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24R,24,25-Dihydroxyvitamin D3 is capable of inducing a minimal intestinal calcium transport response in chicks when compared to an equal amount of 25-hydroxyvitamin D3. 1,24,25-Trihydroxyvitamin D3 is also less active than 1,25-dihydroxyvitamin D3, and its activity is much shorter lived than that of 1,25-dihydroxyvitamin D3. A comparison of the metabolism of 25-hydroxy[26,27-3H]vitamin D3 and 24,25-dihydroxy[26,27-3H]vitamin D3 in the rat and chick shows that 24,25-dihydroxyvitamin D3 and 1,24,25-trihydroxyvitamin D3 disappear at least 10 times more rapidly from the blood and intestine of chicks. Furthermore, examination of the excretory products from both of these species demonstrates that chicks receiving a single dose of 24,25-dihydroxy[26,27-3H]vitamin D3 excrete 66% of the total radioactivity by 48 hours, whereas rats receiving the same dose excrete less than one-half that amount. These results demonstrate that 24,25-dihydroxyvitamin D3 is considerably less biologically active in the chick than in the rat, probably due to more rapid metabolism and excretion.

The ability of low phosphorus diets to stimulate the activity of the 25-hydroxyvitamin D3-1alpha-hydroxylase was tested in the chick. Feeding low phosphorus diets for 2 weeks resulted in a marked increase in enzyme activity relative to chicks fed a normal phosphorus diet. Stimulation of the 25-hydroxyvitamin D3-1alpha-hydroxylase activity by low phosphorus diets, however, was not as great as that observed with a low calcium diet. The low phosphorus and low calcium diets probably results from increased 1,25-dihydroxyvitamin D3 synthesis, whereas the stimulation by phosphate deprivation is only partly the result of increased 1,25-dihydroxyvitamin D3 production.