The activation gate of cardiac Na+ channel modulates voltage- and pH-dependent unbinding of disopyramide

To assess the drug unbinding process from receptor sites in cardiac Na+ channels, we examined the recovery kinetics of disopyramide-blocked Na+ current (INa) in isolated guinea-pig ventricular myocytes using the whole-cell variation of the patch-clamp technique. In the presence of disopyramide (20 microM), the time course of INa recovery from use-dependent block (unbinding) was described by a double exponential function. Although the time constant for the fast phase (tau f) of recovery was unchanged at different membrane voltages, the slow phase (tau s) increased with hyperpolarizing membrane potential: 4.4 +/- 0.2 s at a holding potential of -90 mV and 6.4 +/- 0.3 s at -140 mV (n = 10, P < 0.01). The slow time constant of INa recovery was also increased by acidification. These findings suggest that disopyramide molecules can escape from the receptor site through the hydrophobic pathway after deprotonation, because slowing of recovery from use-dependent block by acidification is caused by a decreased deprotonation rate of receptor-bound drug molecules. In addition to the hydrophobic escape, the roles of the fast inactivation gate and activation gate (m-gate) were evaluated during the recovery process. After inhibition of the fast inactivation process of INa by pretreatment with chloramine-T (2 mM), the fast phase of recovery from use-dependent block by disopyramide was abolished.(ABSTRACT TRUNCATED AT 250 WORDS)