RADIOACTIVE INDICATORS IN BIOCHEMISTRY 



981 



changes with the extra-vascular inorganic ^ip. Owing to these and other 

 interchanges, the ^ap content of the blood plasma rapidly decreases. 

 After the lapse of 60 minutes, the 32p content is reduced to about one- 

 tenth of its original value, the presence of 130 mgm of inorganic P in the 

 blood plasma being now indicated by an activity of 0.1 me. only. The 

 sensitivity of the radioactive indicator is thus increased to 10 limes ils 

 initial value. A similar behaviour is shown hy many other radioactive 

 indicators. 



The sensitivity of acetate ^^U as an indicator of body acetate increases 

 at a remarkable rate, as seen in Fig. 3.^^^) In this figure are plotted the 



20 



n r 



30 45 



Time, mins. 



r 

 60 



80 



Fig. 3. — Change with time in the "C content of brain fractions 

 of mice after intraperitoneal injection of labelled acetate (reprodu- 

 ced, with permission, from Arch, internat. Pharm. Therap. 86, 33 (1943). 



results of experiments in which groups of mice were killed at different 

 times after interperitoneal injection of acetate labelled in the carboxyl 

 group with i^c. The radioactivity of the fat-free brain tissue and also 

 that of the proteins of the brain was then determined, and the figures 

 obtained plotted against time. After only 30 minutes, the activity of 

 the fat-free (acid-soluble + protein) fractions is less than half of the 

 value observed after 10 minutes, the protein fractions showing a slow 

 increase in their i^C content with time. 



The change of sensitivity of the radioactive indicator with time may 

 be disturbing and can make the interpretation of the results obtained 

 very difficult. On the other hand, it may also have great advantages. 

 We shall describe some examples showing both disadvantages and advan- 

 tages which result from the marked time-dependency of the sensitivity 

 of radioactive indicators. 



RENEWAL OF THE MINERAL CONSTITUENTS OF THE SKELETON 



The great inconvenience inherent in the change of the sensitivity of 

 radioactive indicators with time becomes clear from an attempt to deter- 

 mine the rate of renewal of the mineral constituents of the skeleton by 



