1,25-Dihydroxyvitamin D in biological fluids: a simplified and sensitive assay

The effects of 24R,25-dihydroxycholecalciferol and of 1 alpha,25-dihydroxycholecalciferol on ornithine decarboxylase activity and on DNA synthesis in the epiphysis and diaphysis of rat bone and in the duodenum

The effect of cholecalciferol metabolites on ornithine decarboxylase activity and on DNA synthesis in developing long bones was investigated in vitamin D-depleted rats. In the epiphysis there was a 6.4-fold increase in ornithine decarboxylase activity 5 h after a single injection of 24R,25-dihydroxycholecalciferol but not of 24S,25-dihydroxycholecalciferol or other vitamin D metabolites. In comparison, in the diaphysis and duodenum, 1 alpha,25-dihydroxycholecalciferol, but not other vitamin D metabolites, caused a 3-3.5-fold increase in the enzyme activity. The enzyme activity in the tissues examined attained a maximal value at 5 h after the injection of the metabolites. The activity of ornithine decarboxylase in the epiphysial region increased dose-dependently as the result of a single injection of 24R,25-dihydroxycholecalciferol and attained a maximal value at a dose between 30 and 3000 ng. In addition, administration of 24R,25-dihydroxycholecalciferol, but not 24S,25-dihydroxycholecalciferol or other metabolites, caused within 24 h a 1.7-2.0-fold increase in [3H]thymidine incorporation into DNA of the epiphyses of tibial bones. In comparison, 1 alpha,25-dihydroxycholecalciferol caused a 1.5-fold increase in [3H]thymidine incorporation into DNA of the diaphyses and of the duodenum. The present data indicate that 24R,25-dihydroxycholecalciferol is involved in the regulation of epiphyseal growth, whereas 1 alpha,25,dihydroxycholecalciferol stimulates the proliferation of cells in the diaphysis of long bones and in the intestinal mucosa.

Individual quantitation of vitamin D2, vitamin D3, 25-hydroxyvitamin D2, and 25-hydroxyvitamin D3 in human milk

Extraction, lipid-reduction, and chromatographic methods suitable for the resolution and subsequent quantitation of vitamin D2, vitamin D3, 25-hydroxyvitamin D2, and 25-hydroxy-vitamin D3 from human milk are described. This procedure utilizes a methanol:methylene chloride extraction, precipitation of unwanted lipids with cold methanol and ether, backwash with alkaline buffer, silica Sep-Pak preparative chromatography, normal- and reverse-phase high-performance liquid chromatography with final quantitation of the antirachitic sterols by competitive protein binding assay. The described assay was used to determine these antirachitic sterols in milk from women receiving various supplements of vitamin D or undergoing ultraviolet phototherapy.

Do tissues other than the kidney produce 1,25-dihydroxyvitamin D3 in vivo? A reexamination

Recent experiments have shown that 1,25-dihydroxyvitamin D3-like material is produced in cultured nonrenal cells and may be present in the sera of anephric patients. We reexamined the question of whether 1,25-dihydroxyvitamin D3 can be synthesized extrarenally in the rat in vivo. To intact, sham-operated, ureter-ligated, or acutely nephrectomized vitamin D-deficient rats raised on a diet normal in calcium and phosphorus, we gave a physiologic dose of high-specific-activity 25-hydroxy-[3H]vitamin D3 (3.6-3.8 microCi; approximately equal to 25 pmol per rat). Twenty-four hours later we examined their tissues and plasma for the presence of radiolabeled 1,25-dihydroxyvitamin D3. Large amounts of radioactivity that behaved chromatographically as identical with authentic 1,25-dihydroxyvitamin D3 were present in the plasma, bone, and intestine of the intact, sham-operated, or ureter-ligated rats. However, no radioactivity eluting in a manner similar to 1,25-dihydroxyvitamin D3 was found in plasma, bone, or intestine of acutely nephrectomized rats. We conclude that, in the acutely nephrectomized living rat, 1,25-dihydroxyvitamin D3 is not present in plasma, bone, or intestine in quantities detectable by the sensitive techniques we have used. No conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 was observed during a 24-hr period after nephrectomy of vitamin D-deprived rats. This fact casts doubt upon the significance of the in vitro production of 1,25-dihydroxyvitamin D3 by nonrenal cells as an in vivo phenomenon.

