Although a magnesium (Mg)-deficient diet is generally known to induce nephrocalcinosis, our previous study observed that despite the administration of a Mg-deficient diet, the kidney calcium (Ca) and phosphorus (P) concentrations were not increased in male rats. We speculated that this result was due to the P concentration of the experimental diet based on the AIN-93G formula used in the previous study. In the present study, male rats were fed modified AIN-93G diets containing the two different Mg concentrations [0.5 g per kg diet (normal-Mg) or Mg-free (Mg-deficient)] and three different P concentrations [3 (3-P), 5 (5-P) or 7 (7-P) g per kg diet]. By histological examination of the kidney, nephrocalcinosis was not observed in rats fed on the Mg-deficient diet containing 3-P While nephrocalcinosis appeared in rats fed on the Mg-deficient diet containing 5-P and 7-P. The degree of nephrocalcinosis was severe in rats fed on the Mg-deficient diet containing 7-P compared with rats fed on the Mg-deficient diet containing 5-P These results demonstrated that the Mg-deficient diet based on AIN-93G formula dose not induce nephrocalcinosis and that the Mg-deficient diet based on AIN-93G formula with increased dietary P concentrations induces nephrocalcinosis in male rats. We suggest that the onset of nephrocalcinosis could depend on the dietary P concentration in male rats fed on a Mg-deficient diet.

We investigated whether lowering food intake by high phosphorus (P) diet influenced parathyroid hormone (PTH) actions, bone turnover markers, and kidney mineral concentration in rats. Rats in two of the three groups were respectively given free access to a control diet (C group) and a high P diet (HP group) for 21 days. Rats in another group (PF group) were pair-fed the control diet with the HP group. Compared to the C and PF groups, serum PTH concentration, urinary C-terminal telopeptide of type I collagen excretion, and kidney calcium and P concentrations were significantly higher in the HP group. Urinary excretion of cAMP was significantly lower in the HP group than in the C and PF groups. These results suggested that high P diet decreased PTH action in the kidney and increased bone resorption and kidney mineral concentrations independently of lowering food intake.

We investigated the effect of dietary magnesium (Mg) level on protein utilization in rats. Male Wistar rats were fed a control diet (control group) and a Mg-deficient diet (Mg-deficient group) for 28 days. After 28 days, the diet of half of the Mg-deficient group (recovery group) was changed from the Mg-deficient diet to the control diet for either 7 or 14 days. After 28 days, final body weight, weight gain and food efficiency were significantly decreased due to the Mg-deficient diet. Apparent Mg absorption, Mg retention and serum Mg levels were also significantly decreased due to the Mg-deficient diet. Furthermore, the Mg-deficient group showed a significant increase in urinary nitrogen (N) excretion and significant decreases in N retention and serum albumin level. At day 7 and 14 after changing the Mg-deficient diet to the control diet, apparent Mg absorption, Mg retention and serum. Mg levels were significantly increased in the recovery group as compared with those in the Mg-deficient group. However, with regard to final body weight, weight gain and food efficiency, no significant differences were observed between the Mg-deficient group and the recovery group. At day 14 after changing the diet, urinary N excretion was significantly decreased and N retention was significantly increased in the recovery group as compared with the Mg-deficient group. At day 7 and 14 after changing the diet, the serum albumin level was also significantly increased in the recovery group as compared with that in the Mg-deficient group. These results suggest that: 1) the Mg-deficient diet depresses protein utilization; 2) the Mg-deficient diet-induced impairment of protein utilization is reversed by dietary Mg supplementation; and 3) the Mg-deficient diet-induced growth retardation is not completely reversed after 14 days of Mg supplementation.

