24-Hydroxylase: potential key regulator in hypervitaminosis D3 in growing dogs

Age-dependent changes in plasma concentrations of 25-hydroxyvitamin D may complicate vitamin D status assessment of immature cats

Background

Vitamin D deficiency and excess in clinically presented cats conventionally is diagnosed by comparison of patient plasma 25-hydroxyvitamin D (25 (OH)D) concentration with plasma reference intervals determined in healthy adult cats. For immature cats, validity of this vitamin D status assessment method is uncertain.

Objective

The overall objective was determination of whether plasma concentration of 25 (OH) D and other vitamin D metabolites in immature cats markedly change with developmental age as has been reported in other species.

Methods

Four male and 4 female domestic short-hair kittens from weaning were continuously presented a single nutritionally adequate growth-diet. Concentrations of 25 (OH) D and 24,25-dihydroxyvitamin D (24,25 (OH)2D), and calcitriol were quantified in plasma of jugular venous blood collected at 12, 15, 18, and 21 weeks and 1 year of age. Plasma was liquid and solid-phase extracted and fractionation by normal-phase HPLC, and 25 (OH) D and 24,25 OH)2D quantified by reverse-phase HPLC-UV and calcitriol by RIA.

Results

Plasma 3-epi-25 (OH) D and 25 (OH) D concentrations increased (p < 0.001) with age so that by study end the concentrations rose by 1-and 2-fold, respectively. Concentrations of 3-epi-25 (OH) D relative to 25 (OH) D were 30% at 12 weeks and 20% at 1 year. Between ages 12 and 21 weeks, rises in 25 (OH) D concentration were positively correlated with body weight gains (ρ = 0.952, p < 0.001) and 24,25 (OH)2D concentrations were consistently greater than 25 (OH) D concentrations (p < 0.001). At 1 year of age, concentrations of 24,25 (OH)2D declined below those of 25 (OH) D and 3-epi-24,25 (OH)2D consistency occurred in low concentrations. Vitamin D2 metabolites and sex differences in metabolite concentrations were not observed.

Conclusion

Reliance on quantification of plasma 25 (OH) D concentration for vitamin D status assessment in kittens may be confounded by developmental changes in 25 (OH) D independent of vitamin D intake. High 24,25 (OH)2D concentration and occurrence of 3-epi-25 (OH) D in plasma additionally may interfere with the quantification.

Impact of deoxynivalenol in a calcium depletion and repletion nutritional strategy in piglets

This study evaluated the effect of dietary calcium (Ca) levels and deoxynivalenol (DON) contamination on Ca and phosphorus (P) utilization and bone mineralization in piglets. During an initial 13-d depletion phase, 64 piglets (15.7 ± 0.7 kg) received a control (DON-) or DON-contaminated treatment (DON+, 2.7 mg DON/kg) with either a low Ca (Ca-, 0.39%) or normal Ca level (Ca+, 0.65%) with a constant digestible P level (0.40%). A second group of 16 piglets received DON- or DON+ treatments for 9 d for gene expression analysis. During the subsequent 14-d repletion phase, all piglets were fed a Ca+ DON- diet containing 0.65% Ca and 0.35% digestible P without DON. After 5 d of the depletion phase, the absorption of P (DON × Ca; P < 0.05) and Ca was increased by the Ca- (P < 0.01) and DON+ (P < 0.01) diet. After 13 d, feed conversion ratio (P < 0.01) and average daily feed intake (P = 0.06) tended to decrease with the Ca- diet. The bone mineral content (BMC) gain was decreased by Ca, especially with Ca- DON + (DON × Ca, P < 0.05). The P absorption was increased by Ca- DON + (DON × Ca, P < 0.01), although the P retention efficiency was only increased by Ca+ DON + (DON × Ca, P < 0.001). The absorption of Ca was increased by DON+ (P < 0.001), and the Ca efficiency was increased by Ca- DON- (DON × Ca, P < 0.01). After 9 d, the gene expression of intestinal claudin 12 (P < 0.01) and CYP24A1 (P < 0.05), femur cortical RANKL (P < 0.05) and OPG (P = 0.06), and renal calbindin D9K (P < 0.05) and Klotho (P = 0.07) were decreased by DON+. The Ca (P = 0.06) and magnesium (P < 0.01) concentrations were decreased by DON+, and the Ca (P = 0.06) and P digestibility (P < 0.01) were increased. After the repletion phase, Ca- piglets recovered their BMC deficit, but not those receiving DON+ (DON × Ca; P = 0.06). The Ca (P < 0.05) and P (P = 0.06) retention efficiency tended to increase with Ca-. The absorption of Ca and P was increased by Ca- and DON+ (DON × Ca, P < 0.05). The results show that piglets increased their Ca and P utilization efficiency, allowing them to recover the BMC deficit caused by Ca-, but not when the piglets were exposed to DON. Pigs previously receiving Ca-deficient diet with DON still have lower body Ca and P, leading to elevated calcitriol concentrations and enhanced Ca and P intestinal absorption. The fact that DON decreased the expression of genes implicated in Ca intestinal and renal transport and P excretion after 9 d can potentially explain the reduced plasma Ca concentration.

