Limits...
State-dependent inactivation of the alpha1G T-type calcium channel.

Serrano JR, Perez-Reyes E, Jones SW - J. Gen. Physiol. (1999)

Bottom Line: Recovery was similar after 60-ms steps to -20 mV or 600-ms steps to -70 mV, suggesting rapid equilibration of open- and closed-state inactivation.The results are well described by a kinetic model where inactivation is allosterically coupled to the movement of the first three voltage sensors to activate.One consequence of state-dependent inactivation is that alpha1G channels continue to inactivate after repolarization, primarily from the open state, which leads to cumulative inactivation during repetitive pulses.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106, USA.

ABSTRACT
We have examined the kinetics of whole-cell T-current in HEK 293 cells stably expressing the alpha1G channel, with symmetrical Na(+)(i) and Na(+)(o) and 2 mM Ca(2+)(o). After brief strong depolarization to activate the channels (2 ms at +60 mV; holding potential -100 mV), currents relaxed exponentially at all voltages. The time constant of the relaxation was exponentially voltage dependent from -120 to -70 mV (e-fold for 31 mV; tau = 2.5 ms at -100 mV), but tau = 12-17 ms from-40 to +60 mV. This suggests a mixture of voltage-dependent deactivation (dominating at very negative voltages) and nearly voltage-independent inactivation. Inactivation measured by test pulses following that protocol was consistent with open-state inactivation. During depolarizations lasting 100-300 ms, inactivation was strong but incomplete (approximately 98%). Inactivation was also produced by long, weak depolarizations (tau = 220 ms at -80 mV; V(1/2) = -82 mV), which could not be explained by voltage-independent inactivation exclusively from the open state. Recovery from inactivation was exponential and fast (tau = 85 ms at -100 mV), but weakly voltage dependent. Recovery was similar after 60-ms steps to -20 mV or 600-ms steps to -70 mV, suggesting rapid equilibration of open- and closed-state inactivation. There was little current at -100 mV during recovery from inactivation, consistent with

Show MeSH

Related in: MedlinePlus

A kinetic scheme for gating of α1G T-channels. It differs from Kuo and Bean 1994 and Klemic et al. 1998 in assuming that the rates for inactivation and recovery are the same for the rightmost three states (C3, C4, and O). The model includes six rate constants at 0 mV, for voltage sensor movement , channel opening/closing , and inactivation . Three of the rate constants (kV, k−V, and k−O) depend exponentially on voltage, changing e-fold for 25, −18, and −34 mV depolarization (respectively). Inactivation is allosterically coupled to movement of the first three voltage sensors, with factors  for inactivation rate constants and  for recovery. (B) Records of PO,r versus time, from −70 to −20 mV, calculated by dividing currents (recorded by the protocol of Fig. 1 A) by the peak tail current amplitude at each voltage (protocol of Fig. 3). Cell a8612, 3 kHz Gaussian filter. (C) Simulated PO versus time records. The scale bars apply to both B and C.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2230639&req=5

Figure 14: A kinetic scheme for gating of α1G T-channels. It differs from Kuo and Bean 1994 and Klemic et al. 1998 in assuming that the rates for inactivation and recovery are the same for the rightmost three states (C3, C4, and O). The model includes six rate constants at 0 mV, for voltage sensor movement , channel opening/closing , and inactivation . Three of the rate constants (kV, k−V, and k−O) depend exponentially on voltage, changing e-fold for 25, −18, and −34 mV depolarization (respectively). Inactivation is allosterically coupled to movement of the first three voltage sensors, with factors for inactivation rate constants and for recovery. (B) Records of PO,r versus time, from −70 to −20 mV, calculated by dividing currents (recorded by the protocol of Fig. 1 A) by the peak tail current amplitude at each voltage (protocol of Fig. 3). Cell a8612, 3 kHz Gaussian filter. (C) Simulated PO versus time records. The scale bars apply to both B and C.

Mentions: We considered a model where inactivation is coupled allosterically to voltage sensor activation (Fig. 14 A), which has proven successful for describing inactivation for several voltage-dependent channels (Kuo and Bean 1994; Klemic et al. 1998; Patil et al. 1998). The model involves sequential activation of four voltage sensors (presumably the S4 regions), followed by a distinct channel opening step with less voltage dependence. This can describe the observed delay before channel opening, but voltage-independent channel opening (kO) limits the voltage dependence of the time to peak. Channel closing (k−O) must have significant voltage dependence, however, to produce the observed exponential voltage dependence of deactivation (Fig. 5 A). Inactivation is allowed from any of the closed or open states, as in the Hodgkin and Huxley 1952b Na+ channel model, but channel activation favors inactivation (and slows recovery). The rate constants for inactivation and recovery are state dependent, but do not depend directly on voltage.


