Vitamin D: two dihydroxylated metabolites are required for normal chicken egg hatchability

Naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of both C-24R and C-24S epimers

Tritium-labeled 24,25-dihydroxyvitamin D3 was prepared both in vitro, by using chick kidney homogenates, and in vivo in rats from [26,27-methyl-3H]25-hydroxyvitamin D3. These compounds were mixed with synthetic 24(R),25- and 24(S),25-dihydroxyvitamin D3, converted to the corresponding trimethylsilyl ether derivatives, and analyzed by a high-pressure liquid chromatography procedure that separates the derivatized isomers. The tritium-labeled 24,25-dihydroxyvitamin D3 derivatives were found to be a mixture of both the 24(R) and 24(S) epimers; the ratio was found to be 96.4:3.6 in chick kidney homogenates and 96.8:3.2 in the serum of rats under physiological conditions. In addition, nonradioactive 24,25-dihydroxyvitamin D3 isolated from the serum of rats given large doses of vitamin D3 was shown to be an 89.5:10.5 mixture of the 24(R) and 24(S) isomers. When 25-hydroxy-24-oxo-vitamin D3 was utilized as a substrate, it was found to be more selectively reduced to 24(S),25-dihydroxyvitamin D3 than 24(R),25-dihydroxyvitamin D3 by the renal enzyme. The 24(S),25-dihydroxyvitamin D3 has been identified by ultraviolet absorption spectrophotometry, cochromatography with an authentic standard, and mass spectrometry. The reduced metabolites of 25-hydroxy-24-oxo-vitamin D3 were a 1:50 mixture of the 24(R) and 24(S) epimers. There are two known metabolic pathways leading to 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3; one is 24(R)-hydroxylation of 25-hydroxyvitamin D3 and the other is reduction of 25-hydroxy-24-oxo-vitamin D3. In contrast, 24(S),25-dihydroxyvitamin D3 is produced only by reduction of 25-hydroxy-24-oxo-vitamin D3 in the kidney. Therefore, naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of the 24(R) and 24(S) isomers, and not just the 24(R) isomer as reported previously.

Calcium metabolism and its control--a review

It is now established that avians can only utilize the cholecalciferol form of vitamin D, which must be converted to the hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to perform normal calcium metabolism. Although 1,25(OH)2D3 is the final active form of vitamin D, hens fed only this form of vitamin D do not have normal hatchability of eggs. The problem appears to be caused by abnormal calcification and development of the embryonic beak. This appears to be caused by inadequate transport of 1,25(OH)2D3 into the egg. Although 1,25(OH)2D3 is not incorporated into the egg adequately, its precursor, 25-hydroxyvitamin D3 (25-OH-D3), is. The developing embryo however, can utilize 1,25(OH)2D and does so at least as early as Day 10 of incubation. During periods of maximal shell calcification and high circulating estradiol levels, the hen produces high levels of 1,25(OH)2D3. The kidney hydroxylase responsible for the final hydroxylation of the vitamin D hormone can be further stimulated by in vivo or in vitro administration of estradiol and, to a lesser extent, prolactin and parathyroid hormone. When eggs are not produced, as in the senescent or prepubertal stages of life, plasma 1,25(OH)2D3 concentrations are less than half that occurring during periods of active lay. Hens selected for their ability to produce thin or thick shells have 1,25(OH)2D3 concentrations in plasma that are positively correlated to their ability to produce egg shell.

