Magnesium

Jürgen Vormann, Dr. rer. nat.^∗

Magnesium (Mg²⁺) plays an important role in numerous physiological processes. It is a cofactor for various enzymes that are involved in muscle contraction, neurotransmitter release, and the regulation of ion channels. The essential role of Mg²⁺ in fundamental cellular functions is mainly based on several properties of Mg²⁺:

• Its action as an enzyme cofactor

• Its ability to chelate anionic ligands, especially adenosine triphosphate (ATP)

• Its ability to compete with calcium (Ca²⁺) for binding sites, which modulates intracellular and extracellular free Ca²⁺ concentrations

• Its ability to cross-link negatively charged membrane constituents, which stabilizes them

• Its interaction with cell adhesion molecules

Chemistry of Magnesium

Mg²⁺ is a divalent metal ion with an atomic number of 12 and a molecular weight of 24 g/mol. It is one of the most abundant elements in the earth’s crust and is therefore also prevalent in seawater. In vertebrates, it is the fourth most abundant cation in the body and the second most abundant inside cells. Throughout evolution, Mg²⁺ has come to be involved in numerous biological processes.

Absorption, Bioavailability, and Excretion of Magnesium

Mg²⁺ homeostasis in the body is obtained by balancing the intestinal absorption of Mg²⁺ with renal excretion.

Intestinal Mg²⁺ absorption is inversely proportional to the amount ingested. Under normal dietary conditions in healthy individuals, approximately 30% to 50% of ingested Mg²⁺ is absorbed (Fine et al., 1991). Mg²⁺ is absorbed along the entire intestinal tract, but the sites of maximal Mg²⁺ absorption appear to be the distal jejunum and the ileum. The colon absorbs only small amounts of Mg²⁺, which may be important in the context of dietary restriction or compromised Mg²⁺ absorption in the small intestine. When dietary intake is restricted, fractional absorption of Mg²⁺ may increase up to 80%. Conversely, it may be reduced to 20% with a diet high in Mg²⁺. There is evidence that intestinal Mg²⁺ absorption is through paracellular and transcellular pathways (Quamme, 2008) (Figure 33-1). About 90% of normal Mg²⁺ absorption occurs passively through the paracellular pathway between the enterocytes, involving barrier proteins of the claudin family (claudin-16, claudin-19). The rate of Mg²⁺ absorption across the intestinal epithelium is dependent on the transepithelial electrical voltage (which is normally about +5 mV, lumen positive with respect to blood) and the transepithelial concentration gradient. The luminal Mg²⁺ concentration may be about 1.0 to 5.0 mM, depending on the dietary Mg²⁺ content and the presence of anionic chelators. Only the free Mg²⁺ moves through the paracellular pathway so that bound Mg²⁺ does not contribute to the transepithelial gradient. Plasma Mg²⁺ concentration is 0.5 to 0.7 mM, so there is normally a concentration gradient between the lumen and the blood. Various poorly understood hormonal and nonhormonal factors acting through a variety of intracellular signals might influence the passive transport.

FIGURE 33-1 Proposed model of intestinal Mg²⁺ absorption by two independent pathways: passive paracellular absorption involving claudin-16 and claudin-19 and active transcellular transport involving TRPM6.

At a low intraluminal Mg²⁺ concentration, however, absorption occurs primarily via the active transcellular route. Transcellular Mg²⁺ absorption is an active system where the Mg²⁺ entry is mainly mediated by the TRPM6 (transient receptor potential melastatin) channel kinase (see Figure 33-1). TRPM6 protein is localized to the apical membrane of the intestine, thus forming the entry pathway into the enterocyte. Low Mg²⁺ intake induces an increased expression of TRPM6, facilitating Mg²⁺ absorption (Groenestege et al., 2006). The basolateral exit of Mg²⁺ is less well understood. Most probably it consists of a Na⁺/Mg²⁺ antiport system. In addition to these transporters, several other Mg²⁺-sensitive genes have been described that are expressed in intestine and are probably involved in Mg²⁺ transport (Quamme, 2008).

