Oophorectomy in young rats impairs calcium balance by increasing intestinal calcium secretion

Effects of vitamin D2-fortified shiitake mushroom on bioavailability and bone structure

Bioavailability and bone loss inhibitory effects of vitamin D₂ derived from UV-irradiated shiitake mushroom were determined in vivo. The effect of the absence of ovaries on the bioavailability of vitamin D₂ and bone structure was also investigated. Sham operated (sham) and ovariectomized (OVX) rats were divided in 3 groups according to their diets, i.e. control: only vitamin D-deficient diets; UV(X): vitamin D-deficient diets with non-irradiated mushroom powder; UV(O): vitamin D-deficient diets with irradiated mushroom powder. The obtained results showed that vitamin D₂ from shiitake mushroom was able to increase bone mineral density and trabecular bone structure of femur bone as well as its bioavailability. The absence of estrogen induced adverse effects not only on bioavailability of vitamin D₂ but also on trabecular bone. In conclusion, vitamin D₂-fortified shiitake mushroom might help postmenopausal women increase vitamin D₂ bioavailability and retard trabecular bone loss. Abbreviations: OVX: ovariectomized; 25(OH)D: 25-hydroxyvitamin D; 1,25(OH)2D: 1,25-dihydroxyvitamin D; BMD: bone mineral density; micro-CT: micro computed tomography; RSM: response surface methodology; RP-HPLC: Reverse phase-high performance liquid chromatography; MS/MS: tandem mass spectrometry; E2: estradiol; NTx: N-terminal telopeptide of type I collagen; BV/TV: bone volume/total volume; BS/BV: bone surface/bone volume; Tb.Th: trabecular thickness; Tb.Sp: trabecular separation.

Prevention of osteoporosis by oral administration of phytate-removed and deamidated soybean β-conglycinin

Phytate-removed and deamidated soybean β-conglycinin (PrDS) prepared by ion-exchange resins was supplemented to be 4% in the diet administered to ovariectomized rats to investigate its preventive effect on osteoporosis. The apparent calcium absorption rate decreased following ovariectomy and was not replenished by oral administration of phytate-removed soybean β-conglycinin (PrS) or casein. On the other hand, administration of PrDS restored the calcium absorption rate to the same level as the sham group. Markers of bone resorption, such as serum parathyroid hormone (PTH) and urinary deoxypyridinoline (DPD), increased, and the bone mineral density and breaking stress decreased following ovariectomy. However, PrDS supplementation suppressed the changes caused by the decrease in calcium absorption from the small intestine. Therefore, PrDS supplementation shows promise for the prevention of postmenopausal osteoporosis.

Black tea may be a prospective adjunct for calcium supplementation to prevent early menopausal bone loss in a rat model of osteoporosis

The present study was undertaken to find out the ability of black tea extract (BTE) as a suitable alternative of adjunct for calcium supplementation in treating an ovariectomized rat model of early osteoporosis. Female Wistar rats weighing 140-150 g were divided into four groups consisting of six animals in each group: (A) sham-operated control; (B) bilaterally ovariectomized; (C) bilaterally ovariectomized + BTE; (D) bilaterally ovariectomized + 17 β -estradiol. Results suggest that BTE could promote intestinal absorption of calcium significantly (P < 0.01 for duodenum and ileum; and P < 0.05 for jejunum). This was found associated with enhanced activities of two relevant intestinal mucosal enzymes alkaline phosphatase (P < 0.01 for duodenum, jejunum, and ileum) and Ca(2+) activated ATPase (P < 0.01 for duodenum, jejunum, and ileum). Such BTE-mediated promotion of calcium absorption was coupled with increase in serum estrogen titer (P < 0.01) and recovery of all urinary, bone, and serum osteoporotic marker parameters, including bone histological features. Serum parathyroid hormone level, however, was not altered in these animals (P > 0.05). A comparative study with 17 β -estradiol, a well-known adjunct for calcium supplementation, indicated that efficacy of BTE in maintaining skeletal health is close to that of 17 β -estradiol. This study suggests that simultaneous use of BTE is promising as a prospective candidate for adjunctive therapies for calcium supplementation in the early stage of menopausal bone changes.

