Gating charge immobilization caused by the transition between inactivated states in the Kv1.5 channel

Sustained Na+ or Li+ conductance is a feature of the inactivated state in wild-type (WT) and nonconducting Shaker and Kv1.5 channels, and has been used here to investigate the cause of off-gating charge immobilization in WT and Kv1.5-W472F nonconducting mutant channels. Off-gating immobilization in response to brief pulses in cells perfused with NMG(i)(+)/NMG(o)(+) is the result of a more negative voltage dependence of charge recovery (V-1/2 is -96 mV) compared with on-gating charge movement (V-1/2 is -6.3 mV). This shift is known to be associated with slow inactivation in Shaker channels and the disparity is reduced by 40 mV, or similar to 50% in the presence of 135 mM Cs-i(+). Off-gating charge immobilization is voltage-dependent with a V-1/2 of - 12 mV, and correlates well with the development of Na+ conductance on repolarization through C-type inactivated channels (V-1/2 is -11 mV). As well, the time-dependent development of the inward Na+ tail current and gating charge immobilization after depolarizing pulses of different durations has the same time constant (tau = 2.7 ms). These results indicate that in Kv1.5 channels the transition to a stable C-type inactivated state takes only 2-3 ms and results in strong charge immobilization in the absence of Group 1A metal cations, or even in the presence of Na-o(+). Inclusion of low concentrations of Cst delays the appearance of Na+ tail currents in WT channels, prevents transition to inactivated states in Kv1.5-W472F nonconducting mutant channels, and removes charge immobilization. Higher concentrations of Cst are able to modulate the deactivating transition in Kv1.5 channels and prevent the residual slowing of charge return.