VOL. 4 (1950) 



PERMEABILITY AND NERVE FUNCTION, II 



99 



time (hours), Cg and Q concentrations of the ion inside and outside respectively, and a, 

 and a-o are the corresponding activities, d may be assumed to be = 0.05 cm, Cj = 0.32 M 

 (Steinbach and Spiegelman) and Q = 0.013 M. Substituting 40400 cts/min/ml for 

 Rq (from Table I) and 77700 cts/min/g for a^ (from Fig. i) when t = 0.83 h, one obtains 

 a value of 1.25-10"^ cm/h for P, the permeability constant, from the equation above. 



TABLE I 



K'*^ PENETRATION 



Nerves exposed to sea water containing 0.013 M K^^Cl for varying periods of time. Standards (S^ 

 and Sg) : sea water diluted 1:10 and then 0.5 ml evaporated to dryness in duplicate (0.65 micromole 

 K*^/o.5 ml). Counts per min indicate the actual count, uncorrected for time decay of radioactivity. 



Fig. 2 shows the rates of exchange of K against time. It will be noted that the rate 

 is initially high but then drops to a value which is only about one fourth of that of the 

 initial rate. The rate of penetration ap- .s 

 proaches a limiting value of 20 millimoles/ "^^^^ 

 100 g/min (or 2.5 • io~^ mole/cm^/min ^^q 

 assuming an average diameter of 500 ju). -^ 



In a second series of experiments, ^^'^ 

 the nerves were exposed to 0.026 M K*^ ^ 20 

 CI in the bathing sea water (twice the 

 normal K concentration). In carrying 

 out these experiments, a decrease in NaCl 

 concentration was made equivalent to the 

 increase in KCl in order to maintain the 

 isotonicity of the sea water. The data ob- 

 tained are plotted in Fig. 3. 



It is evident from a comparison of Figs i and 3 that the shapes of the curves ob- 

 tained for 0.013 M and 0.026 M KCl are very much alike. However, since the ordinate 

 in Fig. 3 is greater by a factor of two, it can be seen that in the latter case the penetra- 

 tion of K*2 into the fibre reaches a maximal value of 5.3 millimoles/ioo g axpolasm. 

 As in the case of the experiments with 0.013 ^ KCl, exchange of K^^ inside for K^^ 

 outside reaches an equilibrium when the inside concentration of K^^ is twice that of 

 the outside. 



As in the case of Fig. i. Fig. 3 should probably have been resolved into three 

 distinct phases. The considerations applied to the segments of Fig. i are also applicable 

 References p. 114. 



90 



Min. of exposure 

 Fig. 2. Rate of K penetration across the mem- 

 brane of the giant axon of Squid when exposed 

 to 0.013 M K*^C1 in artificial sea water. The rate 

 of penetration of K*^ in micromoles (/<M)/ioo 

 g/min is plotted against time of exposure in min. 



