10 The Chemistry of the Injured Cell 



explain these discrepancies it was suggested that intracellular 

 potassium ions existed in two compartments, one exchangeable 

 with extracellular potassium, the other not exchangeable. However 

 this theory is unattractive and the failure of isotopic K to mix 

 uniformly with intracellular K could be explained better by the 

 view that the diffusion of K inside the cell is not in fact free but is 

 on the contrary restricted. In other words, the movement of ions 

 between intra- and extra-cellular fluids is governed not only by the 

 characteristics of the cell membrane but also by those of the cyto- 

 plasm. 



Harris (1957) has shown that if the cell resembled a model in 

 which a restrictive membrane enclosed a space in which potassium 

 ions moved freely, then for a cell of 80 fx diameter equilibration of 

 intra- and extra-cellular potassium should be 90 per cent complete 

 in less than one second. In fact, mixing occurs very much more 

 slowly. By studying in muscle the uptake of tracer potassium Harris 

 has shown also that the intracellular potassium, although diffusing 

 slowly, possesses uniform diffusion properties. 



These results, and others, indicate that intracellular potassium 

 is not in separate compartments but is in a uniform state. The most 

 popular conception of this state is that the potassium ions are held 

 by absorption on anionic sites, e.g. proteins, nucleic acids, or phos- 

 pholipids. Potassium exchange would then depend on the displace- 

 ment from its locus of one ion by another and would therefore be 

 slow enough to account for the experimental findings in isotopic 

 mixing. 



As far as potassium is concerned, the cell may then be considered 

 as a lattice work or matrix on which are absorbed the potassium 

 ions, while in the interstices of the network move the ions which 

 have been displaced or which have yet to occupy a site. It may be 

 that the endoplasmic reticulum is the structural basis of this ion- 

 binding matrix. 



It seems likely that the movement of sodium ions, too, into and 

 out of the cell is governed not only by membrane permeability but 

 also by limited diffusion within the cell (Harris, 1957) . This 

 suggests that part of the matrix is occupied by sodium. Intracellular 

 sodium and potassium tend to vary inversely so that if the latter 



