74 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



the bone and the age of the animal from which it is taken. There is 

 httle question that this exchange occurs' in vivo between these ions in 

 blood and bone, and the evidence from blood disappearance curves 

 indicates that the reaction is a rapid one (47). The result of this rapid 

 exchange reaction is that the Ca*^ ions introduced into the blood are accu- 

 mulated almost quantitatively in bone. This is primarily due to the fact 

 that the exchangeable-calcium concentration in bone is at least a hundred 

 times the normal calcium concentration in plasma. Thus at equihbrium 

 the specific activity in bone will equal that in plasma, or 



Activity in bone activity in blood 



100 



1 



This means that, shortly after the activity enters the blood, the concen- 

 tration of Ca" in the bone will be at least a hundred times that in the 



Blood 



100Ca 

 100 Ca*^ 



Bone surfaces 



with 



exchangeable 



calcium 



Blood 



\ 



99 Ca 



1Ca' 



I 



1 



100 Ca 



To endogenous 

 secretion and other 

 functions 

 Fig. 2-3. Schematic illustration of how the exchange of calcium ions between blood 

 and bone causes an apparent difference in the behavior of Ca''^ and stable calcium ions 

 in the system. 



plasma. The relative accumulation in bone may exceed this value, 

 because some of the Ca" entering the bone may be rendered less available 

 for exchange by new growth covering the crystal surfaces or by such proc- 

 esses as recrystallization. 



A direct effect upon tracer studies may be illustrated by Fig. 2-3. If 

 100 units of stable calcium ions enters the blood, by injection or absorp- 

 tion from the gastrointestinal tract, it will come in contact with bone sur- 

 faces and undergo exchange. If there is no bone accretion, 100 stable 

 calcium ions will be contained in the blood leaving the bone to go through 

 the process of endogenous excretion or recycling for other metabolic uses. 

 However, if 100 units of labeled calcium ions enters the blood, it will be 

 accumulated in the bone surfaces and, for the most part, replace stable 

 calcium ions, which then follow the same path as do all the Ca ions in the 

 blood leaving the bone. This behavior is not based on any "isotope 

 effect" due to a differential behavior of Ca" ions in blood or bone. It 



