Mechanisms underlying atrial-selective block of sodium channels by Wenxin Keli: Experimental and theoretical analysis

Abstract

Introduction: Atrial-selective inhibition of cardiac sodium channel current (INa) and INa-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study was designed to examine the basis for the atrial-selective actions of Wenxin Keli.

Methods: Whole cell INa was recorded at room temperature in canine atrial and ventricular myocytes. Trains of 40 pulses were elicited over a range of pulse durations and interpulse intervals to determine tonic and use-dependent block. A Markovian model for INa that incorporates interaction of Wenxin Keli with different states of the channel was developed to examine the basis for atrial selectivity of the drug.

Results: Our data indicate that Wenxin Keli does not bind significantly to either closed or open states of the sodium channel, but binds very rapidly to the inactivated state of the channel and dissociates rapidly from the closed state. Action potentials recorded from atrial and ventricular preparations in the presence of 5g/L Wenxin Keli were introduced into the computer model in current clamp mode to simulate the effects on maximum upstroke velocity (Vmax). The model predicted much greater inhibition of Vmax in atrial vs. ventricular cells at rapid stimulation rates.

Conclusion: Our findings suggest that atrial selectivity of Wenxin Keli to block INa is due to more negative steady-state inactivation, less negative resting membrane potential, and shorter diastolic intervals in atrial vs. ventricular cells at rapid activation rates. These actions of Wenxin Keli account for its relatively safe and effective suppression of atrial fibrillation.

Keywords: Antiarrhythmic drugs; Arrhythmias; Atria; Electrophysiology; Pharmacology; Ventricles.

Figure 1

Rate-dependent inhibition of peak sodium channel current (I_Na) in atrial (A and B, n=4) and ventricular (C and D; n=5) cells in response to trains of pulses, −80 mV to −30 mV, 5 (A and C) or 200 ms (B and D) duration delivered with a diastolic interval of 150 ms, in the presence of 10 g/L Wenxin Keli. Peak I_Na recorded in the presence of Wenxin Keli was normalized to peak I_Na recorded during the same trains under control conditions.

Figure 2

Decrease of the peak sodium channel current (I_Na) during the train of 200 ms pulses delivered from −80 mV to −30 mV in the presence of 10 g/L Wenxin Keli and normalized to peak I_Na recorded during the same train in control conditions. Data obtained in atrial cells (top panels; n=4) and in ventricular cells (bottom panels; n=5) using interpulse intervals of 150 ms (A and D), 50 ms (B and E), and 20 ms (C and F).

Figure 3

Effect of Wenxin Keli (WX) on sodium channel availability. Shift of h_∞(V) in the presence of 5 g/L (circles) and 10 g/L WK (triangles) recorded in atrial cells (Panel A) and in ventricular cells (Panel B). Panel C shows the experimentally recorded values of the shift of h_∞(V) in atrial cells (circles) and ventricular cells (squares) together with the best fitted “Bean’s” function[15] (solid line), yielding dissociation constants for WK interaction with the closed state of the channel (~1500 g/L) and with the inactivated state of the channel (~10 g/L).

Figure 4

Effect of Wenxin Keli (WX) on sodium channel activation. There is no change of V_1/2 in the presence of 5 g/L (circles) and 10 g/L WK (triangles) recorded in canine atrial cells (Panel A) and in HEK293 cells expressed with sodium current (Panel B).

Figure 5

Computer simulated decrease of the peak sodium channel current (I_Na) during the train of pulses (5 and 200 ms duration) delivered from −80 mV to −30 mV in the presence of 10 g/L Wenxin Keli using diastolic interval of 150 ms. Peaks of I_Na simulated in the presence of Wenxin Keli were normalized to peaks of I_Na simulated during the same trains in control conditions. Data were simulated using an atrial sodium channel model (top panels; n=4) and using a ventricular sodium channel model (bottom panels; n=5) using pulse durations of 5 ms (A and C) and 200 ms (B and D). Compare with Figure 1.

Figure 6

Computer simulation of decrease in peak sodium channel current (I_Na) during trains of 200 ms pulses delivered from 80 mV to 30 mV in the presence of 10 g/L Wenxin Keli and normalized to peak I_Na simulated during the same train in control conditions. Data were simulated using an atrial sodium channel model (top panels; n=4) and a ventricular sodium channel model (bottom panels; n=5) using interpulse intervals of 150 ms (A and D), 50 ms (B and E), and 20 ms (C and F). Compare with Figure 2.

Figure 7

Comparison of experimentally recorded (data points) and model-generated (lines) steady-state inactivation in atrial (top panels) and ventricular cells (bottom panels) in control (A and D) and in the presence of 5 g/L (B and E) and 10 g/L (C and F) Wenxin Keli.

Figure 8

Simulation of I_Na block by Wenxin Keli using action potential (AP) clamp at 37°C. Trains of APs experimentally recorded from ventricular (right) and atrial (left) cells were applied as a command potential to simulate additional block of I_Na by 5 g/L Wenxin Keli after basic cycle length was changed from 500 to 300 ms in ventricular and atrial cells. From top to bottom: Atrial or ventricular APs, simulated open state probability (~V_max) and fraction of blocked channels.

Figure 9

Panel A shows the partial ionic composition of Wenxin Keli solution (10 g/L) indicating the presence of relatively high concentration of fluorine ions. Panels B and C compare the effects of Wenxin Keli (5 g/L) and fluorine (3 and 20 mg/L) on atrial and ventricular action potential duration (APD) and on effective refractory period (ERP), and post-repolarization refractoriness (PRR) at a cycle length of 500 ms. Dashed lines depict the duration of PRR. PRR was approximated by the difference between ERP and APD measured at 70% (APD₇₀) in atria, and between ERP and APD measured at 90% repolarization (ADP₉₀) in ventricles. Note that ERP corresponds to APD measured at 70–75% repolarization (ADP_70–75) in atria and to APD₉₀ in ventricles. * - p<0.05 vs. control. ** - <0.001 vs. control. † - p <0.001 vs. APD₇₀. Panel B is modified, with permission, from Burashnikov et al.⁴