Effects of vitamin D on calcium regulation in vitamin-D-deficient pigs given a phytate-phosphorus diet

1. Vitamin-D-deficient pigs were fed on a phytate-phosphorus diet and treated with vitamin D3 (+D) to examine the time-course of adaptative changes in plasma minerals, vitamin D metabolites, parathyroid hormone (PTH) and calcium balance and intestinal Ca-binding protein (CaBP). 2. The 5-week vitamin D repletion (25 micrograms cholecalciferol/kg diet) regimen restored plasma Ca, P and alkaline phosphatase (EC 3.1.3.1) to normal, decreased PTH and markedly and rapidly increased plasma 25-hydroxycholecalciferol (25-OHD, sevenfold after 4 d) and 1,25-dihydroxycholecalciferol (1, 25(OH)2D3, 1.8-fold after 4 d). 3. CaBP concentrations were markedly elevated all along the digestive tract, especially in the distal regions. 4. Ca absorption and retention were enhanced (fourfold and sixfold respectively) by the +D diet. 5. The improved Ca absorption, coupled with increased CaBP and 1,25(OH)2D3 levels, suggest that vitamin D metabolism in phytate-P-fed pigs is sensitive to the depressed Ca availability due to phytate feeding. It also indicates that CaBP may play an important role in the adaptation of Ca absorption. 6. Persistent hypercalciuria indicates that mineral metabolism was still affected by the phytate nature of the dietary P in spite of the vitamin D treatment.     

1,25 dihydroxycholecalciferol-mediated calcium absorption and gene expression are higher in female than in male mice

Recent advances in bone and calcium (Ca) metabolism have relied upon genetically modified mice. However, although human studies have identified gender as an important modulator of Ca metabolism, its effect on Ca metabolism has not been examined in mice. Here we examined basal and vitamin D-regulated Ca absorption (in situ ligated loops) and mRNA levels for the apical membrane calcium channel, TRPV6, and the calcium binding protein, calbindin D(9k) (CaBP) mRNA levels (real-time PCR) in duodenum of female and male mice. At 2 mo of age, females fed a 5 g Ca/kg diet had higher Ca absorption (62.3 +/- 4.8 vs. 47 +/- 3.6%) and TRPV6 mRNA levels than males even though plasma 1,25 dihydroxyvitamin D [1,25(OH)(2) D] was not different. In mice fed high (20 g/kg), normal (5 g/kg), or low (0.2 g/kg) Ca diets for 7 d to alter plasma 1,25(OH)(2) D (91 +/- 12, 322 +/- 25, and 587 +/- 43 pmol/L, respectively), the relation between Ca absorption (slope = 0.116 vs. 0.084, P = 0.021) or duodenum TRPV6 mRNA (slope = 0.042 vs. 0.025, P = 0.034) and circulating 1,25(OH)(2) D was steeper in females. After a single 1,25(OH)(2) D injection (200 ng/100 g body weight), peak induction of TRPV6 mRNA was 2-fold greater (at 6 h) and CaBP mRNA was 20% higher in females (at 16 h). Duodenal vitamin D receptor mRNA levels did not differ between genders. Our data indicate that female mice are more sensitive to changes in serum 1,25(OH)(2) D levels than males and that this must be considered when using mice to study calcium and bone biology.     

Dietary short-chain fructooligosaccharides increase calbindin-D9k levels only in the large intestine in rats independent of dietary calcium deficiency or serum 1,25 dihydroxy vitamin D levels

