22f> ADVENTURES IN RADIOISOTOPE RESEARCH 



calcium leaving the circulation. From Ca*^ injected into the circulation 

 of growing hogs, Comar and assoc/^'^^ found only 2% to be present 

 after the lapse of an hour. 



In the experiments of Engfeldt and assoc, three hours was the 

 shortest time after which the ^^P injected rats were sacrificed. The early 

 phase of the experiment, in which a very rapid interchange of the mineral 

 constituents of the bone takes place, is much shorter than three hours. 

 When investigating the uptake of *^Sr by the skeleton of outgrown 

 rabbits during the first 30 seconds, 11.7% were found to be taken up*^-"*, 

 while during the first six hours— thus a 720 times longer period — only 

 about twice as large an uptake was observed. Armstrong and assoc. ^^'* 

 found during the first 20 minutes an interchanges of 4% of the skeletal 

 calcium with plasma calcium, this amount increasing less than three 

 times during the following 160 minutes. 



A change in the concentration of the bone apatite constituting ions, 

 and still more a variation in the concentration of enzymes involved 

 in recristallization of the plasma and the lymph, is bound to influence 

 the rate of recrystallization of the skeleton. Repeated administration 

 of bone phosphate extract by intravenous injection was found to lead 

 to a decrease of the mineral constituents of the bone tissue*^^^\ which are 

 replenished after removal of the excessive phosphatase. Hastings^"', 

 when replacing the plasma of a dog by plasma of low calcium content, 

 found that the mobilization of bone calcium increased the calcium level 

 of the plasma almost momentarily to a normal level. Parathyroid hor- 

 mon is known to exert a direct action on bone*^'^^'''\ In this connection, 

 also Carls ON 's*^^^^ investigation should be mentioned; he found vitamin 

 D deficient rats to be unable to utilize their bone stores for maintaining 

 a normal serum calcium. However, in view of the very great difference 

 in the distribution of the mineral constituents in the bone tissue and the 

 corresponding tendency to remove these differences, the biological 

 recrystallization of the skeleton, as rightly emphasized by Engfeldt 

 and assoc .^^^\ is not due exclusively to these processes^^'*\ 



King raised the idea that, though conventionally, the bony frame- 

 work of the body is regarded as a means of making locomotion possible, 

 it may be that this is no more than a secondary development, the pri- 

 mary function of bone in the body being to act as a reservoir for the 

 maintenance of a constant blood calcium level. 



At a very early date ■^^' ^^' ''^ it has already been observed that the 

 diaphysial phosphate is replaced by administered labelled phosphate 

 at an appreciably lower rate than epiphysial phosphates, and similar 

 observations were made in the investigation of the incorporation of 

 *^0a into the skeleton^^^' ^^\ Since the transition between diaphysial and 

 epiphysial bone tissue is almost continuous, the specific activity of 

 bone phosphorus or bone calcium varies considerably through the whole 