24,25-Dihydroxy Vitamin D and Vitamin D Metabolite Ratio as Biomarkers of Vitamin D in Chronic Kidney Disease

The appropriate management of vitamin D deficiency and hyperparathyroidism is essential to prevent metabolic bone disorder (MBD) and cardiovascular diseases in chronic kidney disease (CKD). Recently, the 24,25-dihydroxyvitamin D [24,25(OH)₂D] and vitamin D metabolite ratio (VMR), i.e., the ratio of 24,25(OH)₂D to 25-hydroxyvitamin D [25(OH)D], have emerged as biomarkers of vitamin D level. We analyzed the usefulness of vitamin D biomarkers for the evaluation of MBD in patients with CKD. We analyzed blood and urine samples from 208 outpatients with CKD stage G2-G5. 25(OH)D showed a poor correlation with the estimated glomerular filtration rate (eGFR). Conversely, the 24,25(OH)₂D level and VMR were significantly correlated with eGFR and the intact parathyroid hormone level. In conclusion, 24,25(OH)₂D and VMR have the potential to be vitamin D biomarkers for the detection of MBD in CKD patients.

The Vitamin D Metabolite Ratio Is Associated With Changes in Bone Density and Fracture Risk in Older Adults

Recent studies have suggested that 25-hydroxyvitamin D (25(OH)D) may be a poor biomarker of bone health, in part because measured levels incorporate both protein-bound and free vitamin D. The ratio of its catabolic product (24,25-dihydroxyvitamin D [24,25(OH)₂ D]) to 25(OH)D (the vitamin D metabolite ratio [VMR]) may provide more information on sufficient vitamin D stores and is not influenced by vitamin D-binding protein concentrations. We evaluated whether the VMR or 25(OH)D are more strongly associated with bone loss and fracture risk in older adults. We performed a retrospective cohort study of 786 community-dwelling adults aged 70 to 79 years who participated in the Health Aging and Body Composition study. Our primary outcomes were annual changes in bone density and incident fracture. The mean age of these participants was 75 ± 3 years, 49% were female, 42% were Black, and 23% had an estimated glomerular filtration rate (eGFR) <60 mL/mL/1.73m² . In fully adjusted models, a 50% lower VMR was associated with 0.3% (0.2%, 0.6%) more rapid decline in total hip bone mineral density (BMD). We found similar relationships with thoracic and lumbar spine BMD. In contrast, 25(OH)D₃ concentrations were not associated with longitudinal change in BMD. There were 178 fractures during a mean follow-up of 10 years. Each 50% lower VMR was associated with a 49% (95% confidence interval [CI] 1.06, 2.08) greater fracture risk, whereas lower 25(OH)D₃ concentrations were not significantly associated with fracture risk (hazard ratio [HR] per 50% lower 1.07 [0.80, 1.43]). In conclusion, among a diverse cohort of community-dwelling older adults, a lower VMR was more strongly associated with both loss of BMD and fracture risk compared with 25(OH)D₃ . Trials are needed to evaluate the VMR as a therapeutic target in persons at risk for worsening BMD and fracture. © 2021 American Society for Bone and Mineral Research (ASBMR).

Pilot Trial of Vitamin D3 and Calcifediol in Healthy Vitamin D Deficient Adults: Does It Change the Fecal Microbiome?

CONTEXT

Experimental studies suggest that vitamin D receptor signaling may benefit the gut microbiome. In humans, whether vitamin D supplementation directly alters the gut microbiome is not well studied.

OBJECTIVE

To determine whether correcting vitamin D deficiency with cholecalciferol (vitamin D3, D3) or calcifediol (25-hydroxyvitamin D3, 25(OH)D3) changes gut microbiome composition.

METHODS

18 adults with vitamin D deficiency (25-hydroxyvitamin D [25(OH)D] <20 ng/mL) received 60 µg/day of D3 or 20 µg/day of 25(OH)D3 for 8 weeks. Changes in serum 25(OH)D, 1,25-diydroxyvitamin D (1,25(OH)2D), and 24,25-dihydroxyvitamin D (24,25(OH)2D) were assessed. We characterized composition of the fecal microbiota using 16S rRNA gene sequencing, and examined changes in α-diversity (Chao 1, Faith's Phylogenetic Diversity, Shannon Index), β-diversity (DEICODE), and genus-level abundances (DESeq2).

RESULTS

Vitamin D3 and 25(OH)D3 groups were similar. After 8 weeks of vitamin D3, mean 25(OH)D and 24,25(OH)2D increased significantly, but 1,25(OH)2D did not (25(OH)D: 17.8-30.1 ng/mL, P = .002; 24,25(OH)2D: 1.1 to 2.7 ng/mL, P =0.003; 1,25(OH)2D: 49.5-53.0 pg/mL, P = .9). After 8 weeks of 25(OH)D3, mean 25(OH)D, 24,25(OH)2D, and 1,25(OH)2D increased significantly (25(OH)D: 16.7-50.6 ng/mL, P < .0001; 24,25(OH)2D: 1.3-6.2 ng/mL, P = .0001; 1,25(OH)2D: 56.5-74.2 pg/mL, P = .05). Fecal microbial α-diversity and β-diversity did not change with D3 or 25D3 supplementation. Mean relative abundance of Firmicutes increased and mean relative abundance of Bacterioidetes decreased from baseline to 4 weeks, but returned to baseline by study completion. DESeq2 analysis did not confirm any statistically significant taxonomic changes.