Extrarenal metabolism of 25-hydroxycholecalciferol in the rat: regulation by 1,25-dihydroxycholecalciferol

To determine the role of the kidney in regulation of 25-hydroxycholecalciferol (25OHD3, metabolism, the effects of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] on 3H-25OHD3 were compared in intact and nephrectomized vitamin D-deficient rats. Sixteen hours after the intravenous administration of 3H-25OHD3, extracts of serum and pooled small intestinal mucosa were fractionated by Sephadex LH-20 column chromatography followed by high performance liquid chromatography. In intact rats, 1,25(OH)2D3 (50 ng/day i.p. for 7 days) increased mean serum 3H-24,25-dihydroxycholecalciferol [3H-24,25(OH)2D3] from 2 +/- 2-210 +/- 80 fmol/ml (mean +/- 1 SD), increased mean serum 3H-25,26-dihydroxycholecalciferol [3H-25,26(OH)2D3] from 2 +/- 2-12 +/- 6 fmol/ml and lowered mean serum 3H-1,25(OH)2D3 from 210 +/- 40-4 +/- 4 fmol/ml. Similarly, in nephrectomized animals, 1,25(OH)2D3 increased mean serum 3H-24,25-(OH)2D3 from 6 +/- 11-115 +/- 30 fmol/ml and increased mean serum 3H-25,26(OH)2D3 from 3 +/- 3-26 +/- 10 fmol/ml. Nephrectomy increased serum 3H-25(OH)D3 in untreated (from 1450 +/- 225-2675 +/- 225 fmol/ml serum) and 1,25(OH)2D3 treated rats (from 1600 +/- 175-3075 +/- 100 fmol/ml). 3H-1,25(OH)2D3 averaged 74 +/- 16% of total radioactivity in intestinal mucosa of untreated intact rats and was not detected in either the serum or intestinal mucosa of nephrectomized animals. The results suggest that in intact animals, extrarenal synthesis can account for substantial 24,25(OH)2D3 production and for most 25,26(OH)2D3 production.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D status after total gastrectomy

Serum levels of 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, 1,25-dihydroxyvitamin D, immunoreactive parathyroid hormone, and urinary excretion of nephrogenous cyclic AMP were measured in 25 patients after total gastrectomy. Two types of reconstruction after total gastrectomy were also compared. Serum 25-hydroxyvitamin D levels were significantly decreased and serum 24,25-dihydroxyvitamin D levels were markedly reduced, whereas serum 1,25-dihydroxyvitamin D levels were significantly increased in the patients. Although serum levels of immunoreactive parathyroid hormone did not show a significant difference, serum alkaline phosphatase levels and urinary excretion of nephrogenous cyclic AMP were significantly increased in the patients. The results suggest that defective vitamin D storage and enhanced vitamin D action coexist in patients after total gastrectomy and that the enhanced vitamin D action, possibly derived from slightly increased parathyroid function, would be a compensatory mechanism to sustained calcium deficiency. No substantial difference of vitamin D status was observed between the two types of reconstruction which differed in passage through the duodenum.

Support of embryonic chick survival by vitamin D metabolites

The provision of 1,25-dihydroxyvitamin D3 as the only source of dietary vitamin D3 to laying hens failed to support normal embryonic development in their fertile eggs. Significant (P less than .001) improvement in embryonic survival to hatching in these eggs resulted from injections of 1,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, 25-hydroxyvitamin D3, or 24,24-difluoro-25-hydroxyvitamin D3 prior to incubation. Maximum embryonic survival with lowest embryonic mortality was observed when 0.20 micrograms/egg of 1,25-dihydroxyvitamin D3 or 0.60 micrograms/egg 25-hydroxyvitamin D3 was injected. These results indicate that several forms of vitamin D, two of which cannot be converted to 24,25-dihydroxyvitamin D3, can provide this activity; and of the vitamin D compounds tested, 1,25-dihydroxyvitamin D3 may be the most active in supporting embryonic survival in the chick when delivered directly by injection.

