Studies on the role of 24-hydroxylation of vitamin D in the mineralization of cartilage and bone of vitamin D-deficient rats

Summary To test the importance of 24-hydroxylation of vitamin D₃ on bone mineralization, rat pups born to vitamin D-deficient females were given either 25-hydroxyvitamin D₃ or 24,24-difluoro-25-hydroxyvitamin D₃ for 16 days beginning at the time of weaning. Following such treatment analysis of blood samples revealed no detectable 24R,25-(OH)₂D₃ and 1,25-(OH)₂D₃ in the rats given the difluoro compound while revealing the expected 24,24-difluoro-25-hydroxyvitamin D₃ and 24,24-difluoro-1,25-dihydroxyvitamin D₃. The rats given 25-hydroxyvitamin D₃ had the expected levels of 25-hydroxyvitamin D₃, 24,25-dihydroxyvitamin D₃, and 1,25-dihydroxyvitamin D₃. Following sacrifice at day 17, postweaning bone mineralization and modeling were studied in long bones using histological methods. Bones taken from vitamin D-deficient rats at the beginning and end of the experimental period had lesions typical of rickets. These included wide growth plates, excessive amounts of osteoid, and metaphyseal fibrosis. Following treatment with either 25-hydroxyvitamin D₃ or 24,24-difluoro-25-hydroxyvitamin D₃, bone mineralization returned to normal. Growth plate widths and the amount of osteoid on bone surfaces were both substantially reduced and to a similar degree in both treatment groups. Normal cartilage core formation and trabecularization of the metaphyseal primary spongiosa were also restored to a similar degree in both groups. In effect, no difference was observed in any bone parameter studied between the 25-hydroxyvitamin D₃- and the 24,24-difluoro-25-hydroxyvitamin D₃-treated animals. These results provide strong evidence that 24-hydroxylation of the vitamin D molecule plays little or no role in the modeling and mineralization of bone.     

Role of vitamin D in maternal skeletal changes during pregnancy and lactation: A histomorphometric study

Summary The effect of vitamin D on bone changes during the reproductive cycle in female rats has been investigated. One group of female rats was maintained on a vitamin D-deficient diet and another group on a vitamin D-replete diet from weaning. Both groups were mated with normal males and changes in their bones were determined histomorphometrically during pregnancy, lactation, and after weaning. All vitamin D-deficient rats had bone changes typical of rickets. Pregnancy caused significant reductions in mineralized tissue of trabecular and cortical bone in the vitamin D-deficient rats. Lactation caused further significant reductions in mineralized tissues of cortical and trabecular bone in both the vitamin D-deficient and vitamin D-replete animals, with the greatest changes seen at weaning. Some restoration of mineralized tissues occurred following weaning. There was an increase in tetracycline-labeled bone surface in the vitamin D-replete animals during lactation, likely due to an increase in bone formation rates. In the vitamin D-deficient animals during lactation, there was a decrease in tetracyclinelabeled bone surface, likely due to severely depressed bone mineralization. These results indicate that the mobilization of calcium from bone to maintain pregnancy and lactation occurs by a mechanism independent of vitamin D.     

Early skeletal mineralization and the role of Vitamin D: An electron probe microanalysis

Summary The effect of vitamin D on the calcium (Ca) and phosphorus (P) contents of metaphyseal trabecula bone (MT) and calcified cartilage (CC) were analyzed by electron probe microanalysis during the periweaning period in rats that had been divided into three groups: group N was fed a normal experimental diet, group R was fed a vitamin D deficient, low P diet, group D was fed the same diet as group R and also injected with 1α-hydroxyvitamin D₃(1α(OH)D₃) (0.2 μg/kg each day). In group R, the Ca and P contents of the CC gradually decreased from weaning until 14 days after weaning when the Ca and P had become minimal. However, in the MT, the Ca and P had decreased significantly at 7 days after weaning. In group D the serum concentrations of Ca and P were lower than in group N until 7 days after weaning, but the Ca and P contents in the CC were restored at 3 days after weaning. However, in the MT, the Ca and P contents in group D were not significantly different from those in group N. These results indicate that (1) vitamin D becomes progressively more important after weaning in mineralization of cartilage and bone and (2) the sensitivity of the CC to vitamin D deficiency is higher than that of MT.     

