1,25-dihydroxyvitamin D
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| Pronunciation |
US: /ˌkælsᵻˈtraɪ.ɒl/; UK: /kælˈsɪtri.ɒl/ |
| Trade names | Rocaltrol, Calcijex, Decostriol |
| MedlinePlus | a682335 |
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Oral, IV, topical |
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| Protein binding | 99.9% |
| Metabolism | Renal |
| Biological half-life | 5–8 hours (adults), 27 hours (children) |
| Excretion | Faeces (50%), urine (16%) |
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| ECHA InfoCard | 100.046.315 |
| Chemical and physical data | |
| Formula | C27H44O3 |
| Molar mass | 416.64 g/mol |
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Calcitriol (INN), also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D3, is the hormonally active metabolite of vitamin D with three hydroxyl groups (abbreviated 1,25-(OH)2D3 or simply 1,25(OH)2D). It was first identified by Michael F. Holick in work published in 1971. Calcitriol increases the level of calcium (Ca2+) in the blood by increasing the uptake of calcium from the gut into the blood, increasing reabsorption of calcium by the kidneys, and possibly increasing the release of calcium into the blood from bone.
Calcitriol usually refers specifically to 1,25-dihydroxycholecalciferol. Because cholecalciferol already has one hydroxyl group, only two are further specified in the nomenclature.
Calcitriol is marketed under various trade names including Rocaltrol (Roche), Calcijex (Abbott), Decostriol (Mibe, Jesalis), Biowoz (Solmarc) and Vectical (Galderma), Rolsical (Sun Pharma).
Calcitriol increases blood calcium levels ([Ca2+]) by:
Calcitriol acts in concert with parathyroid hormone (PTH) in all three of these roles. For instance, PTH also indirectly stimulates osteoclasts. However, the main effect of PTH is to increase the rate at which the kidneys excrete inorganic phosphate (Pi), the counterion of Ca2+. The resulting decrease in serum phosphate causes hydroxyapatite (Ca5(PO4)3OH) to dissolve out of bone thus increasing serum calcium. PTH also stimulates the production of calcitriol (see below).
Many of the effects of calcitriol are mediated by its interaction with the calcitriol receptor, also called the vitamin D receptor or VDR. For instance, the unbound inactive form of the calcitriol receptor in intestinal epithelial cells resides in the cytoplasm. When calcitriol binds to the receptor, the ligand-receptor complex translocates to the cell nucleus, where it acts as a transcription factor promoting the expression of a gene encoding a calcium binding protein. The levels of the calcium binding protein increase enabling the cells to actively transport more calcium (Ca2+) from the intestine across the intestinal mucosa into the blood.
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