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At least 325 records · Page 18Linked to original sources
PubMed1990

Binding properties of plasma vitamin D-binding protein and intestinal 1,25-dihydroxyvitamin D3 receptor in piglets with pseudo-vitamin D-deficiency rickets, type I: treatment effects with pharmacological doses of vitamin D3.

The effective treatment of the rachitic symptoms of pseudo-vitamin D-deficiency rickets, type I (PVDRI) by massive doses of vitamin D3 was examined. For this purpose, the affinities and the maximum binding capacities (Bmax) of the plasma vitamin D-binding protein (DBP) and of the intestinal 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) receptor for vitamin D3, 25-hydroxyvitamin D3 (25-OHD3) and 1,25-(OH)2D3, were investigated in normal piglets and in rachitic piglets that suffered from PVDRI. The piglets were 5 to 10 weeks old and of both sexes. The Bmax of plasma DBP for 25-OHD3 was 6.77 +/- 0.45 microM for PVDRI piglets and 7.30 +/- 0.41 microM for control piglets and showed no differences between the two groups. Equilibrium association constants (Ka) of DBP for 25-OHD3 were 4.3 x 10(8) M-1 for PVDRI piglets and 4.0 x 10(8) M-1 for controls and showed also no differences between the two groups. Similarly the Ka of DBP for 1,25-(OH)2D3 was also the same for rachitic and control piglets (1.45 x 10(7) and 1.54 x 10(7) M-1, respectively). Due to the lower circulating concentration of 1,25-(OH)2D3 in the plasma of rachitic piglets compared to that of controls its free metabolite index was significantly lower in rachitic (0.42 +/- 0.05 x 10(-5)) than in control piglets (3.63 +/- 0.30 x 10(-5)). The Kd and Bmax of the intestinal nuclear receptor for 1,25-(OH)2D3 of rachitic and control piglets were 0.31 +/- 0.05 and 0.33 +/- 0.05 nM and 674 +/- 103 and 719 +/- 123 fmol/mg protein, respectively, and were also not different between the two groups of piglets. It was concluded from these observations that the rachitic symptoms of PVDRI piglets resulted solely from the lower free 1,25-(OH)2D3 concentration in plasma compared to that of normal piglets. The relative affinities of the intestinal 1,25-(OH)2D3 receptor for vitamin D3 and 25-OHD3 were also measured. It was found that 50% displacement of 1,25-(OH)2D3 from the intestinal receptor of PVDRI and control piglets required a 220,000- and 245,000-fold excess of the free concentration of vitamin D3, respectively, and a 20- to 42- and 23- to 71-fold excess of the free concentration of 25-OHD3, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Identification of 8 alpha,25-dihydroxy-9,10-seco-4,6,10(19)-cholestatrien-3-one as a product of metabolism of 25-hydroxycholecalciferol by liver microsomes.

The ability of liver microsomes, sites of synthesis of 25-hydroxycholecalciferol, to further metabolize 25-hydroxycholecalciferol has been assessed. When liver microsomes were incubated with 25-hydroxycholecalciferol in the presence of cytosol, a metabolite was isolated that comigrated with 8 alpha,25-dihydroxy-9,10-seco-4,6,10(19)-cholestatrien-3- one in three different chromatographic systems. The ultraviolet spectrum (220-350 nm) and mass spectrum of the purified metabolite were identical to that of synthetic 8 alpha,25-dihydroxy-9,10-seco-4,6,10(19)-cholestatrien-3-one. This study indicates that liver microsomes convert 25-hydroxycholecalciferol to 8 alpha,25-dihydroxy-9,10-seco-4,6,10(19)-cholestatrien-3-one. The significance of this metabolite, which has been shown previously by others to be produced by alveolar macrophages, has yet to be determined.

Animals↗
PubMed1984

25-Hydroxysterols increase the permeability of liposomes to Ca2+ and other cations.

25-Hydroxycholesterol and 25-hydroxy vitamin D-3 increased the permeability of liposomes to Ca2+ measured by the arsenazo III encapsulation technique. This effect was sensitive to the lipid composition of the membrane, with changes that decreased the motional freedom of phospholipid acyl chains decreasing Ca2+ permeability. The greatest permeability was observed with the zwitter-ionic phospholipids, phosphatidylcholine and phosphatidylethanolamine, whereas the acidic phospholipids, phosphatidylinositol and phosphatidylserine, depressed Ca2+ permeability. The effect was not specific for Ca2+. Other divalent cations were translocated in the order Mn2+ greater than Mg2+ = Ca2+ much greater than Sr2+ = Ba2+. The permeability of liposomes to the monovalent cation, Na+, was also substantially increased. The effect did not appear to be due to ionophoretic properties of the sterols, and it is suggested that perturbation of the membranes by the polar 25-hydroxyl group may play a role in increasing membrane permeability.

