Chloride current density. Mean chloride currents from late-stage (67–93 d) WT (n = 7 fibers; 2 mice) and R6/2 (n = 18 fibers; 5 mice) muscle. (A–D) Voltage-clamp traces for WT (left) and R6/2 (right) fibers. Scale bars are shown in the bottom right. For clarity, only every other voltage and current trace is shown. (A) Voltage protocol. (B–D) Current records normalized to fiber capacitance (µF). (B) Total currents were composed of the chloride current (I_ClC-1) plus leak and capacitive currents. (C) Leak and capacitive currents were measured by blocking chloride channels with 400 µM 9AC. (D) Specific I_ClC-1 records were obtained by subtracting the leak and capacitive currents (C) from the total currents (B). (E) IV relationship of the I_ClC-1 (mean ± SEM) from P2. A significantly lower current (*) was observed from R6/2 mice compared with WT at −140 mV (P = 0.003), −120 mV (P = 0.003), −100 mV (P = 0.002), −80 mV (P = 0.002), −60 mV (P = 0.001), −40 mV (P < 0.001), 0 mV (P < 0.001), 20 mV (P < 0.001), 40 mV (P < 0.001), 60 mV (P < 0.001), 80 mV (P < 0.001), 100 mV (P < 0.001), and 120 mV (P < 0.001), but not at −20 mV (P = 0.223).