This study investigated the effects of moderate magnesium (Mg)-restricted diet on bone formation and bone resorption in rats. Weanling Wistar strain rats were randomly divided into three dietary groups of 6 rats each and fed their respective diets; a control diet containing 0.05% Mg (C), a half Mg diet containing 0.025% Mg (1/2Mg), or a one-fifth Mg diet containing 0.01% Mg (1/5Mg), for 21 days. Serum osteocalcin level was significantly reduced with decreasing dietary Mg level. Urinary excretion of C-terminal telopeptide of type 1 collagen was significantly higher in the 1/5Mg group than in the C group. Serum insulin-like growth factor-1 (IGF-1) level was significantly lower in the 1/2Mg and 1/5Mg groups than in the C group. Serum soluble receptor activator of nuclear factor-kappaB ligand (sRANKL) level was significantly higher in the 1/2Mg and 1/5Mg groups than in the C group. These results showed that a moderate Mg-restricted diet induced a decrease in bone formation and an increase in bone resorption. Furthermore, these changes of bone formation and bone resorption might be caused by serum IGF-1 and sRANKL levels, respectively.

We examined the effects of high calcium (Ca) intake on bone metabolism in magnesium (Mg)-deficient rats. Male Wistar rats were divided into three groups, with each group having a similar mean body weight, and fed a control diet (control group), a Mg-deficient diet (Mg-deficient group) or a Mg-deficient Ca-supplemented diet (Mg-deficient Ca-supplemented group) for 14 d. Femoral Ca content was significantly lower in the Mg-deficient Ca-supplemented group than in the control group and Mg-deficient group. Femoral Mg content was significantly lower in the Mg-deficient group and Mg-deficient Ca-supplemented group than in the control group. Furthermore, femoral Mg content was significantly lower in the Mg-deficient Ca-supplemented group than in the Mg-deficient group. Serum osteocalcin levels (a biochemical marker of bone formation) were significantly lower in the two Mg-deficient groups than in the control group. As a biochemical marker of bone resorption, urinary deoxypyridinoline excretion was significantly higher in the Mg-deficient Ca-supplemented group than in the control group and Mg-deficient group. The results in the present study suggest that high Ca intake had no preventive effect on alteration of bone metabolism in Mg-deficient rats.

The purpose of this study was to investigate the effect of dietary magnesium (Mg) supplementation on bone loss in rats fed a high phosphorus (P) diet. Weanling Wistar strain rats were randomly divided into four dietary groups of 6 rats each and fed their respective diets; a diet containing 0.3% P and 0.05% Mg (C), a diet containing 1.5% P and 0.05% Mg (HP), a diet containing 0.3% P and 0.15% Mg (HMg), or a diet containing 1.5% P and 0.15% Mg (HPMg), for 21 days. Compared to the C and HMg groups, serum parathyroid hormone (PTH) concentration was significantly higher in the HP and HPMg groups. Serum osteocalcin concentration and urinary excretion of C-terminal telopeptides of type I collagen (CTx), markers of bone turnover, were significantly higher in the HP and HPMg groups than in the C and HMg groups. Dietary Mg supplementation had no significant effects on serum PTH and osteocalcin concentrations, while urinary excretion of CTx was significantly lower in the HPMg group than in the HP group. These results suggested that dietary Mg supplementation suppressed bone resorption due to high P diet.

To determine the parathyroid hormone (PTH) action on kidney and bone by high phosphorus (P) diet, this study investigated PTH/PTH-related peptide (PTHrP) receptor mRNA expression in 6-week-old parathyroidectomized (PTX) rats received constant amount of PTH. To maintain serum PTH levels equally to sham operated rats, PTX rats were constantly exposed to rPTH (1-34) and fed a control diet (0.3% P) and a high P diet (1.2% P) for 7 days, respectively. There were no significant differences in serum PTH (1-34) concentration in rats fed the control diet. In sham groups, serum PTH concentrations, both (1-84) and (1-34) fragments, were increased in rats fed the high P diet than in rats fed the control diet. Urinary excretions of P and C-terminal telopeptides of type I collagen were significantly increased in both PTX and sham rats by the high P diet. PTH/PTHrP receptor mRNA expression in kidney and femur was not changed in both PTX and sham rats by the high P diet. In conclusion, high P diet did not change PTH action in PTX rats and increased urinary excretion of P and bone resorption regardless of PTH action.

