Inhibition of late sodium current suppresses calcium-related ventricular arrhythmias by reducing the phosphorylation of CaMK-II and sodium channel expressions.

Cardiac arrhythmias associated with intracellular calcium inhomeostasis are refractory to antiarrhythmic therapy. We hypothesized that late sodium current (I _Na ) contributed to the calcium-related arrhythmias. Monophasic action potential duration at 90% completion of repolarization (MAPD ₉₀ ) was significantly increased and ventricular arrhythmias were observed in hearts with increased intracellular calcium concentration ([Ca ²⁺ ] _i ) by using Bay K 8644, and the increase became greater in hearts treated with a combination of ATX-II and Bay K 8644 compared to Bay K 8644 alone. The prolongations caused by Bay K 8644 and frequent episodes of ventricular tachycardias, both in absence and presence of ATX-II, were significantly attenuated or abolished by late I _Na inhibitors TTX and eleclazine. In rabbit ventricular myocytes, Bay K 8644 increased I _CaL density, calcium transient and myocyte contraction. TTX and eleclazine decreased the amplitude of late I _Na , the reverse use dependence of MAPD ₉₀ at slower heart rate, and attenuated the increase of intracellular calcium transient and myocyte contraction. TTX diminished the phosphorylation of CaMKII-δ and Na _v 1.5 in hearts treated with Bay K 8644 and ATX-II. In conclusion, late I _Na contributes to ventricular arrhythmias and its inhibition is plausible to treat arrhythmias in hearts with increased [Ca ²⁺ ] _i .