The effect of induced metabolic acidosis on vitamin D3 metabolism in rachitic chicks

The metabolism of vitamin D3 was studied in 3-week-old, vitamin D deficient chicks, fed since hatching with a diet containing 3% ammonium chloride, 1% calcium, and 0.7% phosphorus. When kidney homogenates were incubated in vitro with [3H]25-(OH)D3, the production of 1,25-(OH)2D3 was reduced by 40% in acidotic birds. During in vivo experiments, after injection of [3H]D3 (1220 pM/bird), the level of 1,25-(OH)2D3 was also reduced in blood plasma, intestine, and tibiae in acidotic chicks as compared with the controls. As a large increase in plasma phosphate was found during acidosis, these results are discussed in relation to the possible role of phosphorus in the control of 1,25-(OH)2D3 synthesis.

The effect of decreased renal function with and without reduction in renal mass on 1,25-dihydroxycholecalciferol production in rats

Decreased production of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) by the kidney may be responsible for vitamin D resistance in renal failure and contribute to renal osteodystrophy. The present studies investigate the effect of reduced renal mass and reduced renal function on conversion of 3H-25-hydroxycholecalciferol to 3H-1,25-(OH)2D3 in vitamin D-deficient rats. Renal mass was reduced by partial or total nephrectomy. Renal function was reduced by ureteral ligation. At comparable degrees of renal function, reducing renal mass decreased 1,25-(OH)2D3 formation approximately proportional to the amount of renal tissue removed. Ureteral ligation without reduction of renal mass also reduced production within the first 24 hours. This may have been caused by the marked hyperphosphatemia which occurred with ureteral ligation. When partial nephrectomy and ureteral ligation were done together the results were additive. The decreased 1,25-(OH)2D3 formation produced by unilateral nephrectomy was not improved 4 weeks after surgery, although renal function had returned to normal.

The metabolism of [6-3H]1alpha, hydroxycholecalciferol to [6-3H]1alpha,25-dihydroxycholecalciferol in a patient with renal insufficiency

To evaluate whether 1alpha-hydroxycholecalciferol is metabolized to 1alpha,25-dihydroxycholecalciferol in man, [6-3H]1alpha-hydroxycholecalciferol was given intravenously to a patient with renal failure who was maintained daily on 100,000 IU vitamin D and calcium supplements. Using Sephadex LH-20 and high-pressure liquid chromatography, it was clearly demonstrated that 1alpha-hydroxycholecalciferol rapidly disappears from the blood and is metabolized to 1alpha,25-dihydroxycholecalciferol.

The 24-hydroxylation of 1,25-dihydroxyvitamin D3

The production of 1,24,25-trihydroxyvitamin D3 in vivo in vitamin D-deficient rats has been demonstrated from either 25-hydroxyvitamin D3 or 1,25-dihydroxyvitamin D3. The stereochemical configuration of the hydroxyl on the 24 position of 1,24,25-trihydroxyvitamin D3 has also been unambiguously established to be R. Nephrectomy failed to eliminate the conversion of 1,25-dihydroxyvitamin D3 to the 1,24,25-trihydroxyvitamin D3, establishing that the 24-hydroxylase can be demonstrated in at least one organ in addition to kidney in contrast to the 1-hydroxylase. 1,24,25-Trihydroxyvitamin D3 can also be produced in vivo from 1alpha-hydroxyvitamin D3 or from 24-hydroxyvitamin D3, analogs of vitamin D3 which are not naturally occurring. Using chick kidney mitochondrial preparations it has been demonstrated that the 24-hydroxylase is able to utilize 1,25-dihydroxyvitamin D3 as a substrate whereas it is unable to utilize 1alpha-hydroxyvitamin D3. In addition, the chicke kidney 1-hydroxylase is known to convert 24,25-dihydroxyvitamin D3 to the 1,24,25-trihydroxyvitamin D3, but this hydroxylase is unable to act on the 24-hydroxyvitamin D3. These results demonstrate that the renal vitamin D hydroxylases require that a hydroxyl be on the 25 carbon of the vitamin D molecule before it can be 1- or 24-hydroxylated.

