Kinetic analysis of open- and closed-state inactivation transitions in human Kv4.2 A-type potassium channels

1. We studied the gating kinetics of Kv4.2 channels, the molecular substrat e of neuronal somatodendritic A-type currents. For this purpose wild-type a nd mutant channels were transiently expressed in the human embryonic kidney (HEK) 293 cell line and currents were measured in the whole-cell patch-cla mp configuration. 2. Kv4.2 channels inactivated from pre-open closed state(s) with a mean tim e constant of 959 ms at -50 mV. This closed-state inactivation was not affe cted by a deletion of the Kv4.2 N-terminus (Delta2-40). 3. Kv4.2 currents at +40 mV inactivated with triple-exponential kinetics. A fast component (tau = 11 ms) accounted for 73%, an intermediate component (tau = 50 ms) for 23% and a slow component (tau = 668 ms) for 4% of the tot al decay. 4. Both the fast and the intermediate components of inactivation were slowe d by a deletion of the Kv4.2 N-terminus (tau = 35 and 111 ms) and accounted for 33 and 56%, respectively, of the total decay. The slow component was m oderately accelerated by the truncation (tau = 346 ms) and accounted for 11 % of the total Kv4.2 current inactivation. 5. Recovery from open-state inactivation and recovery from closed-state ina ctivation occurred with similar kinetics in a strongly voltage-dependent ma nner. Neither recovery reaction was affected by the N-terminal truncation. 6. Kv4.2 Delta2-40 channels displayed slowed deactivation kinetics, suggest ing that the N-terminal truncation leads to a stabilization of the open 7. Simulations with an allosteric model of inactivation, supported by the e xperimental data, suggested that, in response to membrane depolarization, K v4.2 channels accumulate in the closed-inactivated state(s), from which the y directly recover, bypassing the open state.