Dietary short-chain fructooligosaccharides (Sc-FOS) increase mucosal calbindin-D9k (CaBP) levels in the large intestine whereas levels in the small intestine are decreased in rats. In the present study, we investigated the mechanism by which Sc-FOS induce this increase in CaBP in the large intestine by measuring intestinal CaBP levels in rats fed normal and calcium-deficient diets. Dietary groups included a calcium-containing (0.5%) diet with or without Sc-FOS (100 g/kg diet) and a calcium-deficient (abt. 0.01%) diet with or without Sc-FOS (100 g/kg diet). The rats were fed these diets for 10 days following which they were killed and the intestine removed for collection of the entire mucosa which was divided into four segments, i.e., proximal and distal segments of the small intestine, the cecum and the colorectum. Mucosal CaBP and plasma calcium (Ca), 1,25-dihydroxycholecalciferol (1,25(OH)2D3), 25-hydroxycholecalciferol (25(OH)D3), parathyroid hormone (PTH) and calcitonin levels were measured. Feeding of calcium deficient diet resulted in an increase in CaBP levels in the small intestine, but did not influence levels in the large intestine. Moreover, a significant positive correlation between plasma 1,25(OH)2D3 and CaBP levels in the case of both small intestinal segments (proximal, r = 0.77012, p < 0.00007; distal, r = 0.75056, p < 0.00014) was observed, but not in the case of the large intestinal segments. Sc-FOS increased CaBP levels in the large intestine. These results suggest that the large intestinal CaBP levels do not change in response to dietary calcium conditions and are not regulated by circulating 1,25(OH)2D3 indicating that the effect of Sc-FOS on CaBP levels in the large intestine is independent of the action of 1,25(OH)2D3.     

上皮钙通道基因在维生素D受体和Calbindin-D28k双基因敲除鼠中的变化

目的研究上皮钙通道TRPV5和TRPV6基因与维生素D受体(VDR)和钙结合基因Calbindin-D28k的关系,以及肾钙的重吸收减少是否与TRPV5和TRPV6基因表达减少有关。方法制备VDR和CaBP-D28k双基因敲除小鼠,普通和高钙食物喂养下,检测野生型鼠(WT),CaBP-D28(-/-),VDR(-/-),和VDR(-/-)/CaBP-D28k(-/-)鼠的体重、摄食量及血尿参数值,用实时RT-PCR的方法检测各种鼠肾脏TRPV5和TRPV6的mRNA水平。结果普通饮食下,双基因敲除小鼠尿钙的分泌和血清甲状旁腺激素水平更高。TRPV5和TRPV6两者的表达在CaBP-D28k敲除鼠中均无变化,而在VDR敲除鼠和双基因敲除鼠中下调的幅度相当。高钙饮食下,VDR及双基因敲除小鼠的血离子钙水平正常。所有小鼠这两个基因的表达普遍降低,而在VDR和双基因敲除鼠中的表达更低。结论这些结果证实了上皮钙通道基因受钙和维生素D的调节,CaBP-D28k的缺失对两者无影响。肾钙的重吸收减少与TRPV5和TRPV6基因表达无明显关系。     

High dietary vitamin D prevents hypocalcemia and osteomalacia in CYP27B1 knockout mice

Mice lacking 25-hydroxycholecalciferol [25(OH)D]-1alpha-hydroxylase (CYP27B1) are growth retarded, hypocalcemic, and have poor bone mineralization. We tested whether high dietary cholecalciferol (VD3) could exert effects in the absence of CYP27B1 in vivo. Weanling male wild-type (WT) and CYP27B1 knockout (KO) mice were fed either a 2% calcium (Ca), 20% lactose rescue diet or an AIN93G diet (0.5% Ca, 0.4% phosphorus) containing 1000 (1K, the rodent requirement, 25 microg), 10,000 (10K, 250 microg), or 20,000 (20K, 500 microg) IU VD3/kg diet until 12 wk when blood and tissues were taken. Serum 25(OH)D was >90 nmol/L in the 1K diet group and increased >4-fold in mice fed 10K and 20K diets. The 1K diet impaired growth and caused hypocalcemia in KO mice; the 10K and 20K diets were as effective as the high Ca rescue diet in preventing these outcomes. High VD3 restored expression of vitamin D-regulated genes in intestine (calbindin D(9K)) and kidney (CYP27B1, 24-hydroxylase, calbindin D(9K)) of KO mice. Micro-computed tomography of femora revealed complete recovery of cortical bone in KO mice fed either the rescue or 10K diets but only partial recovery of trabecular bone measures (e.g. 40% lower bone volume, 20% lower trabecular thickness, and 23% increase in trabecular separation). These data show that very high serum 25(OH)D can influence Ca and bone metabolism independent of its conversion to 1,25 dihydroxycholecalciferol. However, neither high dietary Ca nor high dietary VD3 is sufficient to fully recover the phenotype of CYP27B1 KO mice.     

