Demonstration that circulating 1 alpha, 25-dihydroxyvitamin D is loosely regulated in normal children.

The effects of vitamin D, 2.5 mg (100,000 U)/d for 4 d, on serum calcium, serum 25-hydroxyvitamin D (25-OHD) and serum 1 alpha, 25-dihydroxyvitamin D (1 alpha, 25(OH)2D) were compared in 24 normal adults and 12 normal children. The daily dose of vitamin D was 1,500 U/kg body wt in children weighing less than 45 kg. Vitamin D increased mean serum calcium from 9.5 +/- 0.1 to 9.8 +/- 0.1 mg/dl (P less than 0.05), increased mean serum phosphorus from 4.6 +/- 0.1 to 5.0 +/- 0.1 mg/dl (P less than 0.01), increased mean serum 25-OHD from 25 +/- 3 to 34 +/- 4 ng/ml (P less than 0.001), and increased mean serum 1 alpha, 25(OH)2D from 34 +/- 3 to 42 +/- 4 pg/ml (P less than 0.02) in children. In contrast, vitamin D increased mean serum 25-OHD from 18 +/- 2 to 39 +/- 6 ng/ml (P less than 0.001) and did not change mean serum calcium (9.4 +/- 0.1 vs. 9.5 +/- 0.1 mg/dl), mean serum phosphorus (4.0 +/- 0.1 vs. 4.1 +/- 0.1 mg/dl), or mean serum 1 alpha, 25(OH)2D (31 +/- 2 vs. 29 +/- 3 pg/ml) in adults. Mean serum 1 alpha, 25(OH)2D was significantly higher after vitamin D in children than in adults (P less than 0.02). These results provide evidence that circulating 1 alpha, 25(OH)2D is not as tightly regulated in children as it is in adults. This difference in regulation could account in part for the higher values for serum 1 alpha, 25(OH)2D observed in children.     

Evidence that 1,25-dihydroxyvitamin D3 inhibits the hepatic production of 25-hydroxyvitamin D in man

Previous in vitro studies in rachitic rat liver suggested that 1,25-dihydroxyvitamin D inhibits the hepatic production of 25-hydroxyvitamin D (25-OHD). An investigation therefore was carried out in eight normal subjects to determine whether concomitant administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] would alter the response of serum 25-OHD to challenge with vitamin D. In control studies, vitamin D, 100,000 U/d for 4 d, significantly increased mean serum 25-OHD, from 26.3 +/- 2.9 to 66.7 +/- 12.6 ng/ml (P less than 0.01). In contrast, 1,25(OH)2D3, 2 micrograms/d for 4 d, completely prevented an increase in serum 25-OHD in response to the same dose of vitamin D in the same individuals (25.1 +/- 2.2 vs. 27.4 +/- 5.3 ng/ml, NS). In a post-control study in seven of the normal subjects, vitamin D again significantly increased mean serum 25-OHD, from 18.2 +/- 3.1 to 42.8 +/- 4.7 ng/ml (P less than 0.001). In each of the three studies, mean serum calcium, phosphorus, and creatinine did not change and remained within the normal range. Whereas mean urinary calcium did not change in response to vitamin D alone during the 4 d of the two control studies, it increased significantly in the study in which vitamin D and 1,25(OH)2D3 were given together. A dose-response inhibition of the response of serum 25-OHD to vitamin D by 1,25(OH)2D3 was demonstrated in two of the normal subjects. The results provide evidence that 1,25(OH)2D3 inhibits the hepatic synthesis of its precursor 25-OHD in man.     

Evidence for abnormal regulation of circulating 1 alpha,25-dihydroxyvitamin D in patients with sarcoidosis and normal calcium metabolism.

The effects of vitamin D, 2.5 mg (100,000 U)/d for 4 d, on serum calcium, serum 25-hydroxyvitamin D (25-OHD), and serum 1 alpha,25-dihydroxyvitamin D [1 alpha,25(OH)2D] were compared in 17 normal subjects and 6 patients with sarcoidosis who had normocalcemia and no history of hypercalcemia. The diagnosis was confirmed histologically in each of them. Vitamin D increased mean serum 25-PHD from 30 +/- 4 to 99 +/- 15 ng/ml (P < 0.001) and did not change mean serum 1 alpha,25(OH)2D (32 +/- 3 vs. 29 +/- 3 pg/ml) or mean serum calcium (9.5 +/- 0.1 vs. 9.6 +/- 0.1 mg/dl) in the normal subjects. In contrast, vitamin D increased mean serum 25-OHD from 19 +/- 3 to 65 +/- 19 ng/ml (p < 0.05), increased mean serum 1 alpha,25(OH)2D threefold from 40 +/- 7 to 120 +/- 24 pg/ml, and increased mean serum calcium from 9.4 +/- 0.2 to 9.8 +/- 0.2 mg/dl (P < 0.01). There was a significant positive correlation between the serum 1 alpha,25(OH)2D and serum calcium in these individuals (r = 0.663, P < 0.01) but not in the normal subjects. The results (a) provide further evidence for abnormal regulation of circulating 1 alpha,25(OH)2D in sarcoidosis and (b) indicate that the abnormality may exist in patients with normal calcium metabolism. Thus, the defect in vitamin D metabolism in sarcoid apparently is more common than was previously recognized.     

