Cholecalciferol absorption

Ultraviolet light causes a chemical change in dihydrocholesterol to produce cholecalciferol, a precursor of vitamin D. The latter conforms better to the definition of a steroid hormone than a vitamin. Indeed, the classification of vitamin D as a vitamin is an historical accident. The precursor is released from the skin and is further modified in the liver and kidney to form dihydroxycholecalciferol, which is the active form of the hormone (see Chapter 15 for the reactions). It increases calcium absorption from the... 

Vitamin D hormone is derived from vitamin D (cholecalciferol). Vitamin D can also be produced in the body it is formed in the skin from dehydrocholesterol during irradiation with UV light. When there is lack of solar radiation, dietary intake becomes essential, cod liver oil being a rich source. Metaboli-cally active vitamin D hormone results from two successive hydroxylations in the liver at position 25 ( calcifediol) and in the kidney at position 1 ( calci-triol = vit. D hormone). 1-Hydroxylation depends on the level of calcium homeostasis and is stimulated by parathormone and a fall in plasma levels of Ca or phosphate. Vit D hormone promotes enteral absorption and renal reabsorption of Ca and phosphate. As a result of the increased Ca + and phosphate concentration in blood, there is an increased tendency for these ions to be deposited in bone in the form of hydroxyapatite crystals. In vit D deficiency, bone mineralization is inadequate (rickets, osteomalacia). Therapeutic Liillmann, Color Atlas of Pharmacology... 

Cholecalciferol Regulate gene transcription via the vitamin D receptor Stimulate intestinal calcium absorption, bone resorption, renal calcium and phosphate reabsorption decrease parathyroid hormone (PTH) promote innate immunity inhibit adaptive immunity Osteoporosis, osteomalacia, renal failure, malabsorption Hypercalcemia, hypercalciuria the vitamin D preparations have much longer half-life than the metabolites and analogs... 

Cholecalciferol [Vitamin D3] (Delta D) [Vitamin/Dietary Supplement] Uses Dietary supl to Rx vit D deficiency Action T Intestinal Ca2+ absorption Dose 400-1000 Int Units/d PO Caution [A (D doses above the RDA), +] Contra T Ca2+, hypervitaminosis, allergy Disp Tabs SE Vit D tox Interactions T Risk of arrhythmias w/ cardiac glycosides X effects w/ cholestyramine, colestipol, mineral oil, orlistat, phenobarbital, phenytoin EMS Can cause vit D tox (Tin serum Ca2+ weakness, AMS, Gl upset and cardiac arrhythmias) OD May cause T risk of vitD tox give IV fluids... 

With normal exposure to sunlight enough 7-dehydrocholesterol is converted to cholecalciferol in the skin that it was concluded that no dietary vitamin D is required by most adults except during pregnancy. However, recently it has been recognized that old and sick adults probably need 400-600 I.U. per day to maintain calcium absorption arid to prevent osteoporesis and fractures/ " 1 It is usual-... 

As a brief introductory summary, vitamin D substances perform the following fundamental physiological functions (1) promote normal growth (via bone growth) (2) enhance calcium and phosphorus absorption from the intestine (3) serve to prevent rickets (4) increase tubular phosphorus reabsorpiion (5) increase citrate blood levels (6) maintain and activate alkaline phosphatase m bone (7) maintain serum calcium and phosphorus levels. A deficiency of D substances may be manifested in the form of rickets, osteomalacia, and hypoparathyroidism. Vitamin D substances are required by vertebrates, who synthesize these substances in the skin when under ultraviolet radiation, Animals requiring exogenous sources include infant vertebrates and deficient adult vertebrates, Included there are vitamin D (calciferol ergocalciferol) and vitamin D< (activated 7-dehydrocholesterol cholecalciferol). 

Vitamin D regulates calcium and phosphorus absorption and deposition and serum alkaline phosphatase levels. The recommended daily allowance is 5 /xg, increasing to 10 to 15 /xg in older age.109 Vitamin D3 is synthesized under UVB irradiation in the skin where it is stored and released into the circulation in a complex with the vitamin D binding protein. In liver it is hydroxylated to 25(OH)-cholecalciferol, the hormonal precursor, followed by another hydroxylation step in the... 

B. The infrared absorption spectrum of a potassiumbromide dispersion of the sample, in the range of 2 to 12 jxm, exhibits maxima only at the same wavelengths as those of a similar preparation of USP Cholecalciferol Reference Standard. 

C. The ultraviolet absorption spectrum of the sample in a 1 100,000 solution in ethanol exhibits inflections at the same wavelengths as that of USP Cholecalciferol Reference Standard, similarly prepared, and the respective absorptivities at... 

They facilitate the intestinal absorption of fat-solnble vitamins (vitamin A, retinol vitamin D, cholecalciferol vitamin E, tocopherol and vitamin K). 

Early studies showed that, after the administration of [ H]cholecalciferol or ergocalciferol to vitamin D-deficient animals, there is marked accumulation of [ H] calcitriol in the nuclei of intestinal mucosal cells. Physiological doses of vitamin D cause an increase in the intestinal absorption of calcium in deficient animals the response is faster after the administration of calcidiol and faster stUl after calcitriol. There are two separate responses of intestinal mucosal cells to calcitriol a rapid increase in calcium uptake that is due to recruitment of calcium transporters to the cell surface (Section 3.3.2) and a later response from the induction of a calcium binding protein, calbindin-D. 

