Characterization of novel arrhythmogenic patterns arising secondary to heterogeneous expression and activation of Nav1.8

Abstract

Background: Previous studies suggested that SCN10A/Nav1.8 may influence cardiac electrophysiology and the susceptibility to cardiac arrhythmias. Notably, the expression of SCN10A is not uniform, showing variable expression in each cardiac chamber. The present study aims to explore the functional significance of Nav1.8 expression among different cell types present in the ventricular myocardium.

Methods: The effect of the specific Nav1.8 blocker, A-803467, on action potential was recorded from epicardial, mid-myocardial (M cells) and Purkinje tissue slices isolated from the canine left ventricle using standard microelectrode techniques and on late sodium current from Purkinje cells using patch-clamp techniques.

Results: A-803467 treatment did not significantly affect maximum diastolic potential, action potential amplitude or maximum rate of rise of the action potential upstroke in epicardial cells, M cells or Purkinje fibers. Action potential duration (APD) was also unaffected by A-803467 in epicardial cells. However, administration of 1,000 nmol/L A-803467 reduced APD₃₀, APD₅₀, and APD₉₀ during relatively slow pacing rates of 0.2 and 0.5 Hz in M cells. In Purkinje fibers, A-803467 (100 and 1,000 nmol/L) substantially abbreviated APD₅₀ and APD₉₀ at slow pacing rates (0.2 and 0.5 Hz). Moreover, 100 nmol/L A-803467 significantly inhibited the development of early afterdepolarizations induced by 10 nmol/L ATX-II (7/8 vs. 2/8, p < 0.05) as well as the amplitude of late sodium current at 0.2 Hz in Purkinje cells.

Conclusions: The functional significance of Nav1.8 varies among different types of ventricular and conduction system cardiomyocytes. The reduction in I_Na,L and APD, as well as suppression of early afterdepolarizations by Nav1.8 block in Purkinje fibers suggests Nav1.8 as a potential therapeutic target for bradycardia-dependent arrhythmias.

Keywords: Purkinje fibers; SCN10A; arrhythmia; cardiac electrophysiology; pharmacology.

Figure 1

Effect of A-803467 (3–1,000 nmol/L) administration on action potential properties in canine left ventricular epicardial cardiomyocytes (N = number of dogs; n = number of tissue slices). (A–F) Average data for action potential amplitude (APA), maximum diastolic potential (MDP), maximum rate of rise of the action potential upstroke (Vmax), action potential duration at 30%, 50% and 90% repolarization (APD₃₀, APD₅₀ and APD₉₀), before (control) and after administration and washout of A-803467 (N = 5 and n = 5, respectively).

Figure 2

Effect of A-803467 (3–1,000 nmol/L) administration on action potential properties in canine left ventricular M cardiomyocytes (N = number of dogs; n = number of tissue slices). (A–F) Average data for action potential amplitude (APA), maximum diastolic potential (MDP), maximum rate of rise of the action potential upstroke (Vmax), action potential duration at 30%, 50% and 90% repolarization (APD₃₀, APD₅₀ and APD₉₀), before (control) and after administration and washout of A-803467. *p < 0.05 vs. control at 0.5 Hz, **p < 0.01 vs. control at 0.5 Hz, ^#p < 0.05 vs. control at 0.2 Hz, ^##p < 0.01 vs. control at 0.2 Hz (N = 10 and n = 10, respectively).

Figure 3

Effect of A-803467 (3–1,000 nmol/L) administration on action potential properties in canine left ventricular Purkinje fibers (N = number of dogs; n = number of tissue slices). (A–F) Average data for action potential amplitude (APA), maximum diastolic potential (MDP), maximum rate of rise of the action potential upstroke (Vmax), action potential duration at 30%, 50% and 90% repolarization (APD₃₀, APD₅₀ and APD₉₀), before (control) and after administration and washout of A-803467. *p < 0.05 vs. control at 0.5 Hz, **p < 0.01 vs. control at 0.5 Hz, ^#p < 0.05 vs. control at 0.2 Hz, ^##p < 0.01 vs. control at 0.2 Hz (N = 6 and n = 6, respectively).

Figure 4

A-803467 inhibits early afterdepolarizations (EADs) and Nav1.8 induced I_Na,L in Purkinje fibers and cells (N = number of dogs; n = number of tissue slices or cells). (A) The incidence of EADs in Purkinje fibers exposed to ATX-II (10 nmol/L) alone or in combination with 100 nmol/L A-803467. The inset shows an example of A-803467 suppressing the EAD elicited by exposure to ATX-II at 0.5 Hz. *p < 0.05 vs. control + ATX (N = 8 and n = 8, respectively). (B) 100 nmol/L A-803467 inhibits SCN10A-mediated I_Na,L in transfected ND7/23 cells. (C) Summary data of relative I_Na,L at −20 mV in the absence (control) and presence of 100 nmol/L A-803467 in Purkinje cells. ^#p < 0.05 vs. control (N = 6 and n = 8, respectively).