PRINCIPLES OF TRACER METHODOLOGY 3 



studies involving an altered stable-isotope content, restricted only by 

 experimental limitations. 



DIRECT MOVEMENT OF ORGANISMS, ELEMENTS, AND COMPOUNDS 



Obviously, radioisotopes may be used simply to follow movement or 

 transport. If it is physically possible, a radioisotope can be attached to 

 or implanted in the organism. Cobalt 60 and tantalum 182 are especially 

 suited for this purpose, since they are relatively inert chemically and 

 biologicall}^ and emit gamma rays that can be conveniently detected even 

 though considerable mass may be interposed between the source and the 

 detecting device. The depth, location, and lateral movement under- 

 ground of wireworm larvae have been followed by the insertion of Co^" 

 wire in the body cavity, and wireworm beetles have been labeled by plac- 

 ing Ra'-^ under the wings. An alternative procedure with small organ- 

 isms is to use a radioisotope that becomes incorporated metabolically in 

 the body. Malaria, house, and rock-pool moscjuitoes; houseflies, fruit 

 flies, root maggot flies, black flies, and screwworm flies have been labeled 

 usually by mixing P^^ or Sr^^ with the food of the larvae, resulting in the 

 production of adults that could be distinguished by Geiger counting. 

 Cockroaches and flesh flies have been injected with Zr^^ so that they could 

 be detected at a distance. In this manner entomological studies are prac- 

 tical on distribution from breeding sites, flight habits, dispersion, food 

 relationships, food contamination, infestation patterns, longevity in the 

 field, disease transmission, predation, and similar points of interest (15 to 

 24). Bacteria labeled with P^'' have been used to determine the retention 

 and initial distribution of air-borne microorganisms in studies of respira- 

 tory infection (25, 26). 



In the above type of application the radioisotope is not primarily used 

 as a label for a specific chemical element, and the choice of isotope is 

 chiefly dependent on physical characteristics. For general metabolic 

 studies, however, it is necessary that a specific element or compound be 

 labeled and traced. From a nutritional and physiological point of view, 

 the mere knowledge of the movement of an element or compoiuid in a 

 system may not be particularly fruitful. Nevertheless these observations 

 may serve as a good starting point for additional investigation, especially 

 with substances that have been little studied. The fact that most of the 

 important biochemicals are now" available in labeled form has greatly 

 broadened this approach. Just a few areas of interest are cited for illus- 

 trative purposes (the reader will undoubtedly recognize similar applica- 

 tions in his specific field) : (a) absorption from the gastrointestinal tract 

 of nutrients or medicinals as a function of chemical state and physiological 

 conditions, with subsequent tissue distribution and excretion (27, 28) : 



