Figure 6. High-concentration Ca²⁺ blocks the leak current but not MET current.

(A) Monovalent cations Li⁺ and Cs⁺ conducted the leak current. In this experiment, 150 mM NaCl was substituted with 150 mM LiCl or 150 mM CsCl in the external solution. (B) Divalent cations 10 mM Ba²⁺, 75 mM Zn²⁺, 75 mM Co²⁺, 150 mM Mg²⁺, and 75 mM Ca²⁺, conducted the leak current. The 150 mM NaCl was partially or completely replaced with cations as described in the Materials and methods. (C) Representative I_m traces by ramp stimulation for calculation of ionic permeability of the leak channel. The extracellular ion was switched from 150 mM Na⁺ to 75 mM Ca²⁺, and to 150 NMDG⁺, all containing 100 μM DHS. In the intracellular solution, 150 mM CsCl was used. (D) Quantification of ionic permeability calculated from similar recordings in (C). (E) Example trace of I_m of OHCs during perfusion with solutions containing graded concentrations of Ca²⁺ and Na⁺. An 800 nm step deflection was applied to the hair bundle by a glass probe. The glass probe induced MET currents are marked ‘+', accompanying unwanted MET currents and artefacts induced by switching the perfusion system (#). (F) Dose curves of I_BG and I_MET in wild-type OHCs in different Ca²⁺ and Na⁺ concentrations (cell numbers, 9–20). (G) Quantification of dose-dependent background leak current in OHCs from wild-type (black) and Tmc1-knockout (red) mice when bathed in mixed Ca²⁺ and Na⁺. The ions and concentrations used in test external solutions were variable, as described in this figure legend and the Materials and methods. The holding potential was −70 mV. Data are presented as mean ± SEM. N values are shown in each panel. *p<0.05, **p<0.01, ***p<0.001, (B,D) ANOVA.