CONCLUSION

In a small sample of healthy adults with vitamin D deficiency, restoration of vitamin D sufficiency with vitamin D3 or 25(OH)D3 did not lead to lasting changes in the fecal microbiota.

Investigation of the effects of dietary supplementation with 25-hydroxyvitamin D3 and vitamin D3 on indicators of vitamin D status in healthy dogs

OBJECTIVE

To compare the effects of short-term dietary supplementation with vitamin D₃ and 25-hydroxyvitamin D₃ (25[OH]D₃) on indicators of vitamin D status in healthy dogs.

ANIMALS

13 purpose-bred adult dogs.

PROCEDURES

20 extruded commercial dog foods were assayed for 25(OH)D₃ content. Six dogs received a custom diet containing low vitamin D concentrations and consumed a treat with vitamin D₂ (0.33 μg/kg^0.75) plus 1 of 3 doses of 25(OH)D₃ (0, 0.23, or 0.46 μg/kg^0.75) once daily for 8 weeks followed by the alternate treatments in a crossover-design trial. In another crossover-design trial, 7 dogs received a custom diet supplemented with vitamin D₃ or 25(OH)D₃ (targeted content, 3,250 U/kg [equivalent to 81.3 μg/kg] and 16 μg/kg, respectively, as fed) for 10 weeks followed by the alternate treatment. In washout periods before each trial and between dietary treatments in the second trial, dogs received the trial diet without D-vitamer supplements. Dietary intake was monitored. Serum or plasma concentrations of vitamin D metabolites and biochemical variables were analyzed at predetermined times.

RESULTS

25(OH)D₃ concentrations were low or undetected in evaluated commercial diets. In the first trial, vitamin D₂ intake resulted in quantifiable circulating concentrations of 25-hydroxyvitamin D₂ but not 24R,25-dihydroxyvitamin D₂. Circulating 25(OH)D₃ concentration appeared to increase linearly with 25(OH)D₃ dose. In the second trial, circulating 25(OH)D₃ concentration increased with both D vitamer-supplemented diets and did not differ significantly between treatments. No evidence of vitamin D excess was detected in either trial.

CONCLUSIONS AND CLINICAL RELEVANCE

Potency of the dietary 25(OH)D₃ supplement estimated on the basis of targeted content was 5 times that of vitamin D₃ to increase indicators of vitamin D status in the study sample. No adverse effects attributed to treatment were observed in short-term feeding trials.

Differences in 25-Hydroxyvitamin D Clearance by eGFR and Race: A Pharmacokinetic Study

BACKGROUND

Conversion of 25-hydroxyvitamin D (25[OH]D) to the active form of vitamin D occurs primarily in the kidney. Observational studies suggest 25(OH)D clearance from the circulation differs by kidney function and race. However, these potential variations have not been tested using gold-standard methods.

METHODS

We administered intravenous, deuterated 25(OH)D3 (d-25[OH]D3) in a pharmacokinetic study of 87 adults, including 43 with normal eGFR (≥60 ml/min per 1.73 m2), 24 with nondialysis CKD (eGFR <60 ml/min per 1.73 m2), and 20 with ESKD treated with hemodialysis. We measured concentrations of d-25(OH)D3 and deuterated 24,25-dihydroxyvitamin D3 at 5 minutes and 4 hours after administration, and at 1, 4, 7, 14, 21, 28, 42, and 56 days postadministration. We calculated 25(OH)D clearance using noncompartmental analysis of d-25(OH)D3 concentrations over time. We remeasured 25(OH)D clearance in a subset of 18 participants after extended oral vitamin-D3 supplementation.

RESULTS

The mean age of the study cohort was 64 years; 41% were female, and 30% were Black. Mean 25(OH)D clearances were 360 ml/d, 313 ml/d, and 263 ml/d in participants with normal eGFR, CKD, and kidney failure, respectively (P=0.02). After adjustment for age, sex, race, and estimated blood volume, lower eGFR was associated with reduced 25(OH)D clearance (β=-17 ml/d per 10 ml/min per 1.73 m2 lower eGFR; 95% CI, -21 to -12). Black race was associated with higher 25(OH)D clearance in participants with normal eGFR, but not in those with CKD or kidney failure (P for interaction=0.05). Clearance of 25(OH)D before versus after vitamin-D3 supplementation did not differ.

CONCLUSIONS

Using direct pharmacokinetic measurements, we show that 25(OH)D clearance is reduced in CKD and may differ by race.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Clearance of 25-hydroxyvitamin D in Chronic Kidney Disease (CLEAR), NCT02937350; Clearance of 25-hydroxyvitamin D3 During Vitamin D3 Supplementation (CLEAR-PLUS), NCT03576716.

