C. ADRIAN M. HOGBEN l8l 



99). A recent abstract (76) reporting that a silver-silver chloride electrode in 

 the axoplasm of the giant squid axon is 35 mv negative to a similar external 

 electrode infers that chloride is not in electro-chemical equilibrium across this 

 cell wall. However, if the internal chloride concentration were similar to that 

 reported by others (43) and the nerve were in a steady state, the observed po- 

 tential would demand a chloride activity coefficient inside the nerve consider- 

 ably greater than in free solution. Without substantiating information, this 

 observation is insufficient for concluding that chloride is not moving by passive 

 diffusion. 



A fairly complete picture is available of ionic movement through the isolated 

 skin of the frog where, but for the exception of adrenaline stimulation, chloride 

 passively diffuses along the electrical gradient generated by active transport 

 of sodium (54, 66). 



Not infrequently chloride penetration into erythrocytes is cited as an example 

 of simple passive diffusion of chloride. One of the principal arguments is the 

 extremely rapid rate of chloride exchange. But the rate is so rapid that we are 

 prompted to question the assumption that chloride moves only by passive diffu- 

 sion of the free dissociated ion. An ionic flux, if it is simply due to passive diffu- 

 sion, allows calculation of the partial ionic conductance which would be less 

 than the membrane conductance. 



Erythrocyte Cl~ exchange is virtually complete within 2 minutes (69) indi- 

 cating that the exchange half time is less than 20 seconds. Dirken and Mook 

 found that the chloride shift of the ox erythrocyte is almost complete within 

 one second (30). If the anion shift is simply a matter of passive diffusion it is 

 necessary to reckon with both concentration and potential differences. Never- 

 theless, if there were a significant bi-ionic potential during the shift, it would 

 mean that the conductance of one ion, say H+ or HCOs", is increased at the 

 expense of the other ion Cl~. Using an admitted oversimplification for the 

 present argument, the shift will be considered as a dissipation of a concentra- 

 tion difference. If the half-time for decay of a concentration difference between 

 cells and plasma, of equal volume, is known, the flux is given by: 



M = (0.693/2) (Vc/Atx) 



The flux (M) is assumed to be proportional to concentration. The appro .ximate 

 value of the constants are: decay half-time (ti) = 0.5/3600 hr. ; volume (V) = 

 8.8 X io~'^ cm"*; area (A) = 1.4 X lo"^ cm-; and concentration (c) = 80 /uEq 

 cm~l These values yield a flux rate of 12.5 piEq cm~- hr~'. If there were two 

 ions passively diffusing at this rate, the surface resistance would be 40 ohms 

 cm-. 



A satisfactory measure of the electrical resistance of the erythrocyte has 

 not been obtained. The resistance of packed cells is more than 60 times that 

 of serum (35) but it does not allow a useful estimate of the membrane conduct- 