Lack of effect of calcitonin on the regulation of vitamin D metabolism in the rat

The infusion of calcitonin into intact rats increases the accumulation of 1,25-dihydroxy-[26-27-3H]vitamin D3 from 25-hydroxy-[26,27,-3H]vitamin D3 in blood, but has no effect on thyroparathyroidectomized rats using a variety of protocols. Furthermore, the vitamin D status of the animals did not alter the results. Inasmuch as no effect of calcitonin could be found on the accumulation of other vitamin D metabolites as well as 1,25-dihydroxyvitamin D3, it is concluded that calcitonin apparently plays no direct role in the regulation of vitamin D metabolism and that the previous report of an effect of calcitonin on vitamin D metabolism in vivo is probably the result of a secondary response of the parathyroid gland.

Vitamin D metabolism in man. Effect of corticosteroids

This study evaluated the effects of acute intravenous calcium supplementation on vitamin D metabolism in nine patients maintained on long-term prednisone therapy for chronic obstructive lung disease. Vitamin D turnover studies, employing 1,2-(3)H,4(14)C vitamin D3, and coincident measurement of serum 25-hydroxycholecalciferol (25-OHCC) concentrations demonstration (P less than .05) and half-time (P less than .05) in the prednisone-treated patients when compared with the mean 25-OHCC concentration and half-time found in these patients prior to calcium infusion. However, there was no significant difference in the amounts of 24,25-(OH)2CC) or 1,25-dihydroxycholecalciferol (1,25-(OH)2CC) appearing after calcium infusion. The results of the study showed that the intravenous administration of calcium to corticosteroid-treated patients accelerated the disappearance of 25-OHCC from the plasma of these patients without parallel increases in the production of 1,25-(OH)2CC or 24-25-(OH)2CC.

The ionic control of 1,25-dihydroxyvitamin D-3 synthesis in isolated chick renal mitochondria. The role of potassium

In previous studies it was found that change in the concentrations of Ca2+, H+, and HPO2-4 in the incubation medium altered the rates of synthesis of 1,25-dihydroxyvitamin D-3 (1,25(OH)2D-3) by isolated renal mitochondria obtained from D-deficient chicks. The present studies demonstrate that raising the medium concentration of K+ from 1 to 50 mM leads to a 6-fold increase in rate of 1,25(OH)2D-3 synthesis by isolated chick mitochondria; that the magnitnitude of this K+-dependent stimulation is enhaced by optimal concentrations of calcium (pCa = 5) and phosphate (pPi = 3) (3 mM) but not by pH (from 6.8 to 7.4); that the effect is not produced by similar changes in media Na+ concentration; and that the stimulatory effect of K+ is not blocked by ruthenium red, and inhibitor of calcium transport and of the calcium-dependent stimulation of mitochondrial 1,25(OH) 2D-3 synthesis. It was also found that valinomycin, a K+-specific ionophore, enhanced the sensitivity of the mitochondrial 1 alpha-hydroxylase activity to K+. In the presence of valinomycin, an increase of pK+ to 3 was sufficient to cause a significant stimulation of 1,25(OH)2D-3 synthesis. It was concluded that changes in the ion content of the mitochondrial matrix space regulated the activity of the 1 alpha-hydroxylase.

