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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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Slow charge movement is limited by channel activation. (A) Families of Ig evoked by pulses of different duration (0.06–20 ms) to the indicated voltages (HP = −80). Records obtained at +184 and +224 mV were from a different patch than those obtained at +100 and +140 mV. Scale bars represent 100 pA. (B) Qp(t) curves at different voltages from three experiments are fit with double-exponential functions where the time constant and amplitude of the fast component were determined by fitting IgON with an exponential function. Plots from different experiments (different symbols) were normalized to the total fast charge (QTfast) for each patch (see text). The indicated pulse voltages have been corrected based on the Q–V shifts (ΔVh) determined in Fig. 4 A for each experiment. (C) The time constants of QpSlow relaxation (τgSlow) (mean ± SD) are plotted versus voltage and compared with the time constants for IK relaxation (mean ± SD) from Horrigan et al., 1999. τgSlow at +224 mV represents a single measurement. (D) Normalized steady-state Q–Vs (open circles) from four experiments were determined with 20-ms pulses and are fit by a Boltzmann function (dashed line, Vh = 143 mV, z = 0.65 e). Qss–V relationships were corrected for ΔVh determined for these experiments in Fig. 4 A. Averaged Qfast–V and GK–V relationships are plotted for comparison (filled symbols) and are fit by the allosteric voltage-gating scheme (solid lines, zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17).
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Figure 6: Slow charge movement is limited by channel activation. (A) Families of Ig evoked by pulses of different duration (0.06–20 ms) to the indicated voltages (HP = −80). Records obtained at +184 and +224 mV were from a different patch than those obtained at +100 and +140 mV. Scale bars represent 100 pA. (B) Qp(t) curves at different voltages from three experiments are fit with double-exponential functions where the time constant and amplitude of the fast component were determined by fitting IgON with an exponential function. Plots from different experiments (different symbols) were normalized to the total fast charge (QTfast) for each patch (see text). The indicated pulse voltages have been corrected based on the Q–V shifts (ΔVh) determined in Fig. 4 A for each experiment. (C) The time constants of QpSlow relaxation (τgSlow) (mean ± SD) are plotted versus voltage and compared with the time constants for IK relaxation (mean ± SD) from Horrigan et al., 1999. τgSlow at +224 mV represents a single measurement. (D) Normalized steady-state Q–Vs (open circles) from four experiments were determined with 20-ms pulses and are fit by a Boltzmann function (dashed line, Vh = 143 mV, z = 0.65 e). Qss–V relationships were corrected for ΔVh determined for these experiments in Fig. 4 A. Averaged Qfast–V and GK–V relationships are plotted for comparison (filled symbols) and are fit by the allosteric voltage-gating scheme (solid lines, zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17).

Mentions: Although the ON currents in Fig. 3 appear to decay with a single-exponential time course, there is a significant slow component of charge movement. Fig. 5 A plots a family of Ig evoked at +140 mV in response to voltage pulses of different duration (see also Fig. 6 A). The peak amplitude of IgOFF increases rapidly with pulse duration, paralleling the rapid decay of IgON, and then remains relatively constant for pulses longer than 0.5 ms. The total gating charge moved during the pulse (Qp) was determined by integrating IgOFF and is plotted versus pulse duration in Fig. 5 B. Qp increases with a time course that can be fit by a double-exponential function (solid line) with a fast phase (QpFast) corresponding to the rapid decay of IgON, and an additional phase that is roughly 100-fold slower. The slow component (QpSlow) relaxes with a time constant (τgSlow) of 4.22 ms and represents a significant fraction of the total gating charge movement at +140 mV (43%) but is too slow to be observed as a component of IgON. This point is illustrated in Fig. 5 C, which compares IgON evoked at +140 mV to Qp′(t) (dashed line). Qp′(t) is the time derivative of the double-exponential fit to Qp(t) and should represent the time course of IgON (Qp′(t) = dQON/dt = IgON). These two relationships superimpose, demonstrating that observed IgON kinetics are consistent with the presence of a large slow component of ON charge movement.


