Effect of Vitamin D3 and 1,25-Dihydroxyvitamin D3 on Intestinal Transport of Phosphate

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Abstract
Vitamin D plays an important role in the regulation of phosphate (Pi) metabolism. The fact that vitamin D exerts parallel effects on calcium and phosphate in its main target organs (intestine, bone and kidney) has created some difficulty in separating the effects of the vitamin on the cellular transport of these ions. There is now increasing evidence that vitamin D stimulates a phosphate transport mechanism in intestine quite independent of the calcium transport system (l,2). Two facts have eased the recognition of the unique effect of vitamin D on intestinal phosphate absorption. First, maximal stimulation of phosphate absorption occurs in the jejunum (3,4) while the duodenum is known as site of the maximal response of calcium absorption to vitamin D. Second, higher rates of transmural phosphate transport can be maintained in the virtual absence of extracellular calcium, i.e. phosphate does not require a simultaneous movement of calcium in order, to be transported across the gut wall (1,2).