Exchangeable magnesium pool masses reflect the magnesium status of rats

Erythrocyte magnesium fluxes in mice with nutritionally and genetically low magnesium status

Low intracellular magnesium (Mg) contents may be observed in case of severe Mg insufficient intake or because of genetic regulation. This work was conducted to investigate the influence of intracellular Mg content on erythrocyte Mg(2+) influx and efflux in mice with low nutritionally and genetically (MGL and MGH mice) Mg status. C57BL6 mice were fed for 2 wks a diet containing 1000 mg Mg/kg diet Mg (control group), 100 mg Mg/kg diet (Mg-marginal group) or 30 mg Mg/kg diet (Mg deficient group), while mice with low (MGL) and high (MGH) Mg levels were fed a control diet for 2 wks. The quantification of erythrocyte Mg(2+) influx and efflux was performed using a stable isotope of Mg. Our results showed that erythrocyte Mg(2+) influx and efflux were respectively increased and decreased in nutritional Mg deficiency; while in genetically determined Mg status Mg(2+) fluxes were lower in MGL mice compared to MGH mice. Moreover Mg(2+) efflux was significantly correlated to Mg level in erythrocytes in all the mice studied (p < 0.001). In conclusion, erythrocyte Mg(2+) influx and efflux are modulated by low Mg status, namely decreased Mg(2+) efflux compensate for nutritional Mg deficiency, while the genetic regulation of erythrocyte Mg(2+) content depends on modification of Mg(2+) influx.

Stable isotopes in studies of intestinal absorption, exchangeable pools and mineral status: the example of magnesium

Magnesium (Mg) is a biologically essential mineral and Mg deficiency is known to lead to severe biochemical and symptomatic disorders. Radioactive isotopes and, more recently, stable isotopes have been used as research tools to determine intestinal Mg absorption in humans and animals under different nutritional and physiological conditions. Mg isotopes are given orally or orally plus intravenously and analysed in faeces and/or in plasma and urine in order to calculate intestinal Mg absorption and possibly endogenous Mg excretion. Mg isotopes have been used to assess exchangeable pools of Mg under nutritional and physiopathological conditions. Mg isotopes are given intravenously and are analysed in plasma and urine to calculate the size and half-life of the various Mg exchangeable pools. More recently, in vitro isotopic tests have been developed to study the need of cells for Mg in different nutritional and genetic conditions. Whole blood is incubated with Mg isotopes and isotopic blood cell enrichment is measured, which reflects the avidity of cells for Mg and thus its initial status. This paper is a report on the use of stable Mg isotopes and their advantages in these different fields of Mg absorption and metabolism. The studies available have clearly demonstrated that stable isotopes provide a useful research tool for determining intestinal Mg absorption, and represent a precious research tool for the study of Mg metabolism and the assessment of Mg status.

Magnesium metabolism in mice selected for high and low erythrocyte magnesium levels

A genetic control of blood magnesium (Mg) levels has been suggested. To investigate the mechanisms and the biologic significance of this genetic regulation, a mouse model, ie, mice selected for low magnesium level (MGL) and high magnesium level (MGH), was developed. The purpose of this study was to explore the Mg status and Mg metabolism in female MGL and MGH mice. We observed that MGL mice had reduced total and ionized plasma Mg, lower erythrocyte Mg, lower tibia, and kidney Mg levels. In contrast, total urinary Mg and (25)Mg levels were significantly higher in MGL mice. MGL mice had smaller total Mg exchangeable pool masses compared with MGH, and fractional transport rates of Mg (exchange constant) were different. In vitro (25)Mg enrichments in erythrocytes from MGL mice were significantly lower. Moreover, Mg efflux from erythrocytes was significantly higher in MGL. In conclusion, this work demonstrates that MGL mice present lower body stores of Mg than MGH mice and lower body Mg retention. This is confirmed at a cellular level by a lower enrichment of (25)Mg in erythrocytes. The lower retention of Mg by MGL erythrocyte in comparison to MGH appears to be partly due to a higher Mg efflux in MGL erythrocyte. It can be hypothesized that a genetic factor that modulates Na(+)/Mg(2+) exchanger activity may be important in the regulation of Mg metabolism. Further investigations on the mechanisms responsible for differences in Mg retention between MGL and MGH mice could contribute to a better understanding of the genetic regulation of cellular Mg.

Exchangeable magnesium pool masses in spontaneously hypertensive rats

We explored magnesium (Mg) metabolism by determination of exchangeable Mg pool masses and Mg kinetic parameters using stable Mg isotopes in spontaneously hypertensive rats (SHRs). Classical intracellular and extracellular Mg status biomarkers were also measured. Male SHRs and their male Wistar Kyoto (WKY) controls were fed a semipurified diet containing Mg 550 mg /kg for 2 weeks. Each rat received then an intravenous injection of 1.37 mg (25)Mg. The plasma (25)Mg disappearance curve over the next 7 days was used to measure the mass and fractional transport rate of 3 rapidly exchanging Mg metabolic pools, M1, M2, and M3. In the SHRs, plasma and erythrocyte Mg levels and urinary Mg excretion were not modified compared with their control WKYs, but tibia Mg level was significantly lower in the SHRs. Pool M3, the deep tissue pool, was significantly lower in SHRs compared with WKYs, but pools M1 and M2, the extracellular Mg pools, were statistically similar. The fractional transport rate of Mg from M1 to M2 and from M2 to M1 in the SHRs was higher than in the controls. The half-life of M1 was significantly decreased in SHRs compared with WKYs. In conclusion, this work demonstrates a decrease in intracellular Mg stores in SHRs compared with WKYs and disturbance of Mg exchanges in extracellular Mg, confirming a Mg metabolism disturbance in spontaneously hypertensive rats. Further work is now needed to elucidate the origin of the Mg depletion in SHRs and to explore Mg pools in hypertensive patients.

