Effects of vitamin D. A comparison of 1 alpha OHD2 and 1 alpha OHD3 in rats.

The effect of 1 alpha OHD2 and 1 alpha OHD3 have been studied in rachitic, normal, and prednisolone-treated male rats. The healing of rickets, the stimulation of intestinal Ca and P transport, the effect on bone mineral, and the induction of renal calcifications have been examined. The two 1-hydroxylated compounds are equally potent in healing rickets, and by comparison with previous data are equivalent to 100-200 IU/micrograms. 1 alpha OHD2 and 1 alpha OHD3 stimulated intestinal Ca and P transport to the same extent and were antagonistic to prednisolone in this respect. 1 alpha OHD2 reduced the number of osteoclasts and increased bone mineral in normal rats in contrast to 1 alpha OHD3, which increased osteoclasts and slightly reduced bone mineral. Prednisolone-induced osteopenia was more effectively counteracted by 1 alpha OHD2 than by 1 alpha OHD3. Mortality rate was higher in rats intoxicated with 1 alpha OHD3 than for rats given 1 alpha OHD2. LD50 was estimated to be five to fifteen times higher for 1 alpha OHD2. Renal calcification were more pronounced after dosing with 1 alpha OHD3 than after treatment with 1 alpha OHD2. When prednisolone was given together with 1 alpha OHD2 or 1 alpha OHD3, renal calcifications were further increased. These observations demonstrate physiological dissimilarities between vitamin D2 and vitamin D3 in rats which are in accordance with a different metabolism of the two vitamins. The findings, in particular that 1 alpha OHD2 is less toxic than 1 alpha OHD3, are of potential clinical importance.