A Vitamin D-Dependent Serum Factor Promoting Calcium Uptake by Bone

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Abstract
The abilities of sera from normal and vitamin D-deficient rats to support Ca uptake by femurs from normal and vitamin D-deficient rats were determined in vitro and compared with the Ca uptakes of similar bone incubated in Krebs-Ringer solution containing glucose. Ca uptakes by the 2 kinds of bone were similar in each incubation medium. However, Ca uptake was greater when the bone was incubated in normal serum than when it was incubated in serum from vitamin D-deficient rats or in Krebs-Ringer solution. Similar Ca uptakes were observed in the latter 2 media. After vitamin D-deficient rats were refed vitamin D, their serum supported Ca uptake to the same extent as the serum from normal animals. Normal serum contains a vitamin D-dependent factor which stimulates Ca uptake by bone. This factor may be a protein, the synthesis of which is regulated by vitamin D.