Vitamin D metabolism and expression in rats fed on low-calcium and low-phosphorus diets.
1. Cholecalciferol, radioactively labelled with both (14)C and (3)H, was administered weekly for 7 weeks to rats that had been depleted of vitamin D for 4 weeks before repletion with the radioactive vitamin. This permitted measurement of the steady-state effect on vitamin D metabolism of low-calcium and low-phosphorus regimens, as compared with a normal mineral intake. These dietary manoeuvres were carried out during the last 3 weeks of repletion. Cholecalciferol, 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol were determined in plasma, intestine, kidney and bone. Ca(2+)-binding-protein content was measured in intestine and kidneys of comparable animals. 2. In rats on the low-calcium diets, 1,25-dihydroxycholecalciferol concentration was elevated in plasma, bone, kidney and intestine, and intestinal Ca(2+)-binding protein was increased to over twice the concentration found in the control animals. 3. The low-phosphorus regimens led to a decrease in plasma phosphate and 1,25-dihydroxycholecalciferol in all tissues studied, for the latter to the point where it was undetectable in plasma and bone. Intestinal and renal concentrations of Ca(2+)-binding protein were unchanged in the low-phosphate-intake group and decreased in the very-low-phosphate-intake group. 4. It is concluded that in the rat, unlike in the chick, hypophosphataemia is not associated with a stimulation of the production of 1,25-dihydroxycholecalciferol or its expression in the synthesis of Ca(2+)-binding protein. Therefore the plasma phosphate concentration does not appear to be directly involved in the regulation of the functional metabolism of vitamin D.
D000818:Animals; D001842:Bone and Bones / Q000378:metabolism; D002118:Calcium / Q000097:blood; D002136:Calcium, Dietary / Q000008:administration & dosage; D002352:Carrier Proteins / Q000378:metabolism; D002762:Cholecalciferol / Q000378:metabolism; D004032:Diet; D004100:Dihydroxycholecalciferols / Q000378:metabolism; D006996:Hypocalcemia / Q000378:metabolism*; D007413:Intestinal Mucosa / Q000378:metabolism; D007668:Kidney / Q000378:metabolism; D010758:Phosphorus / Q000008:administration & dosage / Q000097:blood / Q000172:deficiency*; D051381:Rats; D014807:Vitamin D / Q000378:metabolism*
D002136:Calcium, Dietary; D002352:Carrier Proteins; D004100:Dihydroxycholecalciferols; D014807:Vitamin D; D002762:Cholecalciferol; D010758:Phosphorus; D002118:Calcium