458 



ADVENTURES IN KADIOISOTOPE RESEARCH 



times that of the activity measured at the end of the experiment, 1 mgm 

 dry resting nerve takes up on an average 0.2 /^gm sodium per minute. 

 In view of the very rapid change in the ^^Na content of plasma which 

 follows intravenous injection, this figure represents only a rough estimate 

 of the amount of sodium taken up by the nerve. 



Potassium Permeability 



In these experiments labelled potassium chloride having a specific 

 activity of 0.3 millicurie was injected. The injection took 1 minute, the 

 cat was killed 2 minutes later. The results obtained are seen in Table 6. 



Table 6. — Uptake of ^^k by Sciatic Nerve 



By a similar consideration as put forward in the case of sodium we 

 arrive to the conclusion that in the course of 1 min. about 0.02 /^gm pot- 

 assium penetrates in 1 mgm dry nerve tissue or 6 /^gm in 1 gm fresh tissue. 



It is of interest to compare this figure with the data recently obtained 

 by HoDGKiN and Huxley (1946) when determining the number of 

 moles of potassium which leak through 1 cm"^ of membrane of the 

 axons from Carcinus maenas in one impulse (1.7 • lO""^^) and the amount 

 of potassium re-absorbed during the period of recovery. When the ex- 

 ternal potassium concentration is increased threefold, tS.lO^i** y jj^qI cm~- 

 sec~^ or 0.7 [xgm potassium per minute were found to be re-absorbed. 



Bromine Permeability 



In these experiments 30 mgm bromine as sodium bromide, with an 

 activity of 0.2 millicurie was injected. 



Table 7. — Uptake of ^^Br by Sciatic Nerve 