A test of 1,25-dihydroxyvitamin D3 secretory capacity in normal subjects for application in metabolic bone diseases

The circulating concentration of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] varies in man in response to a variety of physiological stimuli. In pathological states, random plasma 1,25-(OH)2D3 levels reflect the sum of the underlying pathological process and the homeostatic corrections to those perturbations. Hence random 1,25-(OH)2D3 levels do not allow differentiation as to whether changes in 1,25-(OH)2D3 production or metabolism are primary or secondary to other changes. Similarly random levels do not provide much insight into the reserve of 1,25-(OH)2D3 secretory capacity. We have developed a single infusion of parathyroid extract (PTE) as a test of 1,25-(OH)2D3 secretory capacity. Normal responses in twelve males and six females consisted of an increase in plasma 1,25-(OH)2D3 levels of 78% over basal in the male and 77% over basal in the female subjects. The peak response occurred 18-22 h after the PTE was administered. This test of 1,25-(OH)2D3 secretory capacity should prove useful in the evaluation of patients with metabolic bone disease. As part of this study, the diurnal plasma 1,25-(OH)2D3 level was examined in six males and seven females. There was no significant diurnal rhythm between 8.00 and 22.00 h.

Calcium-phosphate metabolism in autosomal recessive idiopathic strio-pallido-dentate calcinosis and Cockayne's syndrome

In three siblings with autosomal recessive idiopathic strio-pallido-dentate calcinosis (SPDC) and in three other siblings with Cockayne's syndrome (CS) studies on plasma values of calcium and phosphate, intestinal calcium absorption, radiograms of the hands and studies on the influence of parathyroid hormone (PTH) on the renal threshold for phosphate revealed no abnormalities. In one of the SPDC patients and one of the CS patients the effect of PTH on the cyclic adenosine monophosphate (cAMP) concentrations in urine and cerebrospinal fluid (CSF) were determined. In both a normal response of urinary cAMP was noted. In the CS patient the response of CSF cAMP was also normal. The SPDC patient, however, had a significantly decreased response of CSF cAMP. It is suggested that a decreased sensitivity of the cerebral adenylate cyclase complex is involved in the etiology of autosomal recessive idiopathic SPDC. Subsequently this disorder could be considered as cerebral pseudohypoparathyroidism. The etiology of CS remains unknown.

Minerals, vitamin D, and parathyroid hormone in continuous ambulatory peritoneal dialysis

The effects of continuous ambulatory peritoneal dialysis on parathyroid hormone (PTH) and mineral metabolism were evaluated in ten patients. Utilizing a PTH radioimmunoassay, which measures both intact hormone and carboxyl-terminal PTH fragments, it was found that the mean clearance of immunoreactive parathyroid hormone was 1.5 +/- 0.73 ml/min (SEM) yielding a daily net removal of 13.6 +/- 3.2% of estimated total extracellular parathyroid hormone. Gel electrophoresis of the dialysate revealed the presence of both intact parathyroid hormone and fragments in a similar pattern to that of peripheral plasma. Normal levels of 25-(OH) vitamin D and vitamin D binding protein were observed prior to the initiation of continuous ambulatory peritoneal dialysis and following 6 months of treatment. Timed dialysate collections (N = 93) demonstrated a daily calcium influx of only 9.9 +/- 9.7 mg. The daily removal of phosphorus was 308.4 +/- 15.5 mg. Despite elevated serum magnesium levels in all patients, the net daily removal was inadequate (31.2 +/- 15.5 mg). It was concluded that: (1) Unlike chronic hemodialysis, continuous ambulatory peritoneal dialysis removes significant amounts of parathyroid hormone. (2) Normal 25-(OH) vitamin D and vitamin D binding protein levels are maintained with continuous ambulatory peritoneal dialysis despite large protein losses. (3) Substantial amounts of phosphorus are removed with continuous ambulatory peritoneal dialysis but not to an extent that precludes use of phosphorus binders. (4) Dialysate containing lower magnesium and possibly higher calcium concentrations should be made available to improve mineral homeostasis.

