Early and Direct Effect of 1,25-Dihydroxycholecalciferol on Calcium Uptake by Duodena of Rachitic Chicks

Injection of 1,25 dihydroxycholecalciferol (1,25(OH)2D3, 10 μg) directly into the in situligated duodenal loop of rachitic chicks significantly elevated the tissue accumulation of 47Ca within 20–30 min. The transfer of 47Ca from lumen to blood, during the same time period, was not increased nor was there any measurable intestinal calcium-binding protein synthesized. Lesser amounts of 1,25(OH)2D3(1 or 5 μg) did not result in any statistically significant elevation of 47Ca tissue accumulation, nor did they have any effect on 47Ca transfer from lumen to blood (transmural). Ten micrograms of 1,24R,25(OH)3D3was similarly effective in elevating tissue accumulation, whereas 24R,25(OH)2D3and 25(OH)D3were not. These results provide additional evidence for an early and direct action of 1,25(OH)2D3in altering intestinal epithelial membrane transport prior to the induction of synthesis of specific transport proteins.