Quantitation of the main metabolites of vitamin D in a single serum sample II. Determination by uv-absorption and competitive protein binding assays

The present report, together with extraction, separation and purification procedures described previously [1], constitutes a sensitive method for determination of vitamin D and its main metabolites in 2 ml of human serum. By omitting vitamin D itself, 25-OHD, 24,25-(OH)₂D, 25,26-(OH)₂D and 1,25-(OH) ₂D could be determined in less than 0.5 ml serum. The quantitation methods described have detection limits of 1.8 pmol for vitamin D, 20 fmol for 25-OHD, 24,25-(OH)₂D and 25,26-(OH)₂D, and 1 fmol for 1,25-(OH)₂D.     

Radioligand receptor assay for 25-hydroxyvitamin D2/D3 and 1 alpha, 25-dihydroxyvitamin D2/D3.

A competitive protein binding assay for measurement of the plasma concentration of 1 alpha, 25-dihydroxyvitamin D3 [1alpha, 25-(OH)2D3] has been extended to include the immediate precursor of this hormone, 25-hydroxyvitamin D3 (25-OHD3). In addition, the assay system is capable of measuring the two metabolic products of ergocalciferol, namely. 25-hydroxyvitamin D2 (25-OHD2) and 1alpha, 25-dihydroxyvitamin D2 [1alpha, 25-(OH)2D2]. The target tissue assay system consists of a high affinity cytosol receptor protein that binds the vitamin D metabolites and a limited number of acceptor sites on the nuclear chromatin. By utilizing a series of chromatographic purification steps, a single plasma sample can be assayed for any of the four vitamin D metabolites either individually or combined. Therefore, the assay procedure allows for both the quantitative and qualitative assessment of the total active vitamin D level in a given plasma sample. To show that the binding assay was capable of measuring 1alpha, 25-(OH)2D2 as well as 1alpha, 25 (OH)2D3, two groups of rats were raised. One group, supplemented with vitamin D3, produced assayable material that represented 1alpha, 25-(OH)2D3. The other group, fed only vitamin D2 in the diet, yielded plasma containing only 1alpha, 25-(OH)2D2 as the hormonal form of the vitamin. The circulating concentrations of the two active sterols were nearly identical (15 ng/100 ml) in both groups, indicating that the competitive binding assay can be used to measure both hormonal forms in plasma. In a separate experiment, 1alpha, 25-(OH)2D2 was generated in an in vitro kidney homogenate system using 25-OHD2 as substrate. Comparison of this sterol with 1alpha, 25-(OH)2D3 in the assay system showed very similar binding curves; the D2 form was slightly less efficient (77%). Comparison of the respective 25-hydroxy forms (25-OHD2 vs. 25-OHD3) at concentrations 500-fold that of 1alpha, 25-(OH)2D3, again suggested that the binding of the D2 metabolite was slightly less efficient (71%). Finally, the assay was employed to measure the total active vitamin D metabolite pools in the plasma of normal subjects and patients with varying degrees of hypervitaminosis D. The normal plasma levels of 25-OHD and 1alpha, 25-(OH)2D measured in Tucson adults were 25-40 ng/ml and 2.1-4.5 ng/100 ml, respectively. Both sterols were predominately (greater than 90%) in the form of vitamin D3 metabolites in this environment. Typical cases of hypervitaminosis D exhibited approximately a 15-fold increase in the plasma 25-OHD concentration, and a dramatic changeover to virtually all metabolites existing in the form of D2 vitamins. In contrast, the circulating concentration of 1alpha, 25-(OH)2D was not substantially enhanced in vitamin D-intoxicated patients. We therefore conclude that hypervitaminosis D is not a result of abnormal plasma levels of 1alpha, 25-(OH)2D but may be cuased by an excessive circulating concentration of 25-OHD.     

Evidence for extrarenal metabolism of 25-hydroxyvitamin D3 in man

Metabolites of vitamin D3 were measured in the circulation of four patients on chronic haemodialysis (three of whom were surgically anephric) before and during daily ingestion of 40 000 i.u. of cholecalciferol. Circulating 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] was measurable, but abnormally low before treatment; its circulating concentration rose in a substrate dependent manner when serum 25-hydroxyvitamin D3 (25-OHD3) increased, but the response was reduced when compared with the normal relationship. Serum 1,25-hydroxyvitamin D3 [1,25-(OH)2D3] and calcidiol lactone (25-OHD3-lactone) were consistently unmeasurable in sera from these patients before administration of cholecalciferol. However, when serum 25-OHD3 rose with treatment, 1,25-(OH)2D3 became detectable in the sera of three of the four patients and 25-OHD3-lactone could be measured in all of them. These data indicate that extrarenal sites of synthesis of 24,25-(OH)2D3, 25-OHD3-lactone and 1,25-(OH)2D3 exist in chronically dialysed patients but require large amounts of substrate to be significant.     

