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Allosteric voltage gating of potassium channels II. Mslo channel gating charge movement in the absence of Ca(2+).

Horrigan FT, Aldrich RW - J. Gen. Physiol. (1999)

Bottom Line: These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J.Physiol. 114:277-304).The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA.

ABSTRACT
Large-conductance Ca(2+)-activated K(+) channels can be activated by membrane voltage in the absence of Ca(2+) binding, indicating that these channels contain an intrinsic voltage sensor. The properties of this voltage sensor and its relationship to channel activation were examined by studying gating charge movement from mSlo Ca(2+)-activated K(+) channels in the virtual absence of Ca(2+) (<1 nM). Charge movement was measured in response to voltage steps or sinusoidal voltage commands. The charge-voltage relationship (Q-V) is shallower and shifted to more negative voltages than the voltage-dependent open probability (G-V). Both ON and OFF gating currents evoked by brief (0.5-ms) voltage pulses appear to decay rapidly (tau(ON) = 60 microseconds at +200 mV, tau(OFF) = 16 microseconds at -80 mV). However, Q(OFF) increases slowly with pulse duration, indicating that a large fraction of ON charge develops with a time course comparable to that of I(K) activation. The slow onset of this gating charge prevents its detection as a component of I(gON), although it represents approximately 40% of the total charge moved at +140 mV. The decay of I(gOFF) is slowed after depolarizations that open mSlo channels. Yet, the majority of open channel charge relaxation is too rapid to be limited by channel closing. These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J. Cui, and R.W. Aldrich. 1999. J. Gen. Physiol. 114:277-304). The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

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Predictions of the allosteric model. (A) QOFF component amplitudes determined after pulses to +240 mV in 0 Ca2+ are plotted versus pulse duration. The fast component is reduced to <10% of the total OFF charge after a 20-ms pulse. The relaxation of all three components is fit by exponential functions (solid lines) with a τ = 0.91 ms. (B) The decay of QOFF–Qss is plotted on a semilog scale after 0.1- or 20-ms pulses to +160 mV in 60 μM Ca2+ (HP = −80). The 0.1-ms trace is fit by a triple exponential function (solid line, τF = 23.8 μs, τM = 150 μs, τS = 822 μs) and the 20-ms trace is fit with a double-exponential (τM = 150 μs, τS = 822 μs), indicating that the fast component is eliminated when most channels are opened. Dashed lines represent the two components of the 20-ms fit and the fast component of the 0.1-ms fit. (C) Normalized QOFF component amplitudes and total OFF charge (Qp) are plotted versus pulse duration for pulses to +160 mV in 0 Ca2+. OFF components were measured upon repolarization to −80 mV and are normalized to the fast component of ON charge (Qfast) at +160 mV. (D) When QOFF is measured upon repolarization to 0 mV, the Fast component and Qp are unchanged. However, the Medium component decreases and the Slow component increases in a complementary manner. (E) The charge distributions predicted by the allosteric model for Closed (QC) and Open channels (QO) are plotted versus voltage (zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17). Arrows indicate the predicted amplitudes of Medium and Slow OFF components at repolarization voltages of −80 and 0 mV after a pulse to +160 mV (VP).
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Figure 8: Predictions of the allosteric model. (A) QOFF component amplitudes determined after pulses to +240 mV in 0 Ca2+ are plotted versus pulse duration. The fast component is reduced to <10% of the total OFF charge after a 20-ms pulse. The relaxation of all three components is fit by exponential functions (solid lines) with a τ = 0.91 ms. (B) The decay of QOFF–Qss is plotted on a semilog scale after 0.1- or 20-ms pulses to +160 mV in 60 μM Ca2+ (HP = −80). The 0.1-ms trace is fit by a triple exponential function (solid line, τF = 23.8 μs, τM = 150 μs, τS = 822 μs) and the 20-ms trace is fit with a double-exponential (τM = 150 μs, τS = 822 μs), indicating that the fast component is eliminated when most channels are opened. Dashed lines represent the two components of the 20-ms fit and the fast component of the 0.1-ms fit. (C) Normalized QOFF component amplitudes and total OFF charge (Qp) are plotted versus pulse duration for pulses to +160 mV in 0 Ca2+. OFF components were measured upon repolarization to −80 mV and are normalized to the fast component of ON charge (Qfast) at +160 mV. (D) When QOFF is measured upon repolarization to 0 mV, the Fast component and Qp are unchanged. However, the Medium component decreases and the Slow component increases in a complementary manner. (E) The charge distributions predicted by the allosteric model for Closed (QC) and Open channels (QO) are plotted versus voltage (zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17). Arrows indicate the predicted amplitudes of Medium and Slow OFF components at repolarization voltages of −80 and 0 mV after a pulse to +160 mV (VP).

