Enhanced Late Na and Ca Currents as Effective Antiarrhythmic Drug Targets

Figures

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  Figure 1. Enhanced INa-L, initiation of EADs and their selective suppression with specific INa-L blockers (A) shows increased INa-L induced with ATXII in isolated guinea-pig ventricular myocyte initiating EADs and triggered activity (B) (From Isenberg and Ravens, 1984). (C) Shows absence of effect of the selective INa-L blocker, GS-967 on peak INa induced with ATXII in isolated rabbit ventricular myocyte while selectively inhibiting the INa-L (D). (From Belardinelli et al., 2013) (E) Shows 50% reduction of peak INa in WT and (F) in LQT3 mutant Na channel (ΔKPQ) in HEK cells with sub-therapeutic (0.1 μM) concentrations of flecainide while also causing non-specific reduction in the INa−L, (insets) (From Nagatomo et al., 2000). (G) Shows mexiletine's concentration-dependent reduction of the peak INa in isolated rabbit ventricular myocytes with no specific inhibitory effect on the INa-L (From Gao et al., 2013).

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  Figure 2. Enhanced late ICa,Land its selective block with roscovitine but not with verapamil. (A–C) show the time course of the peak and the pedestal (ped) ICa,L during a 300-ms voltage clamp pulse to 0 mV (voltage protocol shown in B). (B). Voltage clamp pulse (above) and superimposed current traces showing ICa,L before (black) and ~5 min after perfusion of 1 mmol/L H2O2 (red). The difference current is shown in the bottom trace. (C), Same as in (B) but with an AP clamp waveform replacing the square voltage clamp pulse (From Xie et al., 2009). (D) Shows an iPSC-derived cardiomyocyte from a patient with TS showing increased late ICa,L (From Yazawa et al., 2011). (E) Shows selective block of the late ICa,L with roscovitine in a cardiomyocyte derived form a TS patient without affecting the peak ICa,L (From Yazawa et al., 2011). (F) Shows lack of selective block of the late ICa,L with verapamil while causing severe depression of the peak ICa,L (From Nawrath et al., 1998).

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  Figure 3. Late ICa,L antiarrhythmic modifications. Class IV antiarrhythmics block the L-type channels indiscriminately reducing peak and late ICa,L, suppressing EC-coupling. ICa,L gating modifications identified by dynamic clamp studies (Madhvani et al., 2011, 2015) specifically reduce late ICa,L, potently abolishing EADs without affecting maximum Cav1.2 open probability; therefore peak ICa,L remains largely preserved. These maneuvers include: (1) pedestal reduction from 10 to 4%; (2) a 5 mV depolarizing shift of steady-state activation; (3) a 5 mV hyperpolarizing shift of SS-inactivation. Roscovitine enantiomers can reduce late ICa,L (pedestal).

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  Figure 4. Oxidative stress with angiotensin II (ANG II) induces early afterdepolarizations (EADs), VT/VF in an aged but not young rat hearts. In all (A–C), simultaneous microelectrode (ME) and pseudo-ECG recordings in a young heart (A) and an aged heart (B,C) exposed to ANG II (2 mol/l). A: representative experiment in a young heart showing that sinus rhythm persists throughout 60 min of ANG II perfusion. (B,C) Recordings from an aged heart showing the emergence of EADs, triggered activity, and VF over the indicated time course. (D) Three isochronal activation maps, with the first being the last sinus beat (beat 1) before the onset of the VF shown at the bottom and indicated as 1 in the ECG (bottom). The sinus beat was followed by two consecutive focal activations that arose from the base of the heart (beats 2 and 3). The red arrows in (D) indicate the direction of propagation. The focal activation lasted for 8–10 beats (the initial VT phase) and then degenerated to VF, as shown in (E) with four optical action potentials (APs; labeled 1–4) shown in the adjacent schema (From Bapat et al., 2012).

