Diastolic transient inward current in long QT syndrome type 3 is caused by Ca2+ overload and inhibited by ranolazine.
Long QT syndrome variant 3 (LQT-3) is a channelopathy in which mutations in SCN5A, the gene coding for the primary heart Na(+) channel alpha subunit, disrupt inactivation to elevate the risk of mutation carriers for arrhythmias that are thought to be calcium (Ca(2+))-dependent. Spontaneous arrhythmogenic diastolic activity has been reported in myocytes isolated from mice harboring the well-characterized Delta KPQ LQT-3 mutation but the link to altered Ca(2+) cycling related to mutant Na(+) channel activity has not previously been demonstrated. Here we have investigated the relationship between elevated sarcoplasmic reticulum (SR) Ca(2+) load and induction of spontaneous diastolic inward current (I(TI)) in myocytes expressing Delta KPQ Na(+) channels, and tested the sensitivity of both to the antianginal compound ranolazine. We combined whole-cell patch clamp measurements, imaging of intracellular Ca(2+), and measurement of SR Ca(2+) content using a caffeine dump methodology. We compared the Ca(2+) content of Delta KPQ(+/-) myocytes displaying I(TI) to those without spontaneous diastolic activity and found that I(TI) induction correlates with higher sarcoplasmic reticulum (SR) Ca(2+). Both spontaneous diastolic I(TI) and underlying Ca(2+) waves are inhibited by ranolazine at concentrations that preferentially target I(NaL) during prolonged depolarization. Furthermore, ranolazine I(TI) inhibition is accompanied by a small but significant decrease in SR Ca(2+) content. Our results provide the first direct evidence that induction of diastolic transient inward current (I(TI)) in Delta KPQ(+/-) myocytes occurs under conditions of elevated SR Ca(2+) load.
Department of Pharmacology, Columbia University College of Physicians and Surgeons, 630 West 168th St., New York, NY 10032, USA.
N
N
Lindegger
B M
BM
Hagen
A R
AR
Marks
W J
WJ
Lederer
R S
RS
Kass
D000083:Acetanilides / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects*; D003971:Diastole / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D015640:Ion Channel Gating / Q000187:drug effects*; D008133:Long QT Syndrome / Q000503:physiopathology*; D051379:Mice; D009154:Mutation / Q000235:genetics; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology*; D000069458:Ranolazine
D000083:Acetanilides; D010879:Piperazines; D000069458:Ranolazine; D002118:Calcium