NS5806 Induces Electromechanically Discordant Alternans and Arrhythmogenic Voltage-Calcium Dynamics in the Isolated Intact Rabbit Heart

Figures

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  Figure 1. Experimental protocol and representative examples of ECG changes induced by NS5806. (A) Langendorff-perfused rabbit hearts with ECG and pacing electrodes (top) and experimental protocol (bottom). Hearts were perfused with Tyrode’s solution. After stabilization and BL recording, hearts were perfused with NS5806 first. Additional drug studies followed: VP and SEA. (B) Representative examples of ECG changes induced by NS5806: BrS-like ECG features, bigeminy, T-wave alternans, VT and VF episodes. All were non-sustained. VF episodes were only observed with pacing. (C) ST segment elevation, QRS duration, and PR interval in different conditions. NS5806 induced QRS widening, ST-segment elevation, and PR prolongation. NS5806 + SEA prolonged PR interval. NS5806 + VP increased ST elevation. Box = 25th and 75th percentiles. Bars = minimum and maximum values. Dots = outliers. p < 0.05 was considered statistically significant. BL, Baseline; VP, Verapamil; SEA, SEA0400.

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  Figure 2. Effects of NS5806 and additional drugs on action potential (AP) and Ca transient dynamics. (A) APD50 (at 50% of repolarization) and APD80 at BL, 5, 15, and 45 μM of NS5806. At 45 μM, both APD50 and APD80 were prolonged. (B) APD50, conduction velocity (CV), and Ca exponential decay (TAU) during pacing at 3 Hz in different conditions. NS5806 significantly increased APD50, reduced CV, and increased TAU. NS5806 + SEA significantly reduced APD50 and increased CV compared to NS5806. NS5806 + VP reduced APD50 compared to NS5806. n = 20 animals in BL and NS5806 group, n = 5 animals in NS5806 + VP and NS5806 + SEA group. Box = 25th and 75th percentiles. Bars = minimum and maximum values. Dots = outliers. p < 0.05 was considered statistically significant. (C) Superimposed AP and Ca tracings during pacing at 3 Hz in different conditions. (D) APD50 and CV restitution in different conditions. NS5806 reached longer range of DI with an APD restitution slope > 1. NS5806 failed to maintain 1:1 capture at shorter DIs. NS5806 dropped CV by half and led to a nearly flat CV restitution curve. DI, Diastolic interval. Other abbreviations as in Figure 1.

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  Figure 3. APD and isochronal maps of paced propagations under different conditions. Spatial variations in APD and CV occurred after infusion of NS5806, particularly at high pacing frequency, which were modulated by the addition of other drugs. This heterogeneous phenomena were augmented by VP and mitigated by SEA. At faster rates, CV slowing and APD prolongation became more apparent in the RV (to the right of the dotted line that represents the left anterior descending artery). Of note, APD maps are shown in same time scale to appreciate the APD prolongation with color.

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  Figure 4. APD rate adaptation. An example of pause-dependent APD shortening during NS5806 infusion (A), APD restitution curves (B), and APD and isochronal map (C) for long DI beats (beat 1 at BL and beat 2 after NS5806). Dotted line marks location of left anterior descending artery. During NS5806 infusion, APD shortening – rather than prolongation – was noted following very long (>1 s) pauses after pacing bursts. (D) Phase diagrams of Ca and APD at BL and under different drugs. In BL control conditions, the first paced beat following a long DI had a long APD. Subsequent paced beats led to APD and Ca alternans, most commonly in phase (AP/Ca concordant). During NS5806 infusion, the first paced beat following a long DI had a short APD, while the Ca transient had normal rate-dependence and was large after long DIs. Thus, the short first APD had still a large Ca transient, which led to APD/Ca alternans discordance after NS5806 administration. NS5806-induced AP/Ca-discordant alternans was unaffected by SEA, but was eliminated by VP.

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  Figure 5. Alternans dynamics. At BL, pacing-induced alternans was predominantly electro-mechanically (AP/Ca) and spatially concordant (A). Faster pacing made alternans spatially discordant with a nodal line (blue dotted line) perpendicular to propagation (black arrow) (B). After NS5806 infusion, alternans was AP/Ca discordant from its onset (C). Faster pacing made alternans spatially discordant with a nodal line not following propagation direction (D). (E) The occurrence of alternans and its spatial and AP/Ca concordance at maximum capturing pacing rate. Con Alt, concordant alternans; Dis Alt, discordant alternans. (F) The cycle length of alternans at onset and maximum capturing pacing rate in different conditions. Lines show mean ± SD. p < 0.05 was considered statistically significant.

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  Figure 6. Spontaneous NS5806-induced alternans is AP/Ca and spatially discordant. (A) ECG of spontaneous T-wave alternans (top), long-short-long-short APD alternans (mid), and small-large-small-large Ca transient amplitude alternans (bottom). (B) Depolarization isochronal maps show a minor alternation in propagation velocity (particularly towards the RV), but the repolarization sequence was dramatically different in alternating beats. Dotted line marks location of left anterior descending artery. (C) Alternans maps show AP/Ca and spatial discordance.

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  Figure 7. Examples of SEA suppresses NS5806-induced ectopy. (A) NS5806-induced spontaneous PVCs with frequent bigeminy. AP and Ca tracing during NS5806 infusion (top), APD and Ca versus DI (mid), and APD and isochronal map (bottom) for beat 1 and beat 2 as indicated in AP tracing (top). Long DIs were followed by short APDs. However, the amplitude of the Ca transient of these beats was consistently large. Ectopic beats initiated at sites in which the Ca transient was still on-going (~30.9% of maximum amplitude) while the AP had already terminated. Dotted line marks location of left anterior descending artery. Of note, APD maps are shown in different time scale to appreciate the spatial distribution with color. (B,C) SEA abolished NS5806-induced ventricular bigeminy and ectopic VT. Arrows indicate the high Ca load sites. Ectopic beats initiated with Ca-driven phase-4 depolarization via NCX. NCX blockade with SEA eliminated phase-4 depolarization.