120 RADIOACTIVITY; BIOLOGICAL TRACERS 



Alternatively, autoradiographic techniques on deep-frozen sections of 

 sacrificed animals can give quantitative information on blood flow at dif- 

 ferent depths in the tissue and at different times. For example, through both 

 superficial and deep cerebral structures the flow rate is about 1.2 cc/min 

 per g of tissue — in all but the white matter, through which the rate of flow 

 may be as low as 0.2. In the spinal cord the flow rate in the gray matter is 

 0.63 cc/min per g; in the white matter it is 0.14. Under light anesthesia 

 these values are reduced about 25 per cent. All these values are given in 

 terms of flow through 1 g of tissue, because there is just no good way to de- 

 termine the number and dimensions of the blood capillaries in these tissues. 



Studies on Metabolism: Time and Space Distribution of 

 Biologically Active Chemicals 



For information subsequently to be used in therapy of one sort or another, 

 tracer studies on metabolism are probably the most important. Every tissue 

 or organ has a definite turnover rate of its molecular components. Every 

 substance which enters through the gastrointestinal tract or through the 

 lungs into the blood stream, or is introduced directly into the body fluids 

 through hypodermic needles, has one or more locations to which it goes, and 

 a definite time (on the average) it stays there before being rejected in favor 

 of new material. In practice, radioactive atoms are introduced into the 

 molecules which compose the material to be studied. 



Where this material goes, and how long it stays there, as well as in what 

 form it is rejected, can all be answered by proper use of isotopic dilution or 

 radioactive labeling technique. For example, studies have been made on the 

 metabolism of proteins, such as the rate of protein synthesis and nitrogen 

 (N 15 ) transfer; on the intermediary carbohydrate metabolism (C 14 and P 32 ); 

 on the intermediary metabolism of lipids — the pathways of fatty-acid oxida- 

 tion and synthesis (H 3 ); on healing of bone fractures; on iodine metabolism 

 (I 131 ) in the liver and in the thyroid; on turnover rate and growth rate of 

 normal** and diseased tissue (C 14 , H 2 , O 18 , Fe 59 , Au 198 ); on the metabolism 

 and turnover in teeth (P 32 ); and on blood circulation in the brain (I 131 )- 



In more detail: the metabolism of nitrogen in the living system has been 

 studied by the introduction of N'Mabeled glycine or other amino acids, 

 ammonia, or nitrates, into food. Measurement of the N 15 — by either activa- 

 tion or mass spectrometry (since N 15 is a stable isotope) — as it appears in 

 the urine, as well as analysis of the molecules in which the nitrogen is con- 

 tained, has shown that the cellular proteins and their constituent amino 

 acids are in a state of ceaseless movement and renewal. The proteins and amino 



**Other isotopes now in use in metabolic studies include: Cr , Na , S , CI , K , Ca , 

 Mn 54 , Zn 65 , Br 82 , Rb 86 , I 128 . 



