RADIOACTIVE IXDICATOKS IX BIOCHEMISTKY 967 



labelled by addition of the short-lived ^^Na, while the long-lived ^ajSTa 

 was added to the Ringer solution facing the outer skin surface. 



Although the terrestrial and meteoric abundance of sodium is some- 

 what greater than that of potassium, yet in the living organism with very 

 few exceptions potassium is by far the more abundant clement. Mam- 

 malia, for example, contain about 3 times as much potassium as sodium. 

 As already mentioned, potassium is located mainly in the tissue cells, 

 sodium in the extracellular body fluid. Before the application of radio- 

 active indicators, the great difference between the sodium content of 

 the muscle cells and the surrounding fluid, or between the red corpuscles 

 and the surrounding plasma, for example, was interpreted as a con- 

 sequence of the impermeability of the phase boundaries to sodium. 

 The application of radio-sodium in permeability studies revealed, how- 

 ever, that sodium penetrates very easily from the blood plasma into 

 the red corpuscles, and vice versa^^^^' (12) ^nd the same applies also to 

 potassium. 



In spite of the marked permeability of the red corpuscles to sodium 

 and potassium, the latter element is strongly (about 30 times) concen- 

 trated in the red corpuscles, while sodium is about 10 times more abundant 

 in the plasma than in the (human) erythrocytes. Such a difference 

 cannot be explained without assuming a distinct difference in the che- 

 mical affinity of sodium and potassium to some type of organic cellular 

 constituents, a difference which makes potassium a more effective com- 

 petitor for the cellular ionic content than sodium. Levi and Ussing^^^) 

 brought important arguments for the view, formerly advocated by 

 LuNDEGARDH^^*^ and by Krogh^^^^ that these hypothetical organic 

 complexes are present in the phase boundary. The red corpuscle mem- 

 brane, though impermeable to sodium ions, contains scattered anions 

 of a substance which forms a stable complex with sodium. Owing to 

 thermal movements these complex molecules will come into contact 

 sometimes with the outside medium and sometimes with the inside 

 medium. If the inside solution contains ^^Na ions, these may exchange 

 with 2^Na in some of the complex molecules, and when these molecules 

 later touch the outside solution, ^^'Nr, will leave the complex in exchange. 



The application of radioactive indicators thus revealed the existence 

 of a new type of permeability which may be denoted as interchange 

 permeability. It exhibits some analogy to the phenomenon of selfdif- 

 fusion. The diffusion process leads usually to changes in the concen- 

 tration of the diffusing substrate: so does permeability. Self-diffusion 



^11^ Hahn, Hevesy and Rebbe, Biochem. J . 33, 1549 (1939). 



^i2)coHN and Cohn, Proc. Soc. Exp. Biol. Med. 41, 445 (1939). 



^^^^ Acta Physiol. Scand. 16, 232 (1948). 



^^*^ Protoplasma 35, 548 (1941). 



(is) Proc. Eoy. Soc. B 133, 140 (194G). 