Impaired stimulation of 25-hydroxyvitamin D-24-hydroxylase in fibroblasts from a patient with vitamin D-dependent rickets, type II. A form of receptor-positive resistance to 1,25-dihydroxyvitamin D3

We describe studies of the molecular defect in 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] action in cultured skin fibroblasts from a patient previously reported to have vitamin D-dependent rickets, type II. Binding of [3H]1,25-(OH)2D3 in fibroblast cytosol was normal with a Bmax (amount of high affinity binding) of 26 fmol/mg protein and a half-maximal saturation of 0.2 nM. Nuclear binding of [3H]1,25-(OH)2D3 following whole cell uptake was 1.5 fmol/micrograms DNA in patient fibroblasts compared with a range of 0.5-2.9 fmol/micrograms DNA in five control strains. The size of the [3H]1,25-(OH)2D3-receptor complex on sucrose density gradients, 3.8 S, was the same as in normal cells. This patient, therefore, appeared to have a receptor-positive form of resistance to 1,25-(OH)2D3. To document resistance to 1,25-(OH)2D3 in the fibroblasts we developed a method for detection of 1,25-(OH)2D3 action in normal skin fibroblasts. Following treatment of normal cell monolayers with 1,25-(OH)2D3 there was more than a 20-fold increase of 25-hydroxy-vitamin D-24-hydroxylase (24-hydroxylase) activity. Treatment of 10 control cell strains with 1,25-(OH)2D3 for 8 h increased the formation of 24,25-dihydroxy-vitamin D3 from 25-hydroxyvitamin D3 in cell sonicates from less than 0.02 to 0.11-0.27 pmol/min per mg protein. When cells from the patient with vitamin D-dependent rickets, type II were treated with 1,25-(OH)2D3 in a similar manner, maximal 24-hydroxylase activity was only 0.02 pmol/min per mg protein, less than a fifth the lower limit of normal. 24-Hydroxylase activity in fibroblasts from the parents of the patient increased normally following treatment with 1,25-(OH)2D3. We conclude that impaired induction of 24-hydroxylase in the presence of normal receptor binding is evidence for postreceptor resistance to the action of 1,25-(OH)2D3.

23,25-Dihydroxy-24-oxovitamin D3: a metabolite of vitamin D3 made in the kidney

Kidney homogenates of rats produced a new metabolite of 25-hydroxyvitamin D3 which has been isolated in pure form after five column chromatographic steps. It was identified as 23,25-dihydroxy-24-oxovitamin D3 by means of ultraviolet and infrared absorption spectrophotometry, mass spectrometry, and proton nuclear magnetic resonance spectrometry. The stereochemistry at the C-23 position is as yet unknown. 25-Hydroxy-24-oxovitamin D3, which also has been isolated in pure form from this system, was found to be the precursor of the new metabolite in vitro. The production of the new metabolite was induced by two different methods: (a) perfusion of the kidneys with 1,25-dihydroxyvitamin D3 contained in the perfusate and (b) injection of 1,25-dihydroxyvitamin D3 in the intact animal. 23,25-Dihydroxy-24-oxovitamin D3 was not biologically active in an assay for intestinal calcium transport and bone calcium mobilization in the vitamin D deficient chick at a dose level of 5.3 nmol. A metabolic pathway is proposed to describe the results; it leads from 25-hydroxyvitamin D3 leads to 24(R),25-dihydroxyvitamin D3 leads to 25-hydroxy-24-oxovitamin D3 leads to 23,25-dihydroxy-24-oxovitamin D3.

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.

