Physiological effects of vitamin D
Vitamin D receptors are present in most cells in the body and are located mainly in the cell nucleus. Similar to steroid and thyroid hormone receptors, vitamin D receptors have hormone-capturing regions and binding sites in DNA.
The active form of vitamin D, 1,25-dihydroxycholecalciferol, has multiple effects on the intestines, kidneys, and bones that increase the absorption of calcium and phosphate into the extracellular fluid and participate in their upregulation. Vitamin D receptors are present in most cells in the body and are located mainly in the cell nucleus. Similar to steroid and thyroid hormone receptors, vitamin D receptors have hormone-binding domains and DNA binding domains. The vitamin D receptor forms a complex with the intracellular receptor, the retinoid-X receptor, and the complex binds DNA and activates transcription in most cases.
However, in some cases, Vitamin D blocks transcription. Although the vitamin receptor binds to many forms of cholecalciferol, its affinity for 1,25-dihydroxycholecalciferol is approximately 1000 times that for 25 hydroxycholecalciferols, which explains their relative biological activity.
The hormone-like effect of vitamin D helps to increase calcium absorption
1,25-Dihydroxycholecalciferol functions as a “hormone” that promotes intestinal calcium absorption. It promotes this absorption mainly by increasing, after a period of 2 days, the formation of calbindin, a protein that helps to capture calcium, in intestinal epithelial cells. The function of this protein in the brush border of cells helps to transport calcium into the cytoplasm. Calcium then moves across the basolateral membrane of the cell by facilitated diffusion. The rate of calcium absorption is proportional to the amount of this calcium-capturing protein. Furthermore, this protein remains in cells for weeks after 1,25-dihydroxycholecalciferol has been eliminated from the body, thereby prolonging the calcium absorption effects of the other 1, 25-dihydroxycholecalciferol may also play a role in increasing calcium absorption by the formation of (1) adenosine calcium stimulated triphosphatase in the brush border of epithelial cells and (2) an alkaline phosphatase (alkaline phosphatase) in the epithelial cells. epithelial cells. The evidence for this effect is still unclear.
Vitamin D supports phosphate absorption in the gut
Although phosphate is normally readily absorbed, phosphate flux across the gastrointestinal epithelium is enhanced by vitamin D. This enhancement is believed to result from the direct action of 1,25-dihydroxycholecalciferol, but it has maybe secondary to this hormone's effect on calcium absorption. Because calcium acts as an intermediate in the transport of phosphates.
Vitamin D reduces urinary calcium and phosphate excretion
Vitamin D also increases calcium and phosphate reabsorption by renal tubular epithelial cells, thereby reducing urinary excretion. However, this effect is weak and probably does not play an important role in regulating concentrations of these substances in the extracellular fluid.
Effects of vitamin D on bone and its relationship to parathyroid hormone activity
Vitamin D plays an important role in bone resorption and bone deposition. Too much vitamin D causes bone destruction. In the absence of vitamin D, the osteolytic effects of PTH are greatly reduced or even prevented. This mechanism of action of vitamin D is not fully understood but is thought to be the result of the effects of 1,25-dihydroxycholecalciferol to increase calcium transport across cell membranes.
Vitamin D in small amounts helps increase bone calcification
One of the ways to promote calcification is to increase the absorption of calcium and phosphate from the gut.
However, even in the absence of this increase, it still enhances bone mineralization. Again, the mechanism of this effect is unclear but is probably also due to the ability of 1,25-dihydroxycholecalciferol to induce the transport of calcium ions across the cell membrane, but in this case, perhaps reverse direction through osteoblasts or osteoclasts.