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  Figure 5. Spontaneous initiation of ventricular tachycardia (VT)/VF in an aged rat heart exposed to 0.1 mM H2O2. (A) ECG showing the last 5 sinus beats before the sudden onset of VT leading to VF. (B) voltage snapshots of the last beat of the VT (beat 1) and of the first 2 beats of the VF (beats 2 and 3). In each snapshot, activation time (in ms) is shown at the bottom right with time 0 (arbitrary) coinciding with the onset of beat 1. The red color in the snapshots represents depolarization (Dep) and the blue repolarization (Rep) as shown in (E). The yellow arrows in the snapshots represent the direction of the wavefront propagation with double horizontal lines denoting the site of conduction block. The VT originates from a focal site at the LV base and propagates as single wavefront toward the apex and undergoes functional conduction block at site 3. The two lateral edges of the front, however, continue to propagate laterally (snapshot, 98 ms) forming figure-eight reentry (snapshot, 108 ms). During the second reentrant wavefront, another wavefront emerges from the apical site of the LV (snapshot, 122 ms), disrupting the activation pattern and signaling the onset of VF. (D) 3 optical…

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  Figure 6. Spontaneous initiation of VF in a middle-aged rabbit exposed to 0.1 mM H2O2. (A) Six epicardial optical APs (V1–V6) recorded from sites shown on the left silhouette of the heart. After nine focal activations arising from the base of the heart (B), the wavefront undergoes block at mid-LV anterior wall after spatially discordant APD alternans emerges (A). The wavefront, however, continues to propagate lateral to the site of block in a clockwise direction causing a reentrant excitation as shown in the snapshots in (B). The numbers under each snapshot is activation time starting with 0 ms (arbitrary) for beats 3 and 4 and then again for beats 10 and 11. (C) microelectrode recording of another VF episode recorded in the same heart showing spontaneous initiation of VF by a mechanism compatible with early afterdepolarization (EAD)-mediated triggered activity (TA) as in the aged rat hearts shown in Figure 6 (From Morita et al., 2009).

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  Figure 7. Suppression of aconitine-induced ventricular tachycardia/ventricular fibrillation (VT/VF) with the specific INa-L blocker, GS967(1 μM) and reversal of its effect on washout. (A) Aconitine-induced VF, which converts to monomorphic VT 3 min after arterial perfusion of GS967 (B) and to sinus rhythm 7 min after GS967 (C). Washout of GS967 with drug-free Tyrode's perfusion causes the reemergence of monomorphic VT that degenerates to VF 8 min after GS967 washout (D). The reintroduction of GS967 in the perfusate suppresses the VF by first converting it to monomorphic VT and then to sinus rhythm (E). Note the marked shortening of the action potential duration after GS967-induced conversion of the monomorphic VT to sinus rhythm despite lengthening of the cycle length (C, E) (from Pezhouman et al., 2014b).

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  Figure 8. Effect of GS967 on use-dependent action potential duration (APD), action potential amplitude (APA), and APD restitution. (A) Baseline microelectrode recordings of action potentials at two pacing cycle lengths (PCLs): 200 ms (top) and100 ms (bottom). (B) Effects of GS967 at these two PCLs, causing 2 and 7% reduction in APA and 32 and 48% reduction in APD, respectively. (C) Superimposed APs showing that GS967 causes greater shortening of APD than APA at faster rates (use-dependent). (D) Mean percent shortening of APA and APD as a function of PCL. (E) Effect of GS967 on slope of dynamic APD restitution curves both at baseline and after aconitine. (F) Superimposed APs at baseline, after aconitine injection just before initiation of VT/VF and after GS967 (From Pezhouman et al., 2014b). *p < 0.01; †p < 0.05.

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  Figure 9. Schema of positive feedback loops promoting intracellular Na and Ca overload, CaMKII activation, and EADs during stress induced with hydrogen peroxide, angiotensin II, and hypokalemia. The enhanced late inward currents potentiate CaMKII activity (positive feedback loop) by prolonging APD and promoting EADs leading to VT/VF. APD indicates action potential duration; CaMKII, Ca-calmodulin kinase II; EAD, early afterdepolarization.