A multi-ion permeation mechanism in neuronal background chloride channels.
Unitary current/voltage relationships of background Cl channels of rat hippocampal neurons were determined for varied gradients and absolute concentrations of NaCl. The channels revealed permeabilities for both Cl and Na ions. A hyperlinear increase of unitary conductance, observed for a symmetrical increase of salt concentration from 300 and 600 mM, indicated a multi-ion permeation mechanism. A variety of kinetic models of permeation were tested against the experimental current/voltage relationships. Models involving a pore occupied by mixed complexes of up to five ions were necessary to reproduce all measurements. A minimal model included four equilibrium states and four rate-limiting transitions, such that the empty pore accepts first an anion and then can acquire one or two cation/anion pairs. Three transport cycles are formed: a slow anion cycle (between the empty and single-anion states), a slow cation cycle (between the one- and three-ion states), and a fast anion cycle (between the three- and five-ion states). Thus, permeant anions are required for cation permeation, and several bound anions and cations promote a high rate of anion permeation. The optimized free-energy and electrical charge parameters yielded a self-consistent molecular interpretation, which can account for the particular order in which the pore accepts ions from the solutions. Although the model describes the mixed anion/cation permeability of the channel observed at elevated concentrations, it predicts a high selectivity for Cl anion at physiological ionic conditions.
Department of Physiology and Biophysics, University of Miami, Florida 33101.
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Franciolini
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Nonner
D000818:Animals; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D018118:Chloride Channels / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism; D017136:Ion Transport; D007700:Kinetics; D008564:Membrane Potentials; D009474:Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D012965:Sodium Chloride / Q000378:metabolism
D018118:Chloride Channels; D002712:Chlorides; D012965:Sodium Chloride; D012964:Sodium