Figure 3. Fast and slow components of K⁺ charge movement are kinetically independent.

(a) Experimental protocol. Superimposed H₂DTG-sensitive currents in response to a voltage jump to −160 mV with step durations of 40, 150, 500 and 1,000 μs. Solid red lines represent monoexponential fits (τ=408 μs) of the current decays on return to 0 mV after each step. The time constant of the 1-ms pulse to −160 mV was 286 μs. (b) Normalized charge quantities moved by the slow and fast components measured at 0 mV. Charge moved by the slow component (Q_s) increases monotonically with the duration of the step to −160 mV. The solid red line represents a monoexponential fit with τ_s=212±9 μs. The fast component (Q_f) remains essentially constant. The presence and magnitude of Q_f are not affected by the monovalent cations used to substitute Na⁺ (that is, TMA or N-methyl D-glucamine), nor by changes in the concentrations of Ca²⁺ or Mg²⁺. Supplementary Fig. 2c shows a potential model and simulations that explain the time course and persistence of Q_f. We noticed that for pulses longer than 1 ms there was a small (∼10–15%) reduction of OFF charge carried by the slow component. Neither the time constant nor the extent of this reduction is influenced by external K⁺, indicating that it is not related to external K⁺ binding and occlusion. n=4; scale bars represent s.d.