In vivo bone mineral analysis throughout skeletal growth in rats: differences due to sex or vitamin D deficiency

This study reports a new technique for the measurement of bone mineral content (BMC) in live rats. A single photon absorptiometric instrument has been adapted for rapid, reproducible measurement of BMC. Four bone sites were selected for use, based on ease of positioning and reproducibility of measurement; these were as follows: proximal femur, midfemur, proximal seventh caudal vertebra, and midseventh caudal vertebra. Significant increases were detected when BMC was measured at biweekly intervals from weaning to 18 weeks of age in normal male rats. Comparison of body growth and changes in BMC of male and female rats with age showed that body weight gain of female rats slowed earlier than that of male rats whereas BMC increased at similar rates in both sexes. Vitamin D deprivation from day 24 of life resulted in decreased BMC at all four measurement sites compared with such measurements in normal control rats. Differences were detectable after 8 weeks of age and occurred despite the maintenance of serum calcium levels within normal range and only slight reduction in body weights of vitamin D deprived rats. These studies demonstrate that single photon absorptiometry can be used to monitor changes in BMC in live rats on a routine basis without harm to the animals. Changes in BMC such as those due to growth or vitamin D deprivation can easily be quantitated using this technique.     

The role of vitamin D metabolites in bone resorption

Two metabolites of vitamin D₃, 25-hydroxycholecalciferol (25-OHD₃) and 1,25-dihydroxy-cholecalciferol (1,25-(OH)₂D₃) are potent stimulators of bone resorption in two test systems whereas vitamin D₃ itself is inactive. These substances were tested (a) by directly comparing their action on bone explants of mouse half-calvariain vitro, and (b) by injecting them into young mice and measuring the degree of resorptionin vitro when explants were made 18 hours atter the injection. In both tests the 1,25-metabolite was about 100 times more potent than 25-OHD₃. The dose-response curve for 1,25-(OH)₂D₃ indicates that doses above about 0.2 ng/g body weight are capable of inducing an increase in bone resorption in normal young mice. These data show that 1,25-(OH)₂D₃ is one of the most potent substances known that affects bone metabolism. The results are discussed in relation to the possible role of 1,25-(OH)₂D₃ in the normal mobilization of calcium from bone.

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Zusammenfassung Bei Anwendung zweier verschiedener Versuchsanordnungen konnte gezeigt werden, daß die beiden Vitamin D₃-Metaboliten 25-Hydroxycholecalciferol (25-OHD₃) und 1,25-Dihydroxycholecalciferol (1,25-(OH)₂D₃) als starke Stimulatoren der Knochenresorption wirken, während sich Vitamin D₃ selbst inaktiv verhält. Diese Substanzen wurden folgendermaßen geprüft: a) durch direkten Vergleich ihrer Wirkung auf Knochenexplantate (Hälften von Mäusecalvarien)in vitro und b) indem die Metaboliten jungen Mäusen injiziert wurden und der Resorptionsgrad an Explantaten 18 Std nach Injektionin vitro gemessen wurde. Bei beiden Versuchsanordnungen war der 1,25-Metabolit etwa 100mal wirksamer als der 25-OHD₃-Metabolit. Aus der Dosiswirkungskurve für 1,25-(OH)₂D₃ geht hervor, daß es möglich ist, mit Dosen über ca. 0,2 ng/g Körpergewicht bei normalen jungen Mäusen bereits eine erhöhte Knochenresorption auszulösen. Diese Resultate zeigen, daß 1,25-(OH)₂D₃ eine der wirksamsten bisher bekannten Substanzen ist, die auf den Knochenmetabolismus einwirken können. Die Ergebnisse werden im Zusammenhang mit der Rolle, die das 1,25-(OH)₂D₃ bei der normalen Freisetzung von Calcium aus dem Knochen spielt, besprochen.

