316 



ADVENTURES IN RADIOISOTOPE RESEARCH 



active indicator, thus, changes very appreciably in the course of the experiment. 

 If we are successful in keeping the activity level of the plasma constant during 

 the experiment, we can eliminate great difficulties otherwise encountered when 

 calculating the turnover rate of organic phosphorus compounds. 



The changes in the activity of the plasma, shown in Fig. 1, can be further 

 reduced by injecting amounts decreasing with time. In our later experiments 

 we have chosen this procedure and varying amounts of labelled P were adminis- 



0,15 - 



.^0,10 



o 

 o 



0,05- 



TT 



150 



200 



250 



mm. 



Fig. 4. Change of the specific activity of the plasma inorganic P 



during continuous subcutaneous injection of labelled 



phosphate to a rabbit. (Specific activity = per cent of the labelled 



P injected, found in 1 mgmP). 



tered by subcutaneous injection. In an experiment taking 12 hours, for exainple, 

 labelled P was injected every 20 min. In experiments taking several weeks, in 

 the later phases of the experiment injections were made twice a day. The change 

 in the plasma activity in such an experiment taking 4 hours is seen in Fig. 4. 

 In experiments taking several hours or days a constant activity level could be 

 easily obtained. 



The determination of the turnover rate of the phosphatides present in the 

 different organs necessitates the determination of the specific activity of the 

 inorganic P and phosphatide P extracted from the organ. This determination 

 was carried out in the following way. At the end of the experiment the animal 

 was killed by bleeding. The organs were at once placed in Hquid air, minced, 

 and extracted with cold 10 per cent trichloracetic acid. The inorganic phosphate 

 present was precipitated as ammonium magnesium phosphate at 0°. Muscle samples 

 were taken before death. To secure the phosphatide present in the organs, these 

 were first dried with cold acetone and then treated with ether, later Avith boiling 

 alcohol. The ether-alcohol extracts were evaporated in vacuo and taken up seve- 

 ral times with petrol-ether; the phosphatides were then converted into phosphate 

 by wet ashing. The procedure appHed when isolating lecithin, cephahn, and sphin- 

 gomyelin will be discussed on page 330. 



