206 ADVENTURES IN RADIOISOTOPE RESEARCH 



RATE OF RENEWAL OF THE SKELETON OF MAMMALIA 



As a result of the administration of labelled phosphate (phosphate 

 containing a minute percentage of the radioactive phosphorus isotope 

 ^^P), the "free" phosphate of the blood plasma soon becomes labelled, 

 and the same applies to the extracellular fluid of the organism in view 

 of the swift passage of phosphate ions through the wall's capillaries. 

 As the plasma and the lymph contain labelled phosphate, all bone apatite 

 formed after the administration of labelled phosphate is bound to be 

 labelled. In the extreme case, when all mineral bone tissue is formed 

 after the administration of labelled phosphate, 1 mgm bone P will have 

 the same ^^P content, and thus the same radioactivity, as 1 mgm plasma P. 

 Thus, the ratio of the specific activity (activity of 1 mgm) of the skeleton 

 P and the specific activity of the plasma P is a measure of the rate of 

 renewal of the skeleton. 



When determining the rate of renewal we must take due regard to 

 the fact that the specific activity of the plasma phosphorus does not 

 remain constant, but decreases strongly in the course of the experiment, 

 owing to successive interchanges of the plasma phosphorus with the 

 phosphorus atoms of the various compounds present in the organs 

 and also to excretion of phosphate. As the rate of interchange is diffe- 

 rent for different compounds and also for different organs, the calculation 

 of the decrease of the specific activity of the plasma P wath time 

 encounters difficulties. The most direct way to eliminate the above 

 mentioned difficulty is to administer repeatedly an appropriate amount 

 of labelled phosphate, and, with the aid of this procedure, to keep the 

 plasma phosphate at a constant level throughout the experiment. The 

 results of such experiments (Hevesy et alia 1940) carried out on 

 rabbits are seen in Table 1. 



As seen from the figures, the degree of renewal of the mineral consti- 

 tuents of the skeleton in the course of 50 days amounts to 30 per cent 



Table 1. Extent of Reneval of the Skeleton of a Rabbit 

 IN THE Course of 50 Days 



Fraction 



Per cent 

 renewed 



Femur epiphysis inorganic P I 29.7 



Femur diaphysis inorganic P I 6.7 



Tibia epiphysis inorganic P 



Tibia diaphysis inorganic P 



Costa inorganic P 



Scapula inorganic P 



43.8 

 Femur epiphysis phosphatide P l 100 



28.6 

 7.6 