Vitamin D Metabolism and Profiling in Veterinary Species

The demand for vitamin D analysis in veterinary species is increasing with the growing knowledge of the extra-skeletal role vitamin D plays in health and disease. The circulating 25-hydroxyvitamin-D (25(OH)D) metabolite is used to assess vitamin D status, and the benefits of analysing other metabolites in the complex vitamin D pathway are being discovered in humans. Profiling of the vitamin D pathway by liquid chromatography tandem mass spectrometry (LC-MS/MS) facilitates simultaneous analysis of multiple metabolites in a single sample and over wide dynamic ranges, and this method is now considered the gold-standard for quantifying vitamin D metabolites. However, very few studies report using LC-MS/MS for the analysis of vitamin D metabolites in veterinary species. Given the complexity of the vitamin D pathway and the similarities in the roles of vitamin D in health and disease between humans and companion animals, there is a clear need to establish a comprehensive, reliable method for veterinary analysis that is comparable to that used in human clinical practice. In this review, we highlight the differences in vitamin D metabolism between veterinary species and the benefits of measuring vitamin D metabolites beyond 25(OH)D. Finally, we discuss the analytical challenges in profiling vitamin D in veterinary species with a focus on LC-MS/MS methods.

Hypervitaminosis D has no positive effects on goat tuberculosis and may cause chronic renal lesions

BACKGROUND

There is evidence for a link between vitamin D deficiency and active tuberculosis (TB). In human beings, several trials have evaluated the role of vitamin D supplementation in TB treatment with conflicting results. However, the role of vitamin D supplementation in animal TB control has received less attention. The authors evaluated the benefit of vitamin D supplementation for preventing mycobacterial infection or reducing TB lesions (TBL) in a controlled trial with goats naturally exposed to Mycobacterium caprae.

METHODS

Two groups of goats, a vitamin D-supplemented group and a non-supplemented control group, were housed for 10 months in direct contact with M caprae-infected adult goats. Upon contact with the infected adult goats, all animals were TB-tested every two months.

RESULTS

No experimental evidence of a protective effect of vitamin D supplementation based on M caprae culture prevalence, TBL prevalence, median TBL score or the proportion of single versus multiple organs presenting TBL was observed.

CONCLUSION

The results indicate that, in the conditions used in this study, vitamin D supplementation in goats does not reduce TB infection risk nor the diffusion and severity of TBL. In addition, vitamin D-supplemented goats presented hyperphosphataemia and renal injury with calcifications suggestive of vitamin D intoxication.

Associations Between Change in Total and Free 25-Hydroxyvitamin D With 24,25-Dihydroxyvitamin D and Parathyroid Hormone

CONTEXT

The physiologic role of free 25-hydroxyvitamin D [25(OH)D] in humans is unclear.

OBJECTIVE

To assess whether rise in total vs free 25(OH)D is associated with change in downstream biomarkers of 25(OH)D entry into target cells in kidney and parathyroid: 24,25-dihyroxyvitamin D [24,25(OH)2D] and PTH, respectively.

DESIGN

16-week randomized controlled trial.

INTERVENTION

60 μg (2400 IU)/d of D3 or 20 μg/d of 25(OH)D3.

SETTING

Academic medical center.

PARTICIPANTS

35 adults age ≥18 years with 25(OH)D levels < 20 ng/mL.

MAIN OUTCOME MEASURES

24,25(OH)2D, 1,25-dihyroxyvitamin D [1,25(OH)2D] and PTH.

RESULTS

At baseline, participants [D3 and 25(OH)D3 groups combined] were 35.1 ± 10.6 years. Mean total 25(OH)D, free 25(OH)D, 24,25(OH)2D, and PTH were 16.6 ng/mL, 4.6 pg/mL, 1.3 ng/mL, and 37.2 pg/mL, respectively. From 0 to 4 weeks, rise in only free 25(OH)D was associated with a concurrent 24,25(OH)2D increase [P = 0.03, adjusted for change in 1,25(OH)2D and supplementation regimen] and PTH decrease (P = 0.01, adjusted for change in calcium and supplementation regimen). Between 4 and 8 weeks, and again from 8 to 16 weeks, rises in free and total 25(OH)D were associated with 24,25(OH)2D increase; in contrast, rise in neither total nor free 25(OH)D was associated with PTH decrease during these time periods.

CONCLUSIONS

Early rise in free 25(OH)D during treatment of vitamin D deficiency was more strongly associated with changes in biomarkers of 25(OH)D entry into target kidney and parathyroid cells, suggesting a physiologic role of free 25(OH)D in humans.

The 24,25 to 25-hydroxyvitamin D ratio and fracture risk in older adults: The cardiovascular health study

