COMPARISON OF THE EFFICIENCY OF NA+/CA2+ EXCHANGER OR NA+/H+ EXCHANGER INHIBITION AND THEIR COMBINATION IN REDUCING CORONARY REPERFUSION-INDUCED ARRHYTHMIAS

During ischaemia/reperfusion, the rise in [Na+](i), induced by simultaneous depression of the Na+/K+-ATPase and activation of the Na+/H+ exchanger (NHE), shifts the Na+/Ca2+ exchanger (NCX) into reverse transport mode, resulting in Ca-i(2+) overload, which is a critical factor in enhancing the liability to cardiac arrhythmias. The inhibition of NHE, and recently NCX has been suggested to effectively protect the heart from reperfusion-induced arrhythmias. In this study, we investigated and compared the efficacy of individual or the simultaneous inhibition of the NHE and NCX against reperfusion-induced arrhythmias in Langendorff-perfused rat hearts by applying a commonly used regional ischaemia-reperfusion protocol.. The NHE and NCX were inhibited by cariporide and SEA0400 or the novel, more selective ORM-10103, respectively. Arrhythmia diagrams calculated for the reperfusion period were analysed for the incidence and duration of extrasystoles (ESs), ventricular tachycardia (VT) and ventricular fibrillation (VF). NHE inhibition by cariporide was highly efficient in reducing the recorded reperfusion-induced arrhythmias. Following the application of SEA0400 or ORM-10103, the number and duration of arrhythmic periods were efficiently or moderately decreased. While both NCX inhibitors effectively reduced ESs, the Most frequently triggered arrhythmias, they exerted limited or no effect on VTs and VFs. Of the NCX inhibitors, ORM-10103 was more effective. Surprisingly, the simultaneous inhibition of the NCX and NHE failed to significantly improve the antiarrhythmic efficacy reached by NCX blockade alone. In conclusion, although principal simultaneous NHE+NCX inhibition should be highly effective against all types of the recorded reperfusion-induced arrhythmias, NCX inhibitors, alone or in combination with cariporide, seem to be moderately suitable to provide satisfactory cardioprotection - at least in the present arrhythmia model. Since ORM-10103 and SEA0400 are known to effectively inhibit after-depolarisations, it is suggested that their efficacy and that of other NCX inhibitors may be higher and more pronounced in the predominantly Ca-i(2+)-dependent triggered arrhythmias.