1 Vitamin D: Revisited 1980

Effect of 25-hydroxyvitamin D3 during prepartum transition and lactation on production, reproduction, and health of lactating dairy cows

We hypothesized that feeding 25-hydroxyvitamin D₃ [25-(OH)D₃] during lactation and prepartum in conjunction with negative dietary cation-anion difference diets would improve milk production, increase the probability of pregnancy, and reduce the incidence of postcalving diseases. Cows from 4 dairies with prepartum transition diets negative in dietary cation-anion difference were used in 2 randomized cohort experiments. In Experiment 1 (Exp. 1), cows were assigned to control [CON; n = 645; no 25-(OH)D₃] or treatment [TRT; n = 537; 2 mg/d of 25-(OH)D₃ from ∼21 d prepartum to parturition and 1 mg/d in lactation] groups at ∼21 d prepartum. Cows were monitored for weekly milk yield, milk composition every 60 d, and health and reproductive measures. In Experiment 2 (Exp. 2), cows (n = 2,064; median 147 d in milk) were assigned to 4 groups and monitored for the same measures as in Exp. 1 to the end of that lactation (L1), the subsequent transition (∼21 d prepartum to parturition), and the next lactation (L2). Groups were as follows, with the amount of 25-(OH)D₃ fed (mg/d) indicated in parentheses for L1, transition, and L2, respectively: (A) control-control (CON-CON; 0-0-0), (B) treatment-treatment (TRT-TRT; 1-2-1), (C) control-treatment (CON-TRT; 0-2-1), and (D) treatment-control (TRT-CON; 1-0-0). For L1, a total of 1,032 cows entered the control groups A or C and a total of 1,032 cows in groups B or D. The number of cows in groups A to D that entered L2 was 521, 523, 273, and 248, respectively. Blood calcium, phosphorus, and 25-(OH)D₃ concentrations were measured from 17 cows/group at 5 times. In Exp. 1, TRT cows had 0.2 lower log somatic cell count than CON cows (4.21 ± 0.045 vs. 4.01 ± 0.050, respectively) and multiparous TRT cows had 41 ± 23% higher probability of pregnancy/day than multiparous CON cows, resulting in a 22-d median decrease in time to pregnancy. Primiparous TRT cows had 1.67 ± 0.40 times greater odds of mastitis/day than primiparous CON cows. In Exp. 2 TRT-TRT cows had between 16 and 29% lower probability to be bred/day than other groups. Multiparous CON-CON and TRT-CON cows had 20 ± 8% and 30 ± 17% greater probability of pregnancy, respectively, than multiparous TRT-TRT cows. Serum calcium concentrations were not affected by group, but phosphorus and 25-(OH)D₃ concentrations were highest in the TRT-TRT cows. The study provides further insights into the use of 25(OH)D₃ in transition and lactation.

Plasma 25-hydroxyvitamin D, more so than its epimer, has a linear relationship to leaner body composition across infancy in healthy term infants

Vitamin D status positively associates with skeletal muscle mass and function in adolescents. The C-3 alpha epimer of 25-hydroxyvitamin D3 (3-epi-25(OH)D3) is high in infants, yet the potential impacts of 25-hydroxyvitamin D3 (25(OH)D3) and 3-epi-25(OH)D3 on skeletal muscle development are largely unexplored. The objective of this study was (i) to explore how the concentrations of 25(OH)D3 and 3-epi-25(OH)D3 track with body composition (lean mass (LM) and fat mass (FM)) and (ii) to determine the association between 25(OH)D3 and 3-epi-25(OH)D3 in infancy. Healthy breastfed infants (n = 132) were followed from 1 to 12 months of age as part of a vitamin D dose–response study (NCT00381914). Anthropometry and diet were assessed. Body composition was measured with dual-energy X-ray absorptiometry. Plasma 25(OH)D3 and 3-epi-25(OH)D3 concentrations were evaluated using liquid chromatography tandem mass spectrometry. Plasma 25(OH)D3 and 3-epi-25(OH)D3 increased from 1 to 3 months of age and decreased thereafter (p < 0.05). Infants with 25(OH)D3 concentrations above 75 nmol/L did not have a higher LM (g or %; p > 0.273) than those below this cutoff. LM was not associated with 25(OH)D3, whereas LM% was positively associated with 25(OH)D3 (β = 0.03; CI: 0.01 to 0.06; p = 0.006), while accounting for sex, weight-for-age Z-score, protein and fat intake, and age. For FM, the variables accounting for a significant amount of the variation were plasma 25(OH)D3 concentration (β = −2.38; CI: −4.35, −0.41; p = 0.019), weight-for-age Z-score, protein and fat intake, and time. In healthy infants, higher vitamin D status associates with leaner body composition, though the effect is smaller in magnitude relative to growth.

