Calcium absorption and kinetics are similar in 7- and 8-year-old Mexican-American and Caucasian girls despite hormonal differences. (1/553)

To assess the possibility of ethnic differences in mineral metabolism in prepubertal children, we compared measures of calcium metabolism in 7- and 8-y-old Mexican-American (MA) and non-Hispanic Caucasian (CAU) girls (n = 38) living in southeastern Texas. We found similar fractional calcium absorption, urinary calcium excretion, calcium kinetic values and total-body bone mineral content in the MA and CAU girls. In contrast, parathyroid hormone (PTH) concentrations were greater in MA girls (4.01 +/- 0.47 vs. 1. 96 +/- 0.50 pmol/L, P = 0.005) than in CAU girls. Serum 25-hydroxyvitamin D concentrations were lower in MA girls (68.9 +/- 7.7 vs. 109.4 +/- 8.4 nmol/L, P = 0.001) than in CAU girls, but 1, 25-dihydroxyvitamin D concentrations did not differ between groups. Seasonal variability was seen for 25-hydroxyvitamin D concentrations in girls of both ethnic groups, but values in all of the girls were >30 nmol/L (12 ng/mL). We conclude the following: 1) greater PTH levels in MA girls than CAU girls are present without evidence of vitamin D deficiency; and 2) differences in 25-hydroxyvitamin D and PTH concentrations between MA and CAU girls do not have a large effect on calcium absorption, excretion or bone calcium kinetics. These data do not provide evidence for adjusting dietary recommendations for mineral or vitamin D intake by MA girls.  (+info)

Enzymatic properties of mouse 25-hydroxyvitamin D3 1 alpha-hydroxylase expressed in Escherichia coli. (2/553)

Renal 25-hydroxyvitamin D3 1 alpha-hydroxylase cDNA cloned from the kidneys of mice lacking the vitamin D receptor was expressed in Escherichia coli JM109. As expected, the bacterially-expressed enzyme catalyzes the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 with a Michaelis constant, K(m), value of 2.7 microM. Unexpectedly, the enzyme also hydroxylates the 1 alpha-position of 24,25-dihydroxyvitamin D3 with a K(m) of 1.3 microM, and a fourfold higher Vmax/K(m) compared with the 25-hydroxyvitamin D3 hydroxylase activity, suggesting that 24,25-dihydroxyvitamin D3 is a better substrate than 25-hydroxyvitamin D3 for 1 alpha-hydroxylase. In addition, the enzyme showed 1 alpha-hydroxylase activity toward 24-oxo-25-hydroxyvitamin D3. However, it showed only slight activity towards 23,25-dihydroxyvitamin D3 and 24-oxo-23,25-dihydroxyvitamin D3, and no detectable activity towards vitamin D3 and 24,25,26,27-tetranor-23-hydroxyvitamin D3. These results suggest that the 25-hydroxyl group of vitamin D3 is essential for the 1 alpha-hydroxylase activity and the 24-hydroxyl group enhances the activity, but the 23-hydroxyl group greatly reduced the activity. Another remarkable finding is that living recombinant E. coli cells can convert the substrates into the 1 alpha-hydroxylated products, suggesting the presence of a redox partner of 1 alpha-hydroxylase in E. coli cells.  (+info)

Specific 1,25(OH)2D3-mediated regulation of transcellular calcium transport in Caco-2 cells. (3/553)

Calcium transport in the apical-to-basolateral (A-to-B) or B-to-A direction was examined in cells treated with 10 nM 1, 25-dihydroxyvitamin D3 [1,25(OH)2D3, calcitriol] for up to 72 h. Net A-to-B calcium transport was positive at all time points and increased from 0.14 +/- 0.06 to 0.50 +/- 0.01 nmol. well-1. min-1 after 72 h of calcitriol treatment. Neither phenol red transport nor transepithelial electrical resistance was altered by calcitriol treatment, suggesting that the increase in net A-to-B calcium transport was not due to paracellular movement. Neither 25-hydroxyvitamin D3 nor 24,25-dihydroxyvitamin D3 (100 nM, 48 h) alters basal or calcitriol-stimulated A-to-B calcium transport. Treatment with the calmodulin antagonist trifluoperazine (50 microM) reduced calcitriol-stimulated A-to-B Ca transport by 56%. The transcription inhibitor actinomycin D inhibited calcitriol-regulated A-to-B calcium transport as well as calbindin D9k and 24-hydroxylase mRNA accumulation. These data demonstrate that calcitriol-mediated A-to-B calcium transport in Caco-2 cells is a specific, transcellular process that requires transcriptional events normally mediated through the vitamin D receptor.  (+info)

Serum vitamin D levels and incident changes of radiographic hip osteoarthritis: a longitudinal study. Study of Osteoporotic Fractures Research Group. (4/553)

