Copyright © 2004, European Society of Cardiology
Intrinsic mechanism of the enhanced rate-dependent QT shortening in the R1623Q mutant of the LQT3 syndrome
aSecond Department of Internal Medicine, University of Occupational and Environmental Health Japan, 1-1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807-8555, Japan
bDepartment of Cardiovascular Medicine, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan
cDepartment of Medicine, Section of Cardiovascular Medicine, University of Wisconsin, Madison, WI 53792, USA
* Corresponding author. Tel.: +81 93 691 7436; fax: +81 93 691 6913. Email address: toshi{at}med.uoeh-u.ac.jp
Objective: In the type 3 long QT syndrome (LQT3), arrhythmia events tend to occur at rest or during sleep. One of the mutations, R1623Q, is located in the voltage sensor of the cardiac sodium channel (hH1), and patients with R1623Q mutation have been also reported to show bradycardia-dependent cardiac events. Although the mutant channel has been characterized by inactivation gating defects, the intrinsic mechanism(s) that might explain why arrhythmia attack is most prevalent at slower heart rates has not been investigated.
Methods: cDNA encoding either the wild-type or the R1623Q mutant of hH1 was stably transfected into HEK293 cells. INa was recorded using a whole-cell patch-clamp technique at 23 °C.
Results: A train of 50 depolarizing pulses from holding potentials (–120 and –80 mV) to –20 mV or a train of 50 action potential waveforms was applied at different frequencies. When using a rectangular waveform voltage clamp protocol, rate-dependent reduction of INa was holding voltage-dependent but was not different between peak INa and late INa. However, using the action potential clamp, preferential rate-dependent reduction of the phase 3 INa was obvious as compared with peak INa. The discrepancy in the rate-dependent reduction between protocols was attributed to accelerated recovery from inactivation under non-equilibrium condition.
Conclusion: The rate dependency of phase 3 INa under non-equilibrium gating is a novel mechanism to explain the enhanced rate-dependent QT-shortening in LQT3 patients. Our findings are important for genotype–phenotype correlations in LQT3 mutants as well as for understanding the function of S4 segment of domain IV region in the cardiac Na+ channel.
KEYWORDS Arrhythmia (mechanisms); Long QT syndrome; Na channel; Membrane currents; Repolarization