Suppression of Early and Late Afterdepolarizations by Heterozygous Knockout of the Na+/ Ca2+Exchanger in a Murine Model

Background The Na+/Ca2+ exchanger (NCX) has been implied to cause arrhythmias. To date, information on the role of NCX in arrhythmogenesis derived from models with increased NCX expression, hypertrophy, and heart failure. Furthermore, the exact mechanism by which NCX exerts its potentially proarrhythmic effect, ie, by promoting early afterdepolarization (EAD) or delayed afterdepolarization (DAD) or both, is unknown. Methods and Results We investigated isolated cardiomyocytes from a murine model with heterozygous knockout of NCX (hetKO) using the patch clamp and Ca2+ imaging techniques. Action potential duration was shorter in hetKO with I-Ktot not being increased. The rate of spontaneous Ca2+ release events and the rate of DADs were unaltered; however, DADs had lower amplitude in hetKO. A DAD triggered a spontaneous action potential significantly less often in hetKO when compared with wild-type. The occurrence of EADs was also drastically reduced in hetKO. I-Ca activity was reduced in hetKO, an effect that was abolished in the presence of the Ca2+ buffer BAPTA. Conclusions Genetic suppression of NCX reduces both EADs and DADs. The following molecular mechanisms apply: (1) Although the absolute number of DADs is unaffected, an impaired translation of DADs into spontaneous action potentials results from a reduced DAD amplitude. (2) EADs are reduced in absolute number of occurrence, which is presumably a consequence of shortened action potential duration because of reduced NCX activity but also reduced I-Ca the latter possibly being caused by a direct modulation of Ca2+-dependent I-Ca inhibition by reduced NCX activity. This is the first study to demonstrate that genetic inhibition of NCX protects against afterdepolarizations and to investigate the underlying mechanisms.