[Monovalent cation selectivity of EDTA-modified calcium channels]

Properties of slow inward sodium currents evoked by membrane depolarization in calcium-free EDTA-containing extracellular solution were studied on isolated intracellularly dialyzed neurons of the mollusc Helix pomatia. The relative permeability of the corresponding ionic channels determined from the shifts of the reversal potential was PNa+ :PLi+ :PNH2NH3+ :POHNH3+ = 1.00:0.80:0.55:0.21. For original sodium channels of the same membrane the relative values were 1.00:1.04:0.44:0.19. The induced sodium current could be blocked by calcium-channels blockers D-600 and niphedipine more effectively than calcium currents in the same membrane (the corresponding dissociation constants were 10(-5)M and 0.8 . 10(-5)M for the induced sodium current comparing with 2.6 . 10(-5)M and 2.3 . 10(-5)M for the calcium current. On the basis of these data a suggestion is made that the calcium channels possess a main selectivity filter which is similar to that of sodium channels, and an additional external binding site which binds divalent cations and prevents monovalent cations from entering the channel. Calcium-chelating agents introduced into calcium-free extracellular solution release this binding site from the occupying ion and thus transform the calcium channel into a sodium one.