122 RADIOACTIVITY; BIOLOGICAL TRACERS 



ated with an increase of the rate of sugar penetration (a process which itself 

 has been followed by C 14 -tagged sugars). Whether the control exercised by 

 insulin is simply by opening the access to pores, or whether it controls in a 

 more subtle manner by increasing the activity of the enzyme (hexokinase) 

 also thought to be adsorbed on the membrane, has not yet been settled. 

 However, it can be seen that the use of radioactive tracers in such a phar- 

 macological problem can make a valuable contribution to our knowledge of 

 the processes involved. 



The pioneering work of Huff and Judd on the quantitative analysis of the 

 time and space distributions of Fe 59 in blood plasma, will be discussed in 

 Chapter 11 as a concrete example of how possible methods of action can be 

 analyzed with a computer if it is fed reliable experimental measurements of 

 where the Fe 59 goes and how long it stays there. We learn a little about what 

 the iron does, and also something about just what processes are interfered 

 with during blood diseases. 



Radioactive Mapping 



Administration of compounds of I 131 , followed bv external measurements 

 of beta-ray intensity in the thyroid region of the neck, has been introduced 

 in some centers as a replacement test for determining whether the thyroid is 

 normal, over-, or under-active. A hyperactive thyroid may absorb up to 80 

 per cent of the tagged iodine; a hypoactive gland may absorb as little as 

 15 per cent before normal biochemical turnover elsewhere in the body re- 

 duces the concentration via excretion. Mapping of the thyroid by I 131 

 scintography is common practice. Both the outline of the organ, and its 

 turnover rate can be obtained from maps made at different time intervals 

 after administration. The maximum activity of the emission is a direct 

 measure of the uptake of iodine by the thyroid. 



The flow of fluids through various critical parts of the system can also be 

 mapped satisfactorily by dissolving in the fluid a small amount of gas which 

 contains a radioactive emitter, and mapping from the outside with a col- 

 limated scintillation counter (Figure 5-6). 



Conclusion 



A great many elementary biochemical reactions are being studied via the 

 tracer technique, and a few physical processes also. Some of these will be 

 found mentioned as examples in different parts of this book. The techniques 

 are reliable and extremely sensitive, and have the unique advantage that the 

 introduction of the radioactive element can be done in such a manner as not 

 to upset the chemistry or the physics of the process in vivo. Already in ex- 

 tensive use in biological research — in his review Kuzin 12 was able to collect 

 358 references to new work published in 1959 alone!— now, led by successes 



