196 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



specific arbitrary counting arrangement at some reference time. The 

 concentration found in a tissue is a direct function of the dose admin- 

 istered to the biological system. For example, if 100 counts/min is given 

 to a 100-g animal and becomes uniformly distributed, the concentration 

 would be 1 count/min/g of tissue. If 200 counts/min had been given, it 

 would act in the same way, and in this instance the concentration would 

 be 2 counts/min/g. Yet the metabolic behavior is the same in both cases. 

 Therefore it is recommended that concentrations be expressed in terms of 

 'percentage of dose, which in the above example would be 1 per cent per 

 gram of tissue for each animal. In all cases the approximate dose should 

 be stated in terms of both radioactivity and mass. This enables the 

 reader to judge whether there might be radiation effects or mass effects 

 and also gives a lead as to the amounts of activity that might be recjuired 

 for similar experiments. 



Another problem that is not so straightforward and touches upon exper- 

 imental procedure is concerned with the ratio of dose to body weight or, 

 more precisely, to the size of the biological system through which the 

 isotope is distributed. For example, if 100 counts/min is given to a 

 100-g animal and becomes uniformly distributed, the concentration is 

 1 per cent of dose per gram of tissue. If a comparative study is made 

 with another animal of 200-g body weight, the concentration becomes 

 0.5 per cent of dose per gram of tissue for the same metabolic behavior. 

 Therefore it would be desirable to take into account the body weight. 

 This may be done by multiplication of the percentage of dose per gram of 

 tissue by the body weight in grams, which would give a value of 100 for 

 each of the above animals. This expression has been called the biological 

 concentration coefficient. The same result can be accomplished by using 

 the simple concentration value of percentage of dose per gram if the dose 

 is adjusted to the body weight of all animals to be compared. Where the 

 tissues are to be analyzed by radioassay, it is usually simpler to use a 

 standard dose and compensate by calculation of the biological concentra- 

 tion coefficient or some other value that takes the body weight into 

 account. However, if cjuantitative autoradiographic comparisons are to 

 be made (see Chap. 7), it is mandatory that in all such cases a constant 

 dose per unit of body weight be used subj ect to the following consideration : 



The above example has been cited in terms of body weight. However, 

 a difficulty arises as to whether the actual dilution of the isotope admin- 

 istered (and this must include excretion losses) is a linear function of the 

 body weight. This can be considered only in terms of the specific meta- 

 bolic behavior of the radioisotope and the effect of body weight on the 

 organs primarily involved. If possible, these uncertainties can be elim- 

 inated by the use of animals of the same body size. However, the experi- 

 mental variables, e.g., age, nutrition, or pathological condition, may often 

 influence the body size. The investigator will have to decide on the basis 



