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Pharmacology: Pharmacodynamics: Mode of action: A major hormonal actions of vitamin D is to maintain serum calcium and phosphorus levels in a healthy physiologic range to sustain a variety of metabolic functions, transcription regulation, and bone metabolism. The interaction of 1,25(0H)2 D with its vitamin D receptor (VDR) in the small intestine increases the efficiency of intestinal calcium absorption from approximately 10-15% up to 30-40% and intestinal phosphorus absorption from approximately 60% to 80%. In the kidneys, 1,25(OH)2D stimulates calcium reabsorption from the glomerular filtrate.
Pharmacokinetics: Cholecalciferol are well absorbed from the gastrointestinal tract, enters the circulation on chylomicrons first and it is only slowly transferred to the specific plasma binding protein (DBP). Vitamin D3 is mainly bound to DBP. The consequence of chylomicron transport of dietary vitamin D is the possibility of uptake by peripheral tissues, such as adipose tissue and muscle, due to the action of lipoprotein lipase. The 25-hydroxylase enzyme in the liver converts vitamin D into 25(OH)D which quickly enters the plasma pool that constitutes the predominant pool, then it is carried to the kidney where to be transformed into active form 1,25-dihydroxycholesterol (calcitriol) by 1-a-hydroxylase enzyme.
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