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Résumé Deux métabolites de la vitamine D₃, le 25-hydroxycholecalciferol (25-OHD₃) et 1,25-dihydroxycholecalciferol (1,25-(OH)₂D₃), stimulent la résorption osseuse dans deux systèmestests alors que la vitamine D₃ est inactive. Ces substances sont testées a) en comparant directement leur action dans les explants osseux de calottes craniennes de sourisin vitro et b) en les injectant dans de jeunes souris et en mesurant le degré de résorptionin vitro, lorsque les explants sont réalisés 18 heures après l'injection. Dans les deux tests, le métabolite 1,25 est environ 100 fois plus puissant que 25-OHD₃. La courbe dose-résponse de 1,25-(OH)₂D₃ indique que des doses au-dessus d'environ 0.2 ng/g de poids corporel sont capables d'induire une augmentation de la résorption osseuse chez de jeunes souris normales. Ces résultats montrent que 1,25-(OH)₂D₃ est une des substances connues les plus actives qui agit sur le métabolisme osseux. Le rôle possible de 1,25-(OH)₂D₃ sur la mobilisation normale du calcium osseux est envisagé.

     

Effect of vitamin D status on the activity of carbonic anhydrase in chicken epiphysis and kidney

Summary Chickens were raised for 6 weeks from the date of hatch under red light on a vitamin D-free diet; controls were given an oral vitamin D supplement. Vitamin D-deficient animals showed decreased total serum calcium concentration and decreased DNA content in epiphysis and kidney homogenates. In calcifying epiphysis, total carbonic anhydrase (CA) activity was decreased, but activity per μg DNA was slightly increased and specific activity was double that of the controls. Polyacrylamide gel isoelectric focusing after preparation of the enzyme showed a picture similar to that seen after parathyroid hormone (PTH) administration in chicks; therefore, this could be considered a secondary hyperparathyroidism. The CA activation was not seen in the kidney which can be explained by induction of an endogenous inhibitor protein of the cyclic AMP-dependent protein kinase exclusively in the kidney in vitamin D deficiency. In an additional experiment, chickens were raised for 3 weeks from the date of hatch under red light on a vitamin D-free diet. Daily oral substitution by different vitamin D metabolites (1,25 (OH)₂D₃, 25OHD₃, 24, 25(OH)₂D₃) over 7 days led to CA activation compared with controls probably by restoring protein kinase activity in the kidney. Our results show that CA activity is inversely correlated with serum calcium concentrations which is in agreement with a regulatory mechanism recently proposed by us.     

维生素D的最新研究进展

维生素D(Vitamin D,VD)是调节骨代谢的重要维生素,婴幼儿及儿童缺乏VD会导致佝偻病,成人及老人缺乏VD会导致骨质疏松症。近期研究表明,VD与高血压、2型糖尿病、血脂紊乱、代谢综合征、过敏性疾病及哮喘、免疫调节和抗炎、抗纤维化、心血管疾病、结核病、慢性肾脏病、各种癌症、感染、甚至死亡等方面密切相关。随着我国老龄化社会的到来和现代生活水平的提高,人们对VD的缺乏越来越重视,因此VD缺乏的患者越来越少。相反,因过量服用VD后中毒的病例却时有发生。安全始终是首要考虑的因素,在中毒剂量范围内补充人体需要的VD,应做好VD摄入水平及VD中毒的健康宣教。外源性途径摄入VD的同时,增加户外活动时间,多进行日光照射产生内源性维生素。在VD缺乏的预防和治疗过程中,应注意掌握VD摄入量和用药周期,密切观察,定期随访,避免VD中毒事件的发生,以期达到降低维生素D缺乏及维生素D缺乏性佝偻病的发生。     