Disposition in the Body. Well absorbed after oral administration and subject to enterohepatic circulation decreased absorption may occur in subjects with impaired liver and biliary function. Metabolised by hydroxylation to active metabolites. The major metabolite is 25-hydroxycholecalciferol which is formed in the liver. This is further metabolised by la- or 24-hydroxylation in the kidneys. Most of a dose is excreted in the bile and eliminated in the faeces about 25% of a dose is excreted as conjugates. Unchanged cholecalciferol does not appear to be excreted in the urine. 

Biochemistry. Vitamin D is introduced into the bloodstream either from the skin after natural synthesis by the irradiation of 7-dehydrocholesterol stored in the epidermis (172) or by ingestion and absorption of vitamin D2 or vitamin D through the gut wall (40). Between 60 and 80% of the vitamin introduced in the blood is taken up by the Hver, where cholecalciferol is transferred from chylomicrons to a vitamin D-binding protein (DBF), an OC-globulin specific for vitamin D and its metaboHtes but one which does not bind with previtamin D in the skin (173). Cholecalciferol is hydroxylated in the Hver at the C-25 position (51,141,174). This hydroxylation occurs in the endoplasmic reticulum and requires NADPH, a flavoprotein, cytochrome P-450, Mg ", and O2 (175). 25-Hydroxylation also occurs in intestinal homogenates of chicks (176), but does not appear to occur outside the... 

Cholecalciferol (or Ergocalciferol) is absorbed from the intestinal tract and requires the presence of bile salts, after absorption it is transported to the hver bound to a specific vitamin D-binding protein (alpha-2 globulin). 

Cholecalciferol is a rodenticide that has been introduced relatively recently and that has been reported to be frequently involved in poisonings of dogs. The compound alters calcium homeostasis by promoting calcium absorption from the gut and also by mobilizing calcium from bone for tissue deposition. Consequently, poisoned animals have increased levels of blood calcium. The calcium is then subsequently deposited in soft tissues like the kidneys, digestive tract mucosa, lungs, heart, liver, and muscle. Mineralization of soft tissues interferes with normal function of these organs. 

Vitamin D occurs in a number of sterol forms, such as vitamin D3 - cholecalciferol - a natural form in foods and made in the skin by the action of UV, and vitamin D2 -ergocalciferol - which is formed in plants by the action of sunlight. These forms are 25-hydroxylated in the kidney, and then la-hydroxylated in the kidney (under the control of parathormone), to make the most active form. This is available as calcitriol (la,25-dihydroxycholecalciferol). Vitamin D facilitates the absorption of calcium and, to a lesser extent, phosphorus from the intestine and promotes deposition into the bones. A deficiency of vitamin D, therefore, results in bone deficiency disorders, e.g. rickets in children. Good food sources include eggs, milk and cheese. 

OH) 2 cholecalciferol Sterol Intestine Bone Facilitation of calcium and phosphorus absorption increase in bone resorption in conjunction with PTH... 

This could include retinol, cholecalciferol/ergocalciferol, a-tocopherol, vitamin K from food, all of which are more readily absorbed when they can be part of a normal mixed micelle process that occurs with lipid digestion and absorption... 

Vitamin D can be obtained from some foods, but its major source is through the action of ultraviolet radiation, which converts 7-dehydrocholesterol to cholecalciferol in the skin. Hydroxylation of this compound in the liver produces 25-hydroxycholecalciferol, which is then converted in the kidney to 1,25-hydroxycholecalciferol, the active form of vitamin D. Vitamin D plays a major role in promoting absorption of calcium and maintaining bone mineralization. Recently, research has focused on immunosuppressive effects of vitamin D. The vitamin D receptor has been detected in lymphocytes and the thymus, and vitamin D plays a role in T cell-mediated immune response (Deluca Cantoma, 2001). 

Calcium homeostasis is modulated by hormones (Fig. 2). Parathyroid hormone (PTH) is the most important calcium regulator. It is a hormone of 84 amino acids, and is secreted from the parathyroid glands in response to a low unbound plasma calcium. PTH causes bone resorption and promotes calcium reabsorption in the renal tubules, preventing loss in the urine. 1,25-dihydroxycholecalciferol (1.23 DHCC) maintains intestinal calcium absorption. This sterol hormone is formed from vitamin D (cholecalciferol). following hydroxylation in the liver (at carbon-25) and kidney (at carbon-1). However.hydroxylation in the kidney is PTH dependent.andsoeven (he absorption of calcium from the gut relies (albeit indirectly) on PTH. 

Vitamin D is involved in phosphorous and calcium metabolism. Cholecalciferol is converted into compounds that are directly involved in the absorption of calcium by the intestines. As with many steroid compoxmds, when cholecalciferol breaks down in the body, the resulting molecules are carried into the nucleus of certain cells and determine or change which genes are turned on or off. Recent reports suggest that the steroid hormone character of vitamin D may provide some anticancer activity. 

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