Demonstration of a lack of change in serum 1 alpha,25-dihydroxyvitamin D in response to parathyroid extract in pseudohypoparathyroidism.

Studies were carried out to compare the effects of parathyroid extract (PTE) on serum and urinary calcium (Ca) and phosphorus (P), serum 25-hydroxyvitamin D (25-OHD), serum 24,25-dihydroxyvitamin D (24,25(OH)2D), serum 1 alpha,25-dihydroxyvitamin D (1 alpha,25(OH)2D), and urinary cyclic AMP in two normal subjects, two patients with hypoparathyroidism (HP) and six patients with pseudohypoparathyroidism (PHP), some of whom were on suboptimal treatment with vitamin D. Two of the patients with PHP were studied while on long-term treatment with 1 alpha,25-(OH)2D3. Before PTE, serum 1 alpha, 25(OH)2D was at the lower limit of normal in one patient and was abnormally low in the other five patients. None of these individuals was on treatment with 1 alpha,25(OH)2D3. Serum 25-OHD and 24,25(OH)2D were either increased or at the upper limit of normal in the patients given vitamin D and were normal in the other patients. PTE lowered the serum P and increased the serum 1 alpha,25(OH)2D, serum and urinary Ca, urinary P, and urinary cyclic AMP in the normal subjects and patients with HP. In individual studies, changes in serum 1 alpha,25(OH)2D and serum Ca occurred in parallel before, during, and after PTE. In contrast, PTE had very little effect in the patients with PHP. Whereas there were highly significant positive correlations between serum 1 alpha,25(OH)2D in each of the normal subjects and patients with HP, there were significant correlations in only one of the patients with PHP. An increase in serum Ca in response to PTE was observed in one of the two patients with PHP who were on long-term treatment with 1 alpha,25(OH)2D3. In these individuals, PTE produced only slight increases in serum 1 alpha,25(OH)2D. Serum 25-OHD and 24,25(OH)2D were not changed by PTE in any of the subjects or patients. The results provide evidence that hypocalcemia in HP and PHP arises in part from low circulating 1 alpha,25-(OH)2D, and indicate that the lack of change in serum 1 alpha,25(OH)2D with PTE in patients with PHP is related to impaired renal adenylate cyclase and phosphaturic responses. These and previous results support the idea that diminished renal production of 1 alpha,25(OH)2D, because of a defect in the parathyroid hormone-responsive adenylate cyclase system, may be a contributing factor in the pathogenesis of the abnormal calcium metabolism in PHP.     

Evidence for alteration of the vitamin D-endocrine system in blacks

As compared with values in white subjects, bone mass is known to be increased and urinary calcium to be diminished in black individuals. To evaluate the possibility that these changes are associated with alterations in the vitamin D-endocrine system, an investigation was performed in 12 black subjects, 7 men and 5 women, and 14 white subjects, 8 men and 6 women, ranging in age from 20 to 35 yr. All of them were hospitalized on a metabolic ward and were given a constant daily diet containing 400 mg of calcium, 900 mg of phosphorus, and 110 meq of sodium. Whereas mean serum calcium, ionized calcium, and phosphate were the same in the two groups, mean serum immunoreactive parathyroid hormone (350 +/- 34 vs. 225 +/- 26 pg/ml, P less than 0.01) and mean serum 1,25-dihydroxyvitamin D (1,25(OH)2D) (41 +/- 3 vs. 29 +/- 2 pg/ml, P less than 0.01) were significantly higher, and mean serum 25-hydroxy-vitamin D (25-OHD) was significantly lower in the blacks than in the whites (6 +/- 1 vs. 20 +/- 2 ng/ml, P less than 0.001). Mean urinary sodium and 24-h creatinine clearance were the same in the two groups, whereas mean urinary calcium was significantly lower (101 +/- 14 vs. 166 +/- 13 mg/d, P less than 0.01) and mean urinary cyclic AMP was significantly higher (3.11 +/- 0.47 vs. 1.84 +/- 0.25 nM/dl glomerular filtrate, P less than 0.01) in the blacks. Further, the blacks excreted an intravenous calcium load, 15 mg/kg body weight, as efficiently as the whites (49 +/- 3 vs. 53 +/- 3%, NS). Mean serum Gla protein was lower in blacks than in whites (14 +/- 2 vs. 24 +/- 3 ng/ml, P less than 0.02), and increased significantly in both groups in response to 1,25(OH)2D3, 4 micrograms/d for 4 d. There was a blunted response of urinary calcium to 1,25(OH)2D3 in the blacks, and mean serum calcium did not change. The results indicate that alteration of the vitamin D-endocrine system with enhanced renal tubular reabsorption of calcium and increased circulating 1,25(OH)2D as a result of secondary hyperparathyroidism may contribute to the increased bone mass in blacks. Their low serum 25-OHD is attributed to diminished synthesis of vitamin D in the skin because of increased pigment.     