Arsenazo III↗
PubMed1987

The phospholipid and fatty acid composition of skeletal muscle cells during culture in the presence of vitamin D-3 metabolites.

The phospholipid and fatty acid composition of primary cultures (24 h) of chick embryo skeletal muscle myoblasts treated for 4-24 h with physiological concentrations of 1,25-dihydroxyvitamin D-3 and 25-hydroxyvitamin D-3 were analyzed. 25-Hydroxyvitamin D-3 did not alter the relative amounts of individual muscle cell phospholipids whereas 1,25-dihydroxyvitamin D-3 significantly increased phosphatidylcholine content, mainly at the expense of a decrease in phosphatidylethanolamine concentration. The increase in phosphatidylcholine occurred at a faster rate during the first 8 h than in the subsequent 8-24 h treatment period. A similar time course in 1,25-dihydroxyvitamin D3-dependent changes in myoblast calcium uptake has been observe. In addition, this metabolite markedly increased (100%) the arachidonate content of myoblast phosphatidylcholine near the fusion stage of the cells (24 h of treatment). The levels of docosahexaenoate, a minor polyunsaturated fatty acid, in phosphatidylcholine and phosphatidylethanolamine were also substantially elevated by 1,25-dihydroxyvitamin D-3. No significant changes in fatty acid composition in response to 25-hydroxyvitamin D-3 were observed. Modifications in phospholipids and polyunsaturated fatty acids may play a role in the effects of 1,25-dihydroxyvitamin D-3 on muscle cell calcium transport and differentiation.

Animals↗
PubMed1976

Absence of regulatory effects of 1alpha25-dihydroxyvitamin D3 on 25-hydroxyvitamin D metabolism in rats constantly infused with parathyroid hormone.

The regulatory role of 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2-D3] in metabolism of 25-hydroxyvitamin D was studied in sham-operated (sham) or thyroparathyroidectomized (TPTX) vitamin D-deficient rats into which calcium and parathyroid hormone (PTH) were constantly infused. A single dose of 325 or 650 pmol of 1alpha,25-(OH)2-D3 caused significant inhibition of 1alpha,25-(OH)2-D3 synthesis in D-deficient sham rats. This inhibition by 1alpha,25-(OH)2-D3, however, was not observed in D-deficient TPTX rats into which PTH was constantly infused. These results can be explained by supposing that the major regulatory effect of 1alpha,25-(OH) 2-D3 on 1alpha,25-(OH)2-D3 synthesis is realized mostly, if not all, by suppressing endogenous secretion of PTH.

Animals↗
PubMed1983

Extremely high circulating levels of 1 alpha,25-dihydroxyvitamin D3 in the marmoset, a new world monkey.

Compared to most mammals, the marmoset, a new world monkey, requires particularly large amounts of vitamin D to maintain normal growth. We compared serum concentrations of vitamin D metabolites in marmosets with rhesus monkeys and humans. The circulating levels of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] in marmosets were 4 to 10 times higher than those in rhesus monkeys and humans. But none of the marmosets exhibited hypercalcemia. In two marmosets which had suffered bone fractures, the 1 alpha,25(OH)2D3 levels were particularly elevated. These results suggest that the marmoset has an end-organ resistance to 1 alpha,25(OH)2D3.

24,25-Dihydroxyvitamin D 3↗
PubMed1983

Rat kidney microsomes convert 25-hydroxyvitamin D3 to an unidentified metabolite.

Kidney microsomes from vitamin D-deficient rats and from thyroparathyroidectomized rats converted 25-hydroxyvitamin D3 to an unidentified metabolite. The addition of the cytosolic fraction enhanced microsomal synthesis of this metabolite two-fold. The kinetics of the conversion in the presence of the cytosolic fraction was allosteric, suggesting that the enzyme responsible for synthesis of this metabolite might serve some role in the regulation of vitamin D metabolism. Microsomes from vitamin D-fed thyroparathyroidectomized rats also produced a second metabolite, tentatively identified as 25,26-dihydroxyvitamin D3 because of its comigration with 25,26-dihydroxyvitamin D3 in three different chromatographic systems.

