| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Department of Physiology, School of Pharmacy and Institute of Nutrition and Food Technology, University of Granada, E-18071 Granada, SPAIN
Address reprint requests to: Juan Llopis, Ph.D., Instituto de Nutrición y Tecnologia de Alimentos, Universidad de Granada, C/Rector Lopez Argüeta s/n, E-18071 Granada, SPAIN
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Methods: Wistar rats were fed a Mg-deficient diet (56 mg/kg) for 70 day. Absorbed Mn, Mn balance and Mn content in plasma, whole blood, skeletal muscle, heart, kidney, liver, femoral bone and sternum were determined after 21, 35 and 70 days.
Results: The Mg-deficient diet significantly increased Mn apparent absorption and Mn balance from week five until the end of the experimental period. This effect was accompanied by a significant increase in the concentration of Mn in heart at all three time points. Whole blood, skeletal muscle and kidney Mn were significantly increased from day 35, and femur Mn content was increased only at the end of the study (day 70). However, Mn concentration in the sternum decreased significantly from day 35. No changes were found in liver Mn content.
Conclusion: Mg deficiency increased Mn absorbed, and this favored the deposition of Mn in all tissues studied except the liver and trabecular bone. The lack of response by the liver to increased Mn absorption may have led to the redistribution of this ion to other tissues.
Key words: Mg deficiency, manganese, rats
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Few studies have investigated the relationship between Mg deficiency and manganese (Mn) metabolism. Yasui et al. [2] found that manganese content in central nervous system tissues and visceral organs was highest in rats fed a low Ca-Mg diet. However, in rats fed a Mg-deficient diet for two weeks, Kimura et al. [3] found decreased Mn levels in several tissues (plasma, brain, spinal cord, lung, spleen, kidney, testis and bone), but not in the adrenal glands or blood. Interactions between Mg and Mn have also been reported in the digestive tract. The addition of high concentrations of Mg to the water or food reduced the absorption in rats of Mn given orally [4]. Manganese uptake and toxicity in Saccharomyces cerevisiae are strongly influenced by intracellular Mg, possibly through the Mg-dependent regulation of divalent-cation transport activity [5].
In addition to these findings there is evidence from epidemiological studies that Mg intake in a large proportion (from 15% to 20%) of the population in industrialized countries is approximately 30% below the Recommended Daily Allowances, and that Mg deficiency, together with inadequate dietary habits, can lead to many disease states [6]. In their study of German children, Schimatschek and Classen [7] found hypomagnesemia in 12% of the children younger than one year of age, and an incidence of 30.5% in adolescents aged 16 to 18 years. According to Wong et al. [8], approximately 10% of patients admitted to large city hospitals are hypomagnesemic.
The aim of the present study was to elucidate the relationship between Mg and Mn and to determine whether the latter indirectly contributes to the development of symptoms related with Mg deficiency. We examined the degree to which an Mg-deficient diet affected the bioavailability of dietary Mn and the distribution of this element in different tissues.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
To study the development of Mg deficiency, seven deficient rats were killed by decapitation on each of experimental days 21, 35 and 70. Blood was collected (with heparin as an anticoagulant) and centrifuged at 3000 g for 15 min to separate plasma; samples of whole blood were taken for digestion before centrifuging. The longissimus dorsi muscle, heart, kidney, liver, spleen, femur and sternum were also removed on each day, weighed and stored at -20°C for analysis. During the last five days of each experimental period, the feces and urine were collected every 24 hours and stored at -20°C for subsequent analysis. The amount of food ingested was recorded.
The results were compared with those for a group of control rats fed the same diet, except that the amount of Mg was adequate to cover their nutritional requirements (450 mg/kg food). Control animals were pair-fed with the deficient rats.
Throughout the 70-day experimental period, the control and Mg-deficient rats were housed in individual metabolic cages in a well-ventilated, temperature-controlled room (21±2°C) with a light/dark period of 12 hours.
We calculated the biological indices absorbed Mn as I-F, and Mn balance as I-(F+U), where I=intake, F=fecal excretion, and U=urinary excretion.
Analytical Techniques
Dry matter was determined as the material remaining after heating to 105±2°C until weight was stable.
Magnesium content in diets and whole blood was determinated by flame ion atomic absorption spectroscopy (AAS) (Perkin Elmer 1100B spectrometer) of samples previously ashed at 450°C (Nabertherm furnace, Germany) until the weight was stable. The resulting residues were extracted with 5 N HCl (Merck) and 0.1% lanthanum chloride (Merck), brought up to an appropriate volume and compared spectrophotometrically against a set of standards.
