946 



ADVENTURES IN RADIOISOTOPE RESEARCH 



with those present in the plasma, and the same applies to the sodium 

 ions. The high concentration of potassium and low concentration of 

 sodium found in the erythrocytes of most animals can thus not be 

 explained as being due to an impermeability of the corpuscle membrane 

 to these ions. 



Not only the resistance of phase boundaries to labelled ions, but also 

 that to molecules of different kinds, can be measured with the aid 

 of isotopic indicators. The rates of interchange of phosphatides present 



10 20 



FiG. 6.— Rate of disappearance of various labelled ions from 



the plasma. 



in the plasma and in different organs were determined in the fol- 

 lowing way^^'^'. Labelled phosphate was administered to a rabbit. 

 After the lapse of 2 days, when the plasma contained an appreciable 

 amount of labelled phosphatides, part of the plasma of another rabbit 

 (rabbit II) was replaced by the labelled plasma. On following the de- 

 crease with time in the activity of the phosphatides extracted from the 

 plasma of rabbit II, it was found that half of the plasma phosphatide 

 molecules had interchanged in the course of few hours with phosphatide 

 molecules present in the organs of the rabbit. An investigation of the 

 activity of the phosphatides isolated from the organs led to the result 

 that a very substantial part of the labelled phosphatide molecules 

 was found in the liver. 