Vitamin D metabolism: physiological regulation in egg-laying Japanese quail

Homogenates of kidney removed from reproductivity active female Japanese quail were incubated with tritiated 25-hydroxyvitamin D3, and the metabolites were extracted and identified by chromatographic methods. Kidneys removed from birds with and without an egg in the oviduct revealed that ovulation results in enhanced production of 1, 25-dihydroxyvitamin D3, the active hormonal form of vitamin D3. Further examination of this phenomenon in relation to the ovulatory cycle revealed that 1, 25-dihydroxy-vitamin D3 production is enhanced throughout the 24 h following ovulation. Particularly important is the finding that its synthesis is already enhanced during the first 6 h after ovulation, at a time before any calcification of the egg shell begins. If, following oviposition, no ovulation occurs, 1, 25-dihydroxy-vitamin D3 production decreases rapidly and significantly within the first 6 h following oviposition. This study has revealed for the first time a physiological state, namely the reproductive period in the female bird, in which endogenous control over 1, 25-dihydroxyvitamin D3 production is exhibited without any previous manipulation, dietary or otherwise, of the animals.

The effect of hepatectomy on the synthesis of 25-hydroxyvitamin D3

The metabolism of [3H]vitamin D3 in hepatectomized vitamin D-deficient rats has been studied. Hepatectomy drastically disrupts vitamin D3 metabolism as revealed by prolonged high levels of [3H] vitamin D3 in the plasma compartment even 12 h after dose in contrast to sham-operated controls. Some conversion of [3H] vitamin D3 to [3H]25-hydroxyvitamin D3 was detected in hepatectomized rats, but the amount was small in spite of the high circulating levels of [3H]vitamin D3. Since the liver initially takes up much of an administered dose in normal animals and the conversion of [3H]vitamin D3 to [3H]25-hydroxyvitamin D3 is small in hepatectomized rats in spite of high circulating [3H]-vitamin D3, it is concluded that the liver plays a major role in the metabolism of vitamin D3 to 25-hydroxy-vitamin D3.

The ionic control of 1,25-dihydroxyvitamin D3 synthesis in isolated chick renal mitochondria. The role of calcium as influenced by inorganic phosphate and hydrogen-ion

Isolated kidney mitochondria prepared from Vitamin D-deficient chicks catalyze the conversion of 25-hydroxyvitamin D3 to 1,25 dihydroxyvitamin D3. It wasfound that changes in the concentrations of Ca-2plus, HPO4-2minus, and Hplus altered synthesis in an interrelated fashion. Increasing the Ca-2plus concentration from 10-6 to 10-5 M caused a four- to fivefold increase in 1 alpha-hydroxylase activity when the medium pH was between 6.5 and 7.0. increasing the [Ca2+] to 10-4 M caused to furhter stimulation. At higher pH values, Ca-2plus had little effect upon 1 alpha-hydroxylase activity. In the absence of calcium [Ca2+] less than or equal to 10-7 M), a change in pH from 6.5 to 7.1 had no effect upon 1 alpha-hydroxylase activity in the presence of 10-5 M calcium, increasing the medium pH had a biphasic effect. An increase in pH from 6.5 to 6.9 caused a 1.5-fold increase in 1 alpha-hydroxylase activity, but a further increase of the pH to 7.1 caused a profound decrease in rate of hydroxylation to approximately 20% of the peak value. Neither 10-5 M LaC13 nor 10 mug/ml of oligomycin altered the effects of Ca2+ upon hydroxylate activity. However, the effect of calcium was blocked by 2.5 times 10-5 M ruthenium red, 0.83 mug/ml of antimycin A, and 500 muM dinitrophenol. The clcium ionophore, A23187, decreased but did not prevent the stimulatory effect of calcium. These data are consistent with the concept that the [Ca2+ in the mitochondrial matrix space is of importance in regulating the 1 alpha-hydroxylase. Phosphate exerted a biphasic effect on 1,25(OH)2D3 production with maximal stimulation (approximately twofold) at 1-3 mM. Calcium enhanced the stimulation by phosphate at all concentrations studied. The presence of potassium modified the interrelated effects of calcium and phosphate in two ways: 10-3 M calcium blocked the stimulation by phosphate; and in the presence of phosphate, 10-3 M calcium resulted in less 1,25(OH)2D3 production by production by isolated mitochondria are qualitatively similar to the effects of these ions on 1,25(OH)2D3 production yb isolated renal tubules.