Protective effect on vitamin D2 on bone apposition from the inhibitory action of hydrocortisone in rats

Summary Using the technique of short interval sequential tetracycline labeling, it was documented that the apposition of mineralized bone matrix in adult male Sprague-Dawley rats was inhibited by hydrocortisone. The inhibition occurred as early as six days after the onset of the treatment and was dose dependent over a dose range of 0.62 to 20 mg per kg body weight per day. Vitamin D₂ supplements by injection protected bone from this hydrocortisone action. 64 I. U. of vitamin D₂ injected daily was able to prevent the inhibition of bone apposition by 20 mg per kg body weight per day of hydrocortisone. The results imply that vitamin D or its metabolites may compete with hydrocortisone in some cellular mechanisms and support the usefulness of vitamin D supplements in the treatment and the prevention of steroid-induced osteoporosis.     

Changes in vitamin D metabolites and bone histology in rats during recovery from rickets

Summary The relative roles of 25-hydroxyvitamin D (25-OHD), 1,25-dihydroxyvitamin D (1,25-(OH)₂D) and 24,25-dihydroxyvitamin D (24,25-(OH)₂D) in bone mineralization are largely unknown. Young vitamin D depleted rats were fed increasing amounts of vitamin D and grouped radiologically in accordance with the rat line test. They ranged from severely rachitic to normal. Radiology was correlated with serum levels of 25-OHD, 1,25-(OH)₂D, 24,25-(OH)₂D, ionized calcium, magnesium, and phosphate, with bone histology, and with the total mineral content of the animals. Serum 1,25-(OH)₂D rose in a linear fashion to supranormal values during bone healing and correlated with the radiological degree of rickets. Serum 25-OHD was below detection limit in the most rachitic and low in the radiologicall normal rats, whereas 24,25-(OH)₂D was low in all groups. These two metabolites showed no correlation with the radiologic, histologic or biochemical parameters. In rachitic rats, 1,25-(OH)₂D appears to play a major role in bone healing and possibly exerts a direct effect on bone cells. It cannot be ruled out, however, that the effect is mediated through a rise in serum levels of calcium and phosphorus, although signs of bone healing were seen in the presence of a subnormal calcium x phosphorus product. Initiation of mineralization can take place with unmeasurable 25-OHD, and 24,25-(OH)₂D seems to be without importance.     

Strontium and bone development under conditions of suboptimal vitamin D

An investigation involving chicks fed diets with 0.5% Ca and 0.5% Sr supplemented with various levels of vitamin D₃ suggests that one-half of the Ca requirement of body growth in the chick is met by Sr as long as the vitamin D₃ level was maintained at 500 to 1000 ICU/kg foodstuff. The entire Ca requirement for bone growth could not, however, be met by Sr. Even though Sr was incorporated into the bone when low levels of vitamin D₃ were fed in the diet, the total ash content, as well as Ca content, was reduced.This work was supported in part by the Division of Radiological Health, U.S.P.H.S. Grant RH 00354.     

Immobilization osteoporosis and active vitamin D: Effect of active vitamin D analogs on the development of immobilization osteoporosis in rats

Summary The therapeutic effects of vitamin D analogs, 1,24(R)-dihydroxycholecalciferol [1,24(R)(OH)₂D₃], 1,24(S)-dihydroxycholecalciferol [1,24(S)(OH)₂D₃], and 1,25-dihydroxycholecalciferol [1,25(OH)₂D₃] on immobilization osteoporosis were studied in rats. The right hind limb was immobilized through application of a plaster cast following the section of the sciatic nerve. The left hind limb was intact. Vitamin D analogs were orally administered for 6 weeks at dose levels of 0.02 and 0.10μg/kg/day, respectively. The mean lengths of the immobilized femurs were not significantly different from those of the intact femurs in all the experimental groups. In the immobilized femur of animals treated with 1,24(R)(OH)₂D₃, 0.10μg/kg, dry and ash weights were heavier and calcium and phosphorus contents greater than those in the nontreated group. Furthermore, the amount of calcified bone mass and the cortical thickness of the femurs of the immobilized limb in 1,24(R)(OH)₂D₃-treated animals were greater than those in the nontreated animals. Treatment with 1,25(OH)₂D₃ at 0.10μg/kg caused an increase of the bone mass in both immobilized and intact femurs when compared with those of the control group. It was concluded that the administration of 1,24(R)(OH)₂D₃ diminished the effect of immobilization in the development of osteoporosis without any side effects.     