25-hydroxyvitamin D [25(OH)D] may not optimally indicate vitamin D receptor activity. Higher concentrations of its catabolic product 24,25-dihydroxyvitmin D [24,25(OH)₂D] and a higher ratio of 24,25(OH)₂D to 25(OH)D (the vitamin D metabolite ratio [VMR]) may provide additional information on receptor activity. We compared the strength of associations of these markers with serum PTH concentrations, hip bone mineral density (BMD), and risk of incident hip fracture in community-living older participants in the Cardiovascular Health Study. Among 890 participants, the mean age was 78years, 60% were women, and the mean 25(OH)D was 28±11ng/ml. In cross-sectional analysis, the strength of association of each vitamin D measure with PTH was similar; a 1% higher 25(OH)D, 24,25(OH)₂D, and VMR were associated with 0.32%, 0.25%, and 0.26% lower PTH, respectively (p<0.05 for all). Among 358 participants with available BMD data, we found no associations of 25(OH)D or VMR with BMD, whereas higher 24,25(OH)₂D was modestly associated with greater hip BMD (1% higher 24,25(OH)₂D associated with 0.04% [95% CI 0.01-0.08%] higher BMD). Risk of incident hip fracture risk was evaluated using a case-cohort design. There were 289 hip fractures during a mean follow up time of 8.4years. Both higher 24,25(OH)₂D and VMR were associated with lower risk of hip fracture (HR per SD higher, 0.73 [0.61, 0.87] and 0.74 [0.61, 0.88], respectively) whereas 25(OH)D was not associated with hip fracture (HR 0.93 [0.79, 1.10]). We conclude that evaluating vitamin D status by incorporating assessment of 24,25(OH)D and the VMR provides information on bone health above and beyond 25(OH)D alone.

Serum 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3 concentrations in adult dogs are more substantially increased by oral supplementation of 25-hydroxyvitamin D3 than by vitamin D3

We previously found a weak response in serum 25-hydroxyvitamin D₃ (25(OH)D₃) concentrations when dogs were supplemented with oral vitamin D₃ (D₃). In the present study, we determined the relative potency of oral 25(OH)D₃ compared with D₃ for increasing vitamin D status in dogs with low serum 25(OH)D concentrations. Four male and three female, 4-year-old, intact, lean, genetically related, Chinese-crested/beagle dogs were studied in a randomised, single cross-over trial. After feeding a low-vitamin D diet (<4 IU/100 g) for 30 d, four dogs received daily D₃ supplementation at 2·3 µg/kg body weight^0·75, while three dogs received a molar equivalency as 25(OH)D₃. The supplements, dissolved in ethanol, were applied to a commercial treat for consumption. Serum 25(OH)D₃ and 24R,25-dihydroxyvitamin D₃ (24R,25(OH)₂D₃) were analysed weekly using a validated HPLC method. Both supplementations increased (P ≤ 0·01) serum 25(OH)D₃ concentrations. However, oral 25(OH)D₃ resulted in greater (P < 0·0001) concentrations than D₃ by week 1, with a difference of 173 % (P < 0·0001) by week 2. The supplementation period was limited to 14 d after serum 25(OH)D₃ concentrations were not appearing to plateau. Thereafter, a washout period of 1 month separated the cross-over. Following 25(OH)D₃, but not D₃ supplementation, serum 24R,25(OH)₂D₃ concentrations increased (P ≤ 0·02), 3 to 5 weeks after initiating supplementation. Vitamin D status, as indicated by serum 25(OH)D₃ and 24R,25(OH)₂D₃ concentrations, is more rapidly and efficiently increased in adult dogs by oral supplementation of 25(OH)D₃ than D₃.

Oral vitamin D supplementation at five times the recommended allowance marginally affects serum 25-hydroxyvitamin D concentrations in dogs

Little is known regarding optimal vitamin D status in adult dogs. To date no studies on vitamin D supplementation for improving vitamin D status have been reported for adult dogs. The aims of this study were to identify dogs with low vitamin D status and evaluate an oral dosage of cholecalciferol (D₃) for effectiveness in increasing vitamin D status. For this, forty-six privately owned dogs were evaluated. Of the dogs, thirty-three (or 71·7 %) had serum 25-hydroxyvitamin D (25(OH)D) concentrations less than 100 ng/ml, a minimum previously suggested for vitamin D sufficiency in dogs. Subsequently, thirteen dogs were enrolled in a supplementation trial. Dogs were given either a D₃ supplement (n 7; 2·3 µg/kg^0·75) or olive oil placebo (n 6) daily with food. Serum concentrations of 25(OH)D were determined at weeks 1, 3 and 6, and at the trial end. Only at the trial end (weeks 9–10) was 25(OH)D significantly greater (P = 0·05) in supplemented v. placebo dogs. Serum concentrations of 24R,25-dihydroxycholecalciferol determined at the trial end were about 40 % of that of 25(OH)D₃ and not significantly different between the groups. Concentrations of parathyroid hormone, ionised Ca, P and creatinine measured in initial and final serum samples indicated supplementation caused no toxicity. We conclude that vitamin D₃ supplementation at a dosage near the National Research Council recommended safe-upper limit was not effective for rapidly raising serum 25(OH)D concentrations in healthy, adult dogs. Further work is needed in evaluating the metabolism of orally administered D₃ in dogs before dosing recommendations can be made.

