Cellular mechanisms of ventricular arrhythmias in a mouse model of Timothy syndrome (long QT syndrome 8)

Abstract Ca 2 + flux through l -type CaV 1.2 channels shapes the waveform of the ventricular action potential (AP) and is essential for excitation–contraction (EC) coupling. Timothy syndrome (TS) is a disease caused by a gain-of-function mutation in the CaV 1.2 channel (CaV 1.2-TS) that decreases inactivation of the channel, which increases Ca 2 + influx, prolongs APs, and causes lethal arrhythmias. Although many details of the CaV 1.2-TS channels are known, the cellular mechanisms by which they induce arrhythmogenic changes in intracellular Ca 2 + remain unclear. We found that expression of CaV 1.2-TS channels increased sarcolemmal Ca 2 + “leak” in resting TS ventricular myocytes. This resulted in higher diastolic [Ca 2 + ]i in TS ventricular myocytes compared to WT. Accordingly, TS myocytes had higher sarcoplasmic reticulum (SR) Ca 2 + load and Ca 2 + spark activity, larger amplitude [Ca 2 + ]i transients, and augmented frequency of Ca 2 + waves. The large SR Ca 2 + release in TS myocytes had a profound effect on the kinetics of CaV 1.2 current in these cells, increasing the rate of inactivation to a high, persistent level. This limited the amount of influx during EC coupling in TS myocytes. The relationship between the level of expression of CaV 1.2-TS channels and the probability of Ca 2 + wave occurrence was non-linear, suggesting that even low levels of these channels were sufficient to induce maximal changes in [Ca 2 + ]i . Depolarization of WT cardiomyocytes with a TS AP waveform increased, but did not equalize [Ca 2 + ]i , compared to depolarization of TS myocytes with the same waveform. We propose that CaV 1.2-TS channels increase [Ca 2 + ] in the cytosol and the SR, creating a Ca 2 + overloaded state that increases the probability of arrhythmogenic spontaneous SR Ca 2 + release.