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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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Current–voltage relations for α1G channels. (A) Sample current records, with 5 kHz Gaussian filtering, from cell e8612. (B) Current–voltage relations averaged from 12 cells, measured at the point of peak current at each voltage.
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Figure 1: Current–voltage relations for α1G channels. (A) Sample current records, with 5 kHz Gaussian filtering, from cell e8612. (B) Current–voltage relations averaged from 12 cells, measured at the point of peak current at each voltage.

Mentions: Currents with the properties expected of T-type calcium currents were recorded from HEK 293 cells stably expressing α1G cDNA. Depolarizations in 10-mV increments from a holding potential of −100 mV elicited transient inward and outward currents (Fig. 1 A). Currents showed voltage-dependent macroscopic activation and inactivation, with faster kinetics at more depolarized voltages. At intermediate voltages, the currents “cross over” as typically observed for Na+ currents and T-currents (Randall and Tsien 1997; Perez-Reyes et al. 1998). The current–voltage (I–V) relationship, measured at the time of peak current during each record, is shown in Fig. 1 B. Detectable current was first observed near −70 mV, with peak inward current near −40 mV.


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

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

Current–voltage relations for α1G channels. (A) Sample current records, with 5 kHz Gaussian filtering, from cell e8612. (B) Current–voltage relations averaged from 12 cells, measured at the point of peak current at each voltage.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 1: Current–voltage relations for α1G channels. (A) Sample current records, with 5 kHz Gaussian filtering, from cell e8612. (B) Current–voltage relations averaged from 12 cells, measured at the point of peak current at each voltage.
Mentions: Currents with the properties expected of T-type calcium currents were recorded from HEK 293 cells stably expressing α1G cDNA. Depolarizations in 10-mV increments from a holding potential of −100 mV elicited transient inward and outward currents (Fig. 1 A). Currents showed voltage-dependent macroscopic activation and inactivation, with faster kinetics at more depolarized voltages. At intermediate voltages, the currents “cross over” as typically observed for Na+ currents and T-currents (Randall and Tsien 1997; Perez-Reyes et al. 1998). The current–voltage (I–V) relationship, measured at the time of peak current during each record, is shown in Fig. 1 B. Detectable current was first observed near −70 mV, with peak inward current near −40 mV.

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