Chronic Vitamin D Intoxication in Captive Iberian Lynx (Lynx pardinus)

To document the biochemical and pathologic features of vitamin D intoxication in lynx and to characterize mineral metabolism in healthy lynx, blood samples were obtained from 40 captive lynx that had been receiving excessive (approximately 30 times the recommended dose) vitamin D3 in the diet, and from 29 healthy free ranging lynx. Tissue samples (kidney, stomach, lung, heart and aorta) were collected from 13 captive lynx that died as a result of renal disease and from 3 controls. Vitamin D intoxication resulted in renal failure in most lynx (n = 28), and widespread extraskeletal calcification was most severe in the kidneys and less prominent in cardiovascular tissues. Blood minerals and calciotropic hormones in healthy lynx were similar to values reported in domestic cats except for calcitriol which was higher in healthy lynx. Changes in mineral metabolism after vitamin D intoxication included hypercalcemia (12.0 ± 0.3 mg/dL), hyperphosphatemia (6.3 ± 0.4 mg/dL), increased plasma calcidiol (381.5 ± 28.2 ng/mL) and decreased plasma parathyroid hormone (1.2 ± 0.7 pg/mL). Hypercalcemia and, particularly, hyperphosphatemia were of lower magnitude that what has been previously reported in the course of vitamin D intoxication in other species. However, extraskeletal calcifications were severe. The data suggest that lynx are sensitive to excessive vitamin D and extreme care should be taken when supplementing this vitamin in captive lynx diets.

The paracrine feedback loop between vitamin D₃ (1,25(OH)₂D₃) and PTHrP in prehypertrophic chondrocytes

The endocrine feedback loop between vitamin D₃ (1,25(OH)₂D₃) and parathyroid hormone (PTH) plays a central role in skeletal development. PTH-related protein (PTHrP) shares homology and its receptor (PTHR1) with PTH. The aim of this study was to investigate whether there is a functional paracrine feedback loop between 1,25(OH)₂D₃ and PTHrP in the growth plate, in parallel with the endocrine feedback loop between 1,25(OH)₂D₃ and PTH. This was investigated in ATDC5 cells treated with 10⁻⁸ M 1,25(OH)₂D₃ or PTHrP, Col2-pd2EGFP transgenic mice, and primary Col2-pd2EGFP growth plate chondrocytes isolated by FACS, using RT-qPCR, Western blot, PTHrP ELISA, chromatin immunoprecipitation (ChIP) assay, silencing of the 1,25(OH)₂D₃ receptor (VDR), immunofluorescent staining, immunohistochemistry, and histomorphometric analysis of the growth plate. The ChIP assay confirmed functional binding of the VDR to the PTHrP promoter, but not to the PTHR1 promoter. Treatment with 1,25(OH)₂D₃ decreased PTHrP protein production, an effect which was prevented by silencing of the VDR. Treatment with PTHrP significantly induced VDR production, but did not affect 1α- and 24-hydroxylase expression. Hypertrophic differentiation was inhibited by PTHrP and 1,25(OH)₂D₃ treatment. Taken together, these findings indicate that there is a functional paracrine feedback loop between 1,25(OH)₂D₃ and PTHrP in the growth plate. 1,25(OH)₂D₃ decreases PTHrP production, while PTHrP increases chondrocyte sensitivity to 1,25(OH)₂D₃ by increasing VDR production. In light of the role of 1,25(OH)₂D₃ and PTHrP in modulating chondrocyte differentiation, 1,25(OH)₂D₃ in addition to PTHrP could potentially be used to prevent undesirable hypertrophic chondrocyte differentiation during cartilage repair or regeneration.

Vitamin D Metabolites; Protective versus Toxic Properties: Molecular and Cellular Perspectives

Vitamin D plays an essential role in bone metabolism. The discovery that the vitamin D receptor (VDR), a member of the nuclear receptor superfamily, is expressed in most tissues led researchers to investigate other biological actions of vitamin D. These effects were found to include anti-inflammatory effects and anti-atherogenesis, decreased renin activity and biosynthesis, induction of cell differentiation, inhibition of cell growth, and immunomodulation. In spite of the plethora of evidence on the protective effects of vitamin D, the reports on its intoxication still are considerably few. Therefore, in this review we aim to summarize the molecular and cellular bases of the protect-ive and toxic vitamin D actions that are mediated mostly by VDR. This review will also shed light on vitamin D metabolites other than the active metabolite calcitriol and particularly 25-hydroxy vitamin D (25(OH)D), putting emphasis on its magnifying role in vitamin D intoxication. One of the important themes we discuss is defining serum levels of beneficial or toxic effects of other exogenous vitamin D administration and its impact on 25(OH)D serum levels in animals and human subjects.

Vitamin D intoxication caused by ingestion of commercial cat food in three kittens

Two siblings, a 6-month-old sexually intact male weighing 2.5 kg (cat 1) and a sexually intact female (cat 2) British Shorthair cat weighing 2.3 kg, were examined because of a 3-week history of polyuria, lethargy and laboured breathing. One year previously, another sibling (cat 3) had been presented because of similar, yet more severe, clinical signs at the age of 5 months. Physical examination revealed lethargy, dehydration and polypnoea with slightly increased inspiratory effort. Diagnostic investigation revealed severe hypercalcaemia (cats 1-3), renal azotaemia (cats 1 and 3) and a radiologically generalised miliary interstitial pattern of the lungs (cats 1-3) attributable to hypervitaminosis D caused by ingestion of commercial cat food. Cat 3 was euthanased. Cats 1 and 2 were treated with isotonic saline solution (180 ml/kg IV daily), sucralfate (30 mg/kg PO q12h), terbutaline (only cat 1: 0.1 mg/kg SC q4h), furosemide (1.5 mg/kg IV q8h) and tapering doses of prednisolone. Cat 2 was normal on day 14. Cat 1 had stable renal disease and was followed up to day 672. The radiological generalised military interstitial pattern of the lungs had improved markedly. Excessive cholecalciferol-containing commercially available cat food poses a great hazard to cats. Supportive treatment may result in long-term survival and improvement of radiological pulmonary abnormalities.