OBJECTIVE: The purpose of this study was to determine the relationship of serum levels of 25-vitamin D and 1,25-vitamin D to incident changes of radiographic hip osteoarthritis (OA) among elderly white women. METHODS: Baseline and followup hip radiographs of 237 subjects were obtained an average of 8 years apart. Hips were scored for individual radiographic features (IRF) and assigned a summary grade based on the number and type of IRF present. Serum 25- and 1,25-vitamin D levels from baseline samples were analyzed by radioimmunoassay. Logistic and linear regression were used to examine the association of 25- and 1,25-vitamin D levels with radiographic changes, adjusting for age, health status, physical activity, weight, vitamin D supplement use, and calcaneal bone mineral density. RESULTS: The risk of incident hip OA defined as the development of definite joint space narrowing was increased for subjects who were in the middle (odds ratio [OR] 3.21, 95% confidence interval [95% CI] 1.06, 9.68) and lowest (OR 3.34, 95% CI 1.13, 9.86) tertiles for 25-vitamin D compared with subjects in the highest tertile. Vitamin D levels were not associated with incident hip OA defined as the development of definite osteophytes or new disease according to the summary grade. No association between serum 1,25-vitamin D and changes in radiographic hip OA was found. CONCLUSION: Low serum levels of 25-vitamin D may be associated with incident changes of radiographic hip OA characterized by joint space narrowing.  (+info)

Autocrine control of vitamin D metabolism in synovial cells from arthritic patients. (5/553)

OBJECTIVE: This study was designed to investigate whether 1, 25-dihydroxyvitamin D3 (1,25-(OH)2D3), produced by activated synovial fluid macrophages, promotes its own catabolism by upregulating vitamin D-24-hydroxylase (24-OHase) in synovial fibroblasts through a vitamin D receptor (VDR) mediated mechanism. METHODS: Synovial macrophages and fibroblasts were derived from patients with rheumatoid arthritis. Expression of VDR and 24-OHase mRNAs was determined using in situ hybridisation. Vitamin D hydroxylase activity was determined by incubating cells with [3H]-25-(OH)D3, or [3H]-1,25-(OH)2D3, and metabolite synthesis quantified using high performance liquid chromatography. RESULTS: 1, 25-(OH)2D3 increased expression of mRNA for both VDR and 24-OHase in fibroblasts by approximately threefold over 24 hours. 1,25-(OH)2D3 increased fibroblast 24-OHase activity, yielding 24-hydroxylated, and more polar, metabolites. In co-culture, fibroblasts were able to catabolise macrophage derived 1,25-(OH)2D3. CONCLUSIONS: 1, 25-(OH)2D3 is produced by macrophages in vitro at biologically relevant concentrations and can increase its own catabolism by synovial fibroblasts; this effect is probably mediated via upregulation of both synovial fibroblast VDR and 24-OHase.  (+info)

Is low plasma 25-(OH)vitamin D a major risk factor for hyperparathyroidism and Looser's zones independent of calcitriol? (6/553)

BACKGROUND: Recent reports suggest that calcitriol might not be the sole active metabolite of vitamin D and that plasma concentrations of 25-(OH)vitamin D (25OHD) are often abnormally low in hemodialysis patients. We have therefore evaluated plasma 25OHD as a risk factor for parathyroid hormone (PTH) hypersecretion and radiological bone disease. We carried out a cross-sectional study during the month of September in an Algerian dialysis center of 113 patients who were not taking supplements of alphacalcidol or calcitriol. METHODS: Plasma 25OHD, calcitriol, PTH, calcium, phosphate, bicarbonate, and aluminum were measured, and x-rays of the hands and pelvis were obtained for evaluation of subperiosteal resorption and Looser's zones. RESULTS: The median plasma 25OHD was 47.5 nmol/liter (range 2.5 to 170.0). Univariate analysis showed that plasma PTH was correlated positively with months on maintenance dialysis and negatively with plasma 25OHD, calcitriol, calcium, bicarbonate and aluminum, but not with that of phosphate. plasma 25OHD was positively correlated with calcium and calcitriol. Using multiple regression analysis, only plasma 25OHD (negative) and the duration on maintenance dialysis (positive) were independently linked to plasma PTH. The prevalence of isolated subperiosteal resorption (ISR) was 34%, and that of the combination of resorption with Looser's zones (CRLZ) was 9%; thus, only 57% of the patients had a normal x-ray appearance. These groups were comparable with regards to age, gender, and duration on dialysis. When the biochemical measurements of the patients with CRLZ were compared with those from patients without radiological lesions, plasma 25OHD was the only parameter to show a statistically significant difference, being significantly lower in the CRLZ group (26 +/- 18 vs. 57 nmol/liter, ANOVA, P < 0.004). Plasma 25OHD was also significantly lower in the ISR group (44, P < 0.05) than in the normal x-ray group, and plasma Ca (P < 0.003) and bicarbonate (P < 0.02) were lower. Logistical analysis showed that the presence of resorption was independently linked only with plasma PTH. Looser's zones and subperiosteal resorption were not seen in patients with plasma 25OHD of more than 40 (Looser's zones) and more than 100 nmol/liter (subperiosteal resorption). The optimal range for intact PTH in hemodialysis patients with mild aluminum overload is 10 to 25 pmol/liter. We found that plasma PTH was inappropriately high only when plasma 25OHD was less than 100 nmol/liter. With a plasma 25OHD of between 100 and 170 nmol/liter, hypercalcemia was present with a plasma PTH of less than 10 pmol/liter in only one case. CONCLUSIONS: This cross sectional study shows that low plasma 25OHD is a major risk factor for hyperparathyroidism and Looser's zones. In dialysis patients, we suggest that the plasma levels of 25OHD are maintained around the upper limit of the reference range of sunny countries.  (+info)

