Citrate in mineralized tissues. III. The effect of purified diets low in calcium and vitamin D on the citrate content of the rat femur

The status of citrate in the hydroxyapatite/collagen complex of bone; and Its role in bone formation

Background

It has been known for more than 70 years that citrate is a major component of bone; comprising 1–2% weight of bone, and a concentration that is ~5–25-fold greater than the citrate concentration of most other tissues. This relationship exists in humans and in all vertebrates; which reveals that it is an indispensible and essential structural/functional component of bone. However, its implications relating to the structure and properties of bone, to the process of bone formation and regeneration, to bone disorders, and other issues have remained largely unknown and unaddressed. Recent studies have identified citrate as a structural component of the apatite nanocrystal/collagen complex, which is essential for imparting the bone properties of stability, strength, and resistance to fracture. This raises the issues of the status of citrate, and its source in normal bone formation.

Methods

The present report investigated the association of citrate with the hydroxyapatite (mineral) component and with the collagen component of human cortical bone preparations. The bone preparations were subjected to demineralization procedures to extract the mineral component; followed by extraction of the collagen component in the residual demineralized bone. The extracts were assayed for citrate, calcium, and collagen.

Results

The results reveal, for the first time, the existence of two major pools of citrate in bone. One pool comprising ~65–80% of the total citrate is associated with the hydroxyapatite component; and another pool comprising ~20–35% of the total citrate is tightly bound to the collagen component of the apatite nanocrystal/collagen complex.

Conclusions

Citrate is an indispensible chemical and structural component of the apatite nanocrystal/collagen complex; and is required for manifestation of the biomechanical properties of bone. These results lead to a new concept of bone formation in which citrate incorporation (“citration”) in concert with mineralization must be included in the process of bone formation. Along with this relationship, osteoblast citrate production has recently been identified as the likely source of citrate. It is now evident that the role of citrate in normal bone formation and its implications in bone disorders and defects, and in bone repair and regeneration, now requires renewed attention and support for much needed research.

The effect of vitamin D in rats maintained on diets with different mineral content but with the same calcium to phosphorus ratio of unity

1. Eight groups of ten weanling rats were maintained for 60 d on diets containing calcium and phosphorus in the ratio 1:1 (w/w) at four different levels of mineral, namely 0.08, 0.12, 0.24 and 0.36% Ca, with and without the addition of ergocalciferol.

2. Provision of vitamin D increased final body-weight except at the highest level of mineral intake. Similarly, increasing the mineral intake increased final body-weight except at the highest level.

3. Vitamin D raised the serum Ca concentration at all levels of mineral intake but in the experiment as a whole had no significant effect on serum P concentration. Serum Ca concentration was significantly higher at the two higher levels of mineral intake than at the two lower levels. Increasing dietary mineral caused a progressive increase in serum P concentration, except at the highest level of intake.

4. In general, the addition of vitamin D to the diets had no effect on the fresh or dry weights of the humeri; there was an indication, however, that at the two lower levels of mineral intake, vitamin D reduced the fresh weight of bone. The addition of the vitamin caused a reduction in bone volume, and an increase in bone length, bone ash and the ratio of the weight of ash to the weight of organic matter in dry, fat-free bone. With increasing dietary mineral there was an increase in the value of all the bone measurements except bone volume, which was decreased.

5. The addition of vitamin D and increasing dietary mineral both caused an increase in tooth mass and ash content.

6. The ratios of tooth mass to bone mass, and tooth ash to bone ash were unaffected by vitamin D, but decreased with increasing mineral intake.

7. Histological examination of the bones showed that the animals receiving 0.08 % of Ca and P were markedly rachitic although the presence of vitamin D ameliorated the condition. When the dietary mineral intake was raised to 0.12% Ca and P the animals were less severely rachitic and in the presence of vitamin D the bones appeared normal.

8. The incisor teeth were abnormal at the two lower levels of mineral intake, but when vitamin D was provided, the histological appearance was improved. When the Ca and P level in the diet was 0.24%, the incisal dentine appeared normal but the apical predentine was slightlywider thannormal; addition of vitamin D produced teeth of normal appearance. The incisor teeth were normal in appearance in the remaining groups.

9. These results are compared with those obtained in two previous studies where the Ca to P ratios were 1:10 and 10:1 (Harrand & Hartles, 1968, 1969).

A study on the effect of vitamin D in rats maintained on diets with different calcium and phosphorus content but with the same high ratio of calcium to phosphorus

1. Eight groups of ten weanling rats were maintained for 60 days on diets containing calcium and phosphorus in the ratio 10:1 at four different levels of mineral, namely 0.8, 1.19, 2.29 and 3.33% Ca.

2. At the two lower levels of mineral intake the provision of vitamin D reduced final body-weight. Increasing the mineral intake increased final body-weight except at the highest level.

3. Increasing the dietary mineral content had no effect on serum Ca, but serum P was higher in the groups receiving the two higher levels of dietary mineral. Vitamin D raised the serum Ca level in the rats receiving the two lower levels of mineral, and serum P was raised by the vitamin at all levels of mineral intake.

4. Provision of vitamin D at the two lower levels of mineral intake decreased the fresh weight, dry weight and volume of the humerus but had no significant effect on the absolute amount of mineral ash in the bone. Consequentially there was an increase in percentage of ash, overall density and the ratio of weight of ash to organic matter in dry fat-free bone (A:R value). With increasing mineral intake there was an increase in all the bone measurements except volume.