The effects of simultaneous increases in dietary phosphorus (P) and magnesium (Mg) concentrations while maintaining a constant P:Mg ratio on nephrocalcinosis and kidney function in female rats was investigated. Female Wistar rats were fed a control diet (3.12 g P, 0.51 g Mg per kg diet) or a diet having either 3 times the control P and Mg concentrations (3-fold diet; 9.25 g P and 1.42 g Mg per kg diet) or 5 times the control concentrations (5-fold diet; 14.97 g P and 2.37 g Mg per kg diet) for 21 d. The three experimental diets all had same P:Mg molar ratios (control diet; 4.81, 3-fold diet; 5.11, 5-fold diet; 4.96). The 3-fold diet had no significant influence on kidney calcium (Ca), Mg or P concentrations. However, kidney Ca, Mg and P concentrations were significantly higher in rats fed the 5-fold diet than in rats fed the control or 3-fold diets. No significant differences in creatinine clearance were observed among the three groups. Urinary albumin and beta 2-microglobulin excretion were higher in rats fed the 5-fold diet than in rats fed the control or 3-fold diets, while the 3-fold diet had no significant influence on the urinary albumin and beta 2-microglobulin excretion. These results suggest that absolute concentrations of dietary P and Mg are important factors with regard to the development of nephrocalcinosis and diminished kidney function.

This study investigated the gender differences in the kidney function of magnesium (Mg)-deficient rats. Male and female rats were fed a control diet or a Mg-deficient diet for 21 d. Mg-deficient diet had no significant effect on kidney calcium (Ca) or phosphorus (P) concentration in male rats, while Ca and P concentrations in female rats were significantly higher in Mg-deficient rats than in the control rats. With regard to indicators of kidney function, no significant differences in creatinine clearance and serum urea nitrogen concentration were observed among the groups. Serum albumin concentrations were significantly lower in rats fed the Mg-deficient diet than in rats fed the control diet. In both sexes, urinary albumin excretion was significantly higher in rats fed the Mg-deficient diet than in rats fed the control diet. Gender differences had no significant influence on creatinine clearance, serum urea nitrogen concentration, serum albumin concentration and urinary albumin excretion. These results suggest that gender differences have no effect on kidney function in Mg-deficient rats under the condition used.

In this study, we ascertained whether the parathyroid hormone (PTH) dominantly regulated the effects of high phosphorus (P) intakes on urinary excretion of P and bone metabolism in rats. To maintain serum PTH level equally, parathyroidectomy (PTX) and sham-operated rats were constantly exposed to rPTH(1-34) and fed both control (0.3% P) and high P (1.2% P) diet for 7 days, respectively. Urinary excretions of P and C-terminal telopeptides of type I collagen were significantly increased in both PTX and sham rats by the high P diet. These results suggest that high P diet increased urinary P excretion while promoting bone resorption regardless of PTH-dependent regulation.

To discover the details of the effects of magnesium (Mg) deficiency on kidney function, the course of changes in N-acetyl-β-D-glucosaminidase (NAG) activity in the urine and in urinary albumin excretion were examined in rats fed a Mg-deficient diet. NAG activity in the urine and urinary albumin excretion in rats fed the Mg-deficient diet significantly increased from 7 d until the end of the feeding period. We suggest that Mg-deficient diet rapidly induces kidney function insufficiency.

We examined sex differences in kidney mineral concentrations and urinary albumin excretion in rats given feed containing various phosphorus (P) levels. With feed that was 0.6%, 0.9%, 1.2%, and 1.5%P, kidney calcium and P concentrations were higher in female rats than in male rats. With 1.2% or 1.5%P, urinary albumin excretion was higher in the female rats. The sex of the animal affected the kidney mineral concentrations and urinary albumin excretion in rats with a high P intake.

