Introduction: As a molecular model of the effect of ischemia on drug block of the transient outward potassium current, the effect of acidosis on the blocking properties of flecainide and quinidine on Kv4.3 currents was studied., Methods and Results: Kv4.3 channels were stably expressed in Chinese hamster ovary cells. Whole-cell, voltage clamp techniques were used to measure the effect of flecainide and quinidine on Kv4.3 currents in solutions of pH 7.4 and 6.0. Extracellular acidosis attenuated flecainide block of Kv4.3 currents, with the IC50 for flecainide (based on current-time integrals) increasing from 7.8 +/- 1.1 microM at pH 7.4 to 125.1 +/- 1.1 microM at pH 6.0. Similar effects were observed for quinidine (IC50 5.2 +/- 1.1 microM at pH 7.4 and 22.1 +/- 1.3 microM at pH 6.0). Following block by either drug, Kv4.3 channels showed a hyperpolarizing shift in the voltage sensitivity of inactivation and a slowing in the time to recover from inactivation/block that was unaffected by acidosis. In contrast, acidosis attenuated the effects on the time course of inactivation and the degree of tonic- and frequency-dependent block for both drugs., Conclusion: Extracellular acidosis significantly decreases the potency of blockade of Kv4.3 by both flecainide and quinidine. This change in potency may be due to allosteric changes in the channel, changes in the proportion of uncharged drug, and/or changes in the kinetics of drug binding or unbinding. These findings are in contrast to the effects of extracellular acidosis on block of the fast sodium channel by these agents and provide a molecular mechanism for divergent modulation of drug block potentially leading to ischemia-associated proarrhythmia.