Flecainide sensitivity of a Na channel long QT mutation shows an open-channel blocking mechanism for use-dependent block.
A long QT mutation in the cardiac sodium channel, D1790G (DG), shows enhanced flecainide use-dependent block (UDB). The relative importance of open and inactivated states of the channel in flecainide UDB has been controversial. We used a modifiable, inactivation-deficient mutant channel that contains the F1486C mutation in the IFM motif to investigate the UDB difference between the wild-type (WT-ICM) and DG (DG-ICM) channels. UDB at 5 Hz was greater in DG-ICM than WT-ICM, and IC50 values for steady-state UDB were 7.19 and 18.06 microM, respectively. When [2-(trimethyammonium) ethyl]methanethiosulfonate bromide (MTSET) was included in the pipette and fast inactivation was disabled, IC50 was 5.04 microM for DG-ICM and 12.63 microM for WT-ICM. We measured open-channel block by flecainide directly in MTSET-treated, noninactivating ICM channels. Steady-state block was higher for DG-ICM than WT-ICM (IC50 was 2.34 microM for DG-ICM and 5.87 microM for WT-ICM), suggesting that open-channel block is an important determinant of flecainide UDB. We obtained association (kon) and dissociation (koff) rates for open-channel block by the Langmuir-isotherm model. They were koff = 31.37 s(-1), kon = 5.83 s(-1).microM(-1), and calculated Kd = 5.38 microM for WT-ICM (where Kd = koff/kon); and koff = 24.88 s(-1), kon = 9.54 s(-1).microM(-1), and calculated Kd = 2.61 microM for DG-ICM. These Kd values were similar to IC50 measured from steady-state open-channel block. Furthermore, we modeled UDB mathematically by using these kinetic rates and found that the model predicted experimental UDB accurately. The recovery from UDB had a minor contribution to UDB. Flecainide UDB is predominantly determined by an open-channel blocking mechanism, and DG-ICM channels appeared to have an altered open-channel state with higher flecainide affinity than WT-ICM.
Zhu
Yujie
Y
Department of Medicine and Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, IL 60612, USA.
Kyle
John W
JW
Lee
Peter J
PJ
D000889:Anti-Arrhythmia Agents / Q000008:administration & dosage; D002460:Cell Line; D003198:Computer Simulation; D004305:Dose-Response Relationship, Drug; D005424:Flecainide / Q000008:administration & dosage*; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects*; D007668:Kidney / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008133:Long QT Syndrome / Q000503:physiopathology*; D008564:Membrane Potentials / Q000187:drug effects*; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D013329:Structure-Activity Relationship
D000889:Anti-Arrhythmia Agents; D015222:Sodium Channels; D012964:Sodium; D005424:Flecainide