COMPARATIVE BIOLOGICAL EFFECTS OF VITAMINS D2 AND D3 AND DIHYDROTACHYSTEROL2 AND DIHYDROTACHYSTEROL3 IN CHICK

The relative biological activities of vitamin D3 (D3), dihydrotachysterol2 (DHT2) and dihydrotachysterol3 (DHT3) were assayed via oral feeding of different levels of the test compounds to chicks raised on a rachitogenic diet. After 28 days the relative growth, per cent of bone ash, and serum calcium levels were determined. The relative dose required to produce 50 per cent of the maximal D3 response for DHT2:DHT3:Da was 96:20:1.0, where 1.0 is equivalent to 0.55 μg D3. Also, a comparison was made of the differing responses of the intestinal calcium absorption system (ICA) and the bone calcium resorption system (BCR) to varying doses of vitamin D2, D3, DHT2 and DHT3. It was found that, although D3 was the most active of the compounds tested in both systems, compounds having the side chain of D3 and DHT3 act preferentially in the ICA system, whereas compounds having the side chain of D2 and DHT2 act preferentially in the BCR system. It is suggested that these results may be an indication of a different detailed mechanism of action of vitamin D in the ICA and BCR systems. © 1969.