Comparison of the Actions of Vitamins D2 and D3 in the Chick with their Retention in Serum, Liver and Intestinal Mucosa

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Abstract
The response of rachitic chicks to single doses of vitamins D2 and D3 was evaluated, 18 hours after dosing, in terms of: (a) the concentration of vitamin D in intestinal mucosa, blood serum and liver, (b) the response of serum calcium and inorganic phosphorus, and (c) the calcium transport by the ileum incubated in vitro in a low Na medium. The vitamin activity, biologically assayed using the calcium transport by the rat duodenum in vitro as a criterion, was similar for both sterols in the intestinal mucosa, but was 3 to 4 times higher in the serum and liver of the chicks dosed with vitamin D3 than of those given vitamin D2. Both sterols lowered the serum inorganic phosphorus of the chick to a similar extent, but vitamin D3 was more effective in increasing serum calcium. Vitamin D3 was over 10 times as effective as vitamin D2 in promoting the in vitro calcium transport by the chick ileum. It is suggested that the lower biological activity of vitamin D2 than of vitamin D3 in the chick is due to reduced physiological effectiveness of the former in the intestinal mucosa and either poorer absorption or more rapid catabolism of the inactive sterol.