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Biomedical subjects

F Risco

Publications and source records attributed to F Risco.

6 recordsLinked to original sources
PubMed2004

Bone specific binding sites for 1,25(OH)2D3 in magnesium deficiency.

It has been reported that some hypoparathyroid patients with magnesium deficiency showed altered responses to vitamin D treatment. In the same way, in vitro bone studies have demonstrated the existence of a decrease in the 1,25-dihydroxyvitamin D3-induced resorption in bone as a result of magnesium deficiency. These findings suggest some kind of alteration in the 1,25(OH)2D3 in bone in magnesium deficiency. In the present work, using a binding assay based on the 1,25(OH)2D3 and 3H-1,25(OH)2D3 competition for the hormone binding sites in rat calvaria homogenates, a significant decrease in the number of 1,25(OH)2D3 specific binding sites has been found in calvaria incubated in magnesium-deficient medium compared to magnesium-replete ones. Alterations in the hormone-receptor affinity were not found. These results suggest that an alteration in the 1,25(OH)2D3 action on magnesium-deficient bone could be due, at least in part, to a decrease in the number of available vitamin D receptors in bone cells.

Animals↗
PubMed1995

Possible alterations of the in vivo 1,25(OH)2D3 synthesis and its tissue distribution in magnesium-deficient rats.

In a previous work we showed that a decrease in the free intracellular magnesium produced alterations in the kinetic behaviour of 1 alpha-hydroxylase which reduced the synthesis rate of 1,25(OH)2D3. This in vitro result strongly supports that magnesium deficiency could also induce in vivo failure of the renal enzyme and then a decrease of 1,25(OH)2D3 serum levels. In the present work we have tested the effect of magnesium deficiency on the in vivo transformation of the substrate 3H-25(OH)D3 to 3H-1,25(OH)2D3 as well as the distribution of synthesized hormone among its different target tissues. We found that magnesium deficiency produced a decrease of both the in vivo synthesis of 3H-1,25(OH)2D3 and the binding of the radioactive hormone to bone tissue. These results may explain the different criteria present in the scientific literature concerning the relationships between magnesium status and vitamin D metabolism.

Animals↗
PubMed1994

Possible involvement of a magnesium dependent mitochondrial alkaline phosphatase in the regulation of the 25-hydroxyvitamin D3-1 alpha-and 25-hydroxyvitamin D3-24R-hydroxylases in LLC-PK1 cells.

It has been demonstrated that dephosphorylation of the ferredoxin component of the mitochondrial 25-hydroxyvitamin D3-1-hydroxylase, as a result of a PTH-cAMP mediated activation, involves a protein phosphatase activity. However, the nature and properties of this phosphatase are uncertain. It has been proved that alkaline phosphatase, a magnesium dependent enzyme, could dephosphorylate in vitro the ferredoxin component of the 25-hydroxyvitamin D3-1-hydroxylase. Moreover, some evidence of mitochondrial localization of some alkaline phosphatases has been published. Although the existence of a levamisole inhibitable alkaline phosphatase activity has been described in renal cells, its role remains to be elucidated. In the present work, the existence of an alkaline phosphatase in mitochondrial membrane preparations from LLC-PK1 cells has been described. This alkaline phosphatase is magnesium dependent and levamisole inhibitable. Preparations of mitochondrial membrane from LLC-PK1 cells also showed 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) and 25-hydroxyvitamin D3-24R-hydroxylase (24-hydroxylase) activities being both enzymes responsive to the 8Br-cAMP mediated regulation. The 8Br-cAMP not only stimulated the 1-hydroxylase and inhibited the 24-hydroxylase activities but also increased the mitochondrial alkaline phosphatase activity. In the same way, the levamisole (specific inhibitor of some alkaline phosphatases) inhibited the mitochondrial alkaline phosphatase and also the 1-hydroxylase activity. In addition, the inhibition of mitochondrial alkaline phosphatase by levamisole avoids the effect of 8Br-cAMP on the 1-hydroxylase and 24-hydroxylase activities. On the other hand, the mitochondrial alkaline phosphatase and the 1-hydroxylase activities showed similar behaviour with respect to the magnesium concentrations in the incubation medium. Taking these results together it could be possible to suggest the implication of the Mg(2+)-dependent mitochondrial alkaline phosphatase activity found in LLC-PK1 cells in the regulation of the 1,25(OH)2D3 and 24,25(OH)2D3 synthesis.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1992

Influence of magnesium on the in vitro synthesis of 24,25-dihydroxyvitamin D3 and 1 alpha, 25-dihydroxyvitamin D3.

The role of magnesium in the regulation of 25(OH)D3-24R-hydroxylase and 25(OH)D3-1 alpha-hydroxylase in vitro remains to be clarified. Using a radiochemical assay, the effect of substrate concentration on both enzymes was measured in rat kidney homogenates as well as the alterations in their kinetic behaviour resulting from changes in the magnesium concentration in the incubation medium. The results showed that magnesium could be a modulator of 24- and 1 alpha-hydroxylases. Variations in magnesium caused alterations in the kinetic behaviour of both enzymes, which altered the synthesis rates of 24,25(OH)2D3 and 1,25(OH)2D3. Moreover, the substrate concentration was clearly involved in the response of both enzymes to the modulation by magnesium.

24,25-Dihydroxyvitamin D 3↗
PubMed1990

Evidence of 1,25-dihydroxyvitamin D synthesis by the rat Walker carcinosarcoma 256.

We tested the existence of 25-hydroxyvitamin D-1 alpha-hydroxylase activity in the Walker carcinosarcoma 256 implanted in rats. This tumour has been shown to induce hypercalcaemia in the host animal. We found this enzyme activity in tumour homogenates, which was in the same range as that in the kidney of tumour-bearing rats. Our results suggest that 1,25-dihydroxyvitamin D synthesized by the Walker tumour might be involved in the mechanism responsible for the hypercalcaemia in the host rat.

25-Hydroxyvitamin D3 1-alpha-Hydroxylase↗