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An examination of the slow inactivation and resting inactivation of Kv1.5 and ShakerIR Cheng, Yen May


External H⁺ and Ni²⁺ ions inhibit Kv1.5 channels by increasing current decay during a depolarizing pulse and reducing the peak current. A similarly accelerated current decay and reduced peak current amplitude have also been described in ShakerIR channels at low extracellular pH, and in the fast inactivating ShakerIR T449K, T449A and Kv1.5 H463G mutants in 0 K⁺₀ at pH 7.4. While the increased current decay may be attributed to an enhancement of the slow inactivation of open channels at depolarized potentials, the reduction of peak current cannot. Using standard whole-cell voltage-clamp techniques and fast perfusion changes, the hypothesis that the decreased peak current is due to the induction (by H⁺, Ni²⁺, or removal of K⁺) of a resting inactivation process was investigated. It was found that exposure of resting Kv1.5 channels to H⁺ or Ni²⁺ causes a decrease in the ability of Ba²⁺ ions to move between the external solution and a deep binding site within the selectivity filter. This result is consistent with an outer pore constriction at rest, possibly similar to that which occurs during slow inactivation. An examination of the time courses for the onset of H⁺- or Ni²⁺-enhanced slow inactivation and resting inactivation of Kv1.5 showed that, compared to slow inactivation at +50 mV, the onset of resting inactivation at −80 mV is a relatively slow process. The bi-exponential recovery following H⁺- or Ni²⁺-enhanced slow inactivation or resting inactivation had time constants similar to those for recovery from control slow inactivation. Due to the greatly accelerated slow inactivation of ShakerIR at low pH and the relatively slower perfusion kinetics of our system, it was difficult to assess the contribution of resting inactivation to the H⁺- induced loss of ShakerIR current. However, analogous kinetic analyses performed on the fast-inactivating ShakerIR T449 and Kv1.5 H463 mutants in 0 K⁺₀ also showed that the recovery time courses for resting inactivation and slow inactivation were similar, suggestive of similar recovery pathways. Together, the results strongly suggest that, under specific conditions, the slow inactivation process in wild-type and mutant Kv1.5 and ShakerIR channels can uncouple from activation and occur at resting potentials.

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