Mentions: The allosteric model predicts that the fast component of OFF charge movement should be eliminated after voltage pulses that open all channels. One way to increase Po is by stepping to more positive voltages. Fig. 8 A plots the time course of QOFF component development at +240 mV. The decay of QOFFfast is more complete than at +140 mV (Fig. 7 F), consistent with a voltage-dependent increase in Po. It is likely that the fast component was not eliminated because, in the absence of Ca2+, mSlo channels are maximally activated only at very positive voltages (greater than +300 mV) (Horrigan et al. 1999). However, in the presence of 60 μM Ca2+, channels can be fully activated at +160 mV. Fig. 8 B compares the relaxation of QOFF–QOFFss after a 0.1- or 20-ms pulse under these conditions. The 0.1-ms trace decays rapidly and is fit by a triple exponential function (τF = 23.8 μs, τM = 150 μs, τS = 822 μs), with the Fast component representing the majority of OFF charge (91%). However, the 20-ms record is well fit by a double-exponential function using only τM and τS. This confirms that the Fast component can be eliminated and that the relaxation of open channels back to the closed state contributes only to the Medium and Slow components of QOFF.


Allosteric voltage gating of potassium channels II. Mslo channel gating charge movement in the absence of Ca(2+).

Horrigan FT, Aldrich RW - J. Gen. Physiol. (1999)

