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Modulation of human ether-à-go-go-related K+ (HERG) channel inactivation by Cs+ and K+
Shetuan Zhang1, Steven J Kehl, David Fedida
1Department of Physiology, University of British Columbia, 2146 Health Sciences Mall, Vancouver, BC, Canada V6T 1Z3.
Insights
External cesium ions significantly slow human ether-à-go-go-related K+ (HERG) channel inactivation, impacting its voltage sensitivity. This suggests cesium and potassium share a common mechanism in HERG channel gating.
Area of Science:
- Molecular biology
- Ion channel physiology
- Cardiovascular research
Background:
- Human ether-à-go-go-related K+ (HERG) channels are crucial for cardiac repolarization.
- Unlike other K+ channels, HERG channels exhibit significant permeability to cesium (Cs+).
- Understanding ion interactions with HERG channels is vital for predicting drug effects and channelopathies.
Purpose of the Study:
- To investigate the effects of external cesium ion (Cs+o) concentration on HERG channel gating kinetics.
- To determine if Cs+o influences HERG channel activation or inactivation.
- To elucidate the mechanism underlying Cs+o's interaction with the HERG channel pore.
Main Methods:
- Utilized patch-clamp electrophysiology to record HERG channel currents in response to varying Cs+o concentrations.
- Quantified changes in inactivation and activation time constants and voltage-dependence of inactivation.
- Performed quantitative analysis to model Cs+o binding within the HERG channel pore.
Main Results:
- Increased Cs+o concentration dramatically slowed HERG channel inactivation without affecting activation.
- Replacement of K+o with Cs+o prolonged inactivation and recovery, shifting steady-state inactivation mid-point by 25 mV.
- Cs+o binding to a voltage-influenced pore site explains the observed slowing of inactivation, particularly at weaker depolarizations.
Conclusions:
- External Cs+ ions selectively modulate HERG channel inactivation, not activation, highlighting ion-specific pore interactions.
- The voltage-dependent binding of Cs+o suggests a shared mechanism with K+o, with additive effects observed.
- These findings underscore the importance of ion-channel interactions in the inactivation process of HERG channels.
Abstract:
Unlike many other native and cloned K+ channels, human ether-à-go-go-related K+ (HERG) channels show significant Cs+ permeability with a PCs/PK (the permeability of Cs+ relative to that of K+) of 0.36 +/- 0.03 (n = 10). Here, we find that raising the concentration of external Cs+ (Cs+o) dramatically slows HERG channel inactivation without affecting activation. Replacement of 5 mM K+o by 135 mM Cs+o increased both inactivation and recovery time constants and shifted the mid-point of the steady-state inactivation curve by 25 mV in the depolarized direction (n = 6, P < 0.01). Raising [Cs+]o also modulated the voltage sensitivity of inactivation gating. With 130 mM Cs+i and 135 mM NMDG+o, the inactivation time constant decreased e-fold per 47.5 +/- 1.1 mV (n = 5), and when 20 mM Cs+ was added to the bath solution, the inactivation time constant decreased e-fold per 20.6 +/- 1.3 mV (n = 5, P < 0.01). A quantitative analysis suggests that Cs+o binds to a site in the pore that is influenced by the transmembrane electrical field, so that Cs+o-induced slowing of HERG inactivation is less prominent at strong depolarizations. K+o has effects that are similar to Cs+o and their effects were additive, suggesting Cs+o and K+o may share a common mechanism of action. The strong effects of Cs+ on inactivation but not on activation highlight the importance of ion and channel interactions during the onset of inactivation in the HERG channel.