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 mu M, respectively. When [2-(trimethyammonium)ethyl]methanethiosulfonate bromide (MTSET) was included in the pipette and fast inactivation was disabled, IC50 was 5.04 mu M for DG-ICM and 12.63 mu M 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 mu M for DG-ICM and 5.87 mu M for WT-ICM), suggesting that open-channel block is an important determinant of flecainide UDB. We obtained association (k(on)) and dissociation (k(off)) rates for open-channel block by the Langmuir-isotherm model. They were k(off) = 31.37 s(-1), k(on) = 5.83 s(-1) . mu M-1, and calculated K-d = 5.38 mu M for WT-ICM (where K-d = k(off)/k(on)); and k(off) = 24.88 s(-1), k(on) = 9.54 s(-1) . mu M-1, and calculated K-d = 2.61 mu M 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.