Physiology of 24,25-dihydroxyvitamin D3 in normal human subjects

We determined the metabolic clearance and production rates of 24,25-dihydroxyvitamin D3 in four normal healthy adults. We also examined the excretion of radioactivity in stool, urine, and bile after the intravenous administration of 24,25-[3H]dihydroxyvitamin D3 to human subjects. 24,25-Dihydroxyvitamin D3 is rapidly cleared from the plasma with a half-life of approximately 390 +/- 25 min (mean +/- SE). The metabolic clearance rate of 24,25-dihydroxyvitamin D3 was 9.2 +/- 1.5 liters/day with a production rate of 26.4 +/- 7.2 micrograms/day (mean +/- SE). Within 1 day 13.0 +/- 4.2% (mean +/- SE) of the administered dose had appeared in the stool; by day 7, 48.8 +/- 2.7% of the dose had appeared in the feces. Within 24 hr, 6.4 +/- 0.8% of the administered dose appeared in the urine; 7.4 +/- 1.8% of the dose had appeared in the urine within 2 days. The biliary excretion of 24,25-dihydroxyvitamin D3 was studied in two subjects. By 8 h, 15.3 +/- 1.3% of the administered dose had appeared in the bile. The metabolites present in bile, feces, and urine were much more polar than 24,25-dihydroxyvitamin D3. These results demonstrate that 24,25-dihydroxyvitamin D3 is rapidly cleared from plasma and is excreted in the feces (probably via the bile) and urine of normal human subjects.

A simple and sensitive assay for 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D in human serum

An improved method is described which permits the simultaneous determination of 25-hydroxyvitamin D [25-(OH)D], 24,25-dihydroxyvitamin D [24,25-(OH)2D] and 1,25-dihydroxyvitamin D [1,25-(OH)2D] in milliliters of human serum. Methodological improvements enabled a rapid and almost complete extraction of the three metabolites from serum and omission of adding labeled internal standards to each serum sample for the calculation of individual recoveries. Commercially available stable chick embryo intestinal mucosa cytosol preparation made the troublesome preparation of cytosol receptor for 1,25-(OH)2D unnecessary. The procedure involves saturation of serum with ammonium carbonate and extraction with methanol/ethyl acetate, followed by separation of 25-(OH) D from the dihydroxy metabolites of vitamin D by Sephadex LH-20 column chromatography and further separation of the dihydroxy metabolites into 24,25-(OH)2D and 1,25-(OH)2D by high-pressure liquid chromatography. This is followed by individual determination of each metabolite by competitive protein-binding assay or radioreceptor assay.

24-hydroxylation of 25-hydroxyvitamin D3: is it required for embryonic development in chicks?

As shown previously, laying hens given 1,25-dihydroxyvitamin D3 as their sole source of vitamin D produce fertile eggs having normal shells, but only 35 to 55 percent of the embryos are normal. Giving these hens additional 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3, or 24,24-difluoro-25-hydroxyvitamin D3 at 1.25 nanomoles per day resulted in 90 to 100 percent normal embryos, and hence, hatchability. Since 24,24-difluoro-25-hydroxyvitamin D3 cannot be 24-hydroxylated, 24-hydroxylation is not required for this function of 25-hydroxyvitamin D3.

Biochemical characterization of 1,25(OH)2D3 receptors in chick embryonal duodenal cytosol

This study presents measurements of serum vitamin D metabolites, calcium and phosphorus as well as measurements of the equilibrium dissociation constant for duodenal 1,25(OH)2D3 receptor in 15-, 18-, 19-, and 20-day chick embryos in comparison to that in 1- and 118-day-old chicks and to vitamin D-deficient chicks. The present results showed that: (a) serum 1,25(OH)2D and 24,25(OH)2D levels rise from 15 and 18 to days 19 and 20 of embryonic development while serum phosphate levels are stable; (b) serum calcium levels rise at hatching to adult levels; (c) the duodenal 1,25(OH)2D3 receptor is detectable in 15-day-old embryo and has a Kd similar to that of 118-day-old vitamin D-replete chicks; and (d) the activity of 1,25(OH)2D3 receptor in chick duodenal cytosol is maximal at hatching.

