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Biophys J, December 2000, p. 3019-3035, Vol. 79, No. 6

Block of Wild-Type and Inactivation-Deficient Cardiac Sodium Channels IFM/QQQ Stably Expressed in Mammalian Cells

Augustus O. Grant,* Rashmi Chandra,* Christopher Keller,* Michael Carboni,* and C. Frank Starmerdagger

 *Departments of Medicine and Pediatrics, Duke University, Durham, NC 27706, and  dagger Department of Biometry, Medical University of South Carolina, Charleston, SC 29426 USA

The role of inactivation as a central mechanism in blockade of the cardiac Na+ channel by antiarrhythmic drugs remains uncertain. We have used whole-cell and single channel recordings to examine the block of wild-type and inactivation-deficient mutant cardiac Na+ channels, IFM/QQQ, stably expressed in HEK-293 cells. We studied the open-channel blockers disopyramide and flecainide, and the lidocaine derivative RAD-243. All three drugs blocked the wild-type Na+ channel in a use-dependent manner. There was no use-dependent block of IFM/QQQ mutant channels with trains of 20 40-ms pulses at 150-ms interpulse intervals during disopyramide exposure. Flecainide and RAD-243 retained their use-dependent blocking action and accelerated macroscopic current relaxation. All three drugs reduced the mean open time of single channels and increased the probability of their failure to open. From the abbreviation of the mean open times, we estimated association rates of ~106/M/s for the three drugs. Reducing the burst duration contributed to the acceleration of macroscopic current relaxation during exposure to flecainide and RAD-243. The qualitative differences in use-dependent block appear to be the result of differences in drug dissociation rate. The inactivation gate may play a trapping role during exposure to some sodium channel blocking drugs.

Biophys J, December 2000, p. 3019-3035, Vol. 79, No. 6
© 2000 by the Biophysical Society   0006-3495/00/12/3019/17  $2.00


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