Vitamin D metabolites in human milk

The concentrations of unconjugated 25-OHD, 24, 25(OH)2D, and 1,25(OH)2D were measured in human milk by competitive protein-binding radioassays following successive preparative Sephadex LH-20 chromatography and HPLC. The mean (+/- SE) concentration of 25-OHD was 0.37 +/- 0.03 ng/ml, of 24,25(OH)2D was 24.8 +/- 1.9 pg/ml, and of 1,25(OH)2D was 2.2 +/-0.1 pg/ml. The concentration of 25-OHD3 in milk as determined by HPLC and UV detection at 254 nm was 0.27 +/- 0.08 ng/ml. The milk concentrations of vitamin D metabolites did not correlate with the maternal serum 25-OHD levels. The total amounts of unconjugated vitamin D metabolites correspond to the known low bioassayable vitamin D antirachitic activity in human milk.

Evidence for 24,25-dihydroxycholecalciferol receptors in long bones of newborn rats

Several reports have appeared that suggest that 24,25-dihydroxycholecalciferol has a possible biological role in bone formation. We have utilized competition studies, saturation analysis, sucrose-density-gradient sedimentation and DEAE-cellulose chromatography to demonstrate that long bones of vitamin D-depleted newborn rats contain cytoplasmic and possibly nuclear receptors that bind 24,25-dihydroxycholecalciferol with specificity and high affinity (Kd = 1.79 nM). Sucrose-density-gradient analysis of the cytoplasmic 24,25-dihydroxycholecalciferol-binding component showed a single binding macromolecule for 24,25-dihydroxycholecalciferol with a sedimentation coefficient of 3.1 S. DEAE-cellulose chromatography showed a [3H]24,25, dihydroxycholecalciferol-macromolecular complex that binds to DEAE-cellulose and elutes between 0.15 and 0.21 M-KCl. The finding of 24,25-dihydroxycholecalciferol receptors in long bones of newborn rats suggests a possible involvement of 24,25-dihydroxycholecalciferol in the metabolism of developing skeletal tissues.

Reduced plasma concentration of 1,25-dihydroxyvitamin D in children with moderate renal insufficiency

We measured the plasma concentration of 1,25-dihydroxyvitamin D (1,25(OH)2D) in 39 children comprising three groups; eight with moderate renal insufficiency (GFR of 25 to 50 ml/min/1.73 M2, seven of whom had tubulointerstitial disease), eight with severe renal insufficiency (on chronic hemodialysis), and 23 healthy control subjects. The mean plasma concentration of 1,25-(OH)2D was reduced by some 40% (P less than 0.002) in the children with moderate renal insufficiency, and by some 80% (P less than 0.001) in the children with severe renal insufficiency. In the children with moderate renal insufficiency, the reduced concentration of 1,25-(OH)2D was associated with increased serum concentrations of immunoreactive parathyroid hormone (iPTH) and reduced serum concentrations of 1,25-(OH)2D was associated with increased serum concentrations of immunoreactive parathyroid hormone (iPTH) and reduced serum concentrations of calcium and phosphorus. When analyzed over the range of renal function from normal through severely impaired, values of iPTH correlate inversely and significantly with those of 1,25-(OH)2D. Growth was impaired in four of the eight children with moderate renal insufficiency. The results of the current study suggest that in children with moderate renal insufficiency, a reduction in the renal synthesis and in the plasma concentration of 1,25-(OH)2D may be important pathogenetic events in disordered metabolism of calcium and phosphorus, including secondary hyperparathyroidism.