The ionic control of 1,25-dihydroxyvitamin D3 production in isolated chick renal tubules

Isolated renal tubules prepared from vitamin D-deficient chicks catalyze the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 (250HD3) in vitro. The effect of calcium and phosphate on the rate of synthesis of the product, 1, 25-dihydroxyvitamin D3 (1,25(OH)2D3), was studied at two levels: the long-term effects of various dietary calcium and phosphate contents on the ability of the tubules to produce 1, 25 (OH)2D3, and the acute effects of different calcium and phosphate concentrations in the incubation medium on the rate of synthesis of 1,25(OH)2D3 by the tubules. Manipulation of dietary calcium and phosphate sufficient to produce marked changes in the concentration of calcium and phosphate in the serum led to altered rates of 1,25(OH)2D3 synthesis by the isolated renal tubules. The renal tubules isolated from chicks raised on a vitamin D-deficient diet containing 0.43% calcium and 0.3% P as inorganic phosphate showed the highest rate of synthesis of 1,25(OH)2D3. Diets containing more or less of either calcium or phosphate produced chicks whose renal tubules had a slower rate of 1,25(OH)2D3 production. The calcium, phosphate, and hydrogen ion content of the incubation medium were manipulated to determine the possible factors concerned with the immediate regulation of 1,25(OH)2D3 production. A calcium concentration of 0.5-1.0 mM was necessary for optimal enzymatic activity. Concentrations of calcium greater than this optimal concentration inhibited 1,25(OH)2D3 production if phosphate was also present, and this inhibition was more pronounced as the phosphate concentration was increased. The stimulation of 1,25(OH)2D3 production by calcium was less at pH 6.7 than at 7.4. Raising the phosphate concentration from 0 to 6 mM in the absence of calcium also stimulated the rate of synthesis of 1,25(OH)2D3. This stimulatory effect was blocked by 4 mM calcium. However, at 1-2 mM calciu, phosphate had a biphasic influence on 1,25(OH)2D3 production; extracellular concentrations of phosphate from 0.6 to 1.2 mM resulted in less 1,25(OH)2D3 production than higher or lower phosphate concentrations. This biphasic effect was seen both at pH 7.4 and 6.8.

The kidney as an endocrine organ involved in the function of vitamin D

Vitamin D3 must be metabilically altered first in the liver to 25-hydroxyvitamin D3 (25 OH-D3) and subsequently in the kidney to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) before it can function. Because 1,25-(OH)2D3 is formed in the kidney and acts in intestine and bone to elevate serum calcium and phosphate concentrations, it can be considered a hormone. The production of 1,25-(OH)2D3 is feedback regulated either directly or indirectly by serum calcium and serum phosphate concentrations. The hypocalcemic regulation is mediated by the parathyroid glands. The hypophosphatemic stimulus, however, does not involve either the parathyroid or thyroid glands. Under conditions whereby the synthesis of 1,25-(OH)2D3 is repressed, 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3 is formed. This metabolite can be converted further to 1,24,25-trihydroxyvitamin D3 (1,24,25-(OH)3D3), which stimulate intestinal calcium transport but not bone calcium mobilization or phosphate transport reactions. A number of vitamin D-resistant bone diseases may be related to defective vitamin D metabolism. For example, bone disease related to choric renal failure likely results from defective formation of 1,25-(OH)2D3 in the kidney. Treatment of this disease with intravenously administered 1,25-(OH)2D3 is effective in correcting the bone lesions. 1Alpha-hydroxyvitamin D3 (1alpha-OH-D3), a new synthetic analog of 1,25-(OH)2D3 which is less expensive to produce than 1,25-(OH)2D3, is effective in anephric animals and may have several advantages over 1,25-(OH)2D3 in treating bone diseases.