We examined whether a difference in potassium dihydrogenphosphate (KH2PO4) and potassium tripolyphosphate (K5P3O10) as dietary phosphorus sources could differentially effect the nephrocalcinosis and proximal tubular function in female rats. Rats were fed on a diet containing KH2PO4 or K5P3O10, at the normal phosphorus level (normal phosphorus diet) or at a high phosphorus level (high-phosphorus diet) for 21 d. Nephrocalcinosis, as confirmed by a histological examination, was apparent in all rats fed on the high-phosphorus diet, and this condition was more severe in those rats fed on K5P3O10 than in those fed on KH2PO4. As indicators of the proximal tubular function, the N-acetyl-beta-D-glucosaminidase activity in urine and the urinary beta2-microglobulin excretion were significantly increased in those rats fed on the high-phosphorus diet containing K5P3O10. These results indicate that the intake of a high-phosphorus diet, more strongly influenced the nephrocalcinosis and proximal tubular function when K5P3O10 rather than KH2PO4 was used as the dietary phosphorus source.

The effects of various phosphate salts as the dietary phosphorus sources on the development of nephrocalcinosis and kidney function were examined in rats fed diets containing monophosphate salts (sodium dihydrogenphosphate, NaH2PO4, or potassium dihydrogenphosphate, KH2PO4) or polyphosphate salts (sodium tripolyphosphate, Na5P3O10, or potassium tripolyphosphate, K5P3O10), at levels representing normal phosphorus (normal phosphorus diet) or high phosphorus (high phosphorus diet) contents for 21 d. High phosphorus diet-feeding increased the kidney calcium and phosphorus concentrations. Kidney calcium and phosphorus concentrations were higher in rats fed the high phosphorus diet containing Na5P3O10 or K5P3O10 than in rats fed the high phosphorus diet containing NaH2PO4 or KH2PO4. Nephrocalcinosis was observed in all rats fed a high phosphorus diet, and the degree of nephrocalcinosis was more severe in rats fed Na5P3O10 or K5P3O10 than in rats fed NaH2PO4 or KH2PO4. In rats fed the high phosphorus diet, creatinine clearance was higher in rats fed Na5P3O10 or K5P3O10 than in rats fed NaH2PO4 or KH2PO4. In rats fed Na5P3O10 or K5P3O10, urinary albumin excretion and N-acetyl-beta-D-glucosaminidase (NAG) activity in the urine were increased in rats fed the high phosphorus diet. These were higher in rats fed the high phosphorus diet containing Na5P3O10 than in rats fed the high phosphorus diet containing NaH2PO4 or KH2PO4. This study observed that the development of nephrocalcinosis and kidney function in rats fed the high phosphorus diet was influenced by the difference in monophosphate or polyphosphate salts provided as the dietary phosphorus source, while the effects of sodium and potassium salts were not evident. We suggest that the development of nephrocalcinosis and kidney function in rats fed a high phosphorus diet was altered depending on the form of phosphate salts provided as the dietary source of phosphorus. Additionally, the development of nephrocalcinosis and diminished kidney function in rats fed the high phosphorus diet was more severe for polyphosphate salts as compared to monophosphate salts.

The effects of long-term high intake of calcium on magnesium utilization were studied in young male rats. The rats were fed a purified diet containing 0.5% calcium as the control diet and 1.5% calcium as the high calcium diet for 10 weeks. Final body weight, weight gain and food efficiency were significantly decreased in rats fed the high calcium diet. The amount and rate of apparent magnesium absorption were significantly lower in rats fed the high calcium diet than in rats fed the control diet. Also, magnesium retention was significantly decreased in rats fed the high calcium diet. Magnesium concentrations in the serum and femur were significantly decreased in rats fed the high calcium diet. We conclude that long-term high intake of calcium induces a reduction of magnesium utilization.