A simple method for the isolation of vitamin D metabolites from plasma extracts

A simple method has been developed using 'SEP-PAK' disposable silica cartridges to separate the major endogenous vitamin D metabolites, namely vitamin D3, 25-hydroxy vitamin D3 (25OHD3), 1,25 dihydroxy vitamin D3 (1.25 (OH)2D3) and 24,25 dihydroxyvitamin D3 (24,25 (OH) 2D3). After extraction of plasma in isopropanol-toluene (25:75) the dried extract is reconstituted in hexane; this is applied to a SEP-PAK column, and stepwise elution carried out under gravity with 0.1 divided by isopropanol in hexane (neutral lipids), 1% isopropanol in hexane (D3), 3 divided by isopropanol in hexane (25OHD3), 3.125 divided by ethanol in dichloromethane (24,25 (OH) 2D3) and 50 divided ethanol in toluene (1, 25(OH) 2D3). Complete separation of these D3 metabolites is achieved by this process and up to 40 samples can be handled at one time. If combined with a suitable ligand binding assay, the system appears to be suitable for preparation of samples prior to the routine assay of vitamin D metabolites.

Circulating vitamin D metabolite concentrations in childhood renal diseases

Vitamin D metabolites were measured in children, untreated with glucocorticoids, who had renal disease. Two groups were defined in relation to endogenous creatinine clearance values: those with impaired clearance , 0 to 48 ml/min per 1.73 m2; and those with unimpaired clearance, 75 to 150 ml/min per 1.73 m2. Serum 1.25(OH)2D was 16 +/- (SD) 12 pg/ml in impaired patients (N=24) and 48 +/- 16 pg/ml in unimpaired patients (N=18). The latter level is not different from healthy childhood controls (43 +/- 12 pg/ml; N=194). Serum samples of 25(OH)D2 and D3 were comparable in each group and not different from control values of 33.2 +/- 10.3 ng/ml. Serum 24,25(OH)2D was 0.6 +/- (SD) 0.14 ng/ml in patients with a clearance of less than 13 ml/min per 1.73 m2, 1.39 +/- 0.54 ng/ml in those with a clearance of 18 to 48 ml/min per 1.73 m2, and 152 +/- 0.91 ng/ml in patients without an impairment of clearance. Only patients with the lowest clearance had values different from control values of 1.70 +/- 0.57 ng/ml. In our study we suggest that a significant reduction in 24,25(OH)2D and 1,25(OH)2D are found at low clearance values in children with tubulointerstitial disease. Our study further suggests that a reduction in renal tubular mass is important in accounting for these changes in vitamin D metabolite values.

Comparison of plasma concentrations of vitamin D and its metabolites in young and aged domestic animals

1. Vitamin D and its metabolites were measured in the plasma of five species of rural domestic animals. 2. Concentration of 1,25-dihydroxyvitamin D was higher (P less than 0.01) in young animals (range 24-118 pg/ml, means +/- SD = 72.0 +/- 30.0) than in adult animals (range 14-67 pg/ml, means +/- SD = 40.2 +/- 22.6). 3. 25-Hydroxyvitamin D3-26,23 lactone was present only in the chick and the pig. 4. Unsheared sheep appeared to be inefficient utilizers of the photochemical conversion of 7-dehydrocholesterol to vitamin D3. 5. Conversion of 25-hydroxyvitamin D to 24,25-dihydroxyvitamin D was most efficient in species with high plasma phosphorus concentrations (pig, sheep).

