1 94 ADVENTURES IN RADIOISOTOPE RESEARCH 



of an exchange between plasma P and bone P will arise. From these considera- 

 tions it follows that an exchange taking place within a long interval cannot be 

 extrapolated from results obtained in experiments of short duration. We have, 

 therefore, carried out experiments in which we kept the activity of the plasma 

 inorganic^ P of rabbits at a constant level for several davs or weeks. 



EXPERIMENT OF LONG DURATION 



To obtain a constant level of the plasma inorganic P, the first day every 30 min 

 and later twice everj? day, labelled sodium phosphate of neghgible weight was 

 administered by subcutaneous injection to rabbits. After removal of the marrow, 

 the bone was first extracted for 12 hr. with hot ether-alcohol. The bone was tlaen 

 treated with hot alkaline glycerol solution for further 6 hr. The fractions obtained 

 were dissolved in HNO3 and their P contents precipitated as molybdate. The 

 molybdate was dissolved in dilute NH3 and precipitated as ammonium magnesium 

 salt. An aliquot of the sample obtained was used in the colorimotric P determina- 

 tion, while another aliquot was reprecipitated as ammonium magnesium phosphat(> 

 and its radioactivity measured with a Geiger counter. In prolonged experiments 

 the analysis of the plasma inorganic P is conveniently replaced by that of thc^ 

 urine P. In Tables 3 and 4 the specific activities of the different bone P fractions 

 are recorded. 



Table 3. Extent of Reji'venation 

 OF THE Tibia in the Course of 9 Days 



Wt. of rabbit: 2 kgm 



Fraction 



% P rejuvenated 

 I (specific activity) 



Epiphysis P 11.2 



Diaphysis P 3.2 



Tibia phosphatide P . . . 74.8 



Marrow phosphatide P . 1 80.1 



In the course of 9 days therefore, only ll°o of ^he epiphysis and 3% of the 

 diaphysis are rejuvenated, while most of the phosphatide molecules present in the 

 marrow and in the bone are newly formed. 



In the course of 50 days, only 29% of the epiphysial and 1% of the diaphysial 

 mineral constituents were replaced (Table 4). The tibia and femur show about the 

 same behaviour. About half of the scapula remained unchanged. The almost 

 complete replacement of the apical and medial parts of the incisor dentine P can 

 hardly be interpreted as due to an exchange between dentine P and plasma P. 

 since the replacement rate of the dentine P was found to be lower than that of the 

 tibia P [Hevesy et al., 1937: Lefevre and Baxe, 1939] and since the tibia P, 

 as seen above, was replaced only to a restricted extent. The high ^sp content of 

 the incisor dentine must bo duo to an actual growth, to a formation by a calcifi- 

 cation process. As a plasma containing ^-P was instrumental in calcifying the 

 newly grown parts of the incisor, the P of the latter was bound to have the same 

 specific activity as shown by the plasma P. As seen in Table 4, the P of the apical 

 part of the incisor dentine investigated has, within the errors of experiment ( + 5%). 



