Inhibition of the K⁺ channel Kv1.4 by acidosis: protonation of an extracellular histidine slows the recovery from N‐type inactivation

Abstract

Acidosis alters the transient outward current, ito, in the heart. We have studied the mechanism underlying the effect of acidosis on one of the K+ channels, Kv1.4 (heterologously expressed in Xenopus laevis oocytes), known to underlie ito. At pH 6.5, wild‐type Kv1.4 current was inhibited during repetitive pulsing, in part as a result of a slowing of recovery from N‐type inactivation. Acidosis still caused slowing of recovery after deletion of just one (either the first or second) of the N‐terminal inactivation ball domains. However, deletion of both the N‐terminal inactivation ball domains greatly reduced the inhibition. As well as the N‐terminus, other parts of the channel are also required for the effect of acidosis, because, whereas the transfer of the N‐terminus of Kv1.4 to Kv1.2 conferred N‐type inactivation, it did not confer acidosis sensitivity. Replacement of an extracellular histidine with a glutamine residue (H508Q) abolished the slowing of recovery by acidosis. Reduction of C‐type inactivation by raising the bathing K+ concentration or by the mutation K532Y also abolished the slowing. It is concluded that binding of protons to H508 enhances C‐type inactivation and this causes a slowing of recovery from N‐type inactivation and, thus, an inhibition of current during repetitive pulsing.

A Variable Residue in the Pore of Kv1 Channels Is Critical for the High Affinity of Blockers from Sea Anemones and Scorpions

“…5, B and D) ), which prevented successful separation of free from bound 125 I-BgK(W5Y/Y26F). Kv1.4 mutants in which residue Lys-532 was replaced by either cysteine or glutamine were previously reported to be functional (27,28). We constructed these mutants and confirmed by Western blots that they were expressed at similar levels as the wild-type channel (data not shown).…”

Proton Sensors in the Pore Domain of the Cardiac Voltage-gated Sodium Channel

“…Previous studies have isolated proton sensor residues within the p-loop of several Na V and K V channels (32,33,37,38). We therefore focused our efforts on the Na V 1.5 channel p-loop regions.…”

Proton block of the pore underlies the inhibition of hERG cardiac K+ channels during acidosis

“…Histidine residues serve as pH sensors for the effects of protons in a number of Kv channels (9,24,31,53). It is reasonable to suppose therefore that histidine residues may act as pH sensors governing the effects of protons in hERG channels.…”

“…cardiac ion channels are affected by acidosis acting via various mechanisms, including: 1) charge-screening effects that bias the closed-open equilibrium toward the closed state as described by Frankenhaeuser and Hodgkin (18); 2) stabilization of channels in nonconducting inactivated states (8,9,31,53); 3) direct block of the conducting pore (6,32,52). Investigations into the effects of acidosis on hERG channel function demonstrate an extracellular proton-induced reduction in peak current amplitude (1,4,27,54,57) and an acceleration of deactivation (1,2,4,25,27,54,57); action potential simulation studies suggest that these actions cause a predisposition to arrhythmia by reducing the protection against premature stimulation (13).…”