Vitamin D: health panacea or false prophet?

Vitamin D deficiency, diagnosed when the serum 25-hydroxyvitamin D (25-OHD(3)) concentration is less than 20 ng/mL, has joined vitamin A deficiency as two of the most common nutrition-responsive medical conditions worldwide. There have been more scientific articles published about vitamin D in the 21st century than about any other vitamin, reflecting the massive expansion of the field of vitamin D research. Adequate vitamin D status has been linked to decreased risks of developing specific cancers, including cancers of the esophagus, stomach, colon, rectum, gallbladder, pancreas, lung, breast, uterus, ovary, prostate, urinary bladder, kidney, skin, thyroid, and hematopoietic system (e.g., Hodgkin's lymphoma, non-Hodgkin's lymphoma, multiple myeloma); bacterial infections; rheumatoid arthritis; Crohn's disease; periodontal disease; multiple sclerosis; asthma; type 2 diabetes; cardiovascular disease; stroke; peripheral artery disease; hypertension; chronic kidney disease; muscle weakness; cognitive impairment; Alzheimer's disease; clinical depression; and premature death. On the other hand, inadequate vitamin D status during human pregnancy may be associated with increased risk for the development of type 1 diabetes in the offspring. However, this point of view may be excessively optimistic. There also is evidence that despite the current heavy reliance on serum 25-OHD(3) concentration for the diagnosis of an individual's vitamin D status, local tissue vitamin D intoxication may be present in individuals with much lower serum 25-OHD(3) concentrations than are currently appreciated. Only rarely are the symptoms of local tissue vitamin D intoxication associated with vitamin D status or intake. An individual's serum 25-OHD(3) concentration may appear to be "low" for reasons totally independent of sunlight exposure or vitamin D intake. Serum 25-OHD(3) concentration is only poorly responsive to increases in vitamin D intake, and the prolonged routine consumption of thousands of international units of vitamin D may interfere with the regulation of phosphate homeostasis by fibroblast growth factor-23 (FGF23) and the Klotho gene product, with consequences that are detrimental to human health. In light of these counterbalancing observations, curbing excessive enthusiasm for universally increasing vitamin D intake recommendations may be in order.

Pharmacokinetics of vitamin D toxicity

Although researchers first identified the fat-soluble vitamin cholecalciferol almost a century ago and studies have now largely elucidated the transcriptional mechanism of action of its hormonal form, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], we know surprisingly little about mechanisms of vitamin D toxicity. The lipophilic nature of vitamin D explains its adipose tissue distribution and its slow turnover in the body (half-life approximately 2 mo). Its main transported metabolite, 25-hydroxyvitamin D(3) [25(OH)D(3)], shows a half-life of approximately 15 d and circulates at a concentration of 25-200 nmol/L, whereas the hormone 1alpha,25(OH)(2)D(3) has a half-life of approximately 15 h. Animal experiments involving vitamin D(3) intoxication have established that 25(OH)D(3) can reach concentrations up to 2.5 mumol/L, at which it is accompanied by hypercalcemia and other pathological sequelae resulting from a high Ca/PO(4) product. The rise in 25(OH)D(3) is accompanied by elevations of its precursor, vitamin D(3), as well as by rises in many of its dihydroxy- metabolites [24,25(OH)(2)D(3); 25,26(OH)(2)D(3); and 25(OH)D(3)-26,23-lactone] but not 1alpha,25(OH)(2)D(3). Early assumptions that 1alpha,25(OH)(2)D(3) might cause hypercalcemia in vitamin D toxicity have been replaced by the theories that 25(OH)D(3) at pharmacologic concentrations can overcome vitamin D receptor affinity disadvantages to directly stimulate transcription or that total vitamin D metabolite concentrations displace 1alpha,25(OH)(2)D from vitamin D binding, increasing its free concentration and thus increasing gene transcription. Occasional anecdotal reports from humans intoxicated with vitamin D appear to support the latter mechanism. Although current data support the viewpoint that the biomarker plasma 25(OH)D concentration must rise above 750 nmol/L to produce vitamin D toxicity, the more prudent upper limit of 250 nmol/L might be retained to ensure a wide safety margin.