Effects of nucleotides, hormones, ions, and 1,25-dihydroxycholecalciferon on 1,25-dihydroxycholecalciferol production in isolated chick renal tubules

1. The production of 1,25-dihydroxycholecalciferol (1,25-DHCC) from tritiated 25-hydroxycholecalciferol (25-HCC) was assessed in renal tubules prepared from vitamin D-deficient chicks.

2. Cyclic AMP and dibutyryl cyclic AMP enhanced the production of 1,25-DHCC.

3. Bovine parathyroid hormone (BPTH) and its synthetic analogue either had no effect or inhibited the production of 1,25-DHCC when calcium was present in the extracellular medium, but enhanced the conversion when calcium was absent. BPTH increased adenylate cyclase activity and cyclic AMP content of the renal tubule cells.

4. Synthetic salmon calcitonin enhanced the production of 1,25-DHCC, whereas human calcitonin had no effect at the concentrations tested. Salmon calcitonin did not increase chick renal adenylate cyclase activity or cyclic AMP levels.

5. A short period of incubation in media containing strontium gluconate (5 mmol/l) significantly increased production of 1,25-DHCC, whereas no effect was seen after more prolonged exposure to strontium. Calcium depletion decreased production of 1,25-DHCC, but when some calcium was present, variation in extracellular concentration of calcium did not affect conversion. Variation in extracellular phosphate concentration did not affect conversion.

6. A small quantity of 1,25-DHCC added to the medium inhibited the conversion of 25-HCC into 1,25-DHCC.

7. The production of 1,25-DHCC is subject to complex control, and intracellular concentrations of calcium, cyclic AMP and 1,25-DHCC may all be important regulatory influences.

Regulation of vitamin D metabolism: factors influencing the rate of formation of 1,25-dihydroxycholecalciferol by kidney homogenates

The formation of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol by chick kidney homogenates is inhibited by increasing concentrations of Ca(2+). The apparent K(m) for the hydroxylation reaction is 1x10(-7)m, significantly lower than that reported for isolated mitochondria. Separated cytoplasmic and particulate fractions are inactive, but on recombination activity is restored, possibly because of the presence in the soluble fraction of a factor with a high affinity for 25-hydroxycholecalciferol.

New ideas on vitamin D

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Regulation by calcium of in vivo synthesis of 1,25-dihydroxycholecalciferol and 21,25-dihydroxycholecalciferol

Tritiated 1,25-dihydroxycholecalciferol accumulates in several tissues, to an extent that varies with dietary calcium content, 12 hr after the administration of 325 pmoles of tritiated 25-hydroxycholecalciferol to rats. As the dietary and serum calcium concentrations increase, the amount of 1,25-dihydroxycholecalciferol is diminished and the concentration of 21,25-dihydroxycholecalciferol increases. This correlation is especially evident in rats given vitamin D(3). In vitamin D-deficient rats, the repression of 1,25-dihydroxycholecalciferol formation occurs with a diet containing 3% calcium and 20% lactose. The results suggest that the production of 1,25-dihydroxycholecalciferol, believed to be the metabolically active form of vitamin D in the intestine, is responsible for the adaptation of calcium absorption to low dietary concentrations of calcium.

A new chromatographic system for vitamin D3 and its metabolites: resoluation of a new vitamin D3 metabolite

A simple yet powerful new chromatographic procedure for vitamin D(3) and its metabolites is described. Liquid-gel partition chromatography on Sephadex LH-20 using a solvent of various percentages of CHCl(3) in Skellysolve B (petroleum ether, bp 67-69 degrees C) permits excellent resolution of vitamin D(3), 25-hydroxyvitamin D(3), and their more polar metabolites. Of special importance is the resolution of the metabolites of vitamin D(3) more polar than 25-hydroxycholecalciferol. Because of this resolution, a new metabolite of vitamin D(3) has been demonstrated in the plasma of rats and in the intestines of chicks given 100 IU of vitamin D(3)-1,2-(3)H.