Abstract
Summary
Chicks regulated to a lowcalcium diet exhibited the expected facultative process regarding the intestinal absorption of calcium. Augmentation of the facultative process was noted when such low-calcium adapted chicks were given a high-calcium diet. This response was transitory, becoming evident 72 hr after initiation of the high-calcium feeding and declining rapidly thereafter. The potentiative response was actinomycin sensitive and could not be produced by the molar substitution of strontium for calcium. Vitamin D was essential for development of the facultative process in lowcalcium fed chicks and the potentiative effect noted in such animals given a high-calcium diet. It is suggested that the additional calcium could have acted on the intestinal cellular processes to accentuate the facultative response by increasing mitotic activity or by inhibiting ribonuclease. It is also theorized that calcium could have effected the facultative response indirectly by controlling synthesis of an active vitamin D metabolite.
Note added in proof: An article by Boyle et al. (25), appearing after the submission of this paper, describes the suppression of 25-HCC metabolism to 1.25-DHCC in high-calcium-fed rats and suggests that 1.25-DHCC is the endogenous factor.
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