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Emerging Concepts in Ion Channel Biophysics
Poster Abstracts
88
68-POS
Board 68
Regulation of Class Ib anti-Arrhythmic Drug Block by the Cardiac Na
+
Channel Voltage-
Sensing Domains
Wandi Zhu,
Jonathan R. Silva
.
Washington University in St. Louis, St. Louis, MO, USA.
Background: Clinical studies have shown that class I anti-arrhythmics are helpful in a subset of
patients while being lethal in others. For example, patients with Long QT mutations respond
differently to mexiletine, a class Ib anti-arrhythmic. We observed the conformational dynamics
of different cardiac Na
+
channel (NaV1.5) domains to discover characteristics that facilitate or
prevent effective mexiletine block.
Methods: NaV1.5 contains four domains (DI-DIV), each with a voltage-sensing domain (VSD).
We previously created four DNA constructs that contain a cysteine within a single VSD.
Channels were expressed in Xenopus oocytes and cysteines were labeled with fluorophores.
Ionic current and VSD-tracking fluorescence emission were simultaneously recorded.
Results: Mexiletine binding to WT channels stabilizes the DIII-VSD in the activated
conformation without affecting the other domains. LQT3 mutant channels show variable
mexiletine sensitivity (R1626P>P1332L>WT=S941N>M1652R). These mutants also show
varying DIII-VSD activation voltage-dependence, despite the distal locations of the mutations.
The DIII-VSD activation shift strongly correlates with mexiletine sensitivity (QT-shortening).
The highly-sensitive mutations stabilize the activated DIII-VSD, while the insensitive mutation
destabilizes it. Thus, an activated DIII-VSD facilitates mexiletine blockade. To test this
hypothesis, we assessed 13 additional mutations, and quantified gating parameters such as
conductance voltage-dependence, inactivation, DIII-VSD activation and drug block. A partial
least square regression (PLSR) model showed that DIII-VSD activation and inactivation
represent components that regulate drug block.
Conclusion: Traditionally, channel activation and inactivation were linked to Class-Ib drug
block. We propose a novel mechanism where DIII-VSD conformation facilitates or impairs
block. By assessing how mutations affect the DIII-VSD, we expect to predict whether a patient
will respond to mexiletine. This mechanism could also be utilized to develop a new type of
combination therapy by stabilizing the DIII-VSD activated state to increase the efficacy of class-
Ib drugs.