Bioavailability

Intestinal Mg²⁺ absorption can be affected by the presence of other nutrients and dietary constituents. High levels of dietary fiber from fruits, vegetables, and grains decrease fractional Mg²⁺ absorption (Siener and Hesse, 1995). However, diets high in vegetables are Mg²⁺-rich, and the high Mg²⁺ content of these diets offsets decreased fractional absorption associated with the higher fiber intake. Many foods high in fiber also contain phytate, which may decrease intestinal Mg²⁺ absorption because Mg²⁺ binds to the phosphate groups on phytic acid. The ability of phosphate to bind Mg²⁺ may explain decreases in intestinal Mg²⁺ absorption in individuals on high-phosphate diets (Franz, 1989). Although dietary calcium has been reported to both decrease and increase Mg²⁺ absorption, human studies have shown no effect (Fine et al., 1991). It is likely that interactions between Ca²⁺ and Mg²⁺ occur at high local concentrations achieved with Mg²⁺ or Ca²⁺ supplements but not with usual dietary supply.

Renal Magnesium Handling

The kidney is the principal organ involved in Mg²⁺ homeostasis (Dimke et al., 2009). Under normal conditions, approximately 80% of the total plasma Mg²⁺ is filtered through the glomerulus, and most of this is reabsorbed (greater than 95%) as the filtrate passes through the nephrons (Figure 33-2). Therefore each day, only about 100 mg Mg²⁺ is excreted in the urine, whereas about 2,500 mg of Mg²⁺ is filtered by the glomeruli.

FIGURE 33-2 Summary of the tubular handling of Mg²⁺. The 100% represents the total filtered load and the other percentage values represent the fraction of the filtered load absorbed at each tubular site.

The major site of Mg²⁺ reabsorption is the cortical segment of the thick ascending limb of the loop of Henle, which accounts for 65% to 75% of renal Mg²⁺ reabsorption. This Mg²⁺ transport is passive and involves claudin-16, with movement from the tubular lumen to the interstitium driven by the lumen-positive transepithelial voltage (Figure 33-3, A). The transepithelial voltage is determined by Na⁺, K⁺, and Cl^– cotransport and active Na⁺ reabsorption. Therefore any changes in these transport mechanisms consequently influence Mg²⁺ (and Ca²⁺) reabsorption. Mg²⁺ reabsorption in the cortical thick ascending limb of the loop of Henle is regulated by a variety of hormones that act mainly by increasing the reabsorption rate. These hormonal responses are mediated by changes in both transepithelial voltage and paracellular permeability. It has been shown that 1,25-dihydroxyvitamin D influences Mg²⁺ transport in the cortical thick ascending limb by downregulating claudin-16 expression at the transcriptional level (Efrati et al., 2005). The amount of reabsorbed Mg²⁺ is mediated by the extracellular Ca²⁺/Mg²⁺-sensing receptor (CaSR), which is expressed in the basolateral membrane. Activation of CaSR by increased blood concentrations of Ca²⁺ or Mg²⁺ leads to inhibition of salt reabsorption and paracellular Ca²⁺ and Mg²⁺ transport in the thick ascending limb, thereby increasing divalent cation excretion. Loop diuretics such as furosemide, which act to inhibit the Cl^– pump and subsequently block Na⁺ reabsorption, may lead to hypomagnesemia, as these drugs have a large effect on transepithelial voltage.

FIGURE 33-3 A, Mg²⁺ reabsorption in the thick ascending limb of Henle loop. Mg²⁺ transport is passive and involves claudin-16, with movement from the tubular lumen to the interstitium driven by the lumen-positive transepithelial voltage. B, Mg²⁺ reabsorption in the distal convoluted tubule. Reabsorption is mediated by an active transcellular transport mechanism and the TRPM6 channel. *NCC*, Sodium chloride cotransporter.

