Role of Ca2+-activated Cl- current during proarrhythmic early afterdepolarizations in sheep and human ventricular myocytes.
OBJECTIVE
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 [ICl(Ca)] is present during phase-2 EADs. Activation of ICl(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.


RESULTS
The contribution of ICl(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 microm noradrenaline. In sheep myocytes, the ICl(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 ICl(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.


CONCLUSIONS
In sheep ventricular myocytes, but not in human ventricular myocytes, ICl(Ca) contributes to phase-1 repolarization of the action potential. In both sheep and human myocytes, ICl(Ca) plays a limited role during phase-2 EADs.
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D018118:Chloride Channels; D005260:Female; D006352:Heart Ventricles / Q000187:drug effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D032342:Muscle Cells / Q000187:drug effects / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D018408:Patch-Clamp Techniques / Q000379:methods; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D012756:Sheep / Q000502:physiology*; D014662:Vasoconstrictor Agents / Q000494:pharmacology; D016276:Ventricular Function
D018118:Chloride Channels; D014662:Vasoconstrictor Agents; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D002118:Calcium; D009638:Norepinephrine