In the negative EOG-generating process a cation which can substitute for Na+ was sought among the monovalent ions, Li+, Rb+, Cs+, NH4+, and TEA+, the divalent ions, Mg++, Ca++, Sr++, Ba++, Zn++, Cd++, Mn++, Co++, and Ni++, and the trivalent ions, Al+++ and Fe+++. In Ringer solutions in which Na+ was replaced by one of these cations the negative EOG's decreased in amplitude and could not maintain the original amplitudes. In K+-Ringer solution in which Na+ was replaced by K+, the negative EOG's reversed their polarity. Recovery of these reversed potentials was examined in modified Ringer solutions in which Na+ was replaced by one of the above cations. Complete recovery was found only in the normal Ringer solution. Thus, it was clarified that Na+ plays an irreplaceable role in the generation of the negative EOG's. The sieve hypothesis which was valid for the positive EOG-generating membrane or IPSP was not found applicable in any form to the negative EOG-generating membrane. The reversal of the negative EOG's found in K+- , Rb+- , and Ba++-Ringer solutions was attributed to the exit of the internal K+. It is, however, not known whether or not Cl- permeability increases in these Na+-free solutions and contributes to the generation of the reversed EOG's.

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