Impaired 24,25-Dihydroxyvitamin D Production in Anephric Human and Pig

Plasma 25-hydroxyvitamin D and 24, 25-dihydroxy-vitamin D [24,25-(OH)₂D] concentrations were measured in normal and chronically dialyzed anephric humans and pigs. Measurement of the 24, 25-(OH)₂D was preceded by three purification steps involving one Sephadex LH-20 column and two high-pressure liquid chromatographic columns. The final high-pressure liquid chromatography step involved resolution of 25-hydroxy-vitamin D₃-26,23 lactone and 25,26-dihydroxy-vitamin D₂ from 24,25-dihydroxyvitamin D₂ and 24,25-dihydroxyvitamin D₃ [24,25-(OH)₂D₃]. The total 25-hydroxyvitamin D [25-hydroxyvitamin D₂ plus 25-hydroxyvitamin D₃ (25-OHD₃)] was 31.7±3.6 ng/ml in the plasma of eight anephric human subjects and 40.1±3.7 ng/ml in five normal human subjects. Six of the eight anephric patients had undetectable (<0.2 ng/ml) 24,25-(OH)₂D concentrations. Two of the eight patients had very low (0.51 and 0.41 ng/ml), but detectable, 24,25-dihydroxyvitamin D₂. The normal human volunteers had plasma 24,25-(OH)₂D concentrations of 2.8±0.7 ng/ml. Chronically dialyzed anephric and normal pigs were given intramuscular injections of massive amounts (5 × 10⁶ IU) of vitamin D₃ immediately after surgery (day 0) and again on day 7. In anephric pigs, plasma 25-OHD₃ progressively rose from 12±4 ng/ml on day 0 to 705±62 ng/ml on day 10. The 25-OHD₃ concentrations in normal pigs rose from 8±2 ng/ml on day 0 to 439±64 ng/ml on day 10. Plasma 25-OHD₃ was higher in anephrics throughout the experiment, and concentrations were significantly higher (P < 0.05) on days 9 and 10. Plasma 24,25-(OH)₂D₃ concentrations declined progressively in anephric pigs from 3.6±0.6 ng/ml on day 0 to 3.2±0.7 ng/ml on day 2. During days 4-10, plasma 24,25-(OH)₂D₃ was not apparent until plasma 25-OHD₃ was >400 ng/ml. In control pigs, plasma 24,25-(OH)₂D₃ was elevated from 4.3±0.6 ng/ml on day 0 to 178±2.7 ng/ml on day 3. Plasma 24,25-(OH)₂D₃ was significantly higher (P < 0.05) in controls on days 1-8. At the end of the experiment (day 10), 24,25-(OH)₂D₃ concentrations were similar and not significantly different in both groups (87.0±18.4 ng/ml in anephric and 110.3±32.1 ng/ml in normal pigs). The identity of the 24,25-(OH)₂D₃ isolated from anephric pig plasma was confirmed by mass spectroscopy. Our data suggest that anephric humans receiving normal dietary levels of vitamin D₃ have little or no ability to produce 24,25-(OH)₂D. However, we have shown that pigs produce 24,25-(OH)₂D₃ when plasma 25-OHD₃ is extremely high (>400 ng/ml).     

Effects of active vitamin D3 and parathyroid hormone on the serum osteocalcin in idiopathic hypoparathyroidism and pseudohypoparathyroidism.

Serum osteocalcin was measured in patients with idiopathic hypoparathyroidism or pseudohypoparathyroidism, before or during the treatment with active vitamin D3 (1,25(OH)2D3 or 1 alpha OHD3). Serum osteocalcin and plasma 1,25(OH)2D were decreased in 11 patients with idiopathic hypoparathyroidism before treatment (2.8 +/- 1.27 ng/ml, P less than 0.001 and 14.3 +/- 4.27 pg/ml, P less than 0.001, respectively). In 24 patients with idiopathic hypoparathyroidism during the treatment, serum osteocalcin and plasma 1,25(OH)2D were within the normal range (4.5 +/- 0.74 ng/ml and 25.7 +/- 5.69 pg/ml, respectively). In five patients with pseudohypoparathyroidism before treatment, plasma 1,25(OH)2D was decreased (15.6 +/- 10.6 pg/ml, P less than 0.001) but serum osteocalcin was normal (7.8 +/- 1.66 ng/ml). In nine patients with pseudohypoparathyroidism during the treatment with active vitamin D3, serum osteocalcin and plasma 1,25(OH)2D were normal (6.8 +/- 1.47 ng/ml and 27.2 +/- 6.0 pg/ml, respectively). Serum PTH in pseudohypoparathyroidism was increased before treatment (0.70 +/- 0.34 ng/ml, P less than 0.05) and was normal during the treatment (0.50 +/- 0.13 ng/ml). In idiopathic hypoparathyroidism, the active vitamin D3 increased serum osteocalcin without PTH. In pseudohypoparathyroidism, PTH may increase serum osteocalcin or modulate the effect of active vitamin D3 on serum osteocalcin.     

