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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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mSlo gating current. (A) mSlo Ig evoked in response to a 0.5-ms pulse to +160 mV from a holding potential of −80 mV. The trace represents the averaged response to eight pulses. IgON and IgOFF are fit by exponential functions (dashed lines). (B) A family of Ig evoked in response to 1-ms pulses to different voltages (0–160 mV in 40-mV steps). (C) QON–V and QOFF–V relationships were obtained by integrating IgON and IgOFF, respectively (from B) over 1-ms intervals. The Qg–V relationship was obtained from Cg measurements at 868 Hz in the same patch. Qfast was determined from an exponential fit to IgON (see below). (D) IgON evoked at +160 mV is compared with the initial time course of IK activation measured at the same voltage from a different experiment. IK is fit with an exponential function (dashed line) from 0.5 to 20 ms after the start of the pulse. The IK scale bar represents 10% of the steady-state amplitude. IgON is also fit with an exponential function, and the shaded area under the fit was used to determine Qfast. (E) IgON and Cg measured from a single patch were integrated to determine Qfast and Qg, respectively, as plotted in F. The Qfast–V relationship is fit with a Boltzmann function (z = 0.57 e, Vh = 136 mV).
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Figure 3: mSlo gating current. (A) mSlo Ig evoked in response to a 0.5-ms pulse to +160 mV from a holding potential of −80 mV. The trace represents the averaged response to eight pulses. IgON and IgOFF are fit by exponential functions (dashed lines). (B) A family of Ig evoked in response to 1-ms pulses to different voltages (0–160 mV in 40-mV steps). (C) QON–V and QOFF–V relationships were obtained by integrating IgON and IgOFF, respectively (from B) over 1-ms intervals. The Qg–V relationship was obtained from Cg measurements at 868 Hz in the same patch. Qfast was determined from an exponential fit to IgON (see below). (D) IgON evoked at +160 mV is compared with the initial time course of IK activation measured at the same voltage from a different experiment. IK is fit with an exponential function (dashed line) from 0.5 to 20 ms after the start of the pulse. The IK scale bar represents 10% of the steady-state amplitude. IgON is also fit with an exponential function, and the shaded area under the fit was used to determine Qfast. (E) IgON and Cg measured from a single patch were integrated to determine Qfast and Qg, respectively, as plotted in F. The Qfast–V relationship is fit with a Boltzmann function (z = 0.57 e, Vh = 136 mV).

Mentions: Fig. 3 A shows Ig evoked in response to a 0.5-ms pulse to +160 mV from a holding potential of −80 mV in 0 Ca2+. The ON current decays rapidly with a time course that is well fit by an exponential function (dashed line) with a time constant of 59 μs, similar to that determined with admittance analysis at +120 mV (70 μs). The OFF current measured at −80 mV decays more quickly, with a time constant of 17 μs. A family of Ig evoked at different voltages (0 to +160 mV) in response to 1-ms pulses is shown in Fig. 3 B. The QON–V and QOFF–V relationships obtained by integrating IgON and IgOFF are plotted in Fig. 3 C (open symbols) together with the Qg–V relationship obtained from capacitance measurements at 868 Hz in the same patch (solid line). At all voltages, QON and QOFF are equivalent, as expected for gating charge. The gating current and capacitance measurements superimpose from 0 to +120 mV but diverge at +160 mV.


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)

mSlo gating current. (A) mSlo Ig evoked in response to a 0.5-ms pulse to +160 mV from a holding potential of −80 mV. The trace represents the averaged response to eight pulses. IgON and IgOFF are fit by exponential functions (dashed lines). (B) A family of Ig evoked in response to 1-ms pulses to different voltages (0–160 mV in 40-mV steps). (C) QON–V and QOFF–V relationships were obtained by integrating IgON and IgOFF, respectively (from B) over 1-ms intervals. The Qg–V relationship was obtained from Cg measurements at 868 Hz in the same patch. Qfast was determined from an exponential fit to IgON (see below). (D) IgON evoked at +160 mV is compared with the initial time course of IK activation measured at the same voltage from a different experiment. IK is fit with an exponential function (dashed line) from 0.5 to 20 ms after the start of the pulse. The IK scale bar represents 10% of the steady-state amplitude. IgON is also fit with an exponential function, and the shaded area under the fit was used to determine Qfast. (E) IgON and Cg measured from a single patch were integrated to determine Qfast and Qg, respectively, as plotted in F. The Qfast–V relationship is fit with a Boltzmann function (z = 0.57 e, Vh = 136 mV).
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Figure 3: mSlo gating current. (A) mSlo Ig evoked in response to a 0.5-ms pulse to +160 mV from a holding potential of −80 mV. The trace represents the averaged response to eight pulses. IgON and IgOFF are fit by exponential functions (dashed lines). (B) A family of Ig evoked in response to 1-ms pulses to different voltages (0–160 mV in 40-mV steps). (C) QON–V and QOFF–V relationships were obtained by integrating IgON and IgOFF, respectively (from B) over 1-ms intervals. The Qg–V relationship was obtained from Cg measurements at 868 Hz in the same patch. Qfast was determined from an exponential fit to IgON (see below). (D) IgON evoked at +160 mV is compared with the initial time course of IK activation measured at the same voltage from a different experiment. IK is fit with an exponential function (dashed line) from 0.5 to 20 ms after the start of the pulse. The IK scale bar represents 10% of the steady-state amplitude. IgON is also fit with an exponential function, and the shaded area under the fit was used to determine Qfast. (E) IgON and Cg measured from a single patch were integrated to determine Qfast and Qg, respectively, as plotted in F. The Qfast–V relationship is fit with a Boltzmann function (z = 0.57 e, Vh = 136 mV).
Mentions: Fig. 3 A shows Ig evoked in response to a 0.5-ms pulse to +160 mV from a holding potential of −80 mV in 0 Ca2+. The ON current decays rapidly with a time course that is well fit by an exponential function (dashed line) with a time constant of 59 μs, similar to that determined with admittance analysis at +120 mV (70 μs). The OFF current measured at −80 mV decays more quickly, with a time constant of 17 μs. A family of Ig evoked at different voltages (0 to +160 mV) in response to 1-ms pulses is shown in Fig. 3 B. The QON–V and QOFF–V relationships obtained by integrating IgON and IgOFF are plotted in Fig. 3 C (open symbols) together with the Qg–V relationship obtained from capacitance measurements at 868 Hz in the same patch (solid line). At all voltages, QON and QOFF are equivalent, as expected for gating charge. The gating current and capacitance measurements superimpose from 0 to +120 mV but diverge at +160 mV.

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