Serum 25 (OH) D and 24,25 (OH)2 levels in childhood nephrosis under different therapeutic regimens of steroid administration

The effect of prednisone therapy on serum levels of 25-hydroxyvitamin D [25(OH)D] and 24,25-dihydroxyvitamin D [24,25(OH)2D] was investigated in 16 children with nephrotic syndrome. These serum levels were significantly lower in patients before prednisone therapy than in age- and season-matched normal subjects. Patients receiving daily prednisone therapy had lower serum levels than those receiving alternate-day prednisone therapy at the time when the total amounts of the steroid administered attained 1,500 or 2,000 mg/m2 of body surface area. Daily doses of 40 mg/m2 of prednisone for 3 days caused a significant decrease in serum 25(OH)D levels. Withdrawal of the steroid for 4 consecutive days was followed by a significant recovery of the serum levels. These results suggest that alternate-day prednisone therapy rather than daily treatment should be used in clinical practice to help maintain normal vitamin D metabolism.

Effect of 1 alpha-hydroxyvitamin D3 on cancellous bone matrix. An experimental study on adult rats

Two groups of adult male rats were treated perorally for 6 weeks with 0.1 microgram and 1.0 microgram of 1a-hydroxyvitamin D3 (1a-OH-D3), respectively. The effect of the treatment on cancellous bone matrix was studied by chemical analysis and morphometric measurements. The effect of the 1.0 microgram dose on the inorganic composition, and on the calcification of the cancellous bone matrix, was significantly more pronounced, decreasing the amount of glycosaminoglycans. The lower dose level, 0.1 microgram of 1a-OH-D3, increased the collagen metabolism, whereas the higher dose level did not. The amount of cancellous bone determined morphometrically increased significantly during treatment with both dose levels. 1a-OH-D3, converted in the organism to the hormonal form 1.25 (OH)2D3, induces new bone formation, probably by direct influence on the cancellous bone tissue itself.

The vitamin D system: a view from basic science to the clinic

Vitamin D produced in the skin and absorbed in the small intestine must be modified metabolically before it can function. It is ultimately converted to a hormone in the kidney that stimulates intestine, bone and kidney to mobilize calcium and phosphorus. This results in normal bone development and normal neuromuscular function. The vitamin D hormone appears to act by a nuclear mechanism to facilitate a target organ response. Finally the vitamin D hormone is produced in response to the need for calcium and phosphorus. The calcium need is interpreted by the parathyroid gland that in turn secretes parathyroid hormone. The parathyroid hormone stimulates production of the vitamin D hormone. This constitutes the vitamin D endocrine system that plays an important role not only in calcium homeostasis but in phosphate homeostasis and in calcium economy of the body. A number of disease states including hypoparathyroidism, pseudohypoparathyroidism, renal osteodystrophy, certain types of vitamin D-resistant rickets and osteoporosis can in part be related to disturbance in the vitamin D endocrine system. Thus measurement of the vitamin D hormone and its precursor will be of great value in diagnosis of metabolic bone disease and most importantly, the availability of new vitamin D compounds will play an important role in the treatment of these bone diseases.

Suppressive effects of 24,25-dihydroxycholecalciferol on bone resorption induced by acute bilateral nephrectomy in rats

The possible suppressive effects of 24,25-dihydroxycholecalciferol on secondary hyperparathyroidism and increased bone resorption were investigated in adult rats raised on a diet normal in calcium, phosphorus, and vitamin D, and subjected to acute bilateral nephrectomy. The animals had received subcutaneous radiocalcium 4 wk before the experiment. 5 h after nephrectomy an increase in serum total calcium, (45)Ca-specific activity, citrate, phosphorus, and magnesium concentrations were observed. Serum immunoreactive parathyroid hormone increased, while serum calcitonin decreased. The osteoclast count in the tibial metaphyses was augmented. The biochemical and histological changes observed were partly parathyroid hormone and calcitonin independent, as they also occurred in parathyroidectomized hypocalcemic rats. Pretreatment with 650 pmol of 24,25-dihydroxycholecalciferol 16 h before nephrectomy prevented bone calcium mobilization and diminished the rise in serum total calcium and citrate both in parathyroid-intact and in parathyroidectomized animals. In parathyroid-intact rats, serum immunoreactive parathyroid hormone and calcitonin remained normal in spite of the fall in serum-ionized calcium, and the number of osteoclasts did not increase. In parathyroidectomized rats, 24,25-dihydroxycholecalciferol did not prevent the postnephrectomy rise in the osteoclast count. This latter observation suggests that this metabolite exerts its effect on bone either by acting on cells other than osteoclasts, i.e., the osteocytes, or by inhibiting cell activity. At equimolar dosage 1,25-dihydroxycholecalciferol had a potent stimulatory effect on bone resorption. This effect of 1,25-dihydroxycholecalciferol was partly blocked by the simultaneous administration of 24,25-dihydroxycholecalciferol. The potential clinical significance of these observations remains to be determined.

