Characterization and influence of cardiac background sodium current in the atrioventricular node

Hongwei Cheng,Jue Li,Andrew F James,Shin Inada,Stéphanie C.M Choisy,Clive H Orchard,Henggui Zhang,Mark R Boyett,Jules C Hancox

doi:10.1016/j.yjmcc.2016.04.014

Hongwei Cheng, Jue Li + Show 7 more

Open Access

https://doi.org/10.1016/j.yjmcc.2016.04.014

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Abstract

Background inward sodium current (IB,Na) that influences cardiac pacemaking has been comparatively under-investigated. The aim of this study was to determine for the first time the properties and role of IB,Na in cells from the heart's secondary pacemaker, the atrioventricular node (AVN). Myocytes were isolated from the AVN of adult male rabbits and mice using mechanical and enzymatic dispersion. Background current was measured using whole-cell patch clamp and monovalent ion substitution with major voltage- and time-dependent conductances inhibited. In the absence of a selective pharmacological inhibitor of IB,Na, computer modelling was used to assess the physiological contribution of IB,Na. Net background current during voltage ramps was linear, reversing close to 0mV. Switching between Tris- and Na+-containing extracellular solution in rabbit and mouse AVN cells revealed an inward IB,Na, with an increase in slope conductance in rabbit cells at −50mV from 0.54±0.03 to 0.91±0.05nS (mean±SEM; n=61 cells). IB,Na magnitude varied in proportion to [Na+]o. Other monovalent cations could substitute for Na+ (Rb+>K+>Cs+>Na+>Li+). The single-channel conductance with Na+ as charge carrier estimated from noise-analysis was 3.2±1.2pS (n=6). Ni2+ (10mM), Gd3+ (100μM), ruthenium red (100μM), or amiloride (1mM) produced modest reductions in IB,Na. Flufenamic acid was without significant effect, whilst La3+ (100μM) or extracellular acidosis (pH6.3) inhibited the current by >60%. Under the conditions of our AVN cell simulations, removal of IB,Na arrested spontaneous activity and, in a simulated 1D-strand, reduced conduction velocity by ~20%. IB,Na is carried by distinct low conductance monovalent non-selective cation channels and can influence AVN spontaneous activity and conduction.

Highlights

The atrioventricular node (AVN) is normally the only site through which electrical activity can pass from atria to ventricles; slow conduction through the AVN facilitates completion of atrial contraction prior to that of the ventricles [1,2,3]
The results of this study demonstrate that an IB,Na is present in AVN cells and that it has the ability to make a substantial contribution to AVN cell electrophysiology
The time-independence of the currents observed during voltage steps enables the use of a voltage-ramp protocol to survey background current rapidly across a wide range of potentials

Summary

Introduction

The atrioventricular node (AVN) is normally the only site through which electrical activity can pass from atria to ventricles; slow conduction through the AVN facilitates completion of atrial contraction prior to that of the ventricles [1,2,3]. The AVN possesses pacemaking properties and should the sinoatrial node (SAN) fail or normal conduction become impaired, the AVN can take over pacemaking of the ventricles [2,3]. In the heart's primary pacemaker, the sinoatrial node, the cellular basis of pacemaking is established to involve both calcium and membrane ‘clocks’, with spontaneous rate influenced by cellular Ca2+ dynamics and by multiple sarcolemmal ionic currents [4,5]. The cellular electrophysiological basis of AVN pacemaking is incompletely understood, though it is clear that this is likely to involve multiple ionic conductances [6,7,8]. There is evidence from both rabbit and dog preparations that intracellular Ca2+ cycling influences AVN pacemaking rate [11,12,13,14], whilst Cav1.3 and 3.1 have been implicated in mouse AVN pacemaking [8]

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