Serum vitamin D metabolites in normal subjects after phototherapy

The serum concentrations of the major vitamin D metabolites (25-hydroxyvitamin D(2 + 3) (25-OH-D), 1,25-dihydroxyvitamin D(2 + 3) (1,25-(OH)2-D), and 24,25-dihydroxyvitamin D(2 + 3) (24,25-(OH)2-D) were studied in 22 healthy male volunteers before and after one or two treatments with whole body UVB or PUVA in conventional doses. The effect of UVB was investigated in the autumn and early spring, whereas the effect of PUVA was investigated only in the autumn. The pre-treatment values of two metabolites were significantly reduced in the spring compared to the autumn level (25-OH-D: 14.0 ng/ml versus 22.0 ng/ml, P less than 0.02; and 24,25-(OH)2-D: 1.23 ng/ml versus 2.74 ng/ml, P less than 0.01), whereas the serum concentration of 1,25-(OH)2-D was not significantly reduced in the spring. After UVB, a small, but not significant, rise in all metabolites was observed in the spring, whereas virtually no changes were measured after UVB or PUVA in the autumn. We conclude that UVB and PUVA do not lead to harmful concentrations of vitamin D metabolites in the blood of healthy subjects.     

Evidence for alteration of the vitamin D-endocrine system in obese subjects.

Serum immunoreactive parathyroid hormone (PTH) is increased in obese as compared with nonobese subjects and declines with weight loss. To determine whether alteration of the vitamin D-endocrine system occurs in obesity and whether ensuing secondary hyperparathyroidism is associated with a reduction in urinary calcium, a study was performed in 12 obese white individuals, five men and seven women, and 14 nonobese white subjects, eight men and six women, ranging in age from 20 to 35 yr. Body weight averaged 106 +/- 6 kg in the obese and 68 +/- 2 kg in the nonobese subjects (P less than 0.01). Each of them were hospitalized on a metabolic ward and were given a constant daily diet containing 400 mg of calcium and 900 mg of phosphorus. Whereas mean serum calcium, serum ionized calcium, and serum phosphorus were the same in the two groups, mean serum immunoreactive PTH (518 +/- 48 vs. 243 +/- 33 pg/ml, P less than 0.001), mean serum 1,25-dihydroxyvitamin D [1,25(OH)2D] (37 +/- 2 vs. 29 +/- 2, P less than 0.01), and mean serum Gla protein (33 +/- 2 vs. 24 +/- 2 ng/ml, P less than 0.02) were significantly higher, and mean serum 25-hydroxyvitamin D (25-OHD) (8 +/- 1 vs. 20 +/- 2 ng/ml, P less than 0.001) was significantly lower in the obese than in the nonobese men and women. Mean urinary phosphorus was the same in the two groups, whereas mean urinary calcium (115 +/- 10 vs. 166 +/- 13 mg/d, P less than 0.01) was significantly lower, and mean urinary cyclic AMP (3.18 +/- 0.43 vs. 1.84 +/- 0.25 nM/dl GF, P less than 0.01) and creatinine clearance (216 +/- 13 vs. 173 +/- 6 liter/d, P less than 0.01) were significantly higher in the obese than in the nonobese individuals. There was a significant positive correlation between percentage of ideal body weight and urinary cyclic AMP (r = 0.524, P less than 0.01) and between percentage of ideal body weight and serum immunoreactive PTH (r = 0.717, P less than 0.01) in the two groups. The results provide evidence that alteration of the vitamin D-endocrine system in obese subjects is characterized by secondary hyperparathyroidism which is associated with enhanced renal tubular reabsorption of calcium and increased circulating 1,25(OH)2D. The reduction of serum 25-OHD in them is attributed to feedback inhibition of hepatic synthesis of the precursor by the increased serum 1,25(OH)2D.     

The role of 1 alpha, 25-dihydroxyvitamin D in the mediation of intestinal hyperabsorption of calcium in primary hyperparathyroidism and absorptive hypercalciuria.