Deficient production of 1,25-dihydroxyvitamin D in elderly osteoporotic patients

There is uncertainty about the adequacy of renal secretion of 1,25-dihydroxyvitamin D(1,25-(OH)2-D) in elderly patients with osteoporosis. To investigate this uncertainty, we stimulated secretion of 1,25-(OH)2-D with a 24-hour intravenous infusion of synthetic human parathyroid hormone fragment 1-34 and compared the results in normal young adults and elderly patients with untreated osteoporosis. Serum levels of 1,25-(OH)2-D were similar in both groups (49 +/- 10 and 42 +/- 9 pg per milliliter [116 +/- 24 and 99 +/- 21 pmol per liter]) before the infusion. However, during the 24-hour infusion, serum levels nearly doubled (P less than 0.01) in the normal volunteers but did not change significantly in the patients. Serum ionized calcium increased and serum inorganic phosphate decreased similarly in both groups during the infusion (P less than 0.05). Although the present study does not establish whether deficient 1,25-(OH)2-D secretory reserve is an effect of age or of osteoporosis, it is possible that such a deficiency will explain the inability of elderly osteoporotic patients to adapt to the low-calcium diets common in this age group. If so, this phenomenon may play a part in the pathogenesis of age-related osteoporosis.

23,25-Dihydroxyvitamin D3: a natural precursor in the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone

To elucidate the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone, two known metabolites of 25-hydroxyvitamin D3--23,25-dihydroxyvitamin D3 and 25,26-dihydroxyvitamin D3--were incubated individually with kidney homogenate prepared from vitamin D-supplemented chickens, a preparation known to produce the lactone from 25-hydroxyvitamin D3. The 25-hydroxyvitamin D3-26,23-lactone produced in vitro was then separated, purified, identified, and quantitated by consecutive straight-phase and reverse-phase high-performance liquid chromatography. 23,25-Dihydroxyvitamin D3 is a far better substrate for production of 25-hydroxyvitamin D3-26,23-lactone than is 25,26-dihydroxyvitamin D3. Production of lactone is highly selective for the natural 23(S)-hydroxy-23,25-dihydroxyvitamin D3 while both epimers of 25,26-dihydroxyvitamin D3 resulted in small amounts of product comigrating with natural lactone. It appears that 23(S),25-dihydroxyvitamin D3, but not 25,26-dihydroxyvitamin D3, is a natural precursor in the synthesis of 25-hydroxyvitamin D3-26,23-lactone; this result also implies that the configuration of the lactone at C-23 is S.

A sensitive competitive protein binding assay for vitamin D in plasma

A sensitive protein binding assay for vitamin D is described. The vitamin D3 was extracted from plasma with diethyl ether and methylene chloride. The lipid extract was purified in Sephadex LH-20 followed by Lipidex 5000 and finally by high pressure liquid chromatography on a Zorbax Sil column (0.79 x 25 cm) developed in 0.25:99.75 isopropanol: methylene chloride. The vitamin D fraction was collected and quantitated by competitive protein binding assay with a 1/50,000 dilution of sheep plasma in 0.05 M potassium phosphate buffer (pH 7.5) containing 0.01% gelatin. [H3]-25-Hydroxyvitamin D3 was used as a radioactive tracer in the assay. We found that under these conditions, sheep plasma had equal affinity for vitamin D2 and vitamin D3 and could detect as little as 0.1 ng of vitamin D. When rat, cow, or human plasma was substituted for the sheep plasma, the decline in sensitivity to vitamin D2 was fivefold to tenfold. With this assay, we found excellent agreement (r = 0.98) between the results obtained by competitive protein binding analysis and direct U.V. absorbance analysis by high pressure liquid chromatography.

Studies on the mechanism of action of 1 alpha, 24-dihydroxyvitamin D3. II. Specific binding of alpha, 24-dihydroxyvitamin D3 to chick intestinal receptor

The binding of vitamin D3 analogues to the chick intestinal cytosol receptor was studied. In intestinal cytosol fraction, receptor proteins having the sedimentation constant of 2.5 S and 3.7 S to which 1 alpha,25-dihydroxyvitamin D3 binds were present, and the latter was specific for the compound. The binding of 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3 to the receptor was also observed, while very weak binding was seen in the case of 24(R)25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3. The binding affinity of 1 alpha,24(R)-dihydroxyvitamin D3 to the 3.7 S receptor was 1.3 times as high as that of 1 alpha,25-dihydroxyvitamin D3, whereas those of 1 alpha,24(S)-dihydroxyvitamin D3, 1 alpha-hydroxyvitamin D3 and 25-hydroxyvitamin D3 were 10, 304 and 652 times lower than 1 alpha,25-dihydroxyvitamin D3, respectively. The dissociation constant of the receptor-1 alpha,25-dihydroxyvitamin D3 complex at 0 degrees C was 3.0 x 10(-11) M, and the dissociation constants were calculated to be 2.4 x 10(-11) M and 2.7 x 10(-10) M for the complexes with 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3, respectively.