Mg²⁺ reabsorption in the distal convoluted tubule is about 5% to 10% and is of crucial importance for regulating the final amount of excreted Mg²⁺, because there is no evidence of Mg²⁺ reabsorption beyond this point. Reabsorption is mediated by an active transcellular transport mechanism, as shown in Figure 33-3, B. Mg²⁺ enters the cell across the apical membrane through the TRPM6 channel. Uptake of Mg²⁺ is driven by the lumen-negative potential difference in this region of the tubule. Extrusion into the interstitium probably occurs by a Na⁺-dependent exchange mechanism. Mutations in TRPM6 were identified in patients with primary hypomagnesemia and with secondary hypocalcemia (Schlingmann et al., 2002; Walder et al., 2002). Mg²⁺ transport rates in the distal convoluted tubule change rapidly upon a decrease in Mg²⁺ availability, which allows efficient Mg²⁺ conservation. Conversely, during hypermagnesemia (or hypercalcemia), fractional excretion rates for Mg²⁺ are increased via activation of the CaSR, which results in inhibition of Mg²⁺ (or Ca²⁺) reabsorption.

The epidermal growth factor (EGF) was identified as a magnesiotropic hormone. EGF stimulates the trafficking of TRPM6 channels to the luminal membrane, increasing the reabsorption of Mg²⁺ through TRPM6 (Thebault et al., 2009). Patients suffering from metastatic colorectal cancer are often treated with antibodies raised against the EGF receptor, such as cetuximab. Anti–EGF receptor monoclonal antibody therapy has been shown to result in hypomagnesemia in a significant number of patients (Tejpar et al., 2007). In addition, changes in acid–base balance significantly influence Mg²⁺ status, because production of TRPM6 is reduced in acidosis (Nijenhuis et al., 2006). In human volunteers a higher dietary acid load increased urinary Mg²⁺ losses (Rylander et al., 2006).

Body Magnesium Content

The normal adult total body Mg²⁺ content is approximately 24 g, or 1,000 mmol, and 50% to 60% of this total resides in bone (Figure 33-4). Part of this Mg²⁺ is on the surface of bone and is in equilibrium with the extracellular Mg²⁺. At reduced plasma concentrations Mg²⁺ can be rapidly released from the bone surface, and at increased plasma concentrations Mg²⁺ is bound to the surface (Elin, 1994). Based on investigations with the stable isotopes ²⁵Mg²⁺ and ²⁶Mg²⁺ in a kinetic model of Mg²⁺ metabolism in healthy men, 24% of the human total Mg²⁺ exchanges rapidly; of this, 79% turns over in 115 hours, representing probably the bone surface pool. The remaining part, which may represent plasma and easily accessible extracellular space, turns over in less than 9 hours (Sabatier et al., 2003). Therefore magnesium in bone represents a reservoir that buffers extracellular magnesium concentration. In humans this magnesium-buffering capacity is reduced with increasing age, because nearly half of the magnesium content of bone is lost over a lifetime (Vormann and Anke, 2002).

FIGURE 33-4 Distribution of Mg²⁺ in the body.

Extracellular Mg²⁺ accounts for only about 1% of total body Mg²⁺. The normal serum Mg²⁺ concentration is 0.75 to 1.0 mmol/L (Weisinger and Bellorin-Font, 1998). Most of the plasma Mg²⁺, approximately 60% to 65%, is ionized or free Mg²⁺. Of the remaining 35% to 40%, 5% to 10% is complexed to anions such as phosphate, citrate, and sulfate, and 30% is bound to proteins (chiefly albumin).

