Limits...
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.

Show MeSH

Related in: MedlinePlus

Estimating open probability from charge movement. The allosteric model predicts a close relationship between Po and the various QOFF components. A, B, and C plot the voltage dependence of these components measured after 20-ms pulses for three experiments. Solid lines indicate predictions of three models (Cases A, B, and C) described in the text. (A) The Fast OFF component should be proportional to the number of closed channels at the end of the pulse. Therefore, [1 − [QOFFfast(VP)/Qfast(VP)] is plotted as an estimate of steady-state Po, where Qfast is the fast component of ON charge. (B) The Slow OFF component should be directly proportional to Po. The quantity (QOFFslow(VP)/QTfast) is plotted where QTfast is the total fast charge estimated by fitting the Qfast–V relationship with a Boltzmann function. (C) The Medium OFF component is normalized by QTfast and plotted versus voltage. (D) The voltage dependence of the Medium OFF component was also examine by fitting IgOFF with a double-exponential function (τF, τM) and plotting the normalized amplitude of IgOFFmed against voltage. IgOFFmed was normalized by fitting the IgOFF–V relationship with a Boltzmann function corresponding to Case C (z = 0.98 e). (E) The Slow component of ON charge (QpSlow) is expected to exhibit a complex voltage dependence (dashed curves) that is highly sensitive to Po. The data, normalized by QTfast, indicate that the slow component is too large to be accounted for by the initial allosteric model parameters (Case A) but a shift in the Po–V relationship (Cases B and C) produces a better fit. A dashed line indicates the charge assigned to the C–O transition (zL) (F) τgSlow determined from the time course of QpSlow and QOFFslow for many experiments are plotted versus voltage. Solid symbols represent mean ± SEM. Dashed and solid lines represent predictions of Case A and B, respectively (Table ).
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2230644&req=5

Figure 10: Estimating open probability from charge movement. The allosteric model predicts a close relationship between Po and the various QOFF components. A, B, and C plot the voltage dependence of these components measured after 20-ms pulses for three experiments. Solid lines indicate predictions of three models (Cases A, B, and C) described in the text. (A) The Fast OFF component should be proportional to the number of closed channels at the end of the pulse. Therefore, [1 − [QOFFfast(VP)/Qfast(VP)] is plotted as an estimate of steady-state Po, where Qfast is the fast component of ON charge. (B) The Slow OFF component should be directly proportional to Po. The quantity (QOFFslow(VP)/QTfast) is plotted where QTfast is the total fast charge estimated by fitting the Qfast–V relationship with a Boltzmann function. (C) The Medium OFF component is normalized by QTfast and plotted versus voltage. (D) The voltage dependence of the Medium OFF component was also examine by fitting IgOFF with a double-exponential function (τF, τM) and plotting the normalized amplitude of IgOFFmed against voltage. IgOFFmed was normalized by fitting the IgOFF–V relationship with a Boltzmann function corresponding to Case C (z = 0.98 e). (E) The Slow component of ON charge (QpSlow) is expected to exhibit a complex voltage dependence (dashed curves) that is highly sensitive to Po. The data, normalized by QTfast, indicate that the slow component is too large to be accounted for by the initial allosteric model parameters (Case A) but a shift in the Po–V relationship (Cases B and C) produces a better fit. A dashed line indicates the charge assigned to the C–O transition (zL) (F) τgSlow determined from the time course of QpSlow and QOFFslow for many experiments are plotted versus voltage. Solid symbols represent mean ± SEM. Dashed and solid lines represent predictions of Case A and B, respectively (Table ).

Mentions: The results discussed thus far are qualitatively consistent with the behavior of the allosteric gating scheme (Fig. 1). Simulations based on the model as shown in Fig. 9, Fig. 10, and Fig. 11 also reproduce the major features of the data. However, the parameters that were ultimately used to fit Ig differ from those used to describe ionic currents (Horrigan et al. 1999). Some of these differences are small and may simply reflect a greater accuracy in characterizing fast voltage-sensor movement with gating currents. Other differences, relating to the slow charge movement, suggest that ionic conditions alter mSlo channel gating.