Gastrointestinal absorption of lead in chicks: involvement of the cholecalciferol endocrine system

The role of dietary calcium and phosphorus in modifying the intestinal absorption of lead and also the effect of lead ingestion on the metabolism of cholecalciferol were studied in chicks. The efficiency of absorption of 203Pb and 47Ca was increased when the animals were fed a low calcium diet and treated with cholecalciferol. The synthesis of the vitamin D-induced calcium-binding protein (CaBP) was correspondingly increased. When the chicks were depleted of vitamin D and repleted with 1,25-dihydroxycholecalciferol [1,25(OH)2D3] as their only source of the vitamin, the absorption of both 47Ca and 203Pb was unaffected by dietary calcium levels, and no change in CaBP levels occurred. Low dietary intake of phosphorus resulted in an increase in 47Ca and 203Pb absorption and in CaBP synthesis when the animals were treated with cholecalciferol. However, when the birds were repleted with 1,25(OH)2D3, the intestinal absorption of 47Ca and of 203Pb was increased, as well as the intestinal CaBP levels. Intracardial injection of increasing doses of 1,25(OH)2D3 to rachitic chicks resulted in a concomitant increase in 203Pb absorption in a manner that correlated with the degree of synthesis of CaBP. Ingestion of lead by the chicks was found to impair growth and renal production of 1,25(OH)2D3, resulting in lowered circulating and intestinal content of the hydroxylated metabolites of cholecalciferol.     

Comparative effects of vitamin K and vitamin D supplementation on calcium balance in young rats fed normal or low calcium diets

We examined the effect of vitamin K and vitamin D supplementation on calcium balance in young rats fed a normal or low calcium diet. Eighty female Sprague-Dawley rats, 6 wk of age, were randomized by the stratified weight method into eight groups with 10 rats in each group: 0.5% (normal) or 0.1% (low) calcium diet, 0.5% or 0.1% calcium diet+vitamin K (vitamin K2, menatetrenone, 30 mg/100 g, food intake), 0.5% or 0.1% calcium diet+vitamin D (25 microg/100 g, food intake), and 0.5% or 0.1% calcium diet+vitamin K+vitamin D. The duration of the study was 10 wk. Vitamin K supplementation promoted the reduction in urinary calcium excretion and retarded the abnormal elevation of serum PTH level in rats fed a low calcium diet, and stimulated intestinal calcium absorption in rats fed a normal calcium diet. Vitamin D supplementation stimulated intestinal calcium absorption with prevention of the abnormal elevation of serum PTH levels and prevented hypocalcemia in rats fed a low calcium diet, and stimulated intestinal calcium absorption in rats fed a normal calcium diet. The stimulation of intestinal calcium absorption was associated with increased serum 1,25-dihydroxyvitamin D levels. An additive effect of vitamin K and vitamin D on intestinal calcium absorption was found only in rats fed a normal calcium diet. This study shows the differential effects of vitamin K and vitamin D supplementation on calcium balance in young rats fed a normal or low calcium diet.     

Insulin-like growth factor-I stimulates renal 1, 25-dihydroxycholecalciferol synthesis in old rats fed a low calcium diet

The adaptive increase in renal proximal tubule 25-hydroxyvitamin D-alpha-hydroxylase activity (1-OHase) during dietary calcium restriction is mediated by an increase in parathyroid hormone (PTH) and is inhibited by aging. Recent studies in mature (3-4 mo) rats demonstrated that insulin-like growth factor-I (IGF-I) restored stimulation of renal 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)] production by low phosphorus diet (LPD), another major stimulus of 1-OHase. These studies were designed to determine whether IGF-I stimulates 1-OHase during low calcium intake in old rats. Male rats were fed a normal calcium diet (NCD, 6 g Ca/kg diet) or low calcium diet (LCD, 0.2 g Ca/kg diet) for 14 d, and recombinant human IGF-I [rhIGF-I, 1.4 mg/(24h 160 kg body wt)] or vehicle was administrated via miniosmotic pump for 72 h before killing. In 4-mo-old male Sprague-Dawley rats, LCD increased in vitro renal 1-OHase activity in the presence but not in the absence of rhIGF-I. LCD increased in vitro1-OHase activity in young (1-mo-old) but not old (24-mo-old) male Fischer 344 rats. RhIGF-I increased 1-OHase activity in 24 mo-old rats fed LCD to levels that were not different from those in 1-mo-old rats fed LCD. The results indicate that the adaptive increase in 1-OHase activity due to a LCD is lost by 4 mo in rats and can be restored by pharmacologic doses of rhIGF-I.     