Analytical measurement and clinical relevance of vitamin D(3) C3-epimer

With an ever-increasing clinical interest in vitamin D insufficiency, numerous automated immunoassays, protein binding assays, and in-house LC-MS/MS methods are being developed for the quantification of 25-hydroxyvitamin D(3) (25(OH)D(3)). Recently, LC-MS/MS methods have identified an epimeric form of 25(OH)D(3) that has been shown to contribute significantly to 25(OH)D(3) concentration, particularly in infant populations. This review describes the metabolic pathway and physiological functions of 3-epi-vitamin D, compares the capability of various 25(OH)D(3) methods to detect the epimer, and highlights recent publications quantifying 3-epi-25(OH)D(3) in infant, pediatric, and adult populations. In total, this review summarizes the information necessary for clinicians and laboratorians to decide whether or not to report/consider the C3-epimer in the analysis and clinical assessment of vitamin D status.

Clinical studies on phosphate handling in hypercalcaemia

Phosphate indices (serum phosphate, tubular reabsorption of phosphate, renal threshold phosphate concentration (TmP/GFR) and index of phosphate excretion) were studied in 88 hypercalcaemic subjects: 64 with primary hyperparathyroidism (HPT) and 24 with hypercalcaemia from other causes, predominantly malignant disease. HPT patients as a group could easily be separated from normal subjects (n = 16) and patients with functional hypoparathyroidism (n = 7) by use of the phosphate variables but these indices were of little discriminating value for the differential diagnosis between HPT and hypercalcaemia from other causes. There was no difference in the urinary cyclic adenosine monophosphate (cAMP) excretion between the two hypercalcaemic patient groups, but HPT patients had clearly elevated serum parathyroid hormone (PTH) levels compared with normal PTH concentrations in patients with other causes of hypercalcaemia. A positive correlation between cAMP and serum calcium and an inverse relationship between cAMP and TmP/GFR were found in patients with hypercalcaemic malignant disease. These findings suggest the existence of a humoral factor with PTH-like effects in malignant disease. Since PTH levels were low, the physiological actions were apparently not mediated by circulating PTH. No difference in the values for phosphate variables, PTH, cAMP or serum calcium was found between renal stone-forming and stone-free patients with HPT.

Secreted frizzled-related protein-4 reduces sodium-phosphate co-transporter abundance and activity in proximal tubule cells

The phosphatonin, secreted frizzled-related protein-4 (sFRP-4), induces phosphaturia and inhibits 25-hydroxyvitamin D 1alpha-hydroxylase activity normally induced in response to hypophosphatemia. To determine the mechanism by which sFRP-4 alters renal phosphate (P(i)) transport, we examined the effect of sFRP-4 on renal brush border membrane (BBMV) Na(+)-dependent P(i) uptake, and the abundance and localization of the major Na(+)-P(i)-IIa co-transporter in proximal tubules and opossum kidney (OK) cells. Infusion of sFRP-4 increased renal fractional excretion of P(i) and decreased renal beta-catenin concentrations. The increase in renal P(i) excretion with sFRP-4 infusion was associated with a 21.9 +/- 3.4% decrease in BBMV Na(+)-dependent P(i) uptake (P < 0.001) compared with a 39.5 +/- 2.1% inhibition of Na(+)-dependent P(i) transport in renal BBMV induced by PTH (P < 0.001). sFRP-4 infusion was associated with a 30.7 +/- 4.8% decrease in Na(+)-P(i)-IIa co-transporter protein abundance (P < 0.01) assessed by immunoblotting methods compared to a 45.4 +/- 8.8% decrease induced by PTH (P < 0.001). In OK cells, sFRP-4 reduced surface expression of a heterologous Na(+)-P(i)-IIa co-transporter. We conclude that sFRP-4 increases renal P(i) excretion by reducing Na(+)-P(i)-IIa transporter abundance in the brush border of the proximal tubule through enhanced internalization of the protein.