补充牛奶和维生素D对SD大鼠骨密度的影响

目的研究补充牛奶和维生素D对老龄大鼠骨密度(BMD)的影响。方法选取80只18月龄的SD大鼠,雌雄各半,作为骨质疏松模型。适应1周,随机分为两组,饮奶组按体重0.083ml·g-1·d-1灌胃并加维生素D36.6IU·kg-1·d-1,正常进食,自由饮水。不饮奶组普通饲料饲喂,自由饮水。灌胃前后使用双能X线骨密度仪测量双侧股骨骨密度(BMD)。结果4周后饮奶组较灌胃前骨密度(BMD)平均增加0.058gcm2,与灌胃前比较差别有统计学意义(P<0.05)。而不饮奶组灌胃后比灌胃前骨密度(BMD)减少0.007gcm2,灌胃前后差别无统计学意义。两组间比较灌胃前后骨密度(BMD)变化显著(P<0.05)。结论牛奶加低剂量维生素D有增加骨密度(BMD)作用。     

High-dose oral vitamin D3 supplementation in rheumatology patients with severe vitamin D3 deficiency

ObjectivesRecent large trials indicate that adherence associated with a daily regimen of vitamin D is low and limits anti-fracture efficacy with vitamin D supplementation. The aim of this report is to describe changes of 25-hydroxyvitamin D (25(OH)D) serum concentrations achieved with a single oral dose of 300 000 IU vitamin D3.MethodsOver a course of 4 months, we identified 33 elderly with severe vitamin D deficiency (25(OH)D < 25 nmol/l) on admission to acute care. Patients were admitted for musculoskeletal pain, bone disease, or gait abnormalities. The mean age was 80.5 years (SD ± 6.1). All patients were treated with a single oral dose of 300 000 IU D3 in combination with 500–1000 mg calcium supplements per day depending on their dietary calcium intake.ResultsBaseline mean 25(OH)D serum concentrations were 15 nmol/l (SD ± 5.5). Mean 25(OH)D serum concentrations increased to 81.4 nmol/l (SD ± 29.7) at 3 months (29 patients) and were still 69.0 nmol/l (SD ± 17.9) at 6 months (26 patients). Mean serum calcium levels were 2.24 mmol/l (SD ± 0.11) at baseline, 2.28 mmol/l (SD ± 0.18) at 3 months, and 2.28 mmol/l (SD ± 0.13) at 6 months. Two patients with mild hypercalcemia (2.69 mmol/l) at 3 months had normal values at 6 months.ConclusionBased on our observations, a single oral dose of 300 000 IU vitamin D3 raises mean 25(OH)D serum concentrations to the target mean of above 75 nmol/l at 3 months and a mean level of 69 nmol/l at 6 months. As calcium absorption is enhanced with higher 25(OH)D serum concentrations, calcium supplementation may need downward adjustment with this regimen to avoid mild hypercalcemia.     

