Reactive oxygen species: the missing link between magnesium deficiency and hypertension?

Introduction

As one who has focused their research efforts on the area of Ca²⁺ homeostasis and vascular function for the better part of their career, I have always been curious about what I consider to be the parallel field of Mg²⁺ and cardiovascular disease. Truth be told, however, I have not really kept up-to-date with the ongoing developments. It is thus with genuine interest and professional distance, that I consider the provocative article by Touyz et al. [¹] in this issue of the journal. The authors show that feeding stroke prone spontaneously hypertensive rats a diet that is deficient in Mg²⁺ (< 0.1% versus 0.21 or 0.75%) accelerates the development of high blood pressure, attenuates endothelium-dependent relaxation of isolated arteries and increases vessel wall hypertrophy. Moreover, the authors provide data consistent with the very exciting hypothesis that Mg²⁺ deficiency exerts these actions through increased formation of reactive oxygen species and an elevation of MAP kinase-dependent signalling.

Magnesium homeostasis and blood pressure

As with the field of Ca²⁺ and blood pressure, there appears to be a large and contradictory literature regarding Mg²⁺ and blood pressure. For example, the Honolulu Heart Study found a strong inverse correlation between Mg²⁺ intake and blood pressure [²] while, in clinical trials, others have found either a modest effect of Mg²⁺ supplementation on blood pressure [³] or no effect of all [^4,5]. Studies performed using animal models of hypertension are similarly contradictory, although Mg²⁺ has been shown to attenuate development of high blood pressure in some animal models including the stroke-prone spontaneously hypertensive rats as reported by Touyz et al. [^1,6]. Moreover, Mg²⁺ deficiencies have been reported in a variety of human conditions including hypertension [⁷]. Hence, the general premise that is emerging is that Mg²⁺ deficiency is associated with an increased incidence of high blood pressure. This concept has received substantial, albeit indirect, support from the results of the DASH trials showing a beneficial effect of consuming diets that are rich in fruits and vegetables and low fat dairy products and which have elevated levels of Mg²⁺ [⁸]. However, a caveat is the fact that the DASH diets did not raise intake of Mg²⁺ alone, but also provided increased intake of other nutrients, including Ca²⁺ and K⁺. Hence, while foods that are rich in Mg²⁺ may lower blood pressure, there is little doubt that they exert their effect in concert with other nutrients.

Given the complexity of the Mg²⁺ supplementation data, it is important to remember that depleting Mg²⁺ by feeding animals a diet devoid of the cation is unlikely to result in the same state of Mg²⁺ balance as that seen in humans with Mg²⁺ deficiency. For example, serum Mg²⁺ levels in humans with apparent dietary deficiencies are in the range of 0.75 mmol/l whereas animals who have undergone Mg²⁺ depletion have serum levels that approach 0.30-0.40 mmol/l, a level where hypomagnesemia is symptomatic in humans and beyond which lower levels are difficult to achieve in view of active renal reabsorption of Mg²⁺ and the large Mg²⁺ store in bone [⁹]. Analogously, completely removing Mg²⁺ from buffer bathing an isolated tissue or from cell culture media likely does not result in the same extracellular milieu as is present in humans with Mg²⁺ deficiency.

Hence, it is only with caution that mechanistic data obtained using animals receiving a Mg²⁺-free diet be extrapolated to the human situation. An alternative approach would be to induce levels of dietary Mg²⁺ deficiency in animal models that are comparable with levels of deficiency observed in humans. Similarly, it would be useful to determine the concentration of Mg²⁺ present in the interstitial compartment of these animals and to use this in experiments performed in vitro. Our own laboratory has been forced by journal reviewers and study sections to use an in-situ microdialysis approach to measure the concentration range over which extracellular Ca²⁺ varies under physiological conditions to guide in-vitro studies of the effect of this cation on vascular function. This approach has been fruitful. For example, while it is recognized that serum ionized Ca²⁺ is maintained at steady levels in mixed venous plasma, we have found that interstitial Ca²⁺ varies between 1 and 2 mmol/l in the duodenal submucosa and renal cortex in response to manipulations that alter Ca²⁺ homeostasis [^10,11].

Mechanistic considerations

A major step forward provided by Touyz et al. [¹] is the linkage of Mg²⁺ deficiency with formation of reactive oxygen species and potential effects on redox sensitive growth factor signalling pathways. Our knowledge in the field of reactive oxygen species (ROS) and vascular function and pathology is currently growing at an exponential rate, and it is becoming clear that these highly reactive molecules are probably not only involved in vascular pathology, but also in normal cell signalling [¹²]. The demonstration by Touyz et al. [¹] that Mg²⁺ deficiency leads to an increase in TBARS, which indicates an increase in ROS and which is reversed by tempol, a superoxide dismutase mimetic, is an important first step in understanding these pathways and will hopefully prompt additional investigation into this area.

A broader question that begs answering is how Mg²⁺ deficiency might induce these effects. The traditional belief is that Mg²⁺ exerts many of its cellular actions by serving as a Ca²⁺ antagonist [¹³] and there is no doubt that Mg²⁺ can compete with Ca²⁺ for binding sites on a variety of cellular targets. It is also thought that Mg²⁺ deficiency may affect cell function by causing the concentration of free intracellular Mg²⁺ to fall below its K_D for key enzymes and hence decrease their activity (i.e. the Mg²⁺-dependent Na⁺ and K⁺-activated ATPase that establishes the Na⁺ and K⁺ gradients across the cell membrane and the Mg²⁺-dependent myosin ATPase which is critical for muscle contraction). This passive mechanism not only seems unlikely from a kinetic point of view when considering rapid effects of altering extracellular Mg²⁺ [¹⁴], but in view of recent developments in the field of Ca²⁺ biology stemming from the discovery and molecular cloning of the G protein-coupled Ca²⁺ sensing receptor from the parathyroid gland [¹⁵], it is somewhat intellectually unsatisfying to believe that Mg²⁺ can only exert its actions through indirect pathways. In contrast, it is tempting to speculate that Mg²⁺ might also exert its effects through a specific receptor akin to what has been proposed by our group to explain the mechanism by which rapid changes in extracellular Ca²⁺ induce sensory nerve-dependent relaxation of isolated arteries [^16,17].

Chicken or egg?

A basic concern that hypertension researchers often face is the question of whether a given manipulation alters blood pressure by affecting vascular function and structure or whether it modifies unidentified pathways that increase blood pressure where the increase in blood pressure in turn causes changes in vascular function and structure. An example is the now two-decade-old observation that platelet Ca²⁺ mobilization is elevated in humans and animals with elevated blood pressure [¹⁸]. This finding has been interpreted by many to indicate that there is a basic defect Ca²⁺ metabolism in platelets which might extend to other reactive cells, including vascular smooth muscle. However, the increase in platelet Ca²⁺ might also result from an increase in shear stress as has been shown in a study by Levenson et al. [¹⁹]. This basic question needs to be addressed for the effect of Mg²⁺ on blood pressure and generation of reactive oxygen species.

Implications and future directions

In view of the continued interest regarding the role of Mg²⁺ in blood pressure regulation and vascular pathology, particularly in light of the hugely successful DASH trials, additional work needs to be conducted in this area. Focus should continue to be directed towards understanding mechanisms of action, with a concerted effort being made towards making the animal models more closely match the human condition, and with an increased focus on molecular mechanisms. The paper by Touyz et al. [¹] provides an exciting step forward with regard to the latter, by providing the first link between an essential dietary nutrient and the key molecular pathways involved in regulating vascular smooth muscle growth and structure.