Characterisation of receptors for 1,25-dihydroxyvitamin D3 in the human testis

Specific high affinity receptors for 1,25-dihydroxyvitamin D3 have been demonstrated in the human testes. The mean binding affinity (Kd +/- SD) of the receptor for 1,25-dihydroxyvitamin D3 was 1.75 +/- 0.32 x 10(-10) M but the binding capacity was low (mean Nmax +/- SD = 0.53 +/- 0.18 fmol/mg protein). Binding was time- and temperature-dependent, with a maximum binding achieved after 1 h at 25 degrees C. Although binding also took place at 4 and 37 degrees C, higher and more rapid binding was found at 25 degrees C. Furthermore, the binding between the ligand and the receptor was specific since only unlabelled 1,25-dihydroxyvitamin D3 competed with the labelled ligand. Binding of 1,25-dihydroxyvitamin D3 was abolished by trypsin and heat. Sucrose density gradient centrifugation revealed a sedimentation coefficient of 3.6S.

Vitamin D status in Crohn's disease: association with nutrition and disease activity

Forty patients with Crohn's disease were divided into undernourished (18) and well nourished (22) groups depending on whether their midarm circumference was below or above 90% of the ideal standard. Plasma 25-(OH)D3 and the dihydroxylated metabolites, 24,25-(OH)2D3 and 1,25-(OH)2D3 were measured in the summer. Results were related to clinical and biochemical parameters and also compared with results from patients with ulcerative colitis and healthy subjects who served as controls. Plasma 25-(OH)D3 was reduced in the undernourished Crohn's group compared with the well nourished Crohn's group, who did not differ from the controls. Over 50% of the undernourished Crohn's group had evidence of secondary hyperparathyroidism and raised alkaline phosphatase concentrations, although concentrations of 1,25-(OH)2D3 were normal. The low 25-(OH)D3 concentrations related to disease activity. It is suggested that undernourished Crohn's patients who have high levels of disease activity are at risk of vitamin D deficiency, and attempts should be made to improve their vitamin D nutrition.

Properties and compartmentalization of the testicular receptor for 1,25-dihydroxyvitamin D3

Adult rat testis contains a specific, high-affinity, low-capacity binding protein for 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) with properties similar to 1,25-(OH)2D3 receptors in other tissues. The receptor sediments at 3.5 +/- 0.2 S20,w in high-salt sucrose density gradients, but aggregates in low-salt gradients. Binding of 1,25-(OH)2D3 was abolished by trypsin, but not by DNase or RNase. Binding was also heavily reduced by the sulfhydryl alkylating agent, N-ethylmaleimide, and by the mercurial reagent, mersalyl, showing that free, reduced SH-groups are necessary for hormone-binding activity. The receptor shows high affinity for 1,25-(OH)2D3 (Kd = 3 X 10(-11) M), but low capacity (Nmax = 8 fmol/mg protein) and is specific for 1,25-(OH)2D3 (Affinity: 1,25-(OH)2D3 greater than 1,24(R),25-(OH)3D3 greater than 25-OH-D3 greater than 1 alpha-OH-D3 greater than 24(R),25-(OH)2D3 much greater than 17 beta-estradiol, testosterone, dexamethasone, R5020, progesterone). With 0.6 nM [3H]1,25-(OH)2D3 and at 0 degrees C, maximum specific binding was achieved after 4 h, and the occupied receptors were stable for more than 24 h. The dissociation of hormone-receptor complexes was temperature-dependent and very slow at low temperature (t1/2 (0 degrees C) much greater than 48 h). At 0 degrees C, the second order association rate constant and the pseudo-first order dissociation rate constant were 2.7 X 10(7) M-1 min-1 and 2 X 10(-5) min-1, respectively. Receptors for 1,25-(OH)2D3 are present in similar amounts in isolated seminiferous tubules and interstitial tissue of adult rats. No specific binding of [3H]1,25-(OH)2D3 could be detected in cultured immature Sertoli cells, cultured immature peritubular (myoid) cells or crude germ cells.

