State-Dependent Inactivation of the α1g T-Type Calcium Channel

We have examined the kinetics of whole-cell T-current in HEK 293 cells stably expressing the α1G 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 [Formula: see text] , but [Formula: see text]. 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 (∼98%). Inactivation was also produced by long, weak depolarizations [Formula: see text] , which could not be explained by voltage-independent inactivation exclusively from the open state. Recovery from inactivation was exponential and fast [Formula: see text] , 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 ≤8% of the channels recovering through the open state. 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 α1G channels continue to inactivate after repolarization, primarily from the open state, which leads to cumulative inactivation during repetitive pulses.