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. 2008 Jan;131(1):13–32. doi: 10.1085/jgp.200709877

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Copyright © 2008, The Rockefeller University Press

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Figure 6. — Mg²⁺ affects voltage sensor deactivation when channels are open. (A) I_g evoked by pulses to +200 mV of different duration (0.05–20 ms) in 0 and 10 mM Mg²⁺. (B) Normalized OFF currents from A decay more slowly as pulse duration increases, especially in the presence of Mg²⁺. Red traces represent 0.05 and 0.1-ms pulses. (C) OFF kinetics at −80 mV following brief (closed) or prolonged (open) pulses are compared by plotting the quantity Q = Q_OFF(t) − Q_OFFSS on a log scale vs. time where Q_OFF is the integral of I_gOFF and Q_OFFSS is the steady-state value of Q_OFF(t). The “closed” traces, representing the average of 0.05 and 0.1-ms records, have an exponential time course with almost identical kinetics (τ_Fast) in 0 Mg²⁺ (15.5 μs) or 10 mM Mg²⁺ (16.3 μs). The “open” traces, representing the average of 10, 15, and 20-ms records, were fit by triple exponential functions (0 Mg²⁺: q_Fast = 6.2 fC, τ_Fast = 15.5 μs, q_MED = 24.9 fC, τ_MED= 54.1 μs, q_SLOW = 11.2 fC, τ_SLOW = 500 μs; 10 Mg²⁺: q_Fast = 2.6 fC, τ_Fast = 16.3 μs, q_MED = 10.6 fC, τ_MED = 127 μs, q_SLOW = 39.1 fC, τ_SLOW = 756 μs). The Med and Slow components of the fits are shown as dotted and solid lines respectively. (D) Scheme 1* depicts the five closed (C_i) and open (O_i) states defined by Scheme 1, where i represents the number of activated voltage sensors (i = 0–4). δ_i and γ_i are forward and reverse rate constants for channel opening while α and β are rates for voltage sensor activation when channels are closed, and f = . Colored arrows represent different pathways of deactivation and Q_OFF components for open channels in 0 Mg²⁺ (top) or 10 mM Mg²⁺ (bottom) at −80 mV. In 10 mM Mg²⁺, channels can close from open states other than O₀ because Mg²⁺ allows voltage sensors in open channels to remain activated at −80 mV (Q_O, Fig. 5 E). The figure illustrates one possible pathway, through the O₂ state. After channels close, voltage sensors can completely deactivate in 10 mM Mg²⁺ (Q_C, Fig. 3 B). This charge movement is limited by the closing rate and therefore contributes to the Slow component of Q_OFF. (E) The amplitude of the three OFF components (Fast, ▴; Med, light blue square; Slow, pink circle) for the experiment in A are plotted vs. pulse duration and fit by exponential functions (0 Mg²⁺, τ = 2.8 ms; 10 Mg²⁺, τ = 2.6 ms).

Inline graphic — Mg²⁺ affects voltage sensor deactivation when channels are open. (A) I_g evoked by pulses to +200 mV of different duration (0.05–20 ms) in 0 and 10 mM Mg²⁺. (B) Normalized OFF currents from A decay more slowly as pulse duration increases, especially in the presence of Mg²⁺. Red traces represent 0.05 and 0.1-ms pulses. (C) OFF kinetics at −80 mV following brief (closed) or prolonged (open) pulses are compared by plotting the quantity Q = Q_OFF(t) − Q_OFFSS on a log scale vs. time where Q_OFF is the integral of I_gOFF and Q_OFFSS is the steady-state value of Q_OFF(t). The “closed” traces, representing the average of 0.05 and 0.1-ms records, have an exponential time course with almost identical kinetics (τ_Fast) in 0 Mg²⁺ (15.5 μs) or 10 mM Mg²⁺ (16.3 μs). The “open” traces, representing the average of 10, 15, and 20-ms records, were fit by triple exponential functions (0 Mg²⁺: q_Fast = 6.2 fC, τ_Fast = 15.5 μs, q_MED = 24.9 fC, τ_MED= 54.1 μs, q_SLOW = 11.2 fC, τ_SLOW = 500 μs; 10 Mg²⁺: q_Fast = 2.6 fC, τ_Fast = 16.3 μs, q_MED = 10.6 fC, τ_MED = 127 μs, q_SLOW = 39.1 fC, τ_SLOW = 756 μs). The Med and Slow components of the fits are shown as dotted and solid lines respectively. (D) Scheme 1* depicts the five closed (C_i) and open (O_i) states defined by Scheme 1, where i represents the number of activated voltage sensors (i = 0–4). δ_i and γ_i are forward and reverse rate constants for channel opening while α and β are rates for voltage sensor activation when channels are closed, and f = . Colored arrows represent different pathways of deactivation and Q_OFF components for open channels in 0 Mg²⁺ (top) or 10 mM Mg²⁺ (bottom) at −80 mV. In 10 mM Mg²⁺, channels can close from open states other than O₀ because Mg²⁺ allows voltage sensors in open channels to remain activated at −80 mV (Q_O, Fig. 5 E). The figure illustrates one possible pathway, through the O₂ state. After channels close, voltage sensors can completely deactivate in 10 mM Mg²⁺ (Q_C, Fig. 3 B). This charge movement is limited by the closing rate and therefore contributes to the Slow component of Q_OFF. (E) The amplitude of the three OFF components (Fast, ▴; Med, light blue square; Slow, pink circle) for the experiment in A are plotted vs. pulse duration and fit by exponential functions (0 Mg²⁺, τ = 2.8 ms; 10 Mg²⁺, τ = 2.6 ms).