The role of vitamin D, calcium absorption in the intestine

The role of vitamin D in the absorption of calcium has long remained unclear. Originally thought vitamin D is a carrier of calcium ions through the intestinal wall. However, subsequent observations showed that he was not able to bind calcium. More successful was the theory of the mechanism of citrate calcium absorption in the intestine (Carllsson, Hollunger, 1954 W. and M. Newman, 1961). According to this hypothesis, vitamin D causes increased formation of citric acid to alkaline border of the intestinal mucosa. Citric acid forms a soluble complex with calcium citrate, calcium, which enters the blood serum, which is oxidized with the release of ionized calcium.
Subsequent work (Schachter, Dowdle, 1960 NE and N. Harrison, 1965) have shown that calcium absorption is the energy-dependent processes associated with the system of oxidative phosphorylation, because the active calcium transport stimulated by vitamin D, requires the presence of oxygen and the exchange of the substrate. The mechanism of absorption due to the ability of vitamin D to increase the permeability of cell membranes lining the small intestine to calcium ions.
According to E. S. Holdsworth (1965), the surface of epithelial cells facing the lumen of the intestine and the serosa, contains forward-volatile kaltsiynasasyvayuschie system (H1 and H2). Cell also performs bidirectional diffuse transport (D1 and D2) of calcium (Fig. 5).
Consequently, the active calcium transport are two directions: from the mucosal cells of the intestinal lumen (H) and the serosa (H2). Thereby decreasing intracellular calcium concentration, facilitates conditions for diffusive transport of the electrolyte. Diffusive transport (D1 and D2) depends on the permeability of the membrane and is in the direction of the concentration gradient (difference between intra-and extracellular calcium content).
A thorough review of the views on the mechanism of action of vitamin D in the body is associated with the work of R. Wasserman and colleagues (1969), LD Wrigth (1969). In experimental studies have convincingly shown that under the influence of vitamin D in the intestinal mucosa rachitic chickens synthesize a specific protein, was directly involved in the absorption of calcium. These results were confirmed by experiments with the use of antibiotics that inhibit protein synthesis in the cell. If animals with signs of rickets simultaneously with vitamin D was administered actinomycin D or puromycin, a specific protein in the intestinal mucosa is not synthesized and calcium absorption is not increased (Bauman, 1971; Krawitt, Kunin, 1971).
It is known that actinomycin D inhibits the transcription of DNA to RNA and. Therefore, scientists have believed that the mechanism of action of vitamin D is also associated with transcription in the nucleus. This conjecture was proved in experiments on incorporation of labeled uridine and orotic acid in a fraction of the nuclear RNA of cells of the intestinal mucosa of animals with rickets after receiving vitamin D3.
According to M. R. Haussler, AW Norman (1967, 1969), the introduction of physiological doses of vitamin D3 rachitic chickens most of the label in the nuclei of intestinal mucosa is directly related to chromatin. They singled out nuclear chromatin acidic protein, which, in their view, the specific receptor of vitamin D3 or its metabolites. The interaction of vitamin D to this receptor causes depression of the gene encoding protein synthesis, which is involved in the transport of calcium.

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