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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

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Cumulative inactivation. (A) The upper panel is two superimposed current records, a single 50-ms depolarization to −20 mV, and four steps to −20 mV for 5 ms each, separated by 10-ms intervals at −100 mV. The lower panel is the predicted open-state inactivation for the four-pulse protocol. Cell a8612, 2 kHz Gaussian filter. (B) Inactivation during a train of four action potential–like depolarizations. Cell e8612, 5 kHz Gaussian filter. The voltage command (shown below) was simulated from the Hodgkin-Huxley (1952b) model for the squid axon, modified to allow spontaneous 50-Hz repetitive firing by shifting the voltage dependence of the “m” gate by 2 mV to more negative voltages. The action potentials were scaled to an initial voltage of −100 mV, with an overshoot of +39 mV. The dashed lines are at 0 and −100 mV.
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Figure 12: Cumulative inactivation. (A) The upper panel is two superimposed current records, a single 50-ms depolarization to −20 mV, and four steps to −20 mV for 5 ms each, separated by 10-ms intervals at −100 mV. The lower panel is the predicted open-state inactivation for the four-pulse protocol. Cell a8612, 2 kHz Gaussian filter. (B) Inactivation during a train of four action potential–like depolarizations. Cell e8612, 5 kHz Gaussian filter. The voltage command (shown below) was simulated from the Hodgkin-Huxley (1952b) model for the squid axon, modified to allow spontaneous 50-Hz repetitive firing by shifting the voltage dependence of the “m” gate by 2 mV to more negative voltages. The action potentials were scaled to an initial voltage of −100 mV, with an overshoot of +39 mV. The dashed lines are at 0 and −100 mV.

Mentions: State-dependent inactivation is often associated with cumulative inactivation, a phenomenon where repetitive pulses produce significant inactivation, even when little or no inactivation is visible during each depolarization (Neher and Lux 1971; Aldrich 1981). We do observe strong cumulative inactivation for brief trains of pulses for α1G, either using square voltage steps (Fig. 12 A) or action potential–like depolarizations (Fig. 12 B).


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

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

Cumulative inactivation. (A) The upper panel is two superimposed current records, a single 50-ms depolarization to −20 mV, and four steps to −20 mV for 5 ms each, separated by 10-ms intervals at −100 mV. The lower panel is the predicted open-state inactivation for the four-pulse protocol. Cell a8612, 2 kHz Gaussian filter. (B) Inactivation during a train of four action potential–like depolarizations. Cell e8612, 5 kHz Gaussian filter. The voltage command (shown below) was simulated from the Hodgkin-Huxley (1952b) model for the squid axon, modified to allow spontaneous 50-Hz repetitive firing by shifting the voltage dependence of the “m” gate by 2 mV to more negative voltages. The action potentials were scaled to an initial voltage of −100 mV, with an overshoot of +39 mV. The dashed lines are at 0 and −100 mV.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 12: Cumulative inactivation. (A) The upper panel is two superimposed current records, a single 50-ms depolarization to −20 mV, and four steps to −20 mV for 5 ms each, separated by 10-ms intervals at −100 mV. The lower panel is the predicted open-state inactivation for the four-pulse protocol. Cell a8612, 2 kHz Gaussian filter. (B) Inactivation during a train of four action potential–like depolarizations. Cell e8612, 5 kHz Gaussian filter. The voltage command (shown below) was simulated from the Hodgkin-Huxley (1952b) model for the squid axon, modified to allow spontaneous 50-Hz repetitive firing by shifting the voltage dependence of the “m” gate by 2 mV to more negative voltages. The action potentials were scaled to an initial voltage of −100 mV, with an overshoot of +39 mV. The dashed lines are at 0 and −100 mV.
Mentions: State-dependent inactivation is often associated with cumulative inactivation, a phenomenon where repetitive pulses produce significant inactivation, even when little or no inactivation is visible during each depolarization (Neher and Lux 1971; Aldrich 1981). We do observe strong cumulative inactivation for brief trains of pulses for α1G, either using square voltage steps (Fig. 12 A) or action potential–like depolarizations (Fig. 12 B).

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