Dietary calcium is a major factor in 1,25-dihydroxycholecalciferol suppression of experimental autoimmune encephalomyelitis in mice.

The active form of vitamin D (1,25-dihydroxycholecalciferol) is a potent immune system regulator. Treating mice with 1, 25-dihydroxycholecalciferol and feeding them diets high in calcium can completely suppress the induction of experimental autoimmune diseases such as experimental autoimmune encephalomyelitis (EAE). Experiments described here were carried out on mice in which development of EAE was induced. Mice were fed diets containing various amounts of calcium and 1,25-dihydroxychole-calciferol. Variables measured were as follows: 1) incidence and severity of EAE; 2) serum calcium concentrations; 3) body weight; 4) total number of cells in the lymph nodes; and 5) interleukin-4 (IL-4) and transforming growth factor-beta1 (TGF-beta1) mRNA levels. When calcium was removed from the diet, the incidence of EAE was reduced 20% in both males and females. Further, the lower the dietary level of calcium, the higher was the dose of 1,25-dihydroxycholecalciferol required to prevent the symptoms. Thus, 1, 25-dihydroxycholecalciferol was found most effective in mice fed a diet adequate or high in calcium. 1,25-Dihydroxycholecalciferol treatment of mice fed high dietary calcium resulted in a decreased number of lymphocytes in the lymph nodes and increased IL-4 and TGF-beta1 mRNA levels. When calcium was omitted from the diet, 1, 25-dihydroxycholecalciferol supplementation increased TGF-beta1 mRNA. Increased IL-4 mRNA and decreased lymphocytes in the lymph nodes in response to 1,25-dihydroxycholecalciferol occurred only when dietary calcium was adequate or high. Our results suggest that dietary calcium and 1,25-dihydroxycholecalciferol are both involved in the prevention of symptomatic EAE.     

Enhanced radiosensitivity in 1,25-dihydroxyvitamin D3 deficient mice

To investigate whether impaired osteogenesis resulting from vitamin D deficiency can influence hematopoiesis recovery after radiation, the 25-hydroxyvitamin D-1α-hydroxylase (1α-hydroxylase) gene knockout (KO) mice and wild type (WT) mice were subjected to different doses of gamma ray. The survival rates, peripheral blood cell counts and bone marrow cellularity were studied after irradiation (IR). The survival rates of the KO mice were significantly lower than that of WT mice after 6 or 8 Gy dose of radiation. The recovery of white blood cells in KO mice was significantly delayed compared with that in WT mice after radiation. The red blood cell number in WT mice was observed to increase more than that in KO mice at days 14 and 28 after radiation. The nadir platelet count in KO mice was nearly half of that in WT mice. Dramatically higher bone marrow cell numbers were found in WT mice compared with KO mice. Our findings demonstrate the enhanced radiosensitivity in 1,25-dihydroxyvitamin D3 (1,25-(OH)(2)D(3)) deficient mice.     