Soft tissue contains about one half of the total body Mg²⁺, approximately 470 mmol. The Mg²⁺ content of soft tissues varies between 2.5 and 9 mmol/kg wet tissue weight. In general, the higher the metabolic activity of the cell, the higher the Mg²⁺ content. Within the cell, significant amounts of Mg²⁺ are in the nucleus, mitochondria, and endoplasmic (or sarcoplasmic) reticulum as well as in the cytosol (Birch, 1993). Most of the Mg²⁺ is bound to proteins and other negatively charged molecules such as nucleoside triphosphates and diphosphates (e.g., ATP and ADP) and nucleic acids (e.g., RNA and DNA). In the cytoplasm, about 80% of the Mg²⁺ is complexed with ATP (Frausto da Silva and Williams, 1991). Only 1% to 5% of the total intracellular pool is free ionized Mg²⁺ (Romani et al., 1993). The concentration of free Mg²⁺ in the cytosol of mammalian cells has been reported to range from 0.2 to 1.0 mmol/L, but values vary with cell type and means of measurement. The free Mg²⁺ in the cell cytosol is maintained at a relatively constant level, even when the Mg²⁺ concentration in the extracellular fluid is experimentally varied above or below the physiological range (Romani, 2007).

The relative constancy of the free Mg²⁺ concentration in the intracellular milieu is attributed to the limited permeability of the plasma membrane to Mg²⁺ and to the operation of specific Mg²⁺ transport systems that regulate the rates at which Mg²⁺ enters or leaves cells. Cellular Mg²⁺ transport has been studied intensively, and it is mediated by a variety of Mg²⁺ transport proteins in different human tissues (Quamme, 2010). In most tissues, Mg²⁺ influx is dependent on TRPM7. This channel is ubiquitously expressed, whereas TRPM6 is only found along the intestine, kidney nephron, lung, and testis. However, total intracellular Mg²⁺ content is determined by a balance of Mg²⁺ influx and Mg²⁺ efflux. Whereas Mg²⁺ influx systems are quite well characterized, much less is known about the mechanisms underlying Mg²⁺ transport out of the cell. Mg²⁺ mainly leaves the cell through a Na⁺/Mg²⁺ exchange mechanism, which depends on Na⁺,K⁺-ATPase activity (Wolf et al., 2008). Various other Mg²⁺ transport systems have been discovered in recent years; however, it is not known how they contribute to intracellular Mg²⁺ homeostasis (Quamme, 2010). It is also not known if polymorphisms of Mg²⁺ transporters influence Mg²⁺ status and affect Mg²⁺-sensitive diseases.

Physiological Roles of Magnesium

Enzyme and ATP Cofactor

The main biological role of Mg²⁺ in mammalian cells is anion charge neutralization (Romani, 2007; Cowan, 2002). Mg²⁺ is particularly found in association with organic polyphosphates such as nucleotide triphosphates and nucleotide diphosphates (e.g., ATP^4– and ADP^3–). Mg²⁺ is also found with other highly anionic species, including multisubstituted phosphates of sugars such as inositol triphosphate, nucleic acids (RNA and DNA), and some carboxylates (e.g., isocitrate-Mg²⁺ as substrate for isocitrate lyase; carboxylate groups on proteins).

Mg²⁺ is normally bound between the β- and γ-phosphates of nucleotide triphosphates, such as ATP, and between the α- and β-phosphates of nucleotide diphosphates, such as ADP (Figure 33-5). This arrangement neutralizes the negative charge density on the ATP or other nucleotide triphosphates or diphosphates, and it facilitates binding of the nucleotide phosphate to the enzymes that use them as substrates. In most reactions in which Mg²⁺ is involved, it is present as a complex with a nucleotide triphosphate or diphosphate, which serves as the substrate. The Mg²⁺ in these complexes does not interact directly with the enzyme in most cases, but it is linked by the substrate in an enzyme–substrate–metal type of substrate-bridged complex. Hence Mg²⁺ plays a dominant role in all enzymatic reactions that require nucleotide triphosphates or diphosphates. This reaction type is widespread in metabolism. Mg²⁺ also is required for binding to some enzymes or other proteins to stabilize them in the active conformation or to induce the formation of a binding site or active site. By these effects, Mg²⁺ is involved in numerous steps in central pathways of carbohydrate, lipid, and protein metabolism and in mitochondrial ATP synthesis. In addition, Mg²⁺ associates with nucleic acids, in which the phosphate groups in the nucleotide chains render the polymers negatively charged overall. Mg²⁺ stabilizes bending of RNA or DNA into particular curved or folded structures. This presumably occurs because of the cross-linking of the oxyanion centers of the phosphate residues by the divalent cation.