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)

Slow charge movement is limited by channel activation. (A) Families of Ig evoked by pulses of different duration (0.06–20 ms) to the indicated voltages (HP = −80). Records obtained at +184 and +224 mV were from a different patch than those obtained at +100 and +140 mV. Scale bars represent 100 pA. (B) Qp(t) curves at different voltages from three experiments are fit with double-exponential functions where the time constant and amplitude of the fast component were determined by fitting IgON with an exponential function. Plots from different experiments (different symbols) were normalized to the total fast charge (QTfast) for each patch (see text). The indicated pulse voltages have been corrected based on the Q–V shifts (ΔVh) determined in Fig. 4 A for each experiment. (C) The time constants of QpSlow relaxation (τgSlow) (mean ± SD) are plotted versus voltage and compared with the time constants for IK relaxation (mean ± SD) from Horrigan et al., 1999. τgSlow at +224 mV represents a single measurement. (D) Normalized steady-state Q–Vs (open circles) from four experiments were determined with 20-ms pulses and are fit by a Boltzmann function (dashed line, Vh = 143 mV, z = 0.65 e). Qss–V relationships were corrected for ΔVh determined for these experiments in Fig. 4 A. Averaged Qfast–V and GK–V relationships are plotted for comparison (filled symbols) and are fit by the allosteric voltage-gating scheme (solid lines, zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17).
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Figure 6: Slow charge movement is limited by channel activation. (A) Families of Ig evoked by pulses of different duration (0.06–20 ms) to the indicated voltages (HP = −80). Records obtained at +184 and +224 mV were from a different patch than those obtained at +100 and +140 mV. Scale bars represent 100 pA. (B) Qp(t) curves at different voltages from three experiments are fit with double-exponential functions where the time constant and amplitude of the fast component were determined by fitting IgON with an exponential function. Plots from different experiments (different symbols) were normalized to the total fast charge (QTfast) for each patch (see text). The indicated pulse voltages have been corrected based on the Q–V shifts (ΔVh) determined in Fig. 4 A for each experiment. (C) The time constants of QpSlow relaxation (τgSlow) (mean ± SD) are plotted versus voltage and compared with the time constants for IK relaxation (mean ± SD) from Horrigan et al., 1999. τgSlow at +224 mV represents a single measurement. (D) Normalized steady-state Q–Vs (open circles) from four experiments were determined with 20-ms pulses and are fit by a Boltzmann function (dashed line, Vh = 143 mV, z = 0.65 e). Qss–V relationships were corrected for ΔVh determined for these experiments in Fig. 4 A. Averaged Qfast–V and GK–V relationships are plotted for comparison (filled symbols) and are fit by the allosteric voltage-gating scheme (solid lines, zJ = 0.55 e, Vh(J) = 155 mV, L = 2 × 10−6, zL = 0.4 e, D = 17).
Mentions: Although the ON currents in Fig. 3 appear to decay with a single-exponential time course, there is a significant slow component of charge movement. Fig. 5 A plots a family of Ig evoked at +140 mV in response to voltage pulses of different duration (see also Fig. 6 A). The peak amplitude of IgOFF increases rapidly with pulse duration, paralleling the rapid decay of IgON, and then remains relatively constant for pulses longer than 0.5 ms. The total gating charge moved during the pulse (Qp) was determined by integrating IgOFF and is plotted versus pulse duration in Fig. 5 B. Qp increases with a time course that can be fit by a double-exponential function (solid line) with a fast phase (QpFast) corresponding to the rapid decay of IgON, and an additional phase that is roughly 100-fold slower. The slow component (QpSlow) relaxes with a time constant (τgSlow) of 4.22 ms and represents a significant fraction of the total gating charge movement at +140 mV (43%) but is too slow to be observed as a component of IgON. This point is illustrated in Fig. 5 C, which compares IgON evoked at +140 mV to Qp′(t) (dashed line). Qp′(t) is the time derivative of the double-exponential fit to Qp(t) and should represent the time course of IgON (Qp′(t) = dQON/dt = IgON). These two relationships superimpose, demonstrating that observed IgON kinetics are consistent with the presence of a large slow component of ON charge movement.

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