Animals↗
PubMed1984

Sarcoid granulomas metabolize 25-hydroxyvitamin D3 in vitro.

Sarcoid granulomas metabolized 25-hydroxyvitamin D3 to two unidentified metabolites during in vitro incubation. A two-step high pressure liquid chromatography system revealed two unique elution positions of these sarcoid-derived metabolites that exactly comigrated with the elution positions of 5(Z)-19-nor-10-oxo-25(OH)D3 and 5(E)-19-nor-10-oxo-25(OH)D3, respectively. These unique metabolites did not bind specifically to a protein receptor for 1,25(OH)2D3.

Biopsy↗
PubMed1986

A role for the cytoskeleton in renal vitamin D metabolism.

Conversion of circulating 25-hydroxyvitamin D3 (25(OH)D3) to its active metabolite 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) occurs in the renal tubule mitochondrion. Recent reports have implicated the cytoskeleton in certain other steroid metabolizing cells as a mediator of a rate-limiting mitochondrial transport step. Whilst the activity of the renal converting enzyme, a typical steroid hydroxylase, is known to be regulated closely by a number of well studied factors, no information is available to indicate whether an analogous transport step is relevant to the regulation of vitamin D metabolism. Cytochalasin B and vinblastine were used as chemical antagonists of the microfilamentous and microtubular elements of the cytoskeleton. Both agents inhibited the conversion of 25(OH)D3 to 1,25(OH)2D3 by isolated vitamin D-deficient chick renal tubules in a dose-dependent manner. At the concentrations required to inhibit 25(OH)D3-1 alpha-hydroxylase activity in whole cells, these agents inhibited neither isolated mitochondrial 1,25(OH)2D3 production, nor 24,25(OH)2D3 synthesis by vitamin D-replete tubules. The cytoskeletal antagonists were found to increase the content of labelled 1,25(OH)2D3 and 25(OH)D3 in a mitochondrial fraction prepared by Percoll fractionation of tubule cells pre-exposed to the antagonists and labelled 25(OH)D3 substrate. The data suggest that disruption of the cytoskeleton may result in inhibition of transport of newly synthesised 1,25(OH)2D3 out of the mitochondrion and through the cell, and accumulating 1,25(OH)2D3 may oppose its further synthesis. This is consistent with a transport process mediated by the cytoskeleton being involved in the regulation of renal vitamin D metabolism.

Animals↗
PubMed1987

25-hydroxylation of vitamin D3 in rat liver: roles of mitochondrial and microsomal cytochrome P-450.

A mitochondrial cytochrome P-450 fraction, which catalyzed 25-hydroxylation of vitamin D3 much more efficiently than intact mitochondria was isolated from livers of male and female rats. For comparison, a microsomal cytochrome P-450 fraction was isolated by the same procedures. The mitochondrial cytochrome P-450 from female rats catalyzed 25-hydroxylation as efficiently as the same material from male rats. The microsomal 25-hydroxylation was male specific. The 25-hydroxylase activity in intact mitochondria and the 25-hydroxyvitamin D3 concentration in serum were similar in male and female rats. There was no correlation between the 25-hydroxylase activity in microsomal cytochrome P-450 and the 25-hydroxyvitamin D3 concentration in serum.

Animals↗
PubMed1988

1,25-Dihydroxyvitamin D3 stimulates Ca-ATPase in a vascular smooth muscle cell line.

I have recently shown that a vascular smooth muscle cell line has receptors for 1,25-dihydroxyvitamin D. To examine a possible role of 1,25-dihydroxyvitamin D in the regulation of cellular calcium homeostasis, effect of the sterol on Ca-ATPase activity was studied. 1,25-dihydroxyvitamin D3 stimulated the enzyme activity in a dose dependent manner at 10(-10) and 10(-9) M when the cells were incubated with the sterol for 10 hours. By contrast, 60 minutes incubation of the cells with 1,25-dihydroxyvitamin D failed to stimulate the enzyme activity. Both 25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 did stimulate the enzyme activity, but at three orders of magnitude higher concentrations. Cycloheximide, at 10(-5)M, abolished the action of 1,25-dihydroxyvitamin D3. These data suggest that 1,25-dihydroxyvitamin D3 stimulates Ca-ATPase activity probably via its receptor and may play a role in regulating cellular calcium homeostasis.

24,25-Dihydroxyvitamin D 3↗