Diastolic transient inward current in Long QT Syndrome type 3 is caused by Ca2+ overload and inhibited by Ranolazine

摘要

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 (Ca²⁺)-dependent. Spontaneous arrhythmogenic diastolic activity has been reported in myocytes isolated from mice harboring the well-characterized ΔKPQ LQT-3 mutation but the link to altered Ca²⁺ cycling related to mutant Na⁺ channel activity has not previously been demonstrated. Here we have investigated the relationship between elevated sarcoplasmic reticulum (SR) Ca²⁺ load and induction of spontaneous diastolic inward current (ITI) in myocytes expressing ΔKPQ Na⁺ channels, and tested the sensitivity of both to the antianginal compound ranolazine. We combined whole cell patch clamp measurements, imaging of intracellular Ca²⁺, and measurement of SR Ca²⁺ content using a caffeine dump methodology. We compared the Ca²⁺ content of ΔKPQ^+/- myocytes displaying ITI to those without spontaneous diastolic activity and found that ITI induction correlates with higher sarcoplasmic reticulum (SR) Ca²⁺. Both spontaneous diastolic ITI and underlying Ca²⁺ waves are inhibited by ranolazine at concentrations that preferentially target INaL during prolonged depolarization. Furthermore, ranolazine ITI inhibition is accompanied by a small but significant decrease in SR Ca²⁺ content. Our results provide the first direct evidence that induction of diastolic transient inward current (ITI) in ΔKPQ^+/- myocytes occurs under conditions of elevated SR Ca²⁺ load.