4 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



(h) permeability and transport through physiologic boundaries, including 

 placental transfer (29) ; (c) labeling and tracing protein antigens and anti- 

 bodies in immunological studies (30 to 32) ; (d) evaluation of movement 

 characteristics of new drugs for compliance with food and drug laws (33) ; 

 (e) movement of substances applied to soil and uptake by roots for trans- 

 location in plants (34 to 35a) ; (/) absorption and translocation in plants 

 of substances applied to foliage, including both nutrients and plant- 

 growth regulators (36, 37) ; and (g) distribution and action of pesticides 

 (38 to 40). General discussions of many of these topics can be found in 

 references (9 to 13). 



A primary advantage of the radioisotope technique in studies of this 

 type is the great sensitivity of measurement usually obtainable. Many 

 of the elements and compounds of interest are normally present in the 

 organism in such small amounts that chemical studies are impossible 

 under physiological conditions. As an example, it is known that a dietary 

 intake of about 1 mg cobalt per day will satisfy the requirements of a 

 1000-lb cow for this element. The chemical measurement of this amount 

 of cobalt distributed through the tissues and excreted by such an animal 

 was impractical. By the use of radiocobalt it was a relatively simple 

 matter to study in detail the distribution of less than 0.1 mg cobalt in the 

 bovine (27). This tremendous sensitivity also allows information to be 

 obtained when the net mass movement of the substance is small. 



The fact that radioisotopes emit radiation that can traverse the tissues 

 and be measured outside the organism has led to a most useful type of 

 measurement called in vivo or surface measurement (see Chap. 5 for more 

 details). The main advantage is that a continuous measurement on the 

 intact organism is available during the course of the experiment, and the 

 same organism may be used repeatedly for observation. This is in con- 

 trast to orthodox procedures, which require samples to be taken and 

 evaluated. Though possible, in vivo methods are difficult to perform 

 quantitatively, and in any case only the radioactivity is measured, so that 

 metabolic information on turnover is not usually available. However, 

 there are many types of studies where this procedure is invaluable. A 

 major application has been in the study of transfer dynamics, which is 

 treated in the next section. The following is a survey listing of metabolic 

 research that has been undertaken by the in vivo method [compiled pri- 

 marily from (41)]: 



Radioisotope In vivo study 



Na^^ Circulation times in normal and diseased humans; arm to arm, arm to 



heart, heart to arm, arm to foot, arm to brain, hand to axilla, foot to 

 groin 



Retention of aerosols 



Accumulation in hands after ingestion 



