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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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Test for closed-state inactivation. Voltage steps to −70 mV (A) or −80 mV (B) for the indicated duration were immediately followed by 5-ms test pulses to −20 mV. Inactivation measured from the test pulses (diamonds) is compared with the expected open-state inactivation (squares) calculated from the integrated current during the step to −70 mV ().  (A),  (B).
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Figure 9: Test for closed-state inactivation. Voltage steps to −70 mV (A) or −80 mV (B) for the indicated duration were immediately followed by 5-ms test pulses to −20 mV. Inactivation measured from the test pulses (diamonds) is compared with the expected open-state inactivation (squares) calculated from the integrated current during the step to −70 mV (). (A), (B).

Mentions: At −70 mV, where channel opening was clearly detectable, the observed inactivation was approximately twice the predicted value (Fig. 9 A). The difference was larger at −80 mV (Fig. 9 B), where inward currents were visible in one or two of the four cells analyzed. If recovery from inactivation were considered, the predicted inactivation would be reduced further, increasing the discrepancy. We conclude that there is excess inactivation that cannot be accounted for by inactivation from the open state, presumably indicating inactivation directly from closed states.


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

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

Test for closed-state inactivation. Voltage steps to −70 mV (A) or −80 mV (B) for the indicated duration were immediately followed by 5-ms test pulses to −20 mV. Inactivation measured from the test pulses (diamonds) is compared with the expected open-state inactivation (squares) calculated from the integrated current during the step to −70 mV ().  (A),  (B).
© Copyright Policy
Related In: Results  -  Collection

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

Figure 9: Test for closed-state inactivation. Voltage steps to −70 mV (A) or −80 mV (B) for the indicated duration were immediately followed by 5-ms test pulses to −20 mV. Inactivation measured from the test pulses (diamonds) is compared with the expected open-state inactivation (squares) calculated from the integrated current during the step to −70 mV (). (A), (B).
Mentions: At −70 mV, where channel opening was clearly detectable, the observed inactivation was approximately twice the predicted value (Fig. 9 A). The difference was larger at −80 mV (Fig. 9 B), where inward currents were visible in one or two of the four cells analyzed. If recovery from inactivation were considered, the predicted inactivation would be reduced further, increasing the discrepancy. We conclude that there is excess inactivation that cannot be accounted for by inactivation from the open state, presumably indicating inactivation directly from closed states.

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