Vitamin D metabolism and calbindin (calcium-binding protein) in aged laying hens

Calbindin (calcium-binding protein) concentration, vitamin D metabolism and shell quality were investigated in young (7- to 9-mo-old) and aged (19- to 21-mo-old) laying hens consuming normal or low levels of calcium (Ca). Although egg weight and percent of cracked eggs were higher and egg production and shell density (mg/cm2) were lower (significantly, P less than 0.01) in aged hens, shell weight, plasma Ca and duodenal and egg shell gland calbindin were similar to those of young hens. Dietary Ca restriction reduced shell weight, shell density and structural bone and plasma Ca in both the young and aged birds. The production of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and its concentration in the plasma were higher in hens fed low dietary levels of Ca than in hens fed normal Ca levels only in the younger hens. However, a slightly higher production of 1,25(OH)2D3 and concentration of duodenal calbindin were also observed in severely Ca-restricted (1.4% Ca for 19 d) aged hens than in the younger hens. The results suggest that the aged hen loses its ability to adapt to changes in Ca intake or needs through mechanisms involving modulation of vitamin D metabolism.     

Plasma 1,25-(OH)2-vitamin D concentrations and net intestinal calcium, phosphate, and magnesium absorption in humans.

We evaluated the relationship between plasma concentrations of the renal hormone 1,25-(OH)2-vitamin D and net intestinal absorption of Ca, PO4, and Mg in vitamin D-replete patients eating similar diets, who had undetectable, normal or elevated plasma 1,25-(OH)2-D levels, Net intestinal Ca absorption was positively correlated to plasma 1,25-(OH)2-D concentrations: percentage dietary Ca absorbed = 10 + 0.17 x plasma total 1,25-(OH)2-3, pmole/liter, r = + 0.58; P less than 0.001. By contrast, there was no significant correlation between PO4 or Mg absorption and plasma 1,25-(OH)2-D concentrations. Moreover, significant quantities of PO4 and Mg were absorbed in the absence of detectable plasma 1,25-(OH)2-D. We conclude that net intestinal Ca absorption is critically dependent upon the availability of the renal hormone 1,25-(OH)2-D in vitamin D-replete humans when dietary Ca intake is normal. By contrast, other factors must play a dominant role in regulating net intestinal PO4 and Mg absorption.     

Zinc nutritional status modulates the response of 1,25-dihydroxycholecalciferol to calcium depletion in rats.

Male Lewis rats (n = 27) were fed a nonpurified diet containing 0.9% calcium, 0.7% phosphorus, and 0.005% zinc until 8 wk of age. At this time rats were assigned randomly to one of two groups. Both groups were fed a low calcium, low zinc, purified diet (0.2% calcium, 0.4% phosphorus, less than 0.00007% zinc), but one group was fed 1.78 mg Zn/(animal.d). The zinc-replete animals were individually matched by weight to the zinc-depleted animals and pari-fed. Balances and plasma concentrations of zinc, calcium, and phosphorus and parathyroid hormone, 25 hydroxycholecalciferol [25(OH)D] and 1,25-dihydroxycholecalciferol [1,25(OH)2D] were determined at the start of calcium depletion and 2 wk later. Calcium and 25(OH)D levels were lower in both groups after calcium depletion. Dietary zinc had no significant effect on calcium or 25(OH)D levels. Phosphorus concentrations were lower after calcium depletion, but phosphorus concentration was higher in the zinc-depleted compared with the zinc-replete group at the end of the experiment. 1,25(OH)2D increased in both groups, but was higher in the zinc-replete than the zinc-depleted group at the end of the experiment. Calcium and phosphorus balances were greater in the zinc-depleted group at the end of the experiment. We conclude zinc depletion diminishes the response of 1,25(OH)2D to calcium depletion in rats. The mechanism is unknown, but may involve nonhormonally mediated changes in gastrointestinal absorption of calcium and phosphorus or an affect of zinc on extraintestinal processes.     