Estrogen deficiency and low-calcium diet increased bone loss and urinary calcium excretion but did not alter arterial stiffness in young female rats

Many epidemiological studies have reported that the severity of arterial diseases such as arterial calcification and stiffness is inversely related to bone loss, i.e., osteoporosis. However, the nature of this relationship is unclear. The purpose of the present study was to examine the influences of estrogen deficiency and/or low-calcium diet (0.1% Ca) on bone metabolism and calcium balance, as well as aortic wall composition and stiffness in young female rats. Twenty-eight 6-week-old female rats were randomized into four groups: OVX-Low calcium (OL) and OVX-Normal calcium groups (ON) were ovariectomized, and Sham-Low calcium (SL) and Sham-Normal calcium groups (SN) were sham-operated. After 12 weeks, the bone mineral density of the lumbar spine and tibial proximal metaphysis were significantly lower in ON than in SN, and also significantly lower in OL than in ON. Additionally, OL rats had significant higher (vs. SN and SL) urinary deoxypyridinoline, but not urinary calcium, excretion at 4 weeks after ovariectomy. However, at 12 weeks after ovariectomy, urinary calcium excretion was significantly higher in OL than in SL, with corresponding increases in two bone turnover markers, bone-type alkaline phosphatase and tartrate-resistant acid phosphatase. Neither estrogen deficiency nor low-calcium diet affected aortic stiffness or elastin degeneration and calcium deposition over the course of the present study, although changes of bone metabolism occurred rapidly. Taken together, these results show that bone loss and arterial stiffness did not progress simultaneously in the present experimental protocol.

Prolonged feeding of difructose anhydride III increases strength and mineral concentrations of the femur in ovariectomized rats

This study demonstrates that feeding difructose anhydride III (DFAIII) improves bone strength and femoral mineral concentrations in a rat model of oestrogen deficiency. We showed the relationship between Ca, Mg and P absorption and bone characteristics in rats. Two groups of female Sprague-Dawley rats (6 weeks old) underwent bilateral ovariectomy (ovariectomized rats, OVX rats) or bilateral laparotomy (sham rats). At 10 weeks old, OVX and sham rats were divided into three subgroups and fed a control, 1·5 % DFAIII or 3 % DFAIII diet for 8 weeks, respectively. Ca but not Mg absorption rates were lowered by ovariectomy; however, ingestion of the 1·5 % and 3 % DFAIII diets similarly restored the reduced Ca absorption in OVX rats at 4 and 8 weeks after feeding of the test diets. DFAIII increased Mg absorption dose-dependently in sham and OVX rats. The bone strength, femoral Ca and Mg concentrations, and distal bone mineral density in the 3 % DFAIII group were higher than those in the control group in OVX rats. The absorption rates of Ca and Mg were significantly correlated with femoral Ca and Mg concentrations and strength, which suggests that increasing both Ca and Mg absorption improves bone characteristics in OVX rats. There were no differences in any of the variables in the femur between the 1·5 % and 3 % DFAIII groups in OVX rats. In conclusion, feeding of a low dose of DFAIII increased intestinal Ca and Mg absorption, and the promotive effect of DFAIII persisted for over 8 weeks. This effect was associated with prevention of ovariectomy-induced osteopenia.

Oophorectomy acutely increases calcium excretion in adult rats

Estrogen deficiency-induced bone loss is associated with complex changes in the calcium fluxes that constitute calcium balance. We studied the effects of oophorectomy on calcium balance and its components within the first 9 wk after the operation. Six-day calcium balance studies were performed on 30-wk-old female Sprague-Dawley rats before either sham operation or oophorectomy (oophx) and at 3-wk intervals for 9 wk postoperation. The rats were fed a diet containing 0.4g Ca/100 g diet and 0.3 g P/100 g diet throughout the study. The postoperative changes in calcium balance (P < 0.05) and net calcium absorption (P < 0.02) were negative in the oophx group compared with the ovary-intact group. The oophx group excreted more calcium via both the kidney (urine Ca, P < 0.05) and the gastrointestinal tract (endogenous fecal Ca, P < 0.05). The postoperation endogenous fecal calcium was higher at 3 wk postoophorectomy than at later times (P < 0.05). Oophorectomy did not affect true calcium absorption up to 9 wk postoophorectomy. Oophorectomy stimulates bone metabolism and our findings indicate that within the first 9 wk after oophorectomy, bone mineral loss is associated with a transient increase in the excretion of calcium by the gastrointestinal tract and the kidney.

