The effect of extracellular Cl⁻ on the magnitude of the depolarization during AP trains. Two electrodes were inserted into the same muscle fiber, and 3.5-s-long 15-Hz trains of AP trigger pulses were injected. Experiments were conducted using extracellular solutions without Cl⁻ (n = 20) or with different Cl⁻ concentrations of 127 (control; n = 11), 80 (n = 23), or 50 mM (n = 14). (A) Representative recordings from different fibers at the four Cl⁻ concentrations. (B) The average change in resting membrane potential during the AP trains at the four Cl⁻ concentrations. Also presented is the G_Cl at the different conditions as reported by Pedersen et al. (2009a) and the resulting G_K/G_Cl ratios. (C) The experimental observations of AP trains at the different Cl⁻ concentrations were simulated, and the change in resting membrane potential was determined. (D) The K⁺ concentration in the deepest t-system compartment during the AP trains under the four conditions.