[2 selective filters in the calcium channel of the somatic membrane of mollusk neurons].

The modification of inward currents in the somatic membrane of mollusc neurons produced by EDTA and other CA-chelating agents was investigated. The results obtained indicate the presence of two selective filters in the calcium channel of this membrane. The first one is located near the outer mouth of the calcium channel. It binds divalent cations in the following sequence--pKCa: pKSr: pKBa: pKMg=6.6 : 5.5 : 4.8 : 4.2. This outer filter controls channel selectivity according to the magnitude of cation charge and presumably contains several carboxylic groups. The second selective filter is located inside the channel and controls its permeability for cations with the same charge. It is assumed that the structure of the inner selective filter resembles very much the structure postulated by Hille for the selective filter of sodium channel and that it contains only one carboxylic group. The investigation of the effect of Ca2+ and Cd2+ ions on sodium currents of the same membrane has shown that the fast sodium current is not blocked by these ions and the observed decrease of its amplitude is connected with the change of the membrane surface potential and corresponding change of near-membrane concentration of carrier ions. On the basis of these experiments, it is suggested that the selectivity filter of sodium channel of this membrane does not contain a carboxylic group.