Long QT syndrome (LQTS) is associated with sudden cardiac death resulting from torsades de pointes (TdP), which are triggered by early afterdepolarizations (EADs). The cardiac Na⁺/Ca²⁺ exchanger (NCX) has been suggested to work as a trigger for EADs.

The purpose of this study was to test the hypothesis that inhibition of NCX with a newly developed selective NCX inhibitor (SEA0400) reduces TdP.

In 34 Langendorff-perfused rabbit hearts, the I_Kr-blocker sotalol (100 μM; n = 18) as well as veratridine (0.5 μM; n = 16), an inhibitor of sodium channel inactivation, led to a significant increase in monophasic action potential (MAP), duration thereby mimicking long QT syndrome-2 and -3. In bradycardic hearts, recordings of eight MAPs demonstrated an increased dispersion of repolarization (sotalol: 67%; veratridine: 89%; P <.01). After lowering of potassium concentration, sotalol (56%) and veratridine (63%) induced TdP. Perfusion with SEA0400 (1 μM) suppressed EADs in 15 of 16 sotalol hearts and in seven of 13 veratridine hearts. SEA0400 significantly shortened MAP duration and reduced dispersion of repolarization in both groups (P <.05). This reduced TdP incidence in the sotalol group (100%) and in the veratridine group (77%). To investigate the effects of NCX inhibition on the cellular level, we used a computer model of the rabbit ventricular myocyte. I_Na and I_Kr were modified to mimic the effects of veratridine and sotalol, respectively. Consistent with our in vitro experiments, reduction of NCX activity accelerated repolarization of the cellular action potential and prevented EADs.

In an intact rabbit heart model of LQT2 and LQT3 as well as in a computer model of the rabbit cardiac myocyte, inhibition of NCX is effective in preventing TdP due to a suppression of EADs, a reversion of action potential prolongation, and a reduction of dispersion of repolarization. Our observations suggest a therapeutic benefit of selective NCX inhibition in LQTS.