Ingestion of water-soluble soybean fiber prevents osteopenia and hypercholesterolemia induced by ovariectomy in rats

This study examined the effects of feeding water-soluble soybean fiber (WSSF), a highly fermentable dietary fiber with low viscosity, on intestinal calcium (Ca) absorption, bone mineral content, and serum total cholesterol levels using female Sprague-Dawley rats (5 weeks old) with or without ovariectomy (OVX). The rats in each group were fed a WSSF (50 g/kg of diet) or WSSF-free diet for 4 weeks. Ca absorption was higher in OVX rats fed the WSSF diet than in OVX rats fed the WSSF-free diet. Femoral Ca content in both sham and OVX rats fed the WSSF diet was higher than that in rats fed the WSSF-free diet. The serum total cholesterol levels were elevated after OVX compared with that in the sham rats, and this elevation was prevented by the WSSF diet. In conclusion, ingestion of WSSF prevented osteopenia and hypercholesterolemia induced by ovariectomy in rats.

Supplemental feeding of difructose anhydride III restores calcium absorption impaired by ovariectomy in rats

In three separate experiments, we examined the effects of feeding difructose anhydride III (DFAIII) on intestinal calcium (Ca) absorption using female Sprague-Dawley rats (6 wk old) with or without ovariectomy (OVX). In Experiment 1, we showed that Ca absorption was lower in OVX rats fed the 2.0 and 3.0 g Ca/kg diets, but not the 1.0 g Ca/kg diet, than in sham-operated rats during a 3-wk test period. In Experiment 2, we demonstrated that Ca absorption rate in sham and OVX rats fed a diet containing 3% DFAIII was higher than that in rats fed a DFAIII-free diet 4 wk after consuming the test diets. Absorptive activities of everted sacs of the colon, but not of the duodenum, in rats fed DFAIII diet for 4 wk were higher than those in rats fed the control diet. In Experiment 3, we determined which of the small and large intestines is responsible for the effects of DFAIII on Ca absorption using OVX rats with cecocolonectomy or transsection and reanastomosis (sham). Both the sham and cecocolonectomized rats were divided into four subgroups and fed a control, polyethylene glycol (PEG), 1.5% DFAIII or 3% DFAIII diet. We set up the PEG group as another control group to observe the effects of shortening transit time of the small intestine in the DFAIII groups. Promotive effects of DFAIII on Ca absorption were abolished by cecocolonectomy. However, in the cecocolonectomized rats, the Ca absorption rate was still higher in the 1.5 and 3% DFAIII groups than in the PEG group. In conclusion, ovarian hormone deficiency impaired Ca absorption, but the reduction of Ca absorption was restored by feeding DFAIII.

Effect of oligofructose or dietary calcium on repeated calcium and phosphorus balances, bone mineralization and trabecular structure in ovariectomized rats*

We investigated the effects of dietary oligofructose and Ca on bone structure in ovariectomized rats, using microradiography and histomorphometry. Ninety-six animals were allocated to seven experimental groups: G1, sham-operated; G2-G7, ovariectomized. Semi-purified diets containing 5 g Ca/kg (recommended content) without oligofructose (G1, G2) or with 25, 50 or 100 g oligofructose/kg (G3, G4, G5) or 10 g Ca/kg (high content) without oligofructose (G6) or with 50 g oligofructose/kg (G7) were fed for 16 weeks. At the recommended level of Ca, high oligofructose (G5) increased femur mineral levels in ovariectomized rats, while medium oligofructose did so at high Ca. Increasing Ca in the absence of oligofructose did not increase femur mineral content. Trabecular bone area (%) analysed in the tibia was 10.3 (sem 1.2) (G1), 7.7 (sem 0.6) (G2), 9.3 (sem 0.7) (G3), 9.4 (sem 1.0) (G4), 9.5 (sem 0.7) (G5), 10.2 (sem 0.8) (G6), and 12.6 (sem 0.8) (G7). At the recommended level of Ca, 25 g oligofructose/kg prevented loss of trabecular area due to increased trabecular thickness, while 50 or 100 g oligofructose/kg increased trabecular perimeter. At high Ca, oligofructose prevented loss of bone area due to increased trabecular number but similar thickness (G7 v. G6). When Ca was raised in the presence of oligofructose (G7), trabecular area and cortical thickness were highest, while loss of trabecular connectivity was lowest of all groups. At the same time, lumbar vertebra Ca was higher; 44.0 (sem 0.8) (G7) compared with 41.6 (sem 0.8) (G2), 41.4 (sem 0.7) (G4), and 40.5 (sem 1.0) mg (G6). We conclude that ovariectomy-induced loss of bone structure in the tibia was prevented but with different trabecular architecture, depending on whether dietary Ca was increased, oligofructose was incorporated, or both. Oligofructose was most effective when dietary Ca was high.