Tissue engineering of a bioartificial renal tubule assist device: in vitro transport and metabolic characteristics. (7/553)

BACKGROUND: Current renal substitution therapy for acute or chronic renal failure with hemodialysis or hemofiltration is life sustaining, but continues to have unacceptably high morbidity and mortality rates. This therapy is not complete renal replacement therapy because it does not provide active transport nor metabolic and endocrinologic functions of the kidney, which are located predominantly in the tubular elements of the kidney. METHODS: To optimize renal substitution therapy, a bioartificial renal tubule assist device (RAD) was developed and tested in vitro for a variety of differentiated tubular functions. High-flux hollow-fiber hemofiltration cartridges with membrane surface areas of 97 cm2 or 0. 4 m2 were used as tubular scaffolds. Porcine renal proximal tubule cells were seeded into the intraluminal spaces of the hollow fibers, which were pretreated with a synthetic extracellular matrix protein. Attached cells were expanded in the cartridge as a bioreactor system to produce confluent monolayers containing up to 1.5 x 109 cells (3. 5 x 105 cells/cm2). Near confluency was achieved along the entire membrane surface, with recovery rates for perfused inulin exceeding 97 and 95% in the smaller and larger units, respectively, compared with less than 60% recovery in noncell units. RESULTS: A single-pass perfusion system was used to assess transport characteristics of the RADs. Vectorial fluid transport from intraluminal space to antiluminal space was demonstrated and was significantly increased with the addition of albumin to the antiluminal side and inhibited by the addition of ouabain, a specific inhibitor of Na+,K+-ATPase. Other transport activities were also observed in these devices and included active bicarbonate transport, which was decreased with acetazolamide, a carbonic anhydrase inhibitor, active glucose transport, which was suppressed with phlorizin, a specific inhibitor of the sodium-dependent glucose transporters, and para-aminohippurate (PAH) secretion, which was diminished with the anion transport inhibitor probenecid. A variety of differentiated metabolic functions was also demonstrated in the RAD. Intraluminal glutathione breakdown and its constituent amino acid uptake were suppressed with the irreversible inhibitor of gamma-glutamyl transpeptidase acivicin; ammonia production was present and incremented with declines in perfusion pH. Finally, endocrinological activity with conversion of 25-hydroxy(OH)-vitamin D3 to 1,25-(OH)2 vitD3 was demonstrated in the RAD. This conversion activity was up-regulated with parathyroid hormone and down-regulated with increasing inorganic phosphate levels, which are well-defined physiological regulators of this process in vivo. CONCLUSIONS: These results clearly demonstrate the successful tissue engineering of a bioartificial RAD that possesses critical differentiated transport, and improves metabolic and endocrinological functions of the kidney. This device, when placed in series with conventional hemofiltration therapy, may provide incremental renal replacement support and potentially may decrease the high morbidity and mortality rates observed in patients with renal failure.  (+info)

Effects of IGF-I on 1,25-dihydroxyvitamin D(3) synthesis by human placenta in culture. (8/553)

The aim of the present study was to assess the effects of insulin-like growth factor I (IGF-I) upon the synthesis of 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] by human placenta trophoblasts in culture. Cytotrophoblastic cells obtained from normal term human placentae were cultured in Dulbecco's modified Eagle's medium with HEPES and glucose (DMEM-HG) during 72 h and further incubated in serum-free DMEM-F12 in the presence of IGF-I prior to the addition of [(3)H]-25-(OH)D(3) used as a precursor. The results showed that 2 h preincubation time with IGF-I was required for maximal production of [(3)H]-1,25-(OH)(2)D(3). Cultures in the presence of increasing concentrations of IGF-I (0-6.5 nmol/l), added 2 h before incubation with the labelled substrate, resulted in a dose-dependent response increment of [(3)H]-1,25-(OH)(2)D(3) production with a maximal conversion rate at the dose of 2.6 nmol/l. Higher doses of IGF-I did not result in further stimulatory effects. Co-incubations in the presence of cycloheximide significantly (P < 0. 0001) inhibited the IGF-I-mediated effects upon [(3)H]-1, 25-(OH)(2)D(3) production. Identity of putative [(3)H]-1, 25-(OH)(2)D(3) produced by human placenta was confirmed by spectral and receptor binding analysis. These results demonstrate the ability of cultured human syncytiotrophoblast cells to convert 25-(OH)D(3) to 1,25-(OH)(2)D(3) and suggest a local protein-dependent regulatory effect of IGF-I upon this biotransformation.  (+info)

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