The cuase for the intestinal hyperabsorptionof calcium (Ca) in various forms of hypercalciurias was explored by a careful measurement of plasma 1 alpha, 25-dihydroxycholecalciferol [1 alpha, 25-(OH)I D] and by an assessment of intestinal Ca absorption and of parathyroid function. In 18 cases of primary hyperparathyroidism (PHPT), the mean plasma concentration of 1 alpha, 25-(OH)2D was significantly increased (4.9 +/- 2.2 SD ng/dl vs. 3.4 +/- 0.9 ng/dl for the control group), and was significantly correlated with fractional Ca absorption (alpha) (r = 0.80, P less than 0.001). Plasma 1 alpha, 25-(OH)2D was also correlated with urinary Ca (P less than 0.05), but not with serum Ca or phosphorus (P), P clearance, urinary cyclic AMP, or serum immunoreactive parathyroid hormone. In 21 cases of absorptive hypercalciuria (AH), plasma 1 alpha, 25-(OH)2D was elevated in one-third of cases, and the mean value of 4.5 +/- 1.1 ng/dl was significantly higher than that of the control group (P less than 0.01). Since relative hypoparathyroidism may be present, the normal absolute value of plasma 1 alpha, 25-(OH)2D, found in two-thirds of cases of AH, may be considered to be inappropriately high. Moreover, in the majority of cases of AH, the data points relating plasma 1 alpha, 25-(OH)2D and alpha fell within 95% confidence limits of values found in non-AH groups (including PHPT). The results suggest that the intestinal hyperabsorption of Ca in PHPT aw AH may be vitamin D dependent. However, the disturbance in vitamin D metabolism may not be the sole cause for the high Ca absorption in AH, since in some patients with AH, the intestinal Ca absorption appears to be inapp     

Chronic 1,25-dihydroxyvitamin D3 administration in the rat reduces the serum concentration of 25-hydroxyvitamin D by increasing metabolic clearance rate.

Administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] can lower the serum concentration of 25-hydroxyvitamin (25-OH-D). To determine if 1,25(OH)2D3 lowers serum 25-OH-D by increasing clearance or reducing production, we directly measured the metabolic clearance rate (MCR) of 25-OH-D in rats chronically infused with 1,25(OH)2D3. Chronic 1,25(OH)2D3 administration (0 to 75 pmol/d) reduced, in a time- and dose-dependent fashion, the serum concentrations of 25-OH-D3 and 24,25(OH)2D3 from 18 +/- 2 to 9 +/- 1 ng/ml and from 4.8 +/- 0.7 to 1.3 +/- 0.3 ng/ml, respectively, and increased sevenfold the in vitro conversion of 25-OH-D to 24,25(OH)2D3 by kidney homogenates. The reduction in serum 25-OH-D3 was completely accounted for by an increase in MCR. No change in production occurred. The influence of 1,25(OH)2D3 on serum 25-OH-D3 and 24,25(OH)2D3 was shown not to be dependent on induction of hypercalcemia. These data suggest that chronic 1,25(OH)2D3 administration lowers serum 25-OH-D by increasing the metabolic clearance of 25-OH-D3 and not by decreasing its production.     

25-hydroxycholecalciferol stimulation of muscle metabolism.

Intact diaphragms from vitamin D-deficient rats were incubated in vitro with [3H]leucine. Oral administration of 10 mug (400 U) of cholecalciferol 7 h before incubation increased leucine incorporation into diaphragm muscle protein by 136% (P less than 0.001) of the preparation from untreated animals. Nephrectomy did not obliterate this response. ATP content of the diaphragm muscle was also enhanced 7 h after administration of the vitamin. At 4 h after administration of cholecalciferol, serum phosphorus concentration was reduced by 0.7 mg/100 ml (P less than 0.025) and the rate of inorganic 32PO4 accumulation by diaphragm muscle was increased by 18% (P less than 0.025) over the untreated animals. Increasing serum phosphate concentration of the vitamin D-deficient animals by dietary supplementation with phosphate for 3 days failed to significantly enhance leucine incorporation into protein. However, supplementation of the rachitogenic, vitamin D-deficient diet with phosphorus for 3 wk stimulated the growth of the animal and muscle ATP levels. This increase in growth and muscle ATP content attributed to the addition of phosphorus to the diet was less than the increase in growth and muscle ATP levels achieved by the addition of both phosphorus and vitamin D to the diet. To eliminate systemic effects of the vitamin, the epitrochlear muscle of the rat foreleg of vitamin D-depleted rats was maintained in tissue culture. Addition of 20 ng/ml of 25-hydroxycholecalciferol (25-OHD3) to the medium enhanced ATP content of the muscle and increased leucine incorporation into protein. Vitamin D3 at a concentration of 20 mug/ml and 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) at a concentration of 500 pg/ml were without effect. Analysis of muscle cytosol in sucrose density gradients revealed a protein fraction which specifically bound 25-OHD3 and which demonstrated a lesser affinity for 1,25-(OH)2D3. These studies suggest that 25-OHD3 may influence directly the intracellular accumulation of phosphate by muscle and thereby play an important role in the maintenance of muscle metabolism and function.     

