Role of Ca2+ -activated Cl− current during proarrhythmic early afterdepolarizations in sheep and human ventricular myocytes

Objectives: The proarrhythmic early afterdepolarizations (EADs) during phase-2 of the cardiac action potential (phase-2 EADs) are associated with secondary Ca2+-release of the sarcoplasmic reticulum. This makes it probable that the Ca2+-activated Cl- current [I-Cl(Ca)] is present during phase-2 EADs. Activation of I-Cl(Ca) during phase-2 of the action potential will result in an outwardly directed, repolarizing current and may thus be expected to prevent excessive depolarization of phase-2 EADs. The present study was designed to test this hypothesis. Methods and Results: The contribution of I-Cl(Ca) during phase-2 EADs was studied in enzymatically isolated sheep and human ventricular myocytes using the patch-clamp methodology. EADs were induced by a combination of a low stimulus frequency (0.5 Hz) and exposure to 1 muM noradrenaline. In sheep myocytes, the I-Cl(Ca) blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 0.5 mM) abolished phase-1 repolarization of the action potential in all myocytes tested. This indicates that I-Cl(Ca) is present in all sheep myocytes. However, DIDS had no effect on phase-2 EAD characteristics. In human myocytes, DIDS neither affected phase-1 repolarization nor phase-2 EAD characteristics. Conclusion: In sheep ventricular myocytes, but not in human ventricular myocytes, I-Cl(Ca) contributes to phase-1 repolarization of the action potential. In both sheep and human myocytes, I-Cl(Ca) plays a limited role during phase-2 EADs.