3.0 



S2.5 



2.0 



g8 M. A. ROTHENBERG VOL. 4 (1950) 



RESULTS 

 A. ION EXCHANGES AT REST 



I. Potassium. In one series of experiments the stellar nerves were exposed to arti- 

 fical sea water in which the K^^ had been replaced by K*^ in the usual sea water concen- 

 tration (0.013 M)- Analysis of axoplasm samples indicated that there was a rapid ex- 

 change of potassium under these conditions. Table I gives a few examples illustrating 

 the size of the axoplasm samples, the magnitude of the radiation measured and the 

 manner in which the standards were prepared. All of the data obtained in this way are 

 presented in Fig. i. Each point on the graph represents a single experiment. The number 

 of millimoles (mM) of K'*^ which penetrated per 100 gm axoplasm (wet weight) is plotted 

 against time of exposure of the nerve fibre to the radioisotopic sea water. It will be 

 noted from Fig. i that the rate of penetration of K*^ through the nerve membrane is 

 initially quite high but it then slows markedly and within 60 min, analyses indicate an 



approach to a maximal value or 2.5 mil- 

 limoles/ioo g asymptotically. If one ac- 

 cepts the values for the potassium content 

 of the axoplasm found in the literature 

 (Steinbach and Spiegelman, 32.1 meq. 

 per cent^ ; Baer and Schmitt, 27 meq. per 

 cent^; Webb and Young 25.3 meq. per 

 cent*) it can be seen that the maximum 

 exchange obtainable under these con- 

 ditions is approximately one tenth of the 

 total K concentration of the axoplasm. 

 In all probability, the curve in Fig. i 

 is a composite of at least two, or possibly 

 more, distinct reactions. The first part 

 of the curve, with the steepest slope, is, 

 in all probability, a true measure of the 

 rate of exchange of K across the nerve membrane. The second phase in which the 

 rate of exchange has slowed down may possibly be ascribed to a movement of the 

 radioactive ions from the inside to the outside after having reached a certain level. 

 Finally, when the inside concentration is about twice that of the outside, there ap- 

 pears to be an equilibrium of the movements in the two directions. 



The expeiiments show that even at rest, there is a dynamic equilibrium between the 

 K inside the fibre and that in its outer environment^. Within 50 min an equilibrium is 

 established. Under such conditions only about one tenth of the total K inside the fibre 

 has exchanged for K*^ in the bathing medium. The K*^ concentration inside the fibre 

 is 2.5 millimoles/ioo g axoplasm against 1.3 millimoles/ioo ml for the sea water. When 

 a steady state of exchange has been attained, it is possible to calculate the permeability 

 constant for this exchange of K at rest by means of Collander's equation as modified 

 by Krogh''. According to Krogh where d is the diameter of the cell (cm), t is 



1.5 



1.0 



0.5 



15 



30 



60 



90 

 Min. of exposure 



Fig. I. K penetration across the membrane of 



the giant axon of Squid when exposed to 0.013 



K^^Cl in artificial sea water. The horizontal 



broken line on the ordinate indicates the K^^ 



concentration outside. The penetration of K*^ 



in millimoles (mM)/ioo g axoplasm (wet weight) 



is plotted against time in minutes. 



P - 0.576 — logio 



C. 



U<- V^n 



References p. 114. 