A new in vitro blood load test using a magnesium stable isotope for assessment of magnesium status

Magnesium (Mg) status is currently assessed by various biochemical biomarkers, most of which, however, have some limitations. We developed an in vitro blood load test as a new Mg biomarker using a Mg stable isotope. This test is based on the hypothesis that cellular Mg uptake is increased in Mg deficiency. For this purpose, Wister male rats were fed either Mg-deficient or Mg-adequate diets for 1 mo and blood was sampled and incubated with the (25)Mg isotope (10 mg/L) for 2 h at 37 degrees C. Erythrocytes, lymphocytes and platelets were isolated and (25)Mg concentrations were determined by inductively coupled plasma/mass spectrometry. The feasibility of this approach was then tested on human blood. (25)Mg enrichments in erythrocytes, lymphocytes and platelets from Mg-deficient rats were greater than those from controls. (25)Mg enrichment was low in human erythrocytes (3%) compared with rat erythrocytes (38%), whereas high (25)Mg enrichments were obtained in human lymphocytes and platelets, suggesting that lymphocytes and platelets may be more appropriate cells than erythrocytes for examining Mg status in humans with this approach. More studies are required to validate the utilization of this test as a Mg status biomarker in humans.

Fractional intestinal absorption of magnesium is directly proportional to dietary magnesium intake in rats

The precise mechanisms of intestinal Mg absorption are still unclear and the possibility of an adaptative rise in the fraction of Mg absorbed as Mg intake is lowered is controversial. Mg deficiency has been studied extensively in rats where it is readily produced by dietary depletion. In this study, we investigated the effect of Mg intake on fractional absorption of Mg acutely and after adaptation to graded Mg intake in rats. For this purpose, male Wistar rats (n = 30) were fed a basal semipurified diet containing 600 mg Mg/kg (MgO) for 7 d. Three groups of 10 rats were then formed and fed the basal semipurified diet with 600, 300 or 150 mg Mg/kg, for 28 d. Apparent and true intestinal absorptions and fecal endogenous excretion of Mg were determined at the beginning and end of the experiment. As expected, plasma Mg levels were lower in the deficient groups than in the control by d 4 and differences were more marked at the end of the experiment. Erythrocyte Mg levels were significantly lower at the end of the experiment in the group fed the diet containing 150 mg Mg/kg. The amounts of Mg absorbed were directly proportional to the dietary Mg intakes in all three experimental groups at both testing periods. This indicated that the apparent and true intestinal absorption percentages of Mg were not different in rats fed the three different levels of dietary Mg. These results also show that fecal endogenous excretion of Mg was nearly directly proportional to the dietary Mg intakes. These results argue in favor of a passive diffusional process for intestinal Mg absorption.

Exchangeable magnesium pool masses in healthy women: effects of magnesium supplementation

BACKGROUND

Studying magnesium pools in the body with use of stable isotopes may be helpful for evaluating magnesium status. Data on the evaluation of magnesium pools in humans are scarce.

OBJECTIVE

We undertook this study to evaluate the effects of a magnesium supplementation program on the size of the exchangeable body pools of magnesium and on classic indexes of magnesium status in healthy women with normal magnesium status.

DESIGN

Ten healthy women participated in a kinetic study with magnesium stable isotopes before and after 8 wk of magnesium supplementation. Each woman received 3 supplements containing 5.08 mmol (122 mg) elemental Mg/d (366 mg/d). Before and at the end of the supplementation period, each woman received an intravenous injection of 1.67 mmol (40 mg) (25)Mg, and the plasma magnesium disappearance curve was followed for the next 7 d. Two methods were used to analyze the exchangeable pools of magnesium: 1) formal multicompartmental modeling and 2) a simplified estimation of the total mass of the rapidly exchangeable magnesium pool (EMgP).

RESULTS

In these healthy women, exchangeable magnesium pools represented 11-12% of total body magnesium on the basis of multicompartmental analysis. The simplified estimation of EMgP overestimated the size of the exchangeable magnesium pools by approximately 45-50%. Eight weeks of magnesium supplementation did not significantly modify the size of the exchangeable magnesium pools, whereas urinary magnesium excretion was significantly higher after 8 wk of supplementation.

CONCLUSION

Women with no clinical evidence of magnesium deficiency may not respond to short-term supplementation with increases in the mass of the exchangeable magnesium body pool or in magnesium turnover rates.