Fig.5. Spontaneous Ca²⁺ oscillation gives rise to EAD.

Panel A shows simultaneous recording of AP (upper trace) and Ca²⁺ concentration (lower trace) under sAP-clamp (n=9cells/5animals). Grey and black lines show traces recorded in the absence and the presence of 100 nM ATX-II, respectively. ATX-II lengthened APD but EADs were seen only when Ca²⁺ was not buffered. Dashed line in panel A shows that the rise in Ca²⁺ signal precedes the turning point of membrane potential indicating that Ca²⁺ drives EAD. Panel B shows that when Ca²⁺ was clamped to 100 nM using BAPTA (n=6cells/3animals), ATX-II lengthened AP but no EAD was observed (grey line control, black line ATX-II). Statistical analysis of the Ca²⁺ transients (Panel C,) reveal that systolic, but not diastolic Ca²⁺ concentration was increased by ATX-II at 0.2 Hz (n=9cells/5animals) and 1.0 Hz (n=8cells/5animals) pacing rate. Panel D shows the percent of cells developing various forms of arrhythmogenic APs in the presence of 100 nM ATX-II. When Ca²⁺ cycling was not buffered (n=11cells/6animals), all cells developed long APD and EADs and some developed DADs and TAPs. When Ca²⁺ was buffered with BAPTA (n=6cells/3animals), the cells developed long APD, but no EAD, DAD or TAP was seen.