102 



M. A. ROTHENBERG 



VOL. 4 (1950) 



6 



CiOA 



tieth of that of the initial rate. This rapid fall in the rate of penetration is further 

 support for the assumption that complete exchange of Na across the membrane occurs 

 within a short period of time. 



Extrapolation of the curve in Fig. 6 to zero time gives a value of 5.8 millimoles/ioo 

 g/min for the initial rate of Na exchange in these nerves. If one carries out a similar 

 operation for the curve of Fig. 2, a value of 0.082 millimole/ioo g/min for K is obtained. 

 These results seem to indicate that the initial rate of exchange of Na is many times 

 greater than of K. These findings do not support the concepts of Conway^ that nerve 

 membranes are impervious to Na although it has to be kept in mind that the obser- 

 vations are limited to the giant axons of Squid. The observations presented are consistent 

 with those of Steinbach and Spiegelman who have been able to demonstrate that Na 



enters these nerves. 



3. Calcium. Table III gives some of 

 the date obtained when nerves were 

 exposed to high specific activity of Ca*^ 

 (0.012 M) in artificial sea water for varying 

 periods of time. All of the date obtained 

 are plotted in the curve of Fig. 7. As in 

 the cases of Na and K, each point on 

 the curve represents a single nerve. The 

 curve has been drawn through the mean 

 of the several values at a given time of 

 exposure. The data obtained were the 

 same when low specific activity Ca*^ was 

 used. 



1.0 



.0.8 



0.2 



1 1 

 " II 



/I I I 1 



10 20 



50 



100 

 Mil), of exposure 



Fig. 7. Ca penetration across the membrane of 

 the giant axon of Squid when exposed to artifical 

 sea water containing 0.012 M Ca'^^Clj. The pene- 

 tration of Ca** inmilHmoles (m!\I)/ioo g axoplasm 

 (wet weight) is plotted against time in minutes. 



TABLE III 



Ca''* PENETRATION 



Nerves exposed to sea water containing 0.012 M Ca^^CIj (high specific activity) for varying periods 

 of time. Standards (S^ and Sj) : sea water diluted 1:200 and then 0.5 ml evaporated in duplicate 

 (0.03 micromole Ca**/o.5 ml). 



It will be noted from Fig. 7 that the Ca*^ inside the nerve seems to reach a maximum 

 value of 0.82 millimole/ioo g within 45 min and then decreases to a value of 0.45 milli- 

 mole/ioo g at 100 min of exposure. It is evident, therefore, that the Ca penetrates into 

 these nerve fibres. The values obtained seem to indicate that the concentration of Ca^^ 

 at 100 min is lower than at 50 min. Further investigations are desirable for an inter- 

 pretation of this observation. 

 References p. 114. 