Serum metabolite profiles and target tissue gene expression define the effect of cholecalciferol intake on calcium metabolism in rats and mice

We studied the effect of cholecalciferol (VD3) intake on VD3 status and markers of calcium (Ca) homeostasis in mice and rats. Serum 25 hydroxycholecalciferol (25OH-VD3) concentrations were increased in animals fed diets containing 400-20,000 international units (IU) VD3/kg (37 nmol.L(-1).1000 IU VD3(-1)), but body weight, serum Ca, and duodenal gene expression were not altered. High-VD3 intake decreased serum 1, 25-dihydroxycholecalciferol [1,25(OH)2-VD3] and renal 25 hydroxycholecalciferol-1alphahydroxylase (CYP27B1) mRNA, suggesting that rodents tolerate high-VD3 intake by suppressing the activity of the VD3 endocrine system. Serum 25OH-VD3 declined when animals were fed diets containing 1000 to 25 IU VD3/kg (9-11 wk, inflection at 200 IU/kg, 4-fold steeper slope below this). Neither body weight nor serum Ca were influenced by low-VD3 intake. However, mice fed the 25-IU/kg diet had lower serum 1,25(OH)2-VD3, duodenal calbindin D9k mRNA, bone mineral density, and renal 25 hydroxycholecalciferol-24 hydroxylase mRNA, whereas renal CYP27B1 mRNA was elevated when rodents were fed < 200 IU VD3/kg. These data reveal a stress on VD3 and Ca metabolism at low dietary VD3 intake. Dietary Ca restriction (0.25 vs. 0.5%, 9 wk) increased serum 1,25(OH)2-VD3 and was 30% greater in rats fed a 10,000-IU VD3/kg diet. High-VD3 intake did not prevent Ca restriction-induced bone loss. Our data show that modeling human VD3 status requires lower intake than the current NRC rodent requirement (1000-IU/kg diet). Also, although rodents are very tolerant of high-VD3 intake, it cannot compensate for moderate Ca restriction.     

Adaptation to low dietary calcium in magnesium-deficient rats

To determine if adaptation to low calcium (Ca) diets is impaired by magnesium (Mg) deficiency, weanling rats were pair-fed control (700 mg Mg/kg diet) or low Mg (70 mg Mg/kg diet) diets containing 5000 mg Ca/g diet for 7 or 10 d. Half of the animals from each group were then killed; the remainder continued on their previous Mg intakes and were subjected to low (500 mg Ca/kg diet) dietary Ca for 7 or 15 d. After 10 d of Mg deficiency, rats fed adequate Ca were hypomagnesemic and hypercalcemic, relative to controls, and had elevated circulating parathyroid hormone (PTH) but normal 1,25-dihydroxycholecalciferol (1,25-DHCC). After 7 d of low dietary Ca, there were no significant differences in plasma Ca, 1,25-DHCC or PTH between control and Mg-deficient rats despite a 50% reduction in plasma Mg in the latter group. After 15 d of low Ca stress, control rats remained normocalcemic, but Mg-deficient rats developed relative hypocalcemia despite similar, high circulating levels of PTH. Plasma 1,25-DHCC in the hypocalcemic, Mg-deficient rats was significantly lower than that of control rats also exposed to low dietary Ca, suggesting an impairment in 1,25-DHCC production during chronic Mg depletion. Significantly elevated renal Ca content was present in the Mg-deficient rats, which was unrelated to dietary Ca and preceded the development of hypocalcemia. The results indicate that both the duration of Mg deficiency and the Ca content of the experimental diet are important determinants of an animal's ability to maintain Ca homeostasis during Mg depletion.     

小鼠Calbindin-D28k基因对钙代谢的影响

目的：探讨Calbindin-D28k对钙代谢的影响及作用。方法：制备维生素D受体（VDR）／Calbindin-D28k双基因剔除小鼠模型，常规及高钙饮食下，检测小鼠体重、摄食量、血尿参数值及甲状旁腺大小等。结果：常规饮食下，双基因剔除小鼠发育更迟缓，体重比VDR单基因剔除小鼠轻42％。尿钙的分泌更高，并发展为严重的继发性甲状旁腺功能亢进。高钙饮食下，VDR及双基因剔除小鼠的血钙离子水平恢复正常。结论：CaBP-D28k对钙代谢平衡起了重要的作用，它的作用大都被CaBP-D9k代偿。     

Changes in serum levels of 1,25-dihydroxyvitamin D3, calcium and phosphorus with age and vitamin D status in chickens