5. The presence of vitamin D had no effect on tooth mass or ash content. Increasing the mineral intake caused an increase in tooth mass and ash except at the highest level of intake.

6. The ratio of tooth ash to bone ash was very much greater at the lower levels of mineral intake than at the higher levels.

7. In the animals on the two lower levels of mineral intake the bones were frankly rachitic, and the presence of vitamin D ameliorated the condition. With increase in mineral intake histological signs of rickets were reduced, but even at the highest level of dietary mineral the epiphyseal discs appeared slightly wider than normal.

8. The incisor teeth of animals on the two lower levels of dietary mineral had wide predentine, and the presence of vitamin D reduced the amount of predentine. At the two higher levels of mineral intake the dentine appeared normal when vitamin D was present in the diet.

9. Quantitatively, the bones were much more affected than the teeth by a low intake of mineral at a Ca to P ratio of 10:1.

A study of the effect of vitamin D in rats maintained on diets with different mineral content but with the same low ratio of calcium to phosphorus

1.Eight groups of weanling rats were maintained for 60 days on diets containing calcium and phosphorus in the ratio 0·1 at four different levels of mineral, namely 0·08, 0·12, 0·23 and 0·32% Ca, and in the presence and absence of added ergocalciferol.

2. Provision of vitamin D increased final body-weight, whereas at the highest mineral intake body-weight was reduced.

3. At each level of dietary mineral the serum Ca concentration was raised by vitamin D, whilst the serum P concentration was lowered except when the mineral intake was lowest.

4. With increasing mineral intake the serum Ca increased until at the highest intake it declined. Serum P was increased only at the highest mineral intake.

5. Measurements were made of bone weights, ash content and volume. Provision of vitamin D increased the fresh weight of bone at all levels of dietary mineral, and the dry weight and ash content were increased except at the lowest level of mineral intake. In general, the higher the dietary mineral the greater was the bone weight and its ash content.

6. The presence of vitamin D resulted in an increase in tooth mass and ash content at each level of mineral intake. Tooth mass and ash increased with increasing dietary mineral except at the highest level when there was a decline.

7. At the lower levels of mineral intake the ratio of tooth mass to bone mass was greater than at the higher levels of intake.

8. Histological examination of the bones revealed no increase in the width of the epiphyseal cartilage in any group. In the group with the lowest levels of Ca and P without vitamin D (0·08% Ca, 0·8% P) the shafts of the femurs appeared thinner and porotic, and the trabeculae were thin; in the group fed the same diets supplemented with vitamin D the shafts were even thinner but less porotic; in all other groups the histological appearance of the bone was within normal limits.

9. Changes in the appearance of the dentine were demonstrated in all groups.

The effect of uncooked and roll-dried maize starch, alone and mixed in equal quantity with sucrose, on dental caries in the albino rat

1. One hundred and ten weanling rats were distributed between five dietary groups in randomized blocks. Each block was constructed from within a litter and each member of the block received a different diet.

2. The main features of the diets were: group I (twenty-six rats) 72% sucrose, group 2 (sixteen rats) 72% uncooked maize starch, group 3 (sixteen rats) 72% roll-dried maize starch, group 4 (twenty-six rats) 36% sucrose and 36% uncooked starch, group 5 (twenty-six rats) 36% sucrose and 36% roll-dried starch.

3. The animals were killed after 20 days on the diets and assessed for dental caries. The rats of group 1 (sucrose) had significantly more caries than all other groups, the rats of group 2 (uncooked starch) had significantly less caries than any of the other groups. Roll-dried starch (group 3) produced significantly more caries than uncooked starch (group 2). The mixture of uncooked starch and sucrose (group 4) was significantly more cariogenic than the mixture of roll-dried starch and sucrose (group 5).

The effect of differing high-carbohydrate diets on dental caries in the albino rat

1. Fifty-six albino rats were distributed between four dietary groups in fourteen randomized blocks of four animals. Each block consisted of siblings and each member of the block received a different dietary treatment. 2. The diets permitted normal growth and the only variations were: group I contained 67% sucrose, group 2 an equivalent amount of uncooked maize starch and groups 3 and 467 % of finely ground Thin Wine and Morning Coffee biscuits respectively. 3. After 31 days on the diet, the animals were killed and assessed for dental caries. The rats of group I had significantly more caries than the other groups. The rats of group 2 had significantly less caries than the other groups. The biscuit diets produced significantly less caries than the sucrose diet, but significantly more caries than the starch diet.

A study in the rat of the interaction between the effects of calcium and phosphorus content of the diet at two different levels and the presence or absence of vitamin D

1. Four groups of weanling rats were maintained on diets containing either 0.12% or 0.24% of both calcium and phosphorus, in the presence and absence of added ergocalciferol. 2. Increase in mineral intake or the provision of vitamin D increased the final body-weight, with a significant interaction between the two factors. 3. Serum Ca and P concentrations were raised significantly by both the vitamin and the increase in mineral intake without signi ficant interaction. 4. Measurements were made of bone and tooth weights and ash values and of the length of the humeri. For only two measurements was there any interaction between the effect of vitamin D and increase in mineral intake, namely the length of the humeri, and their ash values expressed as a percentage of the dry fat-free weight. 5. It is concluded that in relation to the amount of mineral deposited, the incisor tooth is more resistant to nutritional stress than is the bone.