Gating Kinetics of the α1i T-Type Calcium Channel

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Abstract
The α1I T-type calcium channel inactivates almost 10-fold more slowly than the other family members (α1G and α1H) or most native T-channels. We have examined the underlying mechanisms using whole-cell recordings from rat α1I stably expressed in HEK293 cells. We found several kinetic differences between α1G and α1I, including some properties that at first appear qualitatively different. Notably, α1I tail currents require two or even three exponentials, whereas α1G tails were well described by a single exponential over a wide voltage range. Also, closed-state inactivation is more significant for α1I, even for relatively strong depolarizations. Despite these differences, gating of α1I can be described by the same kinetic scheme used for α1G, where voltage sensor movement is allosterically coupled to inactivation. Nearly all of the rate constants in the model are 5–12-fold slower for α1I, but the microscopic rate for channel closing is fourfold faster. This suggests that T-channels share a common gating mechanism, but with considerable quantitative variability.