Effects of vitamin K2 (menatetrenone) on calcium balance in ovariectomized rats

Vitamin K2 (menatetrenone) has been used for the treatment of osteoporosis in Japan. We investigated the effects of ovariectomy (OVX) and vitamin K2 on the calcium (Ca) balance in 20-week-old female Fischer rats. Vitamin K2 (31 mg/kg per day) was given to animals as a dietary supplement. At weeks 4 and 8 after OVX, a Ca balance study was performed for 5 days. The intestinal Ca transport was determined using the everted gut-sac technique at week 9. The Ca balance was poorer in the OVX-control group than in the sham-control group at weeks 4 and 8 after OVX. The Ca balance improved significantly in the vitamin K2 groups as compared with the sham- and OVX-control groups. The intestinal Ca transport decreased due to OVX and was higher in the vitamin K2 administration groups than in the sham- and OVX-control groups, but not to a significant extent. The bone mineral density in the femoral metaphysis as well as the cortical area and cortical thickness in the femoral diaphysis in the OVX-control group were lower than in the sham-control group. The administration of vitamin K2 significantly inhibited an OVX-induced decrease in cortical area and cortical thickness in the femur. These findings suggest that the poor Ca balance observed in ovariectomized rats may be improved by vitamin K2; vitamin K2 may be involved in preventing bone loss in vivo.

Ovariectomized murine model of postmenopausal calcium malabsorption

Two experiments were carried out to examine whether the ovariectomized mouse is a potential in vivo model of intestinal calcium malabsorption as occurs in postmenopausal osteoporosis. In the first experiment, we compared the effects of ovariectomy and 17 beta-estradiol (E2) therapy on calcium absorption in C3H/HeJ (C3H) and C57BL/6J (C57BL) mice, which have high and low peak bone mass, respectively. For each strain of mice, three groups were studied: sham operated, ovariectomized, (OVX), and OVX + E2 (60 micrograms/kg of body weight [bw]/day). Therapy was continued for 35 days and calcium absorption measured. In the C3H mice, ovariectomy caused an increase in fecal calcium (17.6%), and a decrease in the amount (12.8%) and percentage (12.5%) of calcium absorbed. The decrease was prevented by E2 therapy, but the differences in the calcium absorption parameters among the groups were not statistically significant. In contrast, in the C57BL mice, ovariectomy caused a marked increase in fecal calcium (84.5%, p < 0.01), and a marked decrease in the amount (55%, p < 0.01) and percentage (34.8%, p < 0.001) of calcium absorbed, and the decrease in the percentage of calcium absorbed was partially prevented by E2 therapy (p < 0.05). In experiment 2, a dose-response study of the effects of E2 therapy on calcium absorption in OVX C57BL mice was carried out. Five groups of mice were studied: Group 1, sham operated; Group 2, OVX; Group 3, OVX + 60 micrograms of E2/kg of bw/day; Group 4, OVX + 120 micrograms of E2/kg of bw/day; Group 5, OVX + 240 micrograms of E2/kg of bw/day. Therapy was continued for 28 days and calcium absorption measured. Ovariectomy caused a marked increase in fecal calcium (50%, p < 0.0001) and urinary calcium (31%, p < 0.0001) and a marked decrease in calcium absorption (44.9%, p < 0.0001), and these changes were prevented by E2 therapy. The highest level of calcium absorption (109%, p < 0.0001, vs. OVX) was observed in the 120 micrograms of E2 group. Ovariectomy and E2 slightly increased plasma 1,25-dihydroxyvitamin D (1,25(OH)2D) levels but there was no significant correlation between 1,25(OH)2D levels and calcium absorption. The findings in the C57BL mice suggest that estrogen is a physiological regulator of calcium absorption in this strain of mice. Furthermore, these findings share many characteristics with intestinal calcium malabsorption and its reversal by estrogen therapy in hypoestrogenic women. We propose that the OVX C57BL mice warrants further characterization as a potential animal model for investigating issues related to calcium malabsorption in postmenopausal women.