Skin calcium-binding protein: effect of vitamin D deficiency and vitamin D treatment

The amount of skin calcium-binding protein, evaluated using a sensitive radioimmunoassay and indirect immunofluorescence, was decreased in vitamin-D deficient rats and increased after one week vitamin D3 or 1 alpha-hydroxyvitamin D3 treatment. In vitamin D replete and in vitamin D-deficient animals, skin calcium-binding protein was not sensitive to changes in dietary and/or serum calcium concentrations. These results indicate that this protein is different from other calcium-binding proteins such as parvalbumin and calmodulin which are not vitamin D-dependent, and also different from intestinal calcium-binding protein which, in D replete animals, is sensitive to changes in dietary and serum calcium concentrations. Skin calcium-binding protein may, therefore, represent a new class of vitamin D-dependent protein.

Metabolism and binding properties of 24,24-difluoro-25-hydroxyvitamin D3

To evaluate possible functional roles for 24,25-dihydroxyvitamin D3, 24,24-difluoro-25-hydroxyvitamin D3 has been synthesized and shown to be equally as active as 25-hydroxyvitamin D3 in all known functions of vitamin D. The use of the difluoro compound for this purpose is based on the assumption that the C-F bonds are stable in vivo and that the fluorine atom does not act as hydroxyl in biological systems. No 24,25-dihydroxyvitamin D3 was detected in the serum obtained from vitamin D-deficient rats that had been given 24,24-difluoro-25-hydroxyvitamin D3, while large amounts were found when 25-hydroxyvitamin D3 was given. Incubation of the 24,24-difluoro compound with kidney homogenate prepared from vitamin D-replete chickens failed to produce 24,25-dihydroxyvitamin D3, while the same preparations produced large amounts of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. Kidney homogenate prepared from vitamin D-deficient chickens produced 24,24-difluoro-1,25-dihydroxyvitamin D3 from 24,24-difluoro-25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. In binding to the plasma transport protein for vitamin D compounds, 24,24-difluoro-25-hydroxyvitamin D3 is less active than 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3. In binding to the chick intestinal cytosol receptor, 24,24-difluoro-25-hydroxyvitamin D3 is more active than 25-hydroxyvitamin D3 which is itself more active than 24R,25-dihydroxyvitamin D3. The 24,24-difluoro-1,25-dihydroxyvitamin D3 is equal to 1,25-dihydroxyvitamin D3, and both are 10 times more active than 1,24R,25-trihydroxyvitamin D3 in this system. These results provide strong evidence that the C-24 carbon of 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated in vivo, and, further, the 24-F substitution acts similar to H and not to OH in discriminating binding systems for vitamin D compounds.

Drug-induced electrolyte disorders

A wide variety of pharmacologic agents have been implicated in a number of electrolyte disorders. The present review focuses on abnormalities of sodium, potassium, calcium, magnesium, and phosphate. Several mechanisms are involved in the pathogenesis of these disorders. These involve stimulation and modulation of other hormones (e.g., antidiuretic hormone, renin-angiotensin system, parathyroid hormone), damage to renal tubules, and, in some cases, a combination of factors. Recognition of these abnormalities is important because their presence may be life threatening or may aggravate the side effects of the drug itself.

1- but not 24-hydroxylation of vitamin D is required for growth and reproduction in rats

This study examines whether 1,25-dihydroxyvitamin D3 or 24,24-difluoro-25-hydroxyvitamin D3, an analogue of 25-hydroxyvitamin D3 blocked from undergoing 24-hydroxylation, can maintain normal growth and reproduction in the female rat. Vitamin D-deficient weanling rats were maintained from weaning through mating, pregnancy, and lactation with either 1,25-dihydroxyvitamin D3 (given by continuous subcutaneous infusion), 24,24-difluoro-25-hydroxyvitamin D3, 25-hydroxyvitamin D3, or vehicle. Body weight, plasma calcium levels, estrous cycling time, ability to give birth to live pups, litter weight, number of pups per litter, dam plasma calcium level during lactation, and pup growth to 9 wk of age were recorded. No striking differences were observed between the 25-hydroxyvitamin D3 groups and either the 1,25-dihydroxyvitamin D3 group or the 24,24-difluoro-25-hydroxyvitamin D3 group. However, significant differences in most parameters were observed between the vitamin D-deficient and metabolite- or analogue-dosed rats. The results demonstrate that 1,25-dihydroxyvitamin D3 and/or one of its metabolites is sufficient to maintain normal growth, development, and reproductive functions in the female rat. Because 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated at C-24, the 24-hydroxylation of 25-hydroxyvitamin D3 is not essential for normal growth, development, and reproduction in the female rat.

