BASIC DIFFICULTIES IN TRACER METHODOLOGY 75 



merely results from the attainment of equilibrium in a system of high cal- 

 cium concentration iii bone and low concentration in blood; or, expressed 

 mechanistically, the stable calcium ions contact the bone and replace like 

 calcium ions, whereas the labeled calcium ions contact the bone and 

 replace unlike stable calcium ions, because there are so few, if any, labeled 

 calcium ions in the bone to be replaced. Support for this concept has 

 been found in measurements with more than 40 cattle in which the reten- 

 tion of dietary calcium as shown by calcium balance was significantly 

 lower than the retention of dietary labeled calcium (49). This behavior 

 has been utilized to determine endogenous fecal calcium (50). The 

 importance of this reaction in evaluation of the health hazards from 

 bone-seeking radioisotopes will be discussed in Chap. 3. 



The implications, which may also apply to other elements, are sum- 

 marized for calcium as follows: (a) The appearance of administered 

 labeled calcium ions in bone is not necessarily a measure of bone accre- 

 tion, (h) After contact with bone, the Ca" ions in blood are no longer a 

 true tracer for the original calcium ions, (c) Small physiological differ- 

 ences in calcium behavior between control and treated animals may be 

 masked by the large changes in concentration of Ca^^ ions due directly to 

 physical processes. As discussed in Chap. 1, this situation applies also 

 to elements such as Na and K which interchange between compartments. 

 However, the effects are not so marked, since the compartments do not 

 have concentrations differing so w^idely as in the case of calcium, and equi- 

 librium is reached much sooner. 



An example of exchange reactions in plant-nutrition studies is given 

 by Overstreet and Broyer (51), who measured the absorption of stable and 

 radiopotassium ions by barley roots. When plants with a high level of 

 potassium were used, there was an inward movement of radiopotassium 

 into the roots, but there was no simultaneous entry of stable potassium 

 ions. It was concluded that the colloidal phases of the protoplasm and 

 cell wall contained potassium that was capable of rapid exchange with the 

 ions in the culture medium. Under these conditions the movement of 

 radiopotassium into the plant did not represent a net absorption of potas- 

 sium by the plant. 



It should be emphasized that, if the system has reached equilibrium, 

 then the isotope wall act as a true tracer, and the above considerations no 

 longer apply. However, many experimental observations are made 

 before equilibrium is attained. When bone and bone minerals are 

 involved, equilibrium may not be reached for considerable lengths of time, 

 unle.ss the experiment is one in which the animals have been raised on a 

 diet of experimental isotopic composition. Such experiments, based on 

 continuous administration of the radioisotope as opposed to single dosage, 

 offer great possibilities. 