Normal and malignant breast tissue is a target organ for 1,25-(0H)2 vitamin D3

Malignant and benign human breast tumours as well as rabbit breast tissue were examined for specific receptors for 1,25-dihydroxyvitamin D3 using competitive binding studies and sucrose density gradient analysis. Classical high affinity, low capacity receptors for 1,25-dihydroxyvitamin D3 were found in breast and node tissue in seven of ten patients with breast cancer and in all three patients with benign neoplasms. An inflammatory breast mass showed no binding. Similar receptors were found in breast tissue from pregnant and lactating rabbits. Taken with other recent data, these results suggest that 1,25-dihydroxyvitamin D3 may activate calcium transport in the malignant as well as in the normal lactating breast.

In vitro metabolism of 25-hydroxyvitamin D3 by isolated rat kidney cells

Cells were dispersed from rat kidney after enzymatic digestion of the extracellular matrix. When the cells were suspended in a serum-free medium and incubated with (3)H-labeled 25-hydroxyvitamin D(3) (25-OH-D(3)) several polar metabolites, including 1,25-(OH)(2)[(3)H]D(3) and 24,25-(OH)(2)[(3)H]D(3) were produced. The specific activities of the 25-OH-D(3):1- and 24-hydroxylases in isolated rat kidney cells were 10-100 times greater than in avian kidney homogenates. The rates of production of 1,25-(OH)(2)D(3) and 24,25-(OH)(2)D(3) were linear over a wide range in cell densities (0.65-5.0 x 10(6) cells per ml) and substrate concentrations (3.5-70 nM). The rate of production of 24,25-(OH)(2)[(3)H]D(3) from 25-OH-[(3)H]D(3) by cells isolated from rats fed control diet was linear with time for up to 30 min, while the synthesis of 1,25-(OH)(2)[(3)H]D(3) was linear for over 90 min. The specific activity of the 25-OH-D(3):1-hydroxylase was increased in kidney cells from vitamin D-deficient rats (11.5 fmol/min per 10(6) cells) as well as calcium-deficient rats (8.1 fmol/min per 10(6) cells) when compared to cells from rats fed the control diet (2.0 fmol/min per 10(6) cells). Also, the specific activity of the 25-OH-D(3):24-hydroxylase was reduced in cells from the vitamin D-deficient rats (<0.2 fmol/min per 10(6) cells) and calcium-deficient rats (5.1 fmol/min per 10(6) cells) compared to the controls (15.2 fmol/min per 10(6) cells). On the basis of these results, as well as previous in vivo studies, we conclude that the metabolism of 25-OH-D(3) by freshly isolated rat kidney cells reflects the in vivo activities of the renal vitamin D-metabolizing enzymes and may prove useful as an assay.

Vitamin D homeostasis in the perinatal period: 1,25-dihydroxyvitamin D in maternal, cord, and neonatal blood

To investigate vitamin D homeostasis in term pregnancy, we measured 1,25-dihydroxyvitamin D (1,25(OH)2D) in serum samples from 19 term pregnant women and in samples from the placental veins of their infants. Samples were obtained from 14 neonates at 24 hours of age. At delivery, maternal concentrations of 1,25(OH)2D were elevated above normal adult values; placental-vein concentrations in the infants were significantly lower than adult normal or maternal values and bore no relation to maternal values. By the time the infants reached 24 hours of age, their serum concentrations had reached normal adult values, concomitant with a decrease in serum concentration of ionized calcium. We speculate that low 1,25(OH)2D concentrations in utero suggest that there is no need for intestinal calcium absorption in the fetus. Postnatal increase of 1,25(OH)2D may result from its production as a prerequisite to extrauterine requirements for intestinal absorption of calcium and phosphorus.