The content of Mn in diets, whole blood and tissues was determined by AAS of samples previously ashed at 450°C and extracted with a 5 N solution of HCl (Merck).
Magnesium and Mn in plasma and urine were also determined by AAS in samples that were not previously ashed.
Bovine muscle (Certified Reference Material CRM 184, Community Bureau of Reference, Brussels, Belgium) yielded a Mn value 325±30 µg/g (mean±SEM of five determinations; certified value 334±28). This material was used for quality control assays.
Statistical Analyses
The data for control and Mg-deficient animals were compared for each time period with one-way analysis of variance. In both the control an Mg-deficient groups, linear regression analyses were done. All analyses were done with the SPSS software package. Differences were considered significant at the 5% probability level.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
|
|
|
|
In the femur, Mn content was significantly increased on day 70. However, in the sternum we found significant decreases in Mn on days 35 and 70, and there was a clear tendency for Mn concentration in this tissue to decrease during the experimental period (Table 4).
Under our experimental conditions the Mg-deficient diet led to no significant changes in Mn concentration in the liver (Table 4).
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Despite the high correlation between Mn intake and fecal excretion, Mg deficiency led to a decrease in fecal Mn excretion (Table 2). Little is known about the transport of Mn across the intestinal mucosa. Manganese absorption is influenced by many factors such as chemical form, presence of chelating or complexing agents and interactions between different micronutrients. The addition of high concentrations of iron [12–14], calcium or magnesium [4] to the food also reduced the absorption of Mn. Moreover, Mn uptake was enhanced in iron deficiency [14,15]. Under our experimental conditions, the Mg-deficient diet led to an increase in intestinal Fe absorption [16]. It has been suggested that manganese and iron share a common mechanism of absorption and transport in the digestive tract [17]. The effects of Mg deficiency might therefore favor Mn uptake. Magnesium deficiency may also modify Mn absorption indirectly by altering the availability of other divalent cations, such as Ca [18], Zn [19] and Cu [20]. Current theories have suggested competition among some divalent cations for a common transport system [14,21].
In both control and Mg-deficient rats, urinary and fecal Mn excretion showed a similar pattern of changes. Manganese intake showed a significant direct linear correlation with urinary Mn excretion in control animals (r=0.94, p<0.005) and in Mg-deficient rats (r=0.51, p<0.05). In the experimental group, urinary Mn excretion decreased significantly from week five; this effect may be related with the formation of renal deposits of calcium and phosphorus as a result of Mg deficiency [22]. These deposits may have interfered with renal functioning and facilitated the accumulation of Mn ions in the renal tubules. The formation of renal deposits may account for the much lower correlation coefficient in Mg-deficient rats than in controls. Because urinary and fecal excretion of Mn showed similar changes, and because urinary excretion represents a low percentage of the total daily Mn loss, it is not surprising that Mn retention (balance) showed changes similar to those found for Mn absorption. The similarity in the behavior of fecal and urinary excretion was supported by the significant linear correlation between Mn balance and Mn absorption (r=0.71, p<0.001 in controls; p=0.78, p<0.001 in Mg-deficient rats). These results support the hypothesis that intestinal absorption plays an important role in regulating Mn homeostasis [23], whereas renal losses appear not to play a relevant part in Mn regulation.
The greater Mn retention was reflected in the increased Mn concentrations in most tissues studied (Table 4).
Biliary excretion represents an important rout of excretion of Mn from the body [17]. Therefore, the increase in Mn balance in animals fed the Mg-deficient diet, in addition to the factors discussed above, may also have been influenced by a decrease in biliary excretion of Mn.
Increased tissue stores of Mn as a result of Mg deficiency may explain why we found no significant increases in plasma Mn concentrations in the experimental group (Table 4).
In our experiments the depletion of erythrocyte Mg (Table 1) was accompanied by increases in Mn concentration. We assume that the increase in Mn concentrations in whole blood in rats fed the Mg-deficient diet reflects increases in the erythrocyte content of the cation, as the slight changes in plasma Mn levels were insufficient to account for the entire increase. These changes may be related to the facts that Mn2+ is an Mg2+ analog for the Mg2+ carrier of the rat erythrocytes and that Mg2+:Mn2+ exchange is reversible [24]. Moreover, Blackwell et al. [5], who studied Mn uptake in Mg-supplemented and unsupplemented Saccharomyces cerevisiae, observed that Mn2+ uptake was greater in cells previously grown in unsupplemented medium than in those from a Mg-supplemented medium.