The effects of vitamin D3 (D3) on serum levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), ionic calcium, total Ca and phosphorus in chicks were studied from the time of hatching until sexual maturity. Chicks fed on a diet low in D3 showed a serum level of 1,25(OH)2D3 higher than that in chicks on a normal-D3 diet, for both sexes and at any given age. A dramatic increase in the serum level of 1,25(OH)2D3 occurred in female birds approaching sexual maturity and in laying hens raised on the low-D3 diet the level was five times that of their counterparts raised on a normal-D3 diet. The serum 1,25(OH)2D3 level in adult males in the low-D3 groups was seven times that of those on the normal-D3 diet. The serum level of 25-hydroxyvitamin D3 remained relatively unchanged at weeks 2 and 15 in birds on a low D3 intake as well as in those fed on a normal-D3 diet. Nevertheless, the levels of 25-hydroxyvitamin D3 were different between the two groups. No significant change was observed in the level of ionized serum Ca in relation to dietary regimen, but there was an increase in total Ca concentration in females with the onset of reproduction. The serum P level decreased gradually with age, reaching a minimum value 3 and 8 weeks before laying commenced in the groups on low- and normal-D3 diets respectively. An increase was observed when the hens began laying.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vitamin D receptor null mutant mice fed high levels of calcium are fertile

Vitamin D receptor (VDR) null mutant mice provide a model to investigate the possible effect of vitamin D on female reproduction. Infertility in these mice has been reported but it is uncertain whether the infertility results from a lack of VDR or from the hypocalcemia that results from a lack of VDR. VDR null mutant mice and wild-type controls were fed a nonpurified, high calcium or medium calcium diet, plus a diet containing lactose and their reproductive efficiency was examined. VDR null mutant mice fed a nonpurified diet were hypocalcemic and were found to be largely infertile with 14% fertility, while the fertility percentage of normocalcemic VDR null mutant mice and wild-type mice was between 86% and 100%. A high calcium or medium calcium diet maintained 100% fertility in the VDR knockout mice; removal of the lactose from this diet did not diminish reproductive capability. Reproductive capacity of VDR null mutant mice was analyzed when they were fed purified diets containing 0.02-2% calcium. Mutant mice fed a low calcium diet (0.47%) had a lower reproductive efficiency than VDR null mutant mice fed a diet that resulted in normal serum calcium concentrations. Thus, high dietary calcium levels are required for normal reproduction in VDR null mutant female mice. It seems that the defect in reproduction reported previously for VDR null mutant mice is not the lack of a direct effect of 1,25-dihydroxycholecalciferol on reproductive function but is the result of hypocalcemia.     

The effect of dietary calcium on the activity of 25-hydroxycholecalciferol-1-hydroxylase and Ca absorption in vitamin D-replete chicks.

1. As most of the studies on the regulation of renal 25-hydroxycholecalciferol-1-hydroxylase (25-HCC-1-hydroxylase) activity have been done in marginally-vitamin D-deficient animals and as it is known that vitamin D administration suppresses the specific activity of the 25-HCC-1-hydroxylase, it was decided to study the effect of dietary calcium on the activity of 25-HCC-1-hydroxylase and on Ca absorption in vitamin D-replete chicks. 2. Chicks, 10 d old, were given diets differing in their Ca contents (65 nmol cholecalciferol/kg diet) for 10 d and the activity of 25-HCC-1-hydroxylase in kidney homogenates, Ca absorption from the duodenum, Ca-binding protein (CaBP) activity in the duodenal mucosa and plasma Ca and phosphate concentrations were all determined. 3. The CaBP activity and the efficiency of Ca absorption both decreased with increasing dietary intake of Ca. Ca absorption and CaBP activity were significantly correlated (r 0-995, P less than 0-01). 4. The activity of 25-HCC-1-hydroxylase decreased as the dietary level of Ca increased and was significantly correlated with Ca absorption (r 0-900, P less than 0-05). The plasma Ca concentration and the activity of 25-HCC-1-hydroxylase were inversely related (r-0-940, P less than 0-01). 5. It is concluded that in the vitamin D-replete chick the efficiency of duodenal Ca absorption is regulated by the renal 25-HCC-1-hydroxylase activity via production of 1,25-dihydroxycholecalciferol and CaBP synthesis.