State-dependent inactivation of the alpha1G T-type calcium channel.

Serrano JR, Perez-Reyes E, Jones SW - J. Gen. Physiol. (1999)

A kinetic scheme for gating of α1G T-channels. It differs from Kuo and Bean 1994 and Klemic et al. 1998 in assuming that the rates for inactivation and recovery are the same for the rightmost three states (C3, C4, and O). The model includes six rate constants at 0 mV, for voltage sensor movement , channel opening/closing , and inactivation . Three of the rate constants (kV, k−V, and k−O) depend exponentially on voltage, changing e-fold for 25, −18, and −34 mV depolarization (respectively). Inactivation is allosterically coupled to movement of the first three voltage sensors, with factors  for inactivation rate constants and  for recovery. (B) Records of PO,r versus time, from −70 to −20 mV, calculated by dividing currents (recorded by the protocol of Fig. 1 A) by the peak tail current amplitude at each voltage (protocol of Fig. 3). Cell a8612, 3 kHz Gaussian filter. (C) Simulated PO versus time records. The scale bars apply to both B and C.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230639&req=5

Figure 14: A kinetic scheme for gating of α1G T-channels. It differs from Kuo and Bean 1994 and Klemic et al. 1998 in assuming that the rates for inactivation and recovery are the same for the rightmost three states (C3, C4, and O). The model includes six rate constants at 0 mV, for voltage sensor movement , channel opening/closing , and inactivation . Three of the rate constants (kV, k−V, and k−O) depend exponentially on voltage, changing e-fold for 25, −18, and −34 mV depolarization (respectively). Inactivation is allosterically coupled to movement of the first three voltage sensors, with factors for inactivation rate constants and for recovery. (B) Records of PO,r versus time, from −70 to −20 mV, calculated by dividing currents (recorded by the protocol of Fig. 1 A) by the peak tail current amplitude at each voltage (protocol of Fig. 3). Cell a8612, 3 kHz Gaussian filter. (C) Simulated PO versus time records. The scale bars apply to both B and C.
Mentions: We considered a model where inactivation is coupled allosterically to voltage sensor activation (Fig. 14 A), which has proven successful for describing inactivation for several voltage-dependent channels (Kuo and Bean 1994; Klemic et al. 1998; Patil et al. 1998). The model involves sequential activation of four voltage sensors (presumably the S4 regions), followed by a distinct channel opening step with less voltage dependence. This can describe the observed delay before channel opening, but voltage-independent channel opening (kO) limits the voltage dependence of the time to peak. Channel closing (k−O) must have significant voltage dependence, however, to produce the observed exponential voltage dependence of deactivation (Fig. 5 A). Inactivation is allowed from any of the closed or open states, as in the Hodgkin and Huxley 1952b Na+ channel model, but channel activation favors inactivation (and slows recovery). The rate constants for inactivation and recovery are state dependent, but do not depend directly on voltage.

Bottom Line: Recovery was similar after 60-ms steps to -20 mV or 600-ms steps to -70 mV, suggesting rapid equilibration of open- and closed-state inactivation.The results are well described by a kinetic model where inactivation is allosterically coupled to the movement of the first three voltage sensors to activate.One consequence of state-dependent inactivation is that alpha1G channels continue to inactivate after repolarization, primarily from the open state, which leads to cumulative inactivation during repetitive pulses.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106, USA.

ABSTRACT
We have examined the kinetics of whole-cell T-current in HEK 293 cells stably expressing the alpha1G channel, with symmetrical Na(+)(i) and Na(+)(o) and 2 mM Ca(2+)(o). After brief strong depolarization to activate the channels (2 ms at +60 mV; holding potential -100 mV), currents relaxed exponentially at all voltages. The time constant of the relaxation was exponentially voltage dependent from -120 to -70 mV (e-fold for 31 mV; tau = 2.5 ms at -100 mV), but tau = 12-17 ms from-40 to +60 mV. This suggests a mixture of voltage-dependent deactivation (dominating at very negative voltages) and nearly voltage-independent inactivation. Inactivation measured by test pulses following that protocol was consistent with open-state inactivation. During depolarizations lasting 100-300 ms, inactivation was strong but incomplete (approximately 98%). Inactivation was also produced by long, weak depolarizations (tau = 220 ms at -80 mV; V(1/2) = -82 mV), which could not be explained by voltage-independent inactivation exclusively from the open state. Recovery from inactivation was exponential and fast (tau = 85 ms at -100 mV), but weakly voltage dependent. Recovery was similar after 60-ms steps to -20 mV or 600-ms steps to -70 mV, suggesting rapid equilibration of open- and closed-state inactivation. There was little current at -100 mV during recovery from inactivation, consistent with

Show MeSH
Related in: MedlinePlus