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Fig. 2 Characteristics of retrograde conduction in the zebrafish heart at 48 hpf. (A,B) Calcium activation maps for normal anterograde cardiac cycle (A) and spontaneous premature ventricular beat (B). Both anterograde and retrograde AV delay are evident as compressed spacing of isochronal lines (100 msecond intervals). Scale bars: 50 μm. (C) Contemporaneous recordings of atrial and ventricular contraction using intensity-time plots from regions over the respective chambers, during ventricular pacing at 48 hpf. The ladder diagram depicts the resulting Wenckebach-type retrograde ventriculo-atrial block.