1,25-DIHYDROXYCHOLECALCIFEROL EFFECT ON SERUM PHOSPHORUS HOMEOSTASIS IN RATS

1,25-Dihydroxycholecalciferol (1,25-(OH)2D3) increases the serum P concentration of rats on a low-P diet. While studying the biological activity of 1,25-(OH)2D3, under certain circumstances 1,25-(OH)2D3 would decrease the serum P concentration. The analysis of all data obtained in rat experiments during the past 3 yr revealed highly significant linear correlations (P < 0.001) between changes of serum P concentrations after the administration of 1,25-(OH)2D3 (130 pmol/day for 1 or 5 days) and serum P or Ca levels in the animals before injection. Similar correlations could only be found with the higher dose of 25-hydroxycholecalciferol (130 pmol/day for 5 days). Another vitamin D3 metabolite, 24,25-dihydroxycholecalciferol, had no effect on serum P concentrations under these experimental conditions. The 1,25-(OH)2D3 effect on serum P concentration does not require the presence of circulating parathormone and/or calcitonin. 1,25-(OH)2D3 might be an important factor in serum P homeostasis.