Predictions of the allosteric model. (A) QOFF component amplitudes determined after pulses to +240 mV in 0 Ca2+ are plotted versus pulse duration. The fast component is reduced to <10% of the total OFF charge after a 20-ms pulse. The relaxation of all three components is fit by exponential functions (solid lines) with a τ = 0.91 ms. (B) The decay of QOFF–Qss is plotted on a semilog scale after 0.1- or 20-ms pulses to +160 mV in 60 μM Ca2+ (HP = −80). The 0.1-ms trace is fit by a triple exponential function (solid line, τF = 23.8 μs, τM = 150 μs, τS = 822 μs) and the 20-ms trace is fit with a double-exponential (τM = 150 μs, τS = 822 μs), indicating that the fast component is eliminated when most channels are opened. Dashed lines represent the two components of the 20-ms fit and the fast component of the 0.1-ms fit. (C) Normalized QOFF component amplitudes and total OFF charge (Qp) are plotted versus pulse duration for pulses to +160 mV in 0 Ca2+. OFF components were measured upon repolarization to −80 mV and are normalized to the fast component of ON charge (Qfast) at +160 mV. (D) When QOFF is measured upon repolarization to 0 mV, the Fast component and Qp are unchanged. However, the Medium component decreases and the Slow component increases in a complementary manner. (E) The charge distributions predicted by the allosteric model for Closed (QC) and Open channels (QO) are plotted versus voltage (zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17). Arrows indicate the predicted amplitudes of Medium and Slow OFF components at repolarization voltages of −80 and 0 mV after a pulse to +160 mV (VP).
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Figure 8: Predictions of the allosteric model. (A) QOFF component amplitudes determined after pulses to +240 mV in 0 Ca2+ are plotted versus pulse duration. The fast component is reduced to <10% of the total OFF charge after a 20-ms pulse. The relaxation of all three components is fit by exponential functions (solid lines) with a τ = 0.91 ms. (B) The decay of QOFF–Qss is plotted on a semilog scale after 0.1- or 20-ms pulses to +160 mV in 60 μM Ca2+ (HP = −80). The 0.1-ms trace is fit by a triple exponential function (solid line, τF = 23.8 μs, τM = 150 μs, τS = 822 μs) and the 20-ms trace is fit with a double-exponential (τM = 150 μs, τS = 822 μs), indicating that the fast component is eliminated when most channels are opened. Dashed lines represent the two components of the 20-ms fit and the fast component of the 0.1-ms fit. (C) Normalized QOFF component amplitudes and total OFF charge (Qp) are plotted versus pulse duration for pulses to +160 mV in 0 Ca2+. OFF components were measured upon repolarization to −80 mV and are normalized to the fast component of ON charge (Qfast) at +160 mV. (D) When QOFF is measured upon repolarization to 0 mV, the Fast component and Qp are unchanged. However, the Medium component decreases and the Slow component increases in a complementary manner. (E) The charge distributions predicted by the allosteric model for Closed (QC) and Open channels (QO) are plotted versus voltage (zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17). Arrows indicate the predicted amplitudes of Medium and Slow OFF components at repolarization voltages of −80 and 0 mV after a pulse to +160 mV (VP).
Mentions: The allosteric model predicts that the fast component of OFF charge movement should be eliminated after voltage pulses that open all channels. One way to increase Po is by stepping to more positive voltages. Fig. 8 A plots the time course of QOFF component development at +240 mV. The decay of QOFFfast is more complete than at +140 mV (Fig. 7 F), consistent with a voltage-dependent increase in Po. It is likely that the fast component was not eliminated because, in the absence of Ca2+, mSlo channels are maximally activated only at very positive voltages (greater than +300 mV) (Horrigan et al. 1999). However, in the presence of 60 μM Ca2+, channels can be fully activated at +160 mV. Fig. 8 B compares the relaxation of QOFF–QOFFss after a 0.1- or 20-ms pulse under these conditions. The 0.1-ms trace decays rapidly and is fit by a triple exponential function (τF = 23.8 μs, τM = 150 μs, τS = 822 μs), with the Fast component representing the majority of OFF charge (91%). However, the 20-ms record is well fit by a double-exponential function using only τM and τS. This confirms that the Fast component can be eliminated and that the relaxation of open channels back to the closed state contributes only to the Medium and Slow components of QOFF.

Bottom Line: These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J.Physiol. 114:277-304).The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA.

ABSTRACT
Large-conductance Ca(2+)-activated K(+) channels can be activated by membrane voltage in the absence of Ca(2+) binding, indicating that these channels contain an intrinsic voltage sensor. The properties of this voltage sensor and its relationship to channel activation were examined by studying gating charge movement from mSlo Ca(2+)-activated K(+) channels in the virtual absence of Ca(2+) (<1 nM). Charge movement was measured in response to voltage steps or sinusoidal voltage commands. The charge-voltage relationship (Q-V) is shallower and shifted to more negative voltages than the voltage-dependent open probability (G-V). Both ON and OFF gating currents evoked by brief (0.5-ms) voltage pulses appear to decay rapidly (tau(ON) = 60 microseconds at +200 mV, tau(OFF) = 16 microseconds at -80 mV). However, Q(OFF) increases slowly with pulse duration, indicating that a large fraction of ON charge develops with a time course comparable to that of I(K) activation. The slow onset of this gating charge prevents its detection as a component of I(gON), although it represents approximately 40% of the total charge moved at +140 mV. The decay of I(gOFF) is slowed after depolarizations that open mSlo channels. Yet, the majority of open channel charge relaxation is too rapid to be limited by channel closing. These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J. Cui, and R.W. Aldrich. 1999. J. Gen. Physiol. 114:277-304). The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

Show MeSH
Related in: MedlinePlus