467 



Comment on papers 42—47 



When first investigating the rate of exodus of ^^Na from the phusma of the rabbit, 

 we were struck by tlio rapidity with wliich this takes place, by the swiftness of 

 the interchange between vascular and oxtravtiscular sodium. After the lapse of 

 i/g min. about half of the former was replaced by the latter. We know today that 

 the figures obtained in such investigations indicate only the lower limit of the 

 speed with which such an interchange takes place. The injected labelled sodium 

 has first to diffuse into the capillaries before leaving the circulation, and the 

 interchange between vascular and extra vascular sodium which takes place 

 during this early interval is not indicated by the tracer. 



When carrying out our first experiments we had at our disposal potassium 

 of very low specific activity only. To carry out an experiment on the rate of exodus 

 of *-K from the circulation, we had to inject very rapidly 10 ml of physiological 

 potassium c-hloride solution. Though the blocking effect of potassium on heart 

 beat is well known, we were much impressed by the momentary fatal effect of 

 such a rapid injection of a potassium chloride solution. When in possession of 

 potassium of higher specific activity, we compared the rates at which sodium and 

 potassium ions leave the circulation. That the latter was found to take place at 

 a more rapid rate is presumably mainly due to the following fact: The 

 labelled ions leave and re-enter the circulation. The re-entry is faciHtated in the 

 case of sodium by the restricted extravascular pool, sodium being mainly an extra- 

 cellular element in contrast with potassium. Potassium ions which left the plasma 

 and entered tissue cells have less chance of re-entering the plasma than have sodium 

 ions. 



As shown in paper 43 in y^ min half of the labelled water leaves the plasma 

 of the rabbit. This is, for the reason mentioned above, an upper limit of the half- 

 time that the water molecules remain in the plasma. After the lapse of 1 hr an 

 almost complete equipartition of the water molecules between the circulation 

 and the extravascular water was observed. 



Since radio-sulphur was not available at that date for the study of the fate 

 of the sulphate group in the organism, we applied with i^O labelled sulphate 

 (paper 47). 



30* 