Evidence that Increased Circulating 1α,25-Dihydroxyvitamin D is the Probable Cause for Abnormal Calcium Metabolism in Sarcoidosis

Mean plasma 1(alpha),25-dihydroxyvitamin D[1(alpha),25(OH)(2)D] was significantly increased and serum parathyroid hormone was suppressed in three patients with sarcoidosis and hypercalcemia. Prednisone lowered the mean plasma 1(alpha),25(OH)(2)D to normal range and corrected the hypercalcemia. To elucidate the mechanism for the increased sensitivity to vitamin D in this disorder, the effects of orally-administered vitamin D(2) were determined in seven normal subjects, four patients with sarcoidosis and normal calcium metabolism and three patients with sarcoidosis and a history of hypercalcemia who were normocalcemic when studied. Serum and urinary calcium, serum 25-hydroxyvitamin D (25-OHD), plasma 1(alpha),25(OH)(2)D and, in some studies, calcium balance were measured. Vitamin D(2), 250 mug a day for 12 d, produced little, if any, change in mean plasma 1(alpha),25(OH)(2)D and in urinary calcium in the normals and in the patients with normal calcium metabolism. In contrast, vitamin D(2) produced increases in plasma 1(alpha),25(OH)(2)D from concentrations which were within the normal range (20-55 pg/ml) to abnormal values and increased urinary calcium in two patients with abnormal calcium metabolism. In an abbreviated study in the third patient, vitamin D(2), 250 mug a day for 4 d, also increased plasma 1(alpha),25(OH)(2)D abnormally from a normal value. There was a highly significant correlation between plasma 1(alpha),25(OH)(2)D and urinary calcium. Serum 25-OHD and serum calcium remained within the normal range in all subjects and patients. These findings provide evidence that the defect in calcium metabolism in sarcoidosis probably results from impaired regulation of the production and(or) degradation of 1(alpha),25(OH)(2)D. Prednisone may act to correct the abnormal calcium metabolism by reducing circulating 1(alpha),25(OH)(2)D.     

Extrarenal synthesis of 1,25-dihydroxyvitamin D: sensitivity to glucocorticoid treatment

The response of circulating 1,25-dihydroxyvitamin D [1,25-(OH)2D] to challenge with vitamin D treatment both before and after 7-10 days of prednisone therapy (25 mg/day) was investigated in five anephric subjects, six patients with chronic renal failure (CRF), two patients with vitamin D intoxication and four patients with hypoparathyroidism. In anephric subjects serum 25-hydroxyvitamin D [25-(OH)D] rose from 58 +/- 48 (SD) to 377 +/- 221 (SD) nmol/l after administration of 150 micrograms of 25-(OH)D3 for 1 month. Serum 1,25-(OH)2D, which was barely detectable in only two out of five patients under basal conditions, rose to 30 +/- 21 pmol/l after 2 weeks of therapy with 25-(OH)D3, but fell to 10 +/- 5 pmol/l during prednisone treatment. In CRF patients circulating 1,25-(OH)2D rose from 37 +/- 24 to 58 +/- 24 pmol/l during 25-(OH)D3 therapy, but fell to 41 +/- 31 pmol/l during prednisone treatment. In two patients with rheumatoid arthritis, hypercalcaemia due to vitamin D intoxication was associated with raised levels of 1,25-(OH)2D (288 and 317 pmol/l). Administration of prednisone resulted in suppression of 1,25-(OH)2D levels (132 and 96 pmol/l respectively) and reduction of serum calcium to within the normal range. In the hypoparathyroid patients prednisone therapy did not affect circulating 25-(OH)D levels but serum 1,25-(OH)2D fell from 192 +/- 42 to 117 +/- 23 pmol/l and serum calcium from 2.41 +/- 0.21 to 2.20 +/- 0.05 mmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impaired vitamin D metabolism with aging in women. Possible role in pathogenesis of senile osteoporosis.

Calcium absorption decreases with aging, particularly after age 70 yr. We investigated the possibility that this was due to abnormal vitamin D metabolism by studying 10 normal premenopausal women (group A), 8 normal postmenopausal women within 20 yr of menopause (group B), 10 normal elderly women (group C), and 8 elderly women with hip fracture (group D) whose ages (mean +/- SD) were 37 +/- 4, 61 +/- 6, 78 +/- 4, and 78 +/- 4 yr, respectively. For all subjects, serum 25-hydroxyvitamin D [25(OH)D] did not decrease with age, but serum 1,25-dihydroxyvitamin D [1,25(OH)2D], the physiologically active vitamin D metabolite, was lower (P = 0.01) in the elderly (groups C and D; 20 +/- 3 pg/ml) than in the nonelderly (groups A and B; 35 +/- 4 pg/ml). The increase of serum 1,25(OH)D after a 24-h infusion of bovine parathyroid hormone fragment 1-34, a tropic agent for the enzyme 25(OH)D 1 alpha-hydroxylase, correlated inversely with age (r = -0.58; P less than 0.001) and directly with glomerular filtration rate (r = 0.64; P less than 0.001). The response was more blunted (P = 0.01) in elderly patients with hip fracture (13 +/- 3 pg/ml) than in elderly controls (25 +/- 3 pg/ml). We conclude that an impaired ability of the aging kidney to synthesize 1,25(OH)2D could contribute to the pathogenesis of senile osteoporosis.     