Effects of vitamin D metabolites and analogs on bone collagen synthesis in vitro

Summary The effects of selected vitamin D₃ metabolites and analogs on bone collagen synthesis in vitro were examined in organ cultures of neonatal mouse calvarial bone. The incorporation of [³H]proline into the collagenase-digestible fraction of newly synthesized protein was progressively inhibited by 1α,25-dihydroxyvitamin D₃ (1α,25-(OH)₂D₃) (10⁻¹² M to 10⁻⁷ M) in 24-h cultures, and incorporation into noncollagen protein was also blunted at the higher doses employed. The synthetic analog 1α-hydroxyvitamin D₃ (1α-OHD₃) was almost 300-fold less potent an inhibitor of collagen synthesis than was 1α,25(OH)₂D₃, and the natural metabolites 25-hydroxyvitamin D₃ (25OHD₃) and 24R,25-dihydroxyvitamin D₃ (24R,25(OH)₂D₃), 1000-fold less potent, although the dose-response curve for each of these compounds was not parallel with that for 1α,25(OH)₂D₃. The 24S,25(OH)₂D₃ enantiomer was four-fold less potent than 24R,25-(OH)₂D₃ or 25OHD₃, and vitamin D₃ showed less than 2% the activity of 25OHD₃. The responses were unaffected by the substitution of 0.4% bovine albumin for 5% horse serum in the medium, and no stimulation of collagen synthesis was observed in response to 25-hydroxylated metabolites between 2×10⁻¹⁴ and 2×10⁻⁶ M or in cultures treated for up to 96 h with 24R,25(OH)₂D₃ (2×10⁻¹⁰M). The overall results emphasize the similarity of the structural requirements for the inhibition of matrix synthesis and the stimulation of resorption by active vitamin D metabolites in bone. In addition, these studies support the importance of the 1-hydroxyl function to the biologic activity of vitamin D in the skeleton.     

Effects of chronic protein deficiency on skeletal development of young rats

Summary Protein deficiency was produced by freely feeding young rats a 1% lactalbumin diet for 12 weeks in order to study the effects of protein-calorie malnutrition on skeletal development. During the experiment the food and caloric intake and weight of the experimental animals decreased, while those parameters of the control animals progressively increased. However, when gross caloric intake was expressed as a function of the metabolic size of the animal, the caloric consumption was similar for both groups of animals. The protein-deficient animals exhibited micro-radiographic and histological features of an abnormal pattern of endochondral bone formation. Appositional bone growth, as determined by the daily appositional rate and the percentage of endosteal surfaces undergoing active bone formation, was significantly decreased in these animals, as was the percentage of periosteal surfaces exhibiting resorption. Both chemical analyses of the whole bone and electron probe microanalysis in the specific area of actively calcifying bone revealed no significant differences between the mineral content of control and protein-deficient animals. This study distinguishes the effects of protein deficiency from that of combined protein-calorie deprivation and demonstrates that the abnormal skeletal development observed was the result of a decrease in the quantity of bone formed rather than an altered mineral content.     

Bone maturation in the vitamin D,phosphate deficient rat and the response to acid loading

Summary Vitamin D and phosphate deficiency were produced in rats in order (a) to evaluate the degree of bone mineral and matrix maturation using a bromoform/toluene density gradient technique; and (b) to compare the aforementioned bone maturational changes due to vitamin D and phosphate deprivation to those produced with superimposed severe acidosis. Rats were fed a diet deficient in vitamin D and phosphorus (0.2%) from 3 weeks through 7 weeks of age. To examine the additional contribution of dietary calcium, we gave one-half of the animals either a low (0.06%) or high (1.3%) calcium diet. Following the 4 weeks of vitamin D deficiency, one-half of each group was given 1.8% NH₄Cl in the drinking water for 4 succeeding days to induce an acute, severe acidosis. The degree of bone maturation was quantitated via bromoformtoulene density gradient fractionation; total mineral and hydroxyproline (collagen) levels were quantitated as well. The vitamin D-deficient rats deprived of adequate dietary phosphate responded by conserving phosphorus, and as a consequence total bone phosphorus levels were maintained within that level for control rats. This conservation was independent of calcium intake but was extremely sensitive to acute acid loading, where a significant reduction in total bone phosphorus was noted. The bone maturational profile obtained from the vitamin D-phosphate deficient rats, however, revealed a significant accumulation of less mature or dense bone collagen and mineral with a corresponding decrease in the most mature or dense moieties. In contrast to the reduction of the total bone phosphorus content by acute acidosis, the skeletal collagen-mineral maturational profile was not significantly affected by the short-term systemic acidosis. The observed retardations in the bone collagen and mineral maturation of the vitamin D-deficient, phosphate-deprived state provide an additional observation which may well relate to the progressive osteopenia documented in states of chronic, mild acidosis.     