23-Carboxy-24,25,26,27-tetranorvitamin D3 (calcioic acid) and 24-carboxy-25,26,27-trinorvitamin D3 (cholacalcioic acid): End products of 25-hydroxyvitamin D3 metabolism in rat kidney through C-24 oxidation pathway

During the past two and half decades the elucidation of the metabolic pathways of 25OHD(3) and its active metabolite 1alpha,25(OH)(2)D(3) progressed in parallel. In spite of many advances in this area of vitamin D research, the unequivocal identification of the end products of 25OHD(3) metabolism through C-24 oxidation pathway has not been achieved. It is now well established that both 25OHD(3) and 1alpha,25(OH)(2)D(3) are metabolized through the same C-24 oxidation pathway initiated by the enzyme 24-hydroxylase (CYP24A1). Based on the information that the end product of 1alpha,25(OH)(2)D(3) metabolism through C-24 oxidation pathway is 1alpha-OH-23- COOH-24,25,26,27-tetranor D(3) or calcitroic acid; the metabolism of 25OHD(3) into 23-COOH-24,25,26,27-tetranor D(3) has been assumed. Furthermore, a previous study indicated 24-COOH-25,26,27-trinor D(3) as a water soluble metabolite of 24R,25(OH)(2)D(3) produced in rat kidney homogenates. Therefore, 24-COOH-25,26,27-trinor D(3) was also assumed as another end product of 25OHD(3) metabolism through C-24 oxidation pathway. We embarked on our present study to provide unequivocal proof for these assumptions. We first studied the metabolism of 25OHD(3) at low substrate concentration (3x10(-10)M) using [1,2-(3)H]25OHD(3) as the substrate in the perfused rat kidneys isolated from both normal and vitamin D(3) intoxicated rats. A highly polar water soluble metabolite, labeled as metabolite X was isolated from the kidney perfusate. The amount of metabolite X produced in the kidney of a vitamin D intoxicated rat was about seven times higher than that produced in the kidney of a normal rat. We then produced metabolite X in a quantity sufficient for its structure identification by perfusing kidneys isolated from vitamin D intoxicated rats with high substrate concentration of 25OHD(3) (5x10(-6)M). Using the techniques of electron impact and thermospray mass spectrometry, we established that the metabolite X contained both 23-COOH-24,25,26,27-tetranor D(3) and 24-COOH-25,26,27-trinor D(3) in a ratio of 4:1. The same metabolite X containing both acids in the same ratio of 4:1 was also produced when 24R,25(OH)(2)D(3) was used as the starting substrate. Previously, the trivial name of cholacalcioic acid was assigned to 24-COOH-25,26,27-trinorvitamin D(3). Using the same guidelines, we now assign the trivial name of calcioic acid to 23-COOH-24,25,26,27-tetranor D(3). In summary, for the first time our study provides unequivocal evidence to indicate that both calcioic and cholacalcioic acids as the end products of 25OHD(3) metabolism in rat kidney through C-24 oxidation pathway.

Development and evaluation of canine reference genes for accurate quantification of gene expression

In determining relative gene expression by quantitative measurements of mRNA levels using real-time quantitative PCR, internal standards such as reference genes are essential. Large-scale studies evaluating (candidate) reference genes for veterinary research have not been conducted as thoroughly as for human research, although they are equally important. Our goal was to design and evaluate a genome-wide panel of reference genes from different functional classes. First, primers were optimized using mRNA from canine cell lines and from 30 tissues of one dog as template and SYBR green as fluorescent probe. Second, the expression variation and stability of a gene within one specific tissue were determined. Prostate, kidney, mammary gland, left ventricle, and liver tissues from five to nine dogs of different breeds, sexes, ages, body weights, and disease status were used. Averaging relative stabilities over these tissues revealed the usefulness of individual genes as reference genes. Furthermore, according to expression variation of a reference gene within a specific tissue, usually two to four reference genes are sufficient. Taken together, ribosomal protein S19 (RPS19), ribosomal protein S5 (RPS5), beta-2-microglobulin (B2M), and hypoxanthine phosphoribosyltransferase (HPRT) are advocated. However, the optimal set of reference genes depends on the tissue and should be selected and evaluated for each series of experiments.

Different effects of physiologically and pharmacologically increased growth hormone levels on cholecalciferol metabolism at prepubertal age

The aim of the study was to investigate the influence of physiologically and pharmacologically increased plasma growth hormone (GH) levels on cholecalciferol metabolism at prepubertal age. Three groups of dogs raised on the same diet were studied from weaning till 21 weeks of age, i.e., small breed dogs (n = 7, control group); large breed dogs with 15-fold greater growth rates compared to the control group (n = 8, LB-group); and small breed dogs treated with pharmacological doses of growth hormone (n = 6, GH-group; 0.5IU GH per kg body per day) from 12 to 21 weeks of age. Excess of GH had the expected anabolic effect on growth rate and phosphate sparing. Increased plasma GH levels in the LB- and GH-groups versus the control group were accompanied by (1) greater plasma insulin-like growth factor I (IGF-I) levels, (2) greater plasma 1,25-dihydroxycholecalciferol (1,25(OH)(2)D(3)) levels, and (3) lower plasma 24,25(OH)(2)D(3) levels. In the LB-group, excess of GH favored plasma 1,25(OH)(2)D(3) levels by decreasing the clearance of 1,25(OH)(2)D(3), whereas in the GH-group by increasing the production of 1,25(OH)(2)D(3). The lowered plasma 24,25(OH)(2)D(3) levels in the LB- and GH-groups were likely attributed to a competitive inhibition of the production of 24,25(OH)(2)D(3) by GH and/or IGF-I.