Oestrogen deficiency impairs intestinal calcium absorption in the rat

1. The effects of ovariectomy on the relationships between calcium consumption and calcium balance and its components were assessed in adult (10-14 months) sham-operated and ovariectomized (Ovx) rats fed a semi-synthetic diet with the calcium content varying between 0.02 and 0.4%. 2. Adaptation to dietary calcium restriction was monitored for 47 days from commencement of a 0.02 % Ca diet. 3. Response to 1,25-dihydroxyvitamin D (20 ng kg-1 day-1) administration in sham and Ovx rats and oestradiol (E2) (20 microgram kg-1 day-1) replacement in Ovx rats was assessed in rats fed a 0.05% Ca diet. 4. Ovx rats had lower intercepts for the relationships between calcium consumption and both calcium balance (P < 0.005) and intestinal calcium absorption (P < 0.005) compared with sham rats, but 1,25-dihydroxyvitamin D was not reduced in Ovx rats. 5. The magnitude of adaptation to dietary calcium restriction was unaffected by ovariectomy. 6. Intestinal calcium absorption was stimulated by an equivalent amount in sham and Ovx rats following 1, 25-dihydroxyvitamin D administration, although this did not reach statistical significance for sham (sham, t = 1.91, n.s.; Ovx, t = 3. 18, P < 0.05). 7. Oestradiol replacement in Ovx rats induced a marked increase in intestinal calcium absorption (t = 8.25, P < 0. 005), without stimulating circulating 1,25-dihydroxyvitamin D levels and led to a marked increase in calcium balance (t = 6.89, P < 0. 005). 8. These data indicate that the impairment of intestinal calcium absorption following ovariectomy is not the result of reduced circulating 1,25-dihydroxyvitamin D or reduced intestinal responsiveness to 1,25-dihydroxyvitamin D. Moreover E2 stimulates intestinal calcium absorption probably by a direct effect on the intestine.

The effect of (L)-cysteine and (L)-2-oxothiazolidine-4-carboxylic acid (OTZ) on urinary oxalate excretion: studies using a hyperoxaluric rat model

PURPOSE

To determine the efficacy of (L)-cysteine and (L)-2-oxothiazolidine-4-carboxylic acid (OTZ) in reducing urinary oxalate excretion under hyperoxaluric conditions and to determine whether by inclusion of glycolate in a standard diet, cysteine:glyoxylate adduct can be detected in hyperoxaluric rats given either compound.

MATERIALS AND METHODS

Hyperoxaluria (200% above basal) was induced 2 days prior to commencement of the studies and maintained throughout. After a 3 days baseline, animals were randomly allocated to a control or treatment group. Standard diet containing either (L)-cysteine or OTZ was then fed to the treatment groups for 5 days while standard diet alone was fed to the control groups. Urinary oxalate excretion was subsequently monitored and average daily rates were then compared with basal values. Plasma and urine were analyzed for adduct.

RESULTS

Both (L)-cysteine and OTZ significantly reduced urinary oxalate excretion relative to the basal hyperoxaluric level (28.6 +/- 1.5 micromol./day). While (L)-cysteine reduced oxalate excretion over the 5 day treatment period by only 7.82 +/- 1.39 micromol./day (27%), OTZ reduced it by 12.34 +/- 1.58 micromol./day (43%). Adduct could not be detected in plasma or urine in this study.

CONCLUSIONS

This study confirms that both (L)-cysteine and OTZ are effective in reducing urinary oxalate excretion under hyperoxaluric conditions, with OTZ being more effective than (L)-cysteine. These compounds were shown to be 3- to 4-fold more effective in reducing urinary oxalate excretion under hyperoxaluric conditions when compared with the results from previous studies under normooxaluric conditions.

The effect of oophorectomy on calcium homeostasis

Postmenopausal bone loss is associated with a rise in the fasting urine calcium excretion which has been proposed to be a possible cause of increased bone resorption. The oophorectomized rat is widely used as a model of postmenopausal bone loss, but preliminary data suggest that urine calcium excretion is not increased following oophorectomy in this animal model. However the present detailed experimental protocol demonstrated that oophorectomy in adult rats increased the obligatory urine calcium excretion compared with ovary-intact rats (P < 0.001). Importantly urine hydroxyproline excretion was positively correlated with urine calcium only in the oophorectomized rats (P = 0.003). This urine calcium was positively correlated with urine phosphate excretion when dietary sodium was low (P < 0.001) and with urine sodium excretion when dietary sodium was elevated (P = 0.003). Calcium balance studies indicated a greater fall in calcium accretion in growing, oophorectomized rats (6 to 15 weeks of age) compared with ovary-intact animals and intestinal calcium secretion was the major component of the calcium balance to be affected. Losses of calcium in the urine were relatively minor in these growing rats.