An improved method for routine determination of vitamin D and its hydroxylated metabolites in serum from children and adults

A method for routine determination of vitamin D and its major metabolites 25-hydroxyvitamin D (25(OH)D), 24,25-dihydroxyvitamin D (24,25(OH)2D) and 1,25-dihydroxy-vitamin D (1,25(OH)2D) in serum samples from normal children and adults has been developed. Methodological improvements enable a rapid and accurate analysis of 25(OH)D and also the microscale screening of other metabolites present in large concentrations in serum. Vitamin D and its metabolites are extracted from serum samples using hexane/propan-2-ol, which allows a convenient separation of the water soluble and lipid soluble fractions from each other and also from proteins. Preparative silicic acid chromatography was used to separate vitamin D from its metabolites and also from the major portion of co-eluting lipid contaminants. An automated LC solvent delivery and sample introduction system was used to achieve the rapid separation of metabolites. Vitamin D was further purified using adsorption high-performance liquid chromatography and assayed using reverse phase high-performance liquid chromatography connected with UV detection. The 25(OH)D fraction from the preparative chromatography was measured by a competitive protein-binding assay along with 24,25(OH)2D, which was purified by reverse phase high-performance liquid chromatography along with 1,25(OH)2D. A diluted human serum from a pregnant woman (3rd trimester of pregnancy) was used as source of the binding protein for 25(OH)D and 24,25(OH)2D. 1,25(OH)2D was determined by a competitive protein-binding assay using a diluted cytoplasmic 1,25(OH)2D receptor protein isolated from the intestinal mucosa of rachitic chicks. Vitamin D and its metabolite levels were assayed in serum samples from normal children and adults.

Isolation, identification, and biological activity of 25-hydroxy-24-oxovitamin D3: a new metabolite of vitamin D3 generated by in vitro incubations with kidney homogenates

A metabolite of 25-hydroxyvitamin D3 has been isolated in pure form from incubation mixtures containing kidney homogenates of chicks given large doses of vitamin D3. The isolation involved methanol--chloroform extraction and six steps of column chromatography. The metabolite was identified as 25-hydroxy-24-oxovitamin D3 by means of ultraviolet absorption spectrometry, mass spectrometry, infrared spectrometry, nuclear magnetic resonance spectrometry, and specific chemical reactions. Use of a sensitive in situ technique revealed that 25-hydroxy-24-oxovitamin D3 enhances intestinal calcium transport in rats approximately as effectively as 24,25-dihydroxyvitamin D3 does. In contrast, 25-hydroxy-24-oxovitamin D3 appeared to be less active than 24,25-dihydroxyvitamin D3 in chicks 24 h after intravenous injection.

