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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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Voltage-sensor speed and the detection of open-state charge. Ionic and gating currents were simulated in response to a 20-ms pulse to +240 mV (HP = −80 mV) using the allosteric model. Traces labeled 1× were generated using Case B parameters (Table ). 10× and 30× traces indicate the effects of a 10- or 30-fold increase in the time constant of voltage-sensor movement, implemented by decreasing both voltage-sensor rate constants (α, β) and leaving equilibrium constants unchanged. As voltage-sensor movement is slowed (A), the time course of IK activation becomes more sigmoidal. (B) Ig is slowed and the OFF current becomes ‘hooked.’ Most channels are open at the end of the pulse, and the relaxation of OFF charge plotted as QOFF(t)–QOFFss in C is biphasic when voltage-sensor movement is fast (1×) representing O–O (Medium) and O–C (Slow) transitions. However, open-channel charge movement is not evident as a kinetically distinct component of QOFF when voltage sensors are slowed (10×, 30×). (D) Similarly, the decay of IgOFF is much faster than IK deactivation when voltage-sensor movement is fast, but IgOFF and IK decay with a similar time constant for 10× and 30× simulations.
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Figure 13: Voltage-sensor speed and the detection of open-state charge. Ionic and gating currents were simulated in response to a 20-ms pulse to +240 mV (HP = −80 mV) using the allosteric model. Traces labeled 1× were generated using Case B parameters (Table ). 10× and 30× traces indicate the effects of a 10- or 30-fold increase in the time constant of voltage-sensor movement, implemented by decreasing both voltage-sensor rate constants (α, β) and leaving equilibrium constants unchanged. As voltage-sensor movement is slowed (A), the time course of IK activation becomes more sigmoidal. (B) Ig is slowed and the OFF current becomes ‘hooked.’ Most channels are open at the end of the pulse, and the relaxation of OFF charge plotted as QOFF(t)–QOFFss in C is biphasic when voltage-sensor movement is fast (1×) representing O–O (Medium) and O–C (Slow) transitions. However, open-channel charge movement is not evident as a kinetically distinct component of QOFF when voltage sensors are slowed (10×, 30×). (D) Similarly, the decay of IgOFF is much faster than IK deactivation when voltage-sensor movement is fast, but IgOFF and IK decay with a similar time constant for 10× and 30× simulations.

Mentions: Fig. 13 shows that a slowing of voltage-sensor kinetics reduces the ability to detect open-channel charge movement. IK and Ig were simulated in response to a 20-ms pulse to +240 mV as the forward and backward rates for voltage-sensor movement (α, β) were both slowed 10-fold (10×) or 30-fold (30×) relative to those describing mSlo (1×). C–O transition rates and all equilibrium constants were unchanged (relative to Case B parameters). As voltage-sensor movement is slowed, the delay in IK activation increases (Fig. 13 A) and gating currents are slowed (Fig. 13 B). Under these conditions, IK and Ig resemble those evoked from a channel such as Shaker where IK activation kinetics are sigmoidal and ionic and gating currents relax on a similar time scale. Interestingly, a 30-fold slowing of voltage sensor movement also produces a “hook” in IgOFF, a feature that is also observed in Shaker Ig (Bezanilla et al. 1991; Chen et al. 1997; Perozo et al. 1992). The simulation predicts that Po approaches unity at the end of a pulse to +240 mV. Therefore, IgOFF represents the relaxation of open channels. The time course of QOFF(t) plotted on a semilog scale in Fig. 13 C is biphasic when voltage-sensor movement is fast (1×) representing the Medium and Slow components of QOFF. However, kinetically distinct components of QOFF are not evident when voltage-sensor movement is slow (Fig. 13 C, 10× and 30×). Similarly, the decay of IgOFF is much faster than that of IK when voltage sensors are fast (Fig. 13 D, 1×). However, IgOFF and IK decay with similar kinetics when voltage sensors are slow (Fig. 13 D, 10× and 30×). These results demonstrate that OFF charge movement can be limited by the speed of channel deactivation even when multiple open states are present.


