i and stopped when the physiological [Mg²⁺]_i is reached.

The increased plasma insulin concentration in insulin resistance may increase [Mg²⁺]_i. The accompanying hyperglycemia may decrease [Mg²⁺]_i. The antagonizing effects of insulin and hyperglycemia and the regulation of Mg²⁺ influx and Mg²⁺ efflux may explain why [Mg²⁺]_i in skeletal muscle in vivo was unchanged in insulin resistance as measured by ³¹P NMR [112].

To define the role of intracellular Mg²⁺, [Mg²⁺]_i must be known in insulin-sensitive tissues under the conditions of insulin resistance and diabetes type 2 in vivo.

Mg²⁺ in insulin secretion and insulin resistance in human subjects

Studies without Mg²⁺ supplementation

Contrarily, in another study there was no significant association of insulin resistance with the serum Mg²⁺ concentration between 0.75 and 1.0 mM in type 2 diabetics as assessed by euglycemic hyperinsulinemic clamp [82].

Studies with oral Mg²⁺ supplementation

A positive effect of Mg²⁺ supplementation on plasma glucose and insulin concentration may be caused by the retarded hydrolysis of carbohydrates and absorption of glucose in the intestine as discussed above for the effect of Mg²⁺ supplementation in Zucker fat rats.

Membrane lipid fluidity in insulin resistance

Membrane fluidity was increased in erythrocytes of Mg-deficient rats. Their plasma Mg²⁺ concentration was reduced from 0.82 mM to 0.13 mM. Mg deficiency had increased membrane fluidity by 15%, caused by a 9% decrease in cholesterol and a 21% decrease in sphingomyeline [124]. Membrane fluidity and phospholipid composition of hepatocyte plasma membranes of Mg-deficient rats was similarly changed [125].

The effect of membrane fluidity on insulin binding was directly tested. Endothelial cells enriched with monounsaturated or polyunsaturated fatty acids demonstrated no change in insulin binding [126]. Changes in the lipid composition to a higher fluidity by means of incorporating linoleic acid, or to a decreased fluidity with 25-hydroxycholesterol, induced insulin resistance in cultured hepatoma cells [127]. In Ehrlich ascites tumor cells with high membrane fluidity resulting from enrichment with polyunsaturated fatty acids, the number of insulin receptors was increased from 180,000 to 386,000 per cell, and the affinity of insulin binding was decreased. Enrichment with monounsaturated acids and low membrane fluidity reduced the number of insulin receptors to 125,000 and increased the affinity of insulin binding [128]. The activity of the insulin receptor kinase studied in artificial phospholipid vesicles of defined composition is dependent on phospholipids. Insulin receptors incorporated in vesicles consisting of phophatidylcholine and phosphatidylethanolamine lost insulin activation. Kinase activity was restored by phosphatidylserine [129].

There is no evidence that the membrane fluidity of insulin-sensitive cells in insulin resistant subjects with normal or slightly reduced extracellular Mg²⁺ concentration is changed. Moreover, membrane fluidity of erythrocytes was oppositely changed. In diabetics type 2 it was reduced [123], and in Mg deficiency it was increased [124].

Epidemiological studies

Final remarks

[Mg²⁺]_i in skeletal muscle in insulin resistance was not changed. There are no determinations of [Mg²⁺]_i and [Ca²⁺]_i in other insulin-sensitive cell types of subjects with insulin resistance. The essential reactions of insulin signal transduction and insulin resistance are phosphorylations of the protein kinases involved. These reactions rapidly and specifically switch on the activity of these proteins. Phosphorylation of these proteins by serine/threonine protein kinases and protein tyrosine phosphatases can stop these signals. Changes in [Mg²⁺]_i are not suitable for specific regulations. Therefore, [Mg²⁺]_i may have a permissive function in insulin signaling and insulin resistance.

At reduced [Mg²⁺]_o, insulin secretion may be enhanced by an increased Ca²⁺ influx due to reduced competition of extracellular Mg²⁺ with Ca²⁺.

Most important in the development of insulin resistance are various effectors, for example IL-1, Il-6, IL-8, IL-18, TNF-α, β-adrenergics, glucocorticoid, leptin, adiponectin, adipsin, ghrelin, resistin, visfatin, orexin, fibroblast growth factor-21, ROS. Hypomagnesemia may increase the secretion of some of these effectors, thus enhancing insulin resistance. The quantitative contribution of each of these effectors to insulin resistance and their role in hypomagnesemic subjects is not elucidated.

Complex interactions of these effectors, genetic differences and uncontrolled nutritional ingredients may be the reason for controversial results on the relationship between insulin resistance and serum Mg²⁺ concentration or Mg²⁺ intake, as found in some studies with human subjects and epidemiological studies. A positive effect of oral Mg²⁺ supplementation on serum glucose and insulin concentration may be caused by retarded hydrolysis of carbohydrates and glucose absorption in the intestine.

Disclosure

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