Vitamin D metabolism and its possible role in the developing chick embryo

The relationship between bone formation and vitamin D metabolism was investigated in the developing chick embryo. Fertilized White Leghorn eggs were incubated at 38 degrees C in an incubator for 21 days. The fresh weight and calcium content of embryonic tibiae began to increase at day 12 and attained maximal values at day 19. Bone alkaline phosphatase and citrate decarboxylation activities, both of which represent osteoblastic activity, also began to increase at days 10-12, reached maximal values at day 19 and sharply declined thereafter. Both bone enzyme activities were highly correlated with CA2+-binding activity in the chorioallantoic membrane measured by the Chelex 100 assay. When mesonephric and metanephric homogenates were incubated with 25-hydroxy[3H]cholecalciferol, a marked and concomitant increase occurred in the metanephric 1 alpha- and 24-hydroxylase activity after day 14. The production of 1 alpha, 25-dihydroxycholecalciferol attained a maximal value at day 19 and decreased thereafter, whereas that of 24,25-dihydroxycholecalciferol continued to increase until hatching. The production rate of 1 alpha, 25-dihydroxycholecalciferol by the metanephros coincided with the changes in Ca2+-binding activity in the chorioallantoic membrane and osteoblastic activity. Since both intestinal calcium absorption and bone mineral mobilization do not occur in embryonic life, these results support the idea that 1 alpha, 25-dihydroxycholecalciferol may be involved directly in bone formation or induction of a calcium-binding protein in the chorioallantoic membrane.

Relative effectiveness of vitamin D metabolites in increasing bone mineral solubility

Weanling rats were given a vitamin D-deficient diet containing 1.4% calcium and 1.0% phosphorus. After 4 weeks these deficient animals were injected for 7 days with selected doses of one of the following vitamin D metabolites: 25(OH)D3, 1,25(OH)2D3, 24,25(OH)2D3, 25,26(OH)2D3 or the ethanol vehicle. A vitamin D-replete group was placed on the same diet but injected with 50 IU of vitamin D3 once a week for the entire 5-week period. By the use of a modified Ussing chamber [1], the measurements of calcium fluxes into and from the rat calvaria were possible. These data enabled the apparent mineral solubilities to be derived. After 5 weeks on this diet the vitamin D-deficient rats had low levels of serum calcium (1.41 mM) and decreased mineral solubility when compared to the vitamin D-replete group. The apparent solubility of the bone mineral increased toward the vitamin D-replete level in calvaria from vitamin D metabolite-treated rats. However, these changes did not directly reflect the alterations in the level of serum calcium. At any given dose level, 1,25(OH)2D3 was the most effective metabolite in increasing serum calcium. In fact, the high dose (250 pmoles/day) was hypercalcemic. Next in effectiveness was 25(OH)D3. These two metabolites were equally effective in increasing mineral solubility. At a 10 times higher dose, the 24,25(OH)2D3 metabolite was able to normalize serum calcium and improve but not normalize mineral solubility. At the high dose (260 pmoles/day), the 25,26(OH)2D3 metabolite caused no effect on mineral solubility and minimal increases in serum calcium.

The biochemical basis of renal osteodystrophy and post-menopausal osteoporosis: a view from the vitamin D system

The vitamin D endocrine system is now known to play an essential role in the regulation of plasma calcium and phosphorus concentrations and in overall organismal calcium economy. These two basic functions of the vitamin D endocrine system have provided important new insight into several disease states. The two disease states discussed here are the genesis of renal osteodystrophy and of post-menopausal osteoporosis. It seems likely that defects of alternations in the vitamin D system play important roles in the development of these disease states. Successful treatment undoubtedly will involve the vitamin D system and most likely the active form of vitamin D, 1,25- (OH)2D3.

Interaction between 24R,25-dihydroxycholecalciferol and 1,25-dihydroxycholecalciferol on 45Ca release from bone in vitro