The effect of a magnesium-deficient diet on kidney function was studied in young male rats. The rats were fed a purified diet with a magnesium content of either 20.5 (control diet) or 2.6 mmol/kg (magnesium-deficient diet) for 21 d. In rats fed the magnesium-deficient diet, kidney wet and dry weights were significantly increased, and calcium and phosphorus concentrations in the kidney were significantly higher than in rats fed the control diet. Upon histological examination, an increase in the mesangial matrix of the glomeruli and injury to the brush border of the proximal tubules were observed in rats fed the magnesium-deficient diet. Also, a deposition of calcium was observed in the tubules of the corticomedullary junction and medulla of these rats. Total protein and albumin concentrations in serum were significantly decreased in rats fed the magnesium-deficient diet. Urinary albumin excretion was significantly higher, and N-acetyl-beta-D-glucosaminidase activity in the urine was significantly increased in rats fed the magnesium-deficient diet. These findings indicate diminished glomerular and proximal tubular functions. We suggest that a magnesium-deficient diet not only induces nephrocalcinosis, but it also diminishes kidney function.

The purpose of this study was to reveal the improvement effect of switching to a diet of lower phosphorus content on high phosphorus diet-induced nephrocalcinosis and depression of proximal tubular function. The rats were fed a purified diet containing 0.5% phosphorus as the control diet (control group) and 1.5% phosphorus as the high phosphorus diet (damage group) for 14 days. The damage group displayed nephrocalcinosis and depression of proximal tubular function. Subsequently, the rats in the damage group were switched from the high phosphorus diet to the control diet (improvement group) for 21 days. Degree of nephrocalcinosis was reduced in the improvement group compared with the damage group, especially, the improvement group was reduced deposition of calcium in the cortex and in the medulla. As indicators of proximal tubular function, N-acetyl- β -D-glucosaminidase activity in the urine and urinary β2-microglobulin excretion were significantly lower in the improvement group than in the damage group. We conclude that the nephrocalcinosis and depression of proximal tubular function induced by a high phosphorus diet are improved by switching to a diet of lower phosphorus content.

Dietary magnesium preventive effect on high phosphorus diet-induced kidney damage was studied in young male rats. Rats were fed a purified diet containing 0.05%, 0.15% or 0.30% magnesium and 1.5% phosphorus for 21 days. On histological examination, dilatation of the distal tubules and collecting ducts and kidney calcification were observed in all groups. However, the degree of damage was reduced by increased magnesium intake. Tubular necrosis, inflammatory cell infiltration and interstitial edema in the cortex were observed in rats fed a 0.05% or 0.15% magnesium diet, but not shown in those fed a 0.30% magnesium diet. As an indicator of kidney function, serum urea nitrogen concentration was deceased in rats fed a 0.15% magnesium diet or 0.30% magnesium diet. Creatinine and urea clearance were increased with increase in magnesium intake. Urinary albumin excretion and activity of N-acetyl-β-D-glucosam inidase (NAG) in urine were decreased in rats fed a 0.30% magnesium diet. Urinary β2-microglobulin excretion were decreased in rats fed a 0.15% magnesium diet or 0.30% magnesium diet. In conclusion, increased magnesium intake prevented high phosphorus diet-induced kidney damage.

The development of nephrocalcinosis and the time course of changes in kidney function, especially proximal tubular function, were studied in young male rats fed a high-phosphorus diet. The animals were fed a purified diet with a phosphorus content of either 0.5% (normal phosphorus diet) or 1.5% (high-phosphorus diet). In the group fed the high-phosphorus diet, nephrocalcinosis was found in 4 of 42 rats after 1 d of feeding and in all rats of this group at 3 d. The degree of nephrocalcinosis gradually increased with time. Upon histological observation by electron microscopy, vacuoles, lysosomes and swelling of microvilli in the proximal tubules were observed in rats fed the high-phosphorus diet after 1 d of feeding. Giant lysosomes with deposition of calcium and deposition of hydroxyapatite in mitochondria were observed in the proximal tubules of rats fed the high-phosphorus diet at 3 d. Albumin concentration in the urine of these rats was significantly increased at 3 d. The activity of N-acetyl-beta-D-glucosaminidase in the urine was also significantly increased after 1 d of feeding the high-phosphorus diet, and then reached a plateau. The beta 2-microglobulin concentration in the urine of rats fed the high-phosphorus diet was significantly increased at 14 d, and increased more toward 21 d. We concluded that nephrocalcinosis and injury to the proximal tubules are rapidly induced in rats fed a high-phosphorus diet.