Changes in the lipid composition of the erythrocyte membrane have also been found in Mg deficient rats [25]; these modifications may alter membrane fluidity and thus be an additional factor involved in alterations of ion transport systems.
Manganese absorbed is fairly uniformly distributed in the soft tissues, with the liver containing the highest concentration. Part of this Mn is absorbed by the liver and later transferred to the biliary ducts [17]. Under our experimental conditions there was no increase in liver Mn concentrations in Mg-deficient rats, in contrast with what we expected on the basis of the increased Mn absorption in these animals. However, Mn concentrations did increase in skeletal muscle, heart and kidney (Table 4). The discrepant results for liver tissue may be explained, in part, by the interaction between Mn and Fe. We found that on day 21, Mg deficiency had led to the accumulation of significant amounts of Fe in the liver (day 21, 201±11 µg Fe/g dry tissue in controls vs. 402±59 µg Fe/g in Mg-deficient animals; day 35, 205±8 vs. 447±6 µg Fe/g in Mg-deficient animals; day 70, 203±7 in controls vs. 554±6 µg Fe/g in Mg-deficient animals, p<0.001 on all three days). These Fe deposits probably interfered with plasma Mn uptake in the liver. Our findings are consistent with the suggestion that Mn and Fe share mechanisms of transport and cell uptake [23]. The lack of increase in Mn uptake by the liver was reflected by the release of absorbed Mn into the blood stream and its subsequent uptake by different Mg-depleted tissues. These results are consistent with the hypothesis that increased retention may have resulted, in part, from the decrease in biliary Mn excretion.
The slightly higher concentration of Mn in the heart than in skeletal muscle may be related with the greater number of mitochondria in cardiac muscle. Mitochondria have a large capacity for Mn uptake [17]. However, the increase in renal Mn appears to be related with Ca and P deposition as a result of Mg deficiency [22]. These deposits favor the formation of kidney stones and the accumulation of filtrate in the renal tubules. The transport systems of the renal tubules may also have been affected in other ways by Mg deficiency.
Bone tissue contains considerable amounts of Mn, and some researchers have used bone Mn levels as an indicator of the bioavailability of dietary Mn [26]. However, under the experimental conditions used here, we did not find a significant linear correlation between Mn absorption or balance and Mn content in the femur.
However, Mg deficiency had different effects in the sternum in comparison to the femur and soft tissues (Table 4). Because the sternum plays an important role in hematopoiesis, we believe the decrease in Mn concentration was related to an increase in metabolic activity in the bone marrow, as a mechanism to increase erythropoiesis in an attempt to compensate for hemolysis induced by Mg deficiency [27].
Our findings differ from those of Kimura et al. [3], who found that an Mg deficit generally led to depletion of tissue Mn. This discrepancy may be related with the shorter duration of the experiments run by Kimura et al. (14 days), since, under our experimental conditions, increased Mn concentrations were first detected after 35 days of feeding with the Mg-deficient diet.
In conclusion, under our experimental conditions, Mg deficiency increased Mn absorption, which was reflected as increased Mn deposits in all soft and hard tissues studied except the sternum and liver. Our finding that Mn did not accumulate in the liver (the key organ in Mn metabolism) is probably related with the formation of Fe deposits in the liver as a result of Mg deficiency. The lack of response by the liver to increased Mn absorption may have led to the redistribution of this ion to other tissues.
|
ACKNOWLEDGMENTS |
|---|
Received December 1, 1998. Accepted April 1, 1999.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  J. K. Apple, W. J. Roberts, C. V. Maxwell, C. B. Boger, T. M. Fakler, K. G. Friesen, and Z. B. Johnson Effect of supplemental manganese on performance and carcass characteristics of growing-finishing swine J Anim Sci, November 1, 2004; 82(11): 3267 - 3276. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. B. Miller, J. S. Caton, D. M. Schafer, D. J. Smith, and J. W. Finley High Dietary Manganese Lowers Heart Magnesium in Pigs Fed a Low-Magnesium Diet J. Nutr., August 1, 2000; 130(8): 2032 - 2035. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
