Effects produced by the administration of high doses of 1,25-dihydroxycholecalciferol to the chick embryo.

White Leghorn chick embryos were injected on the 15th day of incubation with 70 to 300 pmoles 1,25-(OH)2D3. All doses produced hypercalcemia; with the highest dose, the concentration of calcium in serum started to rise 4 h after the injection, reached a peak 20 h after, and was still high 48 h after. Twenty hours after the injection of the same dose, the concentration of inorganic phosphorus in the serum was significantly lower than in the corresponding controls. The tibias from 17-day-old chick embryos injected with 300 pmoles on day 15 were shorter, lighter, and had a lower ash content than those from controls. Histological signs of resorption appeared to be reduced with respect to controls, but no precise quantitation was conducted. The fact that hypercalcemia was not accompanied by hyperphosphatemia may suggest that the vitamin stimulates resorption of calcium from the shell, which is mainly formed by calcium carbonate rather than from the bone from which calcium and phosphate are usually resorbed together.