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)

Voltage-sensor speed and the detection of open-state charge. Ionic and gating currents were simulated in response to a 20-ms pulse to +240 mV (HP = −80 mV) using the allosteric model. Traces labeled 1× were generated using Case B parameters (Table ). 10× and 30× traces indicate the effects of a 10- or 30-fold increase in the time constant of voltage-sensor movement, implemented by decreasing both voltage-sensor rate constants (α, β) and leaving equilibrium constants unchanged. As voltage-sensor movement is slowed (A), the time course of IK activation becomes more sigmoidal. (B) Ig is slowed and the OFF current becomes ‘hooked.’ Most channels are open at the end of the pulse, and the relaxation of OFF charge plotted as QOFF(t)–QOFFss in C is biphasic when voltage-sensor movement is fast (1×) representing O–O (Medium) and O–C (Slow) transitions. However, open-channel charge movement is not evident as a kinetically distinct component of QOFF when voltage sensors are slowed (10×, 30×). (D) Similarly, the decay of IgOFF is much faster than IK deactivation when voltage-sensor movement is fast, but IgOFF and IK decay with a similar time constant for 10× and 30× simulations.
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Figure 13: Voltage-sensor speed and the detection of open-state charge. Ionic and gating currents were simulated in response to a 20-ms pulse to +240 mV (HP = −80 mV) using the allosteric model. Traces labeled 1× were generated using Case B parameters (Table ). 10× and 30× traces indicate the effects of a 10- or 30-fold increase in the time constant of voltage-sensor movement, implemented by decreasing both voltage-sensor rate constants (α, β) and leaving equilibrium constants unchanged. As voltage-sensor movement is slowed (A), the time course of IK activation becomes more sigmoidal. (B) Ig is slowed and the OFF current becomes ‘hooked.’ Most channels are open at the end of the pulse, and the relaxation of OFF charge plotted as QOFF(t)–QOFFss in C is biphasic when voltage-sensor movement is fast (1×) representing O–O (Medium) and O–C (Slow) transitions. However, open-channel charge movement is not evident as a kinetically distinct component of QOFF when voltage sensors are slowed (10×, 30×). (D) Similarly, the decay of IgOFF is much faster than IK deactivation when voltage-sensor movement is fast, but IgOFF and IK decay with a similar time constant for 10× and 30× simulations.
Mentions: Fig. 13 shows that a slowing of voltage-sensor kinetics reduces the ability to detect open-channel charge movement. IK and Ig were simulated in response to a 20-ms pulse to +240 mV as the forward and backward rates for voltage-sensor movement (α, β) were both slowed 10-fold (10×) or 30-fold (30×) relative to those describing mSlo (1×). C–O transition rates and all equilibrium constants were unchanged (relative to Case B parameters). As voltage-sensor movement is slowed, the delay in IK activation increases (Fig. 13 A) and gating currents are slowed (Fig. 13 B). Under these conditions, IK and Ig resemble those evoked from a channel such as Shaker where IK activation kinetics are sigmoidal and ionic and gating currents relax on a similar time scale. Interestingly, a 30-fold slowing of voltage sensor movement also produces a “hook” in IgOFF, a feature that is also observed in Shaker Ig (Bezanilla et al. 1991; Chen et al. 1997; Perozo et al. 1992). The simulation predicts that Po approaches unity at the end of a pulse to +240 mV. Therefore, IgOFF represents the relaxation of open channels. The time course of QOFF(t) plotted on a semilog scale in Fig. 13 C is biphasic when voltage-sensor movement is fast (1×) representing the Medium and Slow components of QOFF. However, kinetically distinct components of QOFF are not evident when voltage-sensor movement is slow (Fig. 13 C, 10× and 30×). Similarly, the decay of IgOFF is much faster than that of IK when voltage sensors are fast (Fig. 13 D, 1×). However, IgOFF and IK decay with similar kinetics when voltage sensors are slow (Fig. 13 D, 10× and 30×). These results demonstrate that OFF charge movement can be limited by the speed of channel deactivation even when multiple open states are present.

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