Submitted on December 15, 2005
Accepted on February 20, 2006
Flecainide sensitivity of a Na channel long QT mutation shows an open-channel blocking mechanism for use-dependent block
Yujie Zhu1, John W Kyle2, and Peter J Lee3*
1 Medicine, University of Illinois at Chicago, Chicago, IL, USA; Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, IL, USA
2 Medicine, University of Chicago, Chicago, IL, USA
3 Medicine, University of Illinois at Chicago, Chicago, IL, USA; Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, IL, USA; Physiology and Biophysics, University of Illinois at Chicago, Chicago, IL, USA
* To whom correspondence should be addressed. E-mail: PeterL{at}uic.edu.
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, which 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 µM and 18.06 µM, respectively. When MTSET was included in the pipette and fast inactivation was disabled, IC50 were 5.04 µM for DG-ICM and 12.63 µM for WT-ICM. We measured open-channel block by flecainide directly in MTSET-treated, non-inactivating ICM channels. Steady-state block was higher for DG-ICM than WT-ICM (IC50 were 2.34 µM for DG-ICM and 5.87 µM for WT-ICM), suggesting that open channel block is an important determinant of flecainide UDB. We obtained kon and koff rates for open channel block by the Langmuir-isotherm model. They were: koff=31.37 s-1,kon=5.83 s-1µM-1, calculated Kd=5.38 µM for WT-ICM and koff=24.88 s-1, kon=9.54 s-1 µM-1, calculated Kd=2.61 µM for DG-ICM. These Kd values were similar to IC50 measured from steady-state open channel block. Furthermore, we modeled UDB mathematically 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.
