Phosphorus Deprivation: the Metabolism of Vitamin D3 and 25-Hydroxycholecalciferol in Rats

Abstract

The metabolism of intravenously administered ³H-vitamin D₃ (D₂-³H) and ³H-25-hydroxycholecalciferol (25-HCC-³H) was examined in young rats following dietary depletion of phosphorus. Depleted animals, in contrast to controls given phosphate supplements in their drinking water, exhibited poor growth, hypercalcemia, hypophosphatemia, and florid rickets. The acute plasma disappearance and hepatic uptake of a radioactive vitamin D₃ preparation were similar in both groups. A similar lipid and aqueous distribution of plasma radioactive vitamin D₃ metabolites was observed as well. Following intravenous doses of 25-HCC-³H, silicic acid column chromatography of chloroform-extracts of intestinal mucosa and kidney revealed comparable patterns in each of these tissues. In both groups, higher tissue/plasma ratios of 25-HCC-³H were found in kidney compared to intestinal mucosa. The generation of more polar metabolites, previously shown to contain 1,25-hydroxycholecalciferol, was not impaired in test animals. In vitro transfer of ⁴⁵Ca across inverted intestinal loops was significantly greater in phosphorus-deprived animals. These observations suggest that the intestinal transfer of calcium is enhanced and the metabolism of vitamin D₃ apparently unaltered in the phosphorus-deprived rachitic animal in which profound abnormalities of growth and skeletal mineralization occur.