1,25-dihydroxyvitamin-D-receptor in breast cancer cells

A specific receptor for 1,25-dihydroxyvitamin D has been demonstrated in a cultured human breast cancer cell line. This is the first such demonstration in any cancer cell. It may explain the high incidence of metastatic bone destruction and hypercalcaemia in this common malignancy, and the limited success of other steroid-receptor assays in predicting the response of breast cancer to therapy.

Modulation of plasma 1,25-dihydroxyvitamin D in man by stimulation and suppression tests

In normal volunteers infusion of parathyroid extract raised plasma 1,25-dihydroxyvitamin D (1,25-[OH]2D) levels by 80% (peak 8--40 h after infusion) without producing any significant change in plasma calcium, phosphate, or magnesium. An oral calcium load suppressed plasma 1,25-(OH)2D to 63% of control values within 48--72 h. These responses will be useful in the diagnosis and management of disturbances of calcium and mineral metabolism in which vitamin D metabolism is abnormal.

Evidence for extra-renal 1 alpha-hydroxylation of 25-hydroxyvitamin D3 in pregnancy

The kidneys are thought to be the only organs capable of 1 alpha-hydroxylation of vitamin D and its metabolites. We have examined the in vivo conversion of 3H-(25,26)-25-hydroxyvitamin D3(25OHD3) to 3H-(25,26)-1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] in vitamin D-deficient, pregnant and nonpregnant rats. As expected, nephrectomy of nonpregnant, vitamin D-deficient rats prevented the conversion of 25OHD3 to 1 alpha,25(OH)2D3. In contrast, nephrectomy of pregnant, vitamin D-deficient rats reduced but did not abolish the formation of 1 alpha,25(OH)2D3 from its precursor. The identity of the radioactive metabolite formed from 3H-25OHD3 which circulated in nephrectomized, pregnant rats was established as 1 alpha,25(OH)2D3 by comigration with synthetic 1 alpha,25(OH)2D3 on high-pressure liquid chromatography. The simultaneous absence of 1 alpha,25(OH)2D3 in the fetal kidneys indicated that the site of 1 alpha-hydroxylation after nephrectomy of the pregnant rat was probably extra-renal in origin. Two sites of 1 alpha-hydroxylation of 25OHD3, one renal and the other extra-renal, either fetoplacental or maternal, may exist in the pregnant, vitamin D-deficient rat.

The relevance of 25-hydroxycalciferol measurements in sera of patients with renal failure

Iliac crest biopsies and serum specimens were obtained from 36 non-dialyzed uraemic patients. The mean serum 25-hydroxycalciferol concentration of the patients was lower than that of normal subjects and a significant correlation was found between 25-hydroxycalciferol values and the severity of osteomalacia. Parathyroid osteopathy was significantly correlated with serum immunoreactive parathyroid hormone but not with serum 25-hydroxycalciferol values. While normal serum 25-hydroxycalciferol concentrations in uraemic patients do not exclude the presence of osteomalacia, a low concentration is virtually diagnostic of this disorder.

Rickets with alopecia: an inborn error of vitamin D metabolism

Rickets with alopecia, an inborn error of vitamin D metabolism, is described in two sisters. The rachitic disorder began during the first year of life and was refractory to 50,000 IU of vitamin D2/day. Surprisingly, both children had marked elevations in serum concentrations of 1,25-(OH)2D. Although the molecular basis for this disorder is not evident to date, intestinal end-organ unresponsiveness to exceedingly high levels of 1,25-(OH)2D was present, in addition to hyporesponsiveness of bone to these high levels of the hormone, since normocalcemia was maintained despite elevated serum levels of PTH. Therapy with oral 1,25-(OH)2D3 failed to reverse the disorder, but oral phosphorus supplements resulted in significant radiographic and clinical improvement.

