APPLICATIONS OF ISOTOPIC INDICATORS 945 



The problem as to whether and to what extent the P atoms of the 

 dental enamel are renewed has been a subject of extensive invest igations^^^^ 

 which led to the result that, though some '^^p is found to be present 

 in the enamel after administration of labelled phosphate, the extent of 

 renewal of the enamel phosphate is almost negligible. Regarding the 

 extent of replacement of the phosphorus present in the constituents 

 of the dentine, about one millionth part of the food phosphorus was 

 found to be located in the mineral constituents of the dentine of each 

 tooth. 



The bone tissue growing in a labelled organism is bound to become 

 labelled. Of the labelled phosphate administered by mouth, after the 

 lapse of 3 days 2 per cent was found to be present in the rapidly growing 

 incisors of the rat<^^\ As seen in Fig. 5, these phosphate ions are mostly 

 found in the incisal part of the incisor, though a minor part are located 

 at the apical end remote from the pulpa. 



PERMEABILITY INVESTIGATIONS 



The above-mentioned rapid decrease in the plasma activity following 

 intra venal administration of ^sp is to a large extent due to the interchange 

 of plasma phosphate with the phosphate of the extracellular fluid. 

 From this fact it follows that the capillary wall is readily permeable 

 to posphate ; similar results were obtained for the other labelled ions 

 investigated. Sodium ions, which are mainly confined to the extracel- 

 lular space, enter into exchange equiUbrium with the plasma sodium 

 within 20 minutes^^^' ^*' ^^\ This may clearly be seen from Fig. 6.^^^' 

 For potassium and for phosphorus, elements mainly located in 

 the tissue cells, a longer time is required for the attainment of such 

 equilibrium^^*'. The low rate at which exchange equilibrium between 

 the cellular and extracellular phosphorus is reached in the animal orga- 

 nism is mainly due to a low rate of renewal of large parts of the 

 skeleton. 



As seen from the above examples, the method of isotopic indicators 

 can be utilized with advantage in permeability investigations. It is 

 with the aid of isotopic indicators that we best can measure the perme- 

 abihty of phase boundaries, since other methods do not indicate solely 

 the resistance of the phase boundary to the penetration of ions, but a 

 more complex phenomenon. Prior to the application of isotopic indi- 

 cators, the high potassium content of the erythrocytes of most species 

 and their low sodium content were interpreted as being due to the 

 impermeabihty of the erythocyte membrane to potassium and sodium 

 ions. The appHcation of isotopic indicators, however, has disclosed the 

 fact that potassium ions in the erythrocyte interchange quite easily(^-^> 



60 Hevesv 