Vitamin D metabolites in diabetic patients: decreased serum concentration of 24,25-dihydroxyvitamin D

In order to elucidate if changes in vitamin D metabolism play a role for diabetic bone loss, the serum concentrations of the major vitamin D metabolites were studied in 26 adult male ambulatory insulin-treated diabetics, selected to have normal renal function and a duration of diabetes below 11 years. The patients were studied during usual metabolic control and exhibited wide ranges of hyperglycaemia and glycosuria. The serum concentrations of the major metabolites of vitamin D, 25-hydroxyvitamin D(2 + 3) (25OHD), 24,25-dihydroxyvitamin D(2 + 3) (24,25(OH)2D), and 1,25-dihydroxyvitamin D(2 + 3) (1,25(OH)2D), were measured in diabetics, and in age and sex matched controls. The diabetics had slightly decreased serum levels of 25OHD (42.0 nmol/l versus 55.5 nmol/l in normals, P less than 0.05), markedly decreased serum levels of 24,25(OH)2D (2.98 nmol/l versus 5.91 nmol/l, P less than 0.01), but serum levels of 1,25(OH)2D were virtually normal (64.2 pmol/l versus 68.3 pmol/l, ns). The close correlation between serum concentrations of 25OHD and 24,25(OH)2D observed in the normal subjects, was absent in the diabetics. There were no correlations between the serum levels of any of the vitamin D metabolites and the measured indices of glucose and calcium metabolism. It is concluded that insulin-dependent diabetic patients demonstrate definite alterations in serum levels of vitamin D metabolites, the significance of which remains unknown at present.     

The metabolism of a physiological dose of radioactive cholecalciferol (vitamin D3) to its hydroxylated metabolites in man

1. The metabolism of an intravenous pulse-dose of 65 nmol (25 microgram) of double-isotope-labelled cholecalciferol has been studied in 28 individuals. The subjects comprised 19 with serum concentrations of 25-hydroxycalciferol (25-(OH)D) less than or equal to 25 nmol/l, of whom 12 had clinical osteomalacia, and nine with serum 25-(OH)D > 25 nmol/l (30-125 nmol/l). 2. The concentrations in serum of radioactive cholecalciferol, 25-hydroxycholecalciferol (25-(OH)D3) and the three dihydroxylated metabolites: 1,25-, 24,25- and 25,26-dihydroxycholecalciferol (1,25-(OH)2D3, 24,25-(OH)2D3 and 25,26-(OH)2D3) were measured for up to 10 days after the injection. 3. The temporal relationships between the formation of individual radioactive metabolites and factors apparently influencing their production are described and their molar concentrations in serum calculated. 4. Formation of radioactive 1,25-(OH)2D3 was detectable only in vitamin D-deficient subjects. Between individuals, its maximum serum concentration was correlated significantly and inversely with serum calcium but with not other measured variable. In the individual, concentrations of radioactive serum 1,25-(OH)2D3 varied directly with radioactive serum 25-(OH)D3. 5. The failure to detect formation of radioactive 1,25-(OH)2D3 in vitamin D-replete subjects suggests that current estimates of the daily turnover of the hormone in the normal individual may be severalfold too high. 6. Radioactive 25,26-(OH)2D3 was produced rapidly by all subjects and in greater amounts by vitamin D-deficient individuals. Between subjects and in the individual its concentration in serum correlated only with the radioactive serum 25-(OH)D3. Production of this metabolite appeared to be unregulated and dependent solely on the concentration of its precursor. 7. In vitamin D-replete subjects, production of 24,25-(OH)2D3 was also apparently determined by precursor concentration. In vitamin D-depleted subjects, production of radioactive 24,25-(OH)2D3 was variably delayed for up to or more than 10 days. 8. There appeared to be a constraint on the quantitative hepatic production of 25-(OH)D which is not explained by simple feed-back inhibition. 9. If sterols other than 1,25-(OH)2D3 are required to initiate the mineralization of osteomalacic bone, after correction of vitamin D deficiency in man, 25-(OH)D3 and 25,26-(OH)2D3 are produced sufficiently rapidly to meet this hypothetical requirement, but not 24,25-(OH)2D3.