426 ADVENTURES IX RADIOISOTOPE RESEARCH 



it should be possible to measure the rate of passage of sodium chloride 

 through the endothelium. However, when carrying out these experi- 

 ments we meet the following difficulties : (a) Not only does the cir- 

 culation get rid of excess sodium chloride by giving off salt to the extra- 

 cellular space, but also by taking water up from the tissues. Keyes 

 (1937) found, when studying the fate of sucrose intravenously injected 

 into man, that osmotic equilibrium by a shift of water takes place from 

 three to ten times as fast as sucrose exchange. The rate of disappearance 

 of the excess sodium chloride will, thus, not measure the rate of passage 

 of sodium chloride through the capillary wall but a more complex 

 process, (b). The resistance of the endothelium to the passage of 

 sodium and chloride may be quite different, (c) The introduction 

 of appreciable amounts of sodium chloride into the circulation 

 will disturb the normal conditions prevailing in the circulation. When 

 one tries to eliminate this difficulty by introducing small amounts 

 only, the analytical difficulties become almost unsurmountable. All these 

 difficulties can be eliminated by injecting into the veins labelled sodium 

 chloride (sodium chloride containing some radioactive ^^Na of negligible 

 weight) and by measuring the rate of disappearance of the active ions 

 from the plasma, i. e. the decrease of the radioactivity of the plasma. 

 We are not determining in these experiments the rate of influx of excess 

 sodium chloride from the plasma into the extracellular fluid but the rate 

 of exchange between labelled plasma sodium and non-labelled extra- 

 cellular sodium, as the number of sodium (^^Na -j-^^Na) atoms of the 

 plasma remains practically constant all through the experiment. The 

 rate of exchange will be determined by the permeability of the capillary 

 wall to sodium ions and will, thus, be a measure of this permeability. 

 We carried out also experiments with radio-potassium, radio -chlorine, 

 radio-bromine, and radio-phosphate, while heavy water was used as an 

 indicator for water in the study of permeability of the endothelium to 

 water. The measurement of the distribution of radio-sodium between 

 plasma and the extravascular space of the rabbit was previously used 

 to determine the extracellular A'olume of the rabbit (Griffith and 

 Margraith, 1939 ; Hahts- et al., 1939). 



EXPERIMENTAL PROCEDURE 



Radioactive sodium and potassium were prepared by bombarding NaOH and 

 KOH, respectively, with high speed (10 million volts) deuterons. The hydroxydes 

 were neutralized with hydrochloric acid and the solution thus obtained was injected. 

 Radioactive chlorine and broniine were prepared by bombarding NaCl and NaBr, 

 respectively, with deuterons. The active chlorine and bromine obtained were 

 driven off as HCl and HBr, respectively, and were collected in a sodium hydroxyde 

 solution. This procedure was chosen to get rid of the active sodium simultaneously 



