States and sites of actions of flecainide on guinea-pig cardiac sodium channels

Abstract

The states and sites of actions of flecainide on sodium channels were investigated in guinea-pig single cardiac cells, using the whole-cell voltage-clamp technique at 22°C. External application of flecainide caused tonic and use-dependent block of the sodium current (I Na). The tonic block and the steady state use-dependent block increased with increasing drug concentrations. The dose-response curve for the use-dependent block was fitted by the equation for 1:1 drug-receptor binding and yielded a K D of 7.0 μM flecainide. At 5 μM flecainide, the use-dependent block of I Na with 10 and 200 ms depolarizing pulses at an interpulse interval of 400 ms was 31.1 ± 2.7 (mean ± S.E.) and 36.8 ± 2.7%, respectively. The two values were not significantly different. The block developed as a single exponential function with onset rate of 0.041 ± 0.005/pulse. Recovery from flecainide block consisted of two components as reported previously. The mean time constant of the initial fast component was 48 ± 17 ms, which was comparable but significantly longer than that in the absence of the drug. The late slow component was only seen after drug application and the time constant was 26 ± 7 s at −100 mV. Internal application of 5 and 50 μM flecainide for 30 min after rupture of the cell membrane produced a non-significant block and values of 1.7 ± 0.8 and 6.9 ± 2.4%, respectively, for the use-dependent block of I Na. These results indicate that flecainide has a high affinity for the activated state of the channels with slow kinetics, and binds to a specific site not from the cytoplasmic side of the membrane but from the outiside, although 99.0% of the drug molecules are proponated at pH 7.3 (pK a of flecainide = 9.3). We conclude that flecainide interacts with cardiac sodium channels preferentially in the activated state, and that the drug reaches the receptor site from the external rather than the cytoplasmic side.