RADIOACTIVE INDICATORS 207 



The administration of a radioactive indicator of one of the main body 

 constituents is generally followed by a marked decrease in the specific 

 activity of the labeled compound. After the injection, for example, of 

 labeled sodium phosphate into the circulation, the specific activity of 

 the plasma inorganic P rapidly decreases as the radioactive phosphate 

 penetrates into the tissue cells and is incorporated into organic P com- 

 pounds and into the mineral constituents of the skeleton, some P^^ 

 being excreted. Because of these processes the sensitivity of P^^ as an 

 indicator markedly increases during the experiment. If at the start of 

 the experiment the presence of 1 microcurie of P^^ in the plasma in- 

 organic P indicates the presence of 1 mg P^\ in a later stage of the ex- 

 periment the same activity will indicate, quite apart from the decay of 

 the activity of P^^ with time, the presence of much more that 1 mg of 

 inorganic P. 



Consider, for example, the formation of labeled liver lecithin following 

 intravenous injection of P^^. In the first day as much lecithin will 

 be turned over as in the second one, but because of the decrease in P^" 

 in the inorganic phosphate pool with time the incorporation of the same 

 amount of phosphorus into liver lecithin will be followed in the second 

 day by the incorporation of appreciably less P^^ than in the first day. 

 One week before the administration of P^^ one leg of a mouse was ir- 

 radiated with roentgen rays, and the irradiated leg was found to take 

 up only 70 per cent of the amount of P^^ incorporated into the non- 

 irradiated leg (86) . Thus the amount of P^^ removed from the inorganic 

 phosphate pool was reduced by altering the metabolic rate, in this in- 

 stance by exposure to ionizing radiation. Correspondingly the liver 

 lecithin of an irradiated mouse will incorporate more P"^^ in the course 

 of a given time than does liver lecithin in a control mouse. The in- 

 creased specific activity of the lecithin P in the irradiated animal will 

 not be due, as one may be inclined to interpret, to an increased lecithin 

 turnover in the liver of the irradiated animal. It is due to an inter- 

 ference of irradiation with the turnover rate of the mineral constituents 

 of the skeleton, which makes P^^ a less sensitive indicator of phosphorus 

 in the irradiated mouse than in the control. 



Bicarbonate, acetate, succinate, and a great number of other carbon 

 compounds are metabolized at a spectacular rate. For example, in the 

 mouse less than 1 per cent of labeled acetate is present 24 hr after in- 

 jection of acetate, labeled in the carboxyl group. Interference with the 

 normal metabolic pattern can thus be expected to lead to a change of 

 the activity level of the respiratory CO2. Such a change can be ex- 

 pected to reflect itself, for example, in a change of the activity level of 

 glycogen carbon. 