Mechanical stress regulates bone regulatory gene expression independent of estrogen and vitamin D deficiency in rats

Mechanical stress determines bone mass and structure. It is not known whether mechanical loading affects expression of bone regulatory genes in a combined deficiency of estrogen and vitamin D. We studied the effect of mechanical loading on the messenger RNA (mRNA) expression of bone regulatory genes during vitamin D and/or estrogen deficiency. We performed a single bout in vivo axial loading with 14 N peak load, 2 Hz frequency and 360 cycles in right ulnae of nineteen weeks old female control Wistar rats with or without ovariectomy (OVX), vitamin D deficiency and the combination of OVX and vitamin D deficiency (N = 10/group). Total bone RNA was isolated 6 hours after loading, and mRNA expression was detected of Mepe, Fgf23, Dmp1, Phex, Sost, Col1a1, Cyp27b1, Vdr, and Esr1. Serum levels of 25(OH)D, 1,25(OH)₂D and estradiol were also measured at this time point. The effect of loading, vitamin D and estrogen deficiency and their interaction on bone gene expression was tested using a mixed effect model analysis. Mechanical loading significantly increased the mRNA expression of Mepe, and Sost, whereas it decreased the mRNA expression of Fgf23 and Esr1. Mechanical loading showed a significant interaction with vitamin D deficiency with regard to mRNA expression of Vdr and Esr1. Mechanical loading affected gene expression of Mepe, Fgf23, Sost, and Esr1 independently of vitamin D or estrogen, indicating that mechanical loading may affect bone turnover even during vitamin D deficiency and after menopause.     

Effect of aging on vitamin D stores and bone density in women

Summary It has been suggested that the decrease in vitamin D stores with aging is a contributory cause of age-related osteoporosis. We studied this question by measuring bone mineral density (BMD) of the mid-radius, distal radius, and lumbar spine assessed by single and dual photon absorptiometry in 122 women, aged 33–94 years, selected from a random sample of Rochester, MN residents. We measured serum 25-hydroxyvitamin D (25OHD), the major storage from of vitamin D, as well as 25OHD₃ (representing both endogenous and exogenous sources of vitamin D), and 25OHD₂, (representing only exogenous sources). Both baseline serum total 25OHD (r=−0.29,P<0.001) and the metabolite 25OHD₃ (r=−0.41,P<0.001), were negatively associated with age at baseline. After adjusting for the effect of age by multiple regression analysis, there was no association between serum levels of 25OHD₂, 25OHD₃, or total 25OHD and BMD for any of the three skeletal scanning sites. Thus, in a northern American population we cannot demonstrate that reduced bioavailability of vitamin D plays a major role in age-related bone loss.     

The effects of vitamin D analogues on bone resorption

A series of analogues of vitamin D have been tested for their ability to stimulate bone resorption in two test systems used previously to investigate the metabolites of vitamine D. These analogues were tested (a) by directly comparing their action on bone explants of mouse half-calvariain vitro, and (b) by injecting them into young mice and measuring the degree of resorptionin vitro when explants were prepared 18 hours after the injection. It is concluded that the key functional groups concerned with enhancing the activity of vitamin D₃ are the 1- and the 25-hydroxyl,both together; the cis ring structure for ring A appears necessary. 1-Hydroxycholecalciferol (1-OHD₃) is about as active as 25-OHD₃ in the direct test, but its potency is much nearer to that of 1,25-(OH)₂D₃ when tested by the second (indirect) method; it seems likely that 1-OHD₃ is converted into 1,25-(OH)₂D₃ in vivo. The results are discussed in relation to the designing of analogues for clinical and experimental use.