Biological activity of 24,24-difluoro-1,25-dihydroxyvitamin D3

The biological activity of 24,24-difluoro-1,25-dihydroxyvitamin D3 was compared with 1,25-dihydroxyvitamin D3 in the rat. The 24,24-difluoro-1,25-dihydroxyvitamin D3 has a potency of approximately 5-10 times that of 1,25-dihydroxyvitamin D3 in the known in vivo vitamin D responsive systems. These systems include intestinal calcium transport, bone calcium mobilization, calcification of epiphyseal plate cartilage, and elevation of plasma calcium and phosphorus concentrations. Thus, 24,24-difluoro-1,25-dihydroxyvitamin D3 is the first known analogue with higher potency than 1,25-dihydroxyvitamin D3 in vivo.

Vitamin D metabolism in hyperthyroidism

The serum concentrations of 25-hydroxycholecalciferol (25 OH D3), 24,25-dihydroxycholecalciferol [24,25(OH)-2D3] and 1,25-dihydroxycholecalciferol [1,25(OH)2D3] were measured in twenty-one patients with untreated hyperthyroidism. Compared with control subjects, 25 OH D3 concentrations were not altered, 24,25(OH)2D3 concentrations were increased, although not significantly and 1,25(OH)2D3 concentrations were decreased (P = 0.01). Following oral carbimazole therapy, 24,25(OH)2D3 concentrations fell (P less than 0.01), 1,25(OH)2D3 concentrations increased (P less than 0.01) and 25 OH D3 concentrations were unchanged. The altered 1,25(OH)2D3 and 24,25(OH)2D3 concentrations found in hyperthyroidism are probably due to the effects of thyroid hormone on bone and mineral metabolism. Increased serum calcium and phosphate concentrations with secondary hypoparathyroidism result in stimulation of the renal 24-hydroxylase and suppression of the 1-hydroxylase enzymes. In addition, serum 24,25(OH)2D3 concentrations were significantly correlated with serum triiodothyronine levels (T3) (r = 0.66, P less than 0.002) before treatment. This may indicate a direct stimulatory effect of T3 on 24-hydroxylase activity. No relationship was found between serum 1,25(OH)2D3 concentrations before therapy and serum T3.

23,24,25-Trihydroxyvitamin D3, 24,25,26-trihydroxyvitamin D3, 24-keto-25-hydroxyvitamin D3, and 23-dehydro-25-hydroxyvitamin D3: new in vivo metabolites of vitamin D3

Four new in vivo metabolites of vitamin D3 were isolated from the blood plasma of chicks given large doses of vitamin D3. The metabolites were isolated by methanol-chloroform extraction and a series of chromatographic procedures. By use of mass spectrometry, ultraviolet absorption spectrophotometry, and specific chemical reactions, the metabolites were identified as 23,24,25-trihydroxyvitamin D3, 24,25,26-trihydroxyvitamin D3, 24-keto-25-hydroxyvitamin D3 and 23-dehydro-25-hydroxyvitamin D3.

[Vitamin D 1980--a stocktaking (author's transl)]

Vitamin D3 and its various metabolites have a central position in the regulation of the calcium- and phosphorus metabolism. Of special importance is 1,25-dihydroxyvitamin D3, which is almost exclusively synthesized in the kidney. Its production is dependent upon several hormonal stimuli and of course occurs normally only in intact renal tissue. The multifactorial regulation of its formation implies that a variety of diseases could cause disturbances in the metabolism of vitamin D. 1,25-dihydroxyvitamin D3 exerts its biological effects not only at the intestine, bone and kidney, but as well presumably in organs as the parathyroid glands, hypophysis, pancreas and skin. 24,25-dihydroxyvitamin D3--although its biological value is still in debate--represents possibly a second vitamin D-hormone of importance. Inspite of intensive research work the metabolic fate of vitamin D3 and its full biological significance is not yet entirely understood.