APPLICATIONS OF ISOTOPIC INDICATORS 953 



EXCRETION STUDIES 



Chemical analyses of the food and of the excreta permit the deter- 

 mination of the extent to which the organism is in balance. Chemical 

 methods, however, cannot determine to what degree the substances 

 found in the faeces originate from undigested food and to what extent 

 they have been carried into the digestive tract, coming from the body 

 proper in the form of digestive juices. This problem can be solved under 

 strictly physiological conditions with the aid of isotopic indicators. 



The simplest procedure is the following^^*^ At a suitable time after 

 administration of labelled sodium phosphate, we determine the specific 

 activity of the urine P and that of the faeces P. Both originate from the 

 blood plasma and, provided that we wait for a sufficient time, the spe- 

 cific activity of the P compounds carried into the digestive tract from 

 the body will be about equal to that of the urine P. If the faeces P were 

 entirely of endogenous origin, it should show a specific activity equal 

 to that of the urine P. If we find the faeces P to be less active than the 

 urine P, the active faeces P of endogenous origin must have been diluted 

 by non-active P. Since the sole source of nonactive P is the diet, the 

 ratio of the specific activities of the faeces P and urine P tells us to what 

 extent the endogenous faeces P has been diluted by food P. 



The ratio (100 x specific activity of faeces P/specific activity of urine 

 P) gives the percentage of P in the faeces which originates from the 

 body proper. In the case of human subjects, 3/4 to 4/5 of the P present 

 in the faeces was found to originate from non-absorbed P. 



LABELLED RED CORPUSCLES 



As seen in Fig. 7,^"^^ labelled phosphate penetrates at a fairly slow rate 

 into the red corpuscles. On entering the corpuscles, however, the newly- 

 arrived phosphate ions participate rapidly in the formation of acid- 

 soluble organic phosphorus molecules which occur in a comparatively 

 high concentration in the corpuscles. The formation of new acid-soluble 

 phosphorus compounds in the corpuscles is largely associated with gly- 

 colytic processes occurring there and is attended by the destruction of 

 an equal or almost equal number of "old" molecules. As a result of these 

 processes, the specific activity of the labile P atoms of adenosine triphos- 

 phoric acid and that of the P of some other compounds w ill soon acquire 

 a specific activity almost as high as that shown by the free P of the cor- 

 puscles, but much lower than that of the free P of the plasma. This fact 

 and the fairly slow rate of penetration of phosphate through the cor- 

 puscle wall explain the low rate of loss of ^sp by labelled corpuscles 

 when brought into contact with unlabelled plasma, and make possible 



