The role of the Na+/Ca2+ exchanger, I(Na) and I(CaL) in the genesis of dofetilide-induced torsades de pointes in isolated, AV-blocked rabbit hearts.
OBJECTIVE
The Na+/Ca2+ exchanger (NCX) may contribute to triggered activity and transmural dispersion of repolarization, which are substrates of torsades de pointes (TdP) type arrhythmias. This study examined the effects of selective inhibition of the NCX by SEA0400 on the occurrence of dofetilide-induced TdP.


METHODS
Effects of SEA0400 (1 micromol x L(-1)) on dofetilide-induced TdP was studied in isolated, Langendorff-perfused, atrioventricular (AV)-blocked rabbit hearts. To verify the relevance of the model, lidocaine (30 micromol x L(-1)) and verapamil (750 nmol x L(-1)) were also tested against dofetilide-induced TdP.


RESULTS
Acute AV block caused a chaotic idioventricular rhythm and strikingly increased beat-to-beat variability of the RR and QT intervals. SEA0400 exaggerated the dofetilide-induced increase in the heart rate-corrected QT interval (QTc) and did not reduce the incidence of dofetilide-induced TdP [100% in the SEA0400 + dofetilide group vs. 75% in the dofetilide (100 nmol x L(-1)) control]. In the second set of experiments, verapamil further increased the dofetilide-induced QTc prolongation and neither verapamil nor lidocaine reduced the dofetilide-induced increase in the beat-to-beat variability of the QT interval. However, lidocaine decreased and verapamil prevented the development of dofetilide-induced TdP as compared with the dofetilide control (TdP incidence: 13%, 0% and 88% respectively).


CONCLUSIONS
Na+/Ca2+ exchanger does not contribute to dofetilide-induced TdP, whereas Na+ and Ca2+ channel activity is involved in TdP genesis in isolated, AV-blocked rabbit hearts. Neither QTc prolongation nor an increase in the beat-to-beat variability of the QT interval is a sufficient prerequisite of TdP genesis in rabbit hearts.
Farkas
Attila S
AS
2nd Department of Internal Medicine and Cardiology Centre, University of Szeged, Szeged, Hungary. farkasa@in2nd.szote.u-szeged.hu
Makra
Péter
P
Csík
Norbert
N
Orosz
Szabolcs
S
Shattock
Michael J
MJ
Fülöp
Ferenc
F
Forster
Tamás
T
Csanády
Miklós
M
Papp
Julius Gy
JG
Varró
András
A
Farkas
András
A
D000818:Animals; D020746:Calcium Channels, L-Type / Q000502:physiology*; D002908:Chronic Disease; D003326:Coronary Circulation / Q000187:drug effects; D005260:Female; D006327:Heart Block / Q000503:physiopathology; D006339:Heart Rate / Q000187:drug effects; D066298:In Vitro Techniques; D010627:Phenethylamines / Q000009:adverse effects*; D026902:Potassium Channel Blockers / Q000009:adverse effects*; D011817:Rabbits; D015222:Sodium Channels / Q000502:physiology*; D019831:Sodium-Calcium Exchanger / Q000502:physiology*; D013449:Sulfonamides / Q000009:adverse effects*; D016171:Torsades de Pointes / Q000139:chemically induced* / Q000378:metabolism / Q000503:physiopathology
D020746:Calcium Channels, L-Type; D010627:Phenethylamines; D026902:Potassium Channel Blockers; D015222:Sodium Channels; D019831:Sodium-Calcium Exchanger; D013449:Sulfonamides; C063533:dofetilide