Interaction among vitamin D3 metabolites on bone receptor sites is not known. Therefore, interaction between the most potent vitamin D3 metabolite, 1,25(OH)2D3, and the most abundant dihydroxymetabolite, 24R,25(OH)2D3, was studied on isolated rat fetal bone by measuring 45Ca release from prelabeled bones. 24R,25(OH)2D3 at concentrations of 10-50 ng/ml caused marked inhibition of the bone-resorbing activity of 1,25(OH)2D3 at concentrations of 10-50 pg/ml. 24S,25(OH)2 (unnatural enantiometer), on the other hand, at a concentration of 100 ng/ml did not inhibit the bone-resorbing effect of 10 pg/ml 1,25(OH)2D3. 24R,25(OH)2D3 at a concentration of 20 ng/ml did not inhibit the 45Ca-releasing effect of a submaximal concentration of PTH (500 ng/ml). Therefore, the inhibitory effect of 24R,25(OH)2D3 on the bone response to 1,25(OH)2D3 appeared to be specific and probably due to a competitive inhibitory effect. In addition, the inhibitory effect of 24R,25(OH)2D3 was weak, since it could be partially overcome by increasing the concentration of 1,25(OH)2D3.

Subcellular mechanisms involving vitamin D

During the past 15 years a vitamin D endocrine system has been demonstrated in which vitamin D produced normally in the skin is activated first by conversion in the liver and subsequently in the kidney to a hormonal form, 1,25-(OH)2D3. The production of the hormonal form of vitamin D3, is regulated, and much has been learned regarding the molecular mechanism of the hydroxylations of vitamin D and regarding the physiologic regulators of the 25-OH-D-1-hydroxylase. Much remains to be learned regarding the mechanism whereby the 1-hydroxylase is modulated. 1,25-(OH)2D3 appears to function in the target organs of bone, intestine, kidney, and elsewhere by a nucleus-mediated process. Receptors for 1,25-(OH)2D3 have been clearly demonstrated and characterized in crude form. How the receptor and ligand interact with the nucleus is not clear, nor are the gene products that result from this interaction known. One product, a calcium binding protein, is known but its role in calcium transport is in debate. Although much has been learned in the last decade and a half, much remains to be learned regarding the molecular mechanisms whereby vitamin D brings about its remarkable changes in mineral metabolism.

Vitamin D deficiency inhibits pancreatic secretion of insulin

The effects of a vitamin D deficiency on insulin and glucagon release was determined in the isolated perfused rat pancreas by radioimmunoassay of the secreted proteins. During a 30-minute period of perfusion with glucose and arginine, pancreases from vitamin D-deficient rats exhibited a 48 percent reduction in insulin secretion compared to that for pancreases from vitamin D-deficient rats that had been replenished with vitamin D. Vitamin D status had no effect on pancreatic glucagon secretion. This result, along with the previously demonstrated presence in the pancreas of a vitamin D-dependent calcium-binding protein and cytosol receptor for the hormonal form of vitamin D, 1,25-dihydroxyvitamin D3, indicates an important role for vitamin D in the endocrine functioning of the pancreas.

Effects and interactions of 24R,25(OH)2D3 and 1,25(OH)2D3 on bone

The effects of various combinations of therapy with 1 alpha,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24R,25-dihydroxycholecalciferol (24R,25(OH)2D3) on structural and dynamic parameters of bone were evaluated in 40 chicks raised on a vitamin D-deficient diet from time of hatching and supplemented with the dihydroxylated metabolites. The results showed that: 1) the maintenance of volumetric density of bone is dependent on the presence of 1,25(OH)2D3, 2) lack of 1,25(OH)2D3 is associated with an increase in the number of osteocytes per unit volume of bone, most probably due to decreased amounts of bone formed by each osteoblast before becoming an osteocyte, 3) adequate quantities of either 24R,25(OH)2D3 or 1,25(OH)2D3 are needed to prevent accumulation of osteoid or the production of endosteal fibrosis, and 4) maintenance of normal tetracycline label width requires both hydroxylated compounds with one of them in sufficient amounts. The data of this study demonstrate that the integrity of certain parameters of bone structure could be maintained only with 1,25(OH)2D3, others with either dihydroxylated metabolites, and still others with a combination of both. These data underscore the biological activity of 24R,25(OH)2D3 by demonstrating its effectiveness on bone.