title: Extracellular zinc ion inhibits ClC-0 chloride channels by facilitating slow gating.
abstract: Extracellular Zn2+ was found to reversibly inhibit the ClC-0 Cl- channel. The apparent on and off rates of the inhibition were highly temperature sensitive, suggesting an effect of Zn2+ on the slow gating (or inactivation) of ClC-0. In the absence of Zn2+, the rate of the slow-gating relaxation increased with temperature, with a Q10 of approximately 37. Extracellular Zn2+ facilitated the slow-gating process at all temperatures, but the Q10 did not change. Further analysis of the rate constants of the slow-gating process indicates that the effect of Zn2+ is mostly on the forward rate (the rate of inactivation) rather than the backward rate (the rate of recovery from inactivation) of the slow gating. When ClC-0 is bound with Zn2+, the equilibrium constant of the slow-gating process is increased by approximately 30-fold, reflecting a 30-fold higher Zn2+ affinity in the inactivated channel than in the open-state channel. As examined through a wide range of membrane potentials, Zn2+ inhibits the opening of the slow gate with equal potency at all voltages, suggesting that a two-state model is inadequate to describe the slow-gating transition. Following a model originally proposed by Pusch and co-workers (Pusch, M., U. Ludewig, and T.J. Jentsch. 1997. J. Gen. Physiol. 109:105-116), the effect of Zn2+ on the activation curve of the slow gate can be well described by adding two constraints: (a) the dissociation constant for Zn2+ binding to the open channel is 30 microM, and (b) the difference in entropy between the open state and the transition state of the slow-gating process is increased by 27 J/ mol/ degreesK for the Zn2+-bound channel. These results together indicate that extracellular Zn2+ inhibits ClC-0 by facilitating the slow-gating process.
authors.affiliation: Department of Physiology, National Yang-Ming University, Taipei 11221, Taiwan. tychen@ym.edu.tw
authors.forename: T Y
authors.initials: TY
authors.lastname: Chen
mesh_terms: D000818:Animals; D002413:Cations, Divalent / Q000494:pharmacology; D018118:Chloride Channels / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D009865:Oocytes / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013696:Temperature; D014101:Torpedo; D014981:Xenopus; D015032:Zinc / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D002413:Cations, Divalent; D018118:Chloride Channels; D011994:Recombinant Proteins; D015032:Zinc