FIGURE 33-5 Physiological forms of Mg²⁺-ATP and Mg²⁺-ADP.

The transcription, translation, and replication of nucleic acids (RNA and DNA) require enzymes that catalyze the hydrolysis and formation of phosphodiester bonds. Almost all these enzymes require Mg²⁺ for optimal activity.

Replicating cells must be able to synthesize new protein, and all cells must continually replace protein that is degraded. Protein synthesis has been reported to be highly sensitive to Mg²⁺ depletion because it is required for virtually every step of protein biosynthesis. Formation of the aminoacyl–transfer RNA species (which requires Mg²⁺-ATP), maintenance of its conformation (which is required for recognition by messenger RNA), and maintenance of the ribosomes all require Mg²⁺. It is also required for structure and activity of elongation factor–guanosine triphosphate (GTP) complexes that allow protein synthesis to begin and for the GTPase activities that occur during elongation and termination of protein biosynthesis.

Second Messenger Systems

Many hormones, neurotransmitters, and other cellular effectors regulate cellular activity via the adenylate cyclase system. As is the case for other ATP-utilizing enzymes, the actual substrate for adenylate cyclase is Mg²⁺-ATP (Figure 33-6). There is also evidence for a Mg²⁺ binding site on adenylate cyclase, through which Mg²⁺ directly increases enzyme activity.

Another group of hormones and neurotransmitters exert their effects by raising the ionized calcium (Ca²⁺) concentration in the cytosol of their target cells through the activation of the phosphoinositol cycle (see Figure 33-6). One of the principal mechanisms by which this is thought to occur is by receptor-mediated activation of phospholipase C, which hydrolyzes a specific phospholipid in the plasma membrane, PIP₂ (phosphatidylinositol 4,5-bisphosphate), to yield two biologically active products, diacylglycerol and inositol 1,4,5-triphosphate (IP₃). Diacylglycerol activates protein kinase C, and IP₃ triggers calcium release from the endoplasmic reticulum. The IP₃ is rapidly inactivated by dephosphorylation. It appears that Mg²⁺ is essential for the normal functioning of this phosphoinositol cycle because the kinase that forms the PIP₂ and also the enzymes that inactivate IP₃ require Mg²⁺ at physiological concentrations (Volpe et al., 1990). In contrast, higher Mg²⁺ concentrations may decrease intracellular Ca²⁺ by two mechanisms: (1) noncompetitive inhibition of IP₃ binding to its receptor, and (2) inhibition of the release of Ca²⁺ via IP₃-gated channels (Volpe et al., 1990).

Ca²⁺ Channels

Mg²⁺ has been called “nature’s physiological calcium channel blocker” (Iseri and French, 1984). During Mg²⁺ depletion, intracellular Ca²⁺ rises. This may be caused by both uptake from extracellular Ca²⁺ and release from intracellular Ca²⁺ stores. Mg²⁺ has been demonstrated to decrease the inward Ca²⁺ flux through slow Ca²⁺ channels (O’Rourke, 1993; White and Hartzell, 1989). In addition, Mg²⁺ will decrease the transport of Ca²⁺ out of the endoplasmic reticulum into the cell cytosol. Consequently, because Ca²⁺ plays an important role in skeletal and smooth muscle contraction, a state of Mg²⁺ depletion may result in muscle cramps, hypertension, and coronary and cerebral vasospasm (Altura and Altura, 1995).