Robust anti-arrhythmic efficacy of verapamil and flunarizine against dofetilide-induced TdP arrhythmias is based upon a shared and a different mode of action

Abstract

Background and purpose: The high predisposition to Torsade de Pointes (TdP) in dogs with chronic AV-block (CAVB) is well documented. The anti-arrhythmic efficacy and mode of action of Ca(2+) channel antagonists, flunarizine and verapamil against TdP were investigated.

Experimental approach: Mongrel dogs with CAVB were selected based on the inducibility of TdP with dofetilide. The effects of flunarizine and verapamil were assessed after TdP and in different experiments to prevent dofetilide-induced TdP. Electrocardiogram and ventricular monophasic action potentials were recorded. Electrophysiological parameters and short-term variability of repolarization (STV) were determined. In vitro, flunarizine and verapamil were added to determine their effect on (i) dofetilide-induced early after depolarizations (EADs) in canine ventricular myocytes (VM); (ii) diastolic Ca(2+) sparks in RyR2(R4496+/+) mouse myocytes; and (iii) peak and late I(Na) in SCN5A-HEK 293 cells.

Key results: Dofetilide increased STV prior to TdP and in VM prior to EADs. Both flunarizine and verapamil completely suppressed TdP and reversed STV to baseline values. Complete prevention of TdP was achieved with both drugs, accompanied by the prevention of an increase in STV. Suppression of EADs was confirmed after flunarizine. Only flunarizine blocked late I(Na). Ca(2+) sparks were reduced with verapamil.

Conclusions and implications: Robust anti-arrhythmic efficacy was seen with both Ca(2+) channel antagonists. Their divergent electrophysiological actions may be related to different additional effects of the two drugs.

Figure 1

Upper panel: anti-arrhythmic effects of flunarizine (suppression) against dofetilide-induced TdP (left) and ectopic activity (right; as single ectopic beats, SEB and multiple ectopic beats, MEB) is shown with an individual example (middle part) of lead II electrocardiogram (ECG) and left ventricular monophasic action potential (LV MAP) recordings (printed at 10 mm·s⁻¹ speed and calibrated at 1 mV per cm for ECG and 20 mV for the MAP recording) on scale paper in baseline (left), with TdP (middle) and after flunarizine. Lower panel illustrates continuous short-term variability (STV_LV) quantification for this experiment. *P < 0.05 versus baseline. TdP, Torsade de Pointes.

Figure 2

TdP prevention (upper panel) with flunarizine is presented in two serial experiments, first dofetilide alone (left) and with flunarizine pretreatment (right). The effects on QT/QTc (middle part) and short-term variability (STV_LV) in individuals as well as average (lower part) are plotted. TdP, Torsade de Pointes.

Figure 3

Anti-arrhythmic effects of flunarizine in isolated ventricular myocytes of the chronic AV-block (CAVB) dog are depicted. (A) 20 superimposed consecutive action potentials (APs) in baseline (left) and after flunarizine (right) as well as the time course of APD (dots) and short-term variability (STV_APD, continuous red line), baseline and with flunarizine perfusion are shown. (B) Similar, 20 superimposed APs in baseline (left), with dofetilide-induced EADs (arrow in middle panel) and after EADs suppression with flunarizine (right) and the temporal behaviour of APD and STV_APD are shown for this experiment. EADs, early after depolarizations.

Figure 4

Effects of flunarizine (left) and verapamil (right) on late I_Na: representative recordings of late I_Na from a single cell in the absence of drug (black line), during superfusion with 3 nM ATX-II (ATX, grey line) and during superfusion with 1 µM flunarizine (left; red line) or 10 µM verapamil (right; blue line). Insets: expanded traces (last 50 ms following depolarizing pulse) of late I_Na in the absence (black line) and presence of flunarizine (red line) or verapamil (blue line) respectively.

Figure 5

(A) Left, representative line-scan images of spontaneous Ca²⁺ sparks recorded in a RyR2^R4496C+/+ ventricular myocyte in the absence (top) or presence (bottom) of 1 µM flunarizine. Right, Ca²⁺ spark occurrence before (control) and during flunarizine (n= 8 cells). (B) Similar, images of spontaneous Ca²⁺ sparks in the absence (top) or presence (bottom) of 10 µM verapamil. Right panel shows the average data in control and with verapamil (n= 11 cells). *P < 0.05 versus control. RyR, ryanodine receptor.

Figure 6

Effects of flunarizine (left), verapamil (middle) and the combination of verapamil and lidocaine on baseline short-term variability (STV_LV). Effects of these drugs on baseline STV_LV is shown for individual animals (thin lines) and as a mean value for each group (thick line).