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L. J. MULLIXS 



values of size and, possibly, by the tendency of Ca^* to plug the channels. 

 The application of slowly increasing outward electric currents allows the mem- 

 brane macromolecules to assume new positions with respect to the through 

 channels being formed, without large increases in Na+ flux because the rate of 

 through channel creation and the rate of 'flow' of macromolecules may be 

 assumed to be comparable. It might be expected that membranes would differ 

 in what might be called their 'miscroscopic viscosity' or their resistance to 

 flow or redistribution of their structural elements, and that this would be 

 apparent as a difference in accommodation. 



A variety of other arrangements for the transformation of the membrane 

 from a relatively Na+ impermeable-permeable structure suggest themselves 

 but, in some way or another, appear less attractive than that described above. 

 One might set the mean interspace distribution at the size of (Na+)i , but 

 consider that practically all these sites are occupied by Ca ++ at Ringer con- 

 centration. With P Na properly adjusted, P K and P c i could be given proper 

 values. Outward current could then be thought of as removing the Ca++ block- 

 ing the Na + channels and permitting the required inward Na + flux, but it is 

 hard to see why this should not continue as long as the membrane is depolarized. 



Further improvements in the model should consider that ions on both sides 

 of the membrane will tend to distort the interspace size distribution from its 

 mean value for an ion-free structure, to one with a mean close to the size of a 

 particular ion. At the inner surface of the membrane the mean interspace size 

 is probably close to that of (K+)i , other cations or anions being relatively 

 ineffective competitors because of their size or low concentration, while at the 

 outer surface of the membrane Na + and Cl~ compete but the actual mean may 

 be expected to be close to CI - in size. If, in the resting membrane, the partition 

 coefficients are maximal for CI - on the outside and K + on the inside, coefficients 

 for the other ions at each surface can be obtained from a distribution curve and 

 relative figures for the mobilities of ions moving in either direction can be given 

 as shown in Table II. 



In a non-through channel the possibility exists that ions of different size 

 are alternately loaded owing to the continuous variation in channel size that 

 is presumed to be taking place. This effect may be expected to result in very 



TABLE II 



