1,25-Dihydroxycholecalciferol effect on serum phosphorus homeostasis in rats

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