64-1 G. N.^JENKINS AND C. J)AWES 



TABLE I. Composition of Rat Femurs on Various Diets 

 (From Hartles and Leaver, 1961) 



Ca P Citric acid 



(%) (%) (%) 



Conclusion: Calcium readily mobilized from bone with high citrate but not 

 from bone with low citrate. 



bone with the low citrate was unable to supply sufficient calcium 

 to avoid tetany. Studies on the extraction of citrate from bone show 

 that some fractions of it are more readily dissolved out than others, 

 and Hartles and Leaver tentatively suggest that some of it may be 

 regarded as a "constant fraction" and some as a "variable fraction." 

 This work suggests that one of the functions of vitamin D is to in- 

 crease the citrate in the variable fraction and make the calcium 

 available to parathyroid influence. How this happens \^ not known, 

 but it might be speculated that the simultaneous presence in bone 

 of the citrate and calcium facilitates the release of a calcium citrate 

 complex. The suggestion is, then, that the citrate in the bone, rather 

 than citrate newly formed by bone cells, is an important factor in 

 calcium mobilization. 



Hartles and Leaver seem to have overlooked another possible ex- 

 planation of their results, namely, that vitamin D prevented tetany 

 by increasing the absorption of calcium from the low concentrations 

 in the diet and digestive juices. 



Incidentally, Hartles and Leaver point out that their results with 

 diets containing normal levels of vitamin D but low in calcium sup- 

 ported Neuman and Neuman's hypothesis on vitamin D action in 

 that the bone citrate was raised under conditions which would be 

 expected to stimulate the parathyroid. In the groups without vitamin 

 D, however, the hypothesis would suggest that with both calcium 

 and vitamin D low in the diet (i.e. with parathyroid stimulated), 

 the citrate should be higher than in a diet with normal calcium and 

 low in vitamin D, The reverse was found. 