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)

Estimating open probability from charge movement. The allosteric model predicts a close relationship between Po and the various QOFF components. A, B, and C plot the voltage dependence of these components measured after 20-ms pulses for three experiments. Solid lines indicate predictions of three models (Cases A, B, and C) described in the text. (A) The Fast OFF component should be proportional to the number of closed channels at the end of the pulse. Therefore, [1 − [QOFFfast(VP)/Qfast(VP)] is plotted as an estimate of steady-state Po, where Qfast is the fast component of ON charge. (B) The Slow OFF component should be directly proportional to Po. The quantity (QOFFslow(VP)/QTfast) is plotted where QTfast is the total fast charge estimated by fitting the Qfast–V relationship with a Boltzmann function. (C) The Medium OFF component is normalized by QTfast and plotted versus voltage. (D) The voltage dependence of the Medium OFF component was also examine by fitting IgOFF with a double-exponential function (τF, τM) and plotting the normalized amplitude of IgOFFmed against voltage. IgOFFmed was normalized by fitting the IgOFF–V relationship with a Boltzmann function corresponding to Case C (z = 0.98 e). (E) The Slow component of ON charge (QpSlow) is expected to exhibit a complex voltage dependence (dashed curves) that is highly sensitive to Po. The data, normalized by QTfast, indicate that the slow component is too large to be accounted for by the initial allosteric model parameters (Case A) but a shift in the Po–V relationship (Cases B and C) produces a better fit. A dashed line indicates the charge assigned to the C–O transition (zL) (F) τgSlow determined from the time course of QpSlow and QOFFslow for many experiments are plotted versus voltage. Solid symbols represent mean ± SEM. Dashed and solid lines represent predictions of Case A and B, respectively (Table ).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230644&req=5

Figure 10: Estimating open probability from charge movement. The allosteric model predicts a close relationship between Po and the various QOFF components. A, B, and C plot the voltage dependence of these components measured after 20-ms pulses for three experiments. Solid lines indicate predictions of three models (Cases A, B, and C) described in the text. (A) The Fast OFF component should be proportional to the number of closed channels at the end of the pulse. Therefore, [1 − [QOFFfast(VP)/Qfast(VP)] is plotted as an estimate of steady-state Po, where Qfast is the fast component of ON charge. (B) The Slow OFF component should be directly proportional to Po. The quantity (QOFFslow(VP)/QTfast) is plotted where QTfast is the total fast charge estimated by fitting the Qfast–V relationship with a Boltzmann function. (C) The Medium OFF component is normalized by QTfast and plotted versus voltage. (D) The voltage dependence of the Medium OFF component was also examine by fitting IgOFF with a double-exponential function (τF, τM) and plotting the normalized amplitude of IgOFFmed against voltage. IgOFFmed was normalized by fitting the IgOFF–V relationship with a Boltzmann function corresponding to Case C (z = 0.98 e). (E) The Slow component of ON charge (QpSlow) is expected to exhibit a complex voltage dependence (dashed curves) that is highly sensitive to Po. The data, normalized by QTfast, indicate that the slow component is too large to be accounted for by the initial allosteric model parameters (Case A) but a shift in the Po–V relationship (Cases B and C) produces a better fit. A dashed line indicates the charge assigned to the C–O transition (zL) (F) τgSlow determined from the time course of QpSlow and QOFFslow for many experiments are plotted versus voltage. Solid symbols represent mean ± SEM. Dashed and solid lines represent predictions of Case A and B, respectively (Table ).
Mentions: The results discussed thus far are qualitatively consistent with the behavior of the allosteric gating scheme (Fig. 1). Simulations based on the model as shown in Fig. 9, Fig. 10, and Fig. 11 also reproduce the major features of the data. However, the parameters that were ultimately used to fit Ig differ from those used to describe ionic currents (Horrigan et al. 1999). Some of these differences are small and may simply reflect a greater accuracy in characterizing fast voltage-sensor movement with gating currents. Other differences, relating to the slow charge movement, suggest that ionic conditions alter mSlo channel gating.

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