Reduced serum 1,25-(OH)2 vitamin D3 levels in prednisone-treated adolescents with systemic lupus erythematosus

The serum levels of 1,25-(OH)2 vitamin D3 were assayed in samples from 12 adolescent patients with SLE. Subnormal levels were observed in 7 of these 12 patients. Low levels of the metabolically active polar metabolite of vitamin D3 may contribute to the development of osteopenia observed in this disease. The cumulative effects of the osteoporotic and anti vitamin D effects of long term steroid therapy in children with SLE may require the cautious administration of supplemental vitamin D.

Reduction of serum-1, 25-dihydroxyvitamin-D3 in children receiving glucocorticoids

Serum-1,25-dihydroxyvitamin-D3 (1,25-[OH]2D3) was subnormal in children receiving long-term glucocorticoid treatment for various glomerular diseases, including nephrotic syndrome. In children with chronic glomerulonephritis not treated with glucocorticoids who had similar serum-creatinine with glucocorticoids who had similar serum-creatinine concentrations, serum-1,25-dihydroxyvitamin-D3 concentrations resembled those in healthy controls, indicating that glomerular renal disease per se does not account for reduced serum-1,25(OH)2DE concentrations in steroid-treated patients. The reduction in concentration of this most active vitamin-D metabolite correlated with the dose of steroid administered and with reduction in forearm bone mineral content measured by the photon absorption technique. Reduced serum-1,25-(OH)2D3 concentration may be important in the pathogenesis of steroid-induced osteopenia.

Serum 1,25-dihydroxyvitamin D levels in normal subjects and in patients with hereditary rickets or bone disease

The serum concentration of 1,25-dihydroxylvitamin D (1,25-[OH]2D) in normal children and in children with inherited diseases of bone was compared by use of a competitive binding assay. Observed values were: in 12 normal children and adolescents, 37.1 +/- 1.9 pg per milliliter (mean +/- S.D.); in 14 patients with X-linked hypophosphatemic rickets treated with vitamin D2 and phosphate supplements, 15.6 +/- 7.8 (P less than 0.01 versus control); in six patients with autosomal recessive vitamin D dependency treated with vitamin D2, 9.5 +/- 2.9 (P less than 0.01 versus control); and in four untreated patients with autosomal dominant hypophosphatemic (non-rachitic) bone disease, 30.2 +/- 6.3 (not significantly different from the controls). The difference in bone disease between X-linked hypophosphatemia (severe) and hypophosphatemic bone disease (mild) at comparable low serum levels of phosphate implies that 1,25-(OH)2D and phosphate may have independent roles in the pathogenesis of defective bone mineralization.

Resistance to parathyroid hormone in renal failure: role of vitamin D metabolites

Resistance to the calcemic action of parathyroid extract (PTE) was shown in thyroparathyroidectomized rats after 5 hr of renal failure that was induced by either bilateral nephrectomy (NPX) or ureter ligation (UL). Studies were carried out to investigate the relationship of parathyroid resistance to the vitamin D status of the animal. Concentrations of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24,25-dihydroxycholecalciferol (24,25(OH)2D3) were similar in pooled sera samples from rats either UL or sham-operated and treated with PTE. Pretreatment with oral 25-hydroxycholecalciferol or with a combination of i.v. 24,25(OH)2D3 and 1,25(OH)2D3 prior to UL failed to alter the resistance. Resistance was also present in a group in vitamin-D-deficient rats. A similar group given 1 microgram of vitamin D2 showed more parathyroid resistance than did the group not given vitamin D2. In chronic renal failure of 28 day's duration, parathyroid resistance was again demonstrated, but, in contrast to the acute renal failure models, this was partly corrected by prior 1,25(OH)2D3 administration. These studies show that parathyroid resistance is not caused by an abnormality of vitamin D metabolism in the acute renal failure model, and we suggest that the phenomenon is due to the accumulation of one or more uremic factors.