Dual Pathway Electrophysiology In The Atrioventricular Node And Its ...

Cav1.2, an L-type calcium channel, is the most abundantly expressed calcium ion channel in the AM and VM. It is also expressed in the AVN cells but at much lower levels.28,37 In the absence of sodium channels, it has been theorised that ion channels with a more negative activation threshold are required for cellular excitation. Cav1.3, another L-type calcium channel, meets this requirement.38 Cav1.3 mRNA is expressed at significantly higher levels in the human AVN, particularly in the INE, CN and PB regions, relative to AM and VM.27,28 Together, Cav1.2 and Cav1.3 contribute to ICa,L in the AVN.

Cav3.1 and Cav3.3 are the major T-type calcium ion channels in the human AVN. Cav3.2 mRNA was reported to be undetectable in the human heart.39 Cav3.1 is highly expressed in the AVN, particularly in CN, PB and INE cells, at levels significantly higher than in AM and VM.27,28 Cav3.3 has a more uniform distribution through the AVN region and its expression is also comparable to AM and VM.27,28

Calcium Handling Proteins

In the working myocardium, the primary purpose of calcium handling proteins in the cells is for facilitating contraction. However, this is not the case in the nodal and conduction system cells. It has been proposed that the purpose of calcium handling proteins in AVN (and sinoatrial node) cells is primarily to regulate the calcium clock and the pacemaking function of these cells.40 The level of sodium calcium exchanger isoform 1 mRNA was reported to be similar in AVN tissue relative to AM and VM; however, there was a slight upregulation in the CN and a small downregulation in the INE regions specifically.27,28 The sarcoplasmic reticulum calcium release and uptake channel expression was also previously investigated. While Greener et al.28 reported a lower abundance of ryanodine receptor 2 in AVN cells versus myocytes, Dobrzynski et al.27 reported an increase in ryanodine receptor 2 mRNA but a decrease in ryanodine receptor 2 protein in human AVN relative to VM. Ryanodine receptor 3 mRNA expression was higher in the TC and INE cells but similar elsewhere. Finally, the expression of the sarcoplasmic reticulum calcium pump SERCA2a mRNA was uniform throughout the AM, VM and AVN cells.27,28

Potassium Channels

Potassium channels are responsible for repolarisation of the cardiac cells. Based on the specific current they contribute to, they can be divided into three components: transient outward potassium current (Ito), delayed rectifier currents and inward rectifier current (IK1).41

Ito is responsible for the repolarisation during phase 1 of the AP. The major contributor to Ito in humans is voltage-dependent potassium channel Kv4.3.35,42 The expression of Kv4.3 mRNA in the human AVN is similar to the working myocardium. Other Ito contributors, such as Kv1.4 and Kv4.2, have significantly higher expression in the CN and PB relative to the working myocardium.27,28

Delayed rectifier potassium currents are ––of three types: ultra-rapid delayed rectifier (IK,ur), rapid delayed rectifier (IK,r) and slow delayed rectifier (IK,s). The ion channels associated with these currents are Kv1.5, human ether-a-go-go (hERG) and KvLQT (the LQT-like subfamily of voltage gated potassium channels), respectively.41,43 Kv1.5 mRNA expression was high in the AM but very less in the AVN cells and VM. hERG mRNA expression in the AM, AVN cells and VM were reported to be uniform by Greener et al.28 but Dobrzynski et al.27 reported that hERG mRNA expression was significantly higher in VM. Finally, KvLQT was uniformly expressed in AM, VM and all AVN regions except for the INE where it was significantly reduced.27,28

IK1 is responsible for maintaining the resting membrane potential (RMP) in most cardiac cells. However, the major IK1 ion channel, Kir2.1, is significantly reduced in the AVN, which could account for its more positive RMP.27,28,44,45 Other potassium channel isoforms responsible for IK1 are more uniformly distributed in the AM, AVN and VM.27,28

Autonomic Nervous System

Both sympathetic and parasympathetic nervous systems exert control over the AVN.16,46 Sympathetic innervation is achieved through the beta-adrenergic receptors (AR), which when stimulated can increase the rate of junctional rhythm in the human AVN.16 The mRNA of the two most commonly expressed beta-AR isoforms, beta1-AR and beta2-AR, are expressed throughout the AVN region at similar levels. The expression profile is mostly uniform between the AM, VM and AVN regions, except for a slightly lower abundance of beta1-AR expression in the PB and beta2-AR expression in the INE.27,28

Parasympathetic innervation in the human AVN is achieved by the activation of the acetylcholine-activated potassium channel current (IK,Ach). These channels are heteromers that are formed by the Kir3.1 and Kir3.4 potassium channel isoforms and are upregulated in the CN and PB cells.27,28 This could be an indication of the importance of parasympathetic stimulation of the AVN. Activation of the IK,Ach channels can then reduce AV junctional rhythm rate in the AVN.16

Thus, the AVN is under very tight autonomic control that can modulate its conduction and pacemaking properties, which determine AV delay and AV functional rhythm, respectively. Therefore, even though it is not the lead pacemaker in a healthy heart, autonomic control of the AVN is crucial to maintain normal conduction delays (<0.1 ms) that allow sufficient time for ventricular filling and also prevent re-entrant arrhythmias.16,47

In addition to AV junctional rhythm rate, autonomic innervation can alter the lead pacemaker site within the human AVN.16 It has been reported that during control conditions, the intrinsic pacemaker site is at the proximal end of the BoH. When the AVN is treated with isoproterenol (sympathetic stimulation), the leading pacemaking site shifts to the CN region. This could be a result of the heterogeneous beta-ARs in this region. Similarly, when the human AVN is treated with acetylcholine (parasympathetic stimulation), the leading pacemaker site shifts to the TC area.

Functional Heterogeneity

The vast differences in molecular profiles of the AVN and its surrounding region underly the complex electrophysiological heterogeneities of the human AVN. The pacemaking capability, dual conduction pathways, refractory period variations and the other specialised functions and characteristics of the AVN stem from its unique structural and molecular composition. A few of these distinct features are discussed in detail here.

Action Potential

The differences in ion channel expression can result in differing AP morphologies in various compartments of the AVN. This morphological variation was observed by optical mapping of the human AVN.15,16 These AP morphologies closely match those recorded by patch clamping of the rabbit AVN, which also reported varying RMP and even major ionic currents contributing to the AVN AP.44 For example, while the AM and VM RMPs were more negative, the AVN cells had an RMP of around –50 mV. TC, which are intermediate cells between AM and AVN cells, had an RMP similar to atrial cells (–70 mV) whereas PB cells have an RMP closer to that of CN cells. These RMP variations closely follow the expression of the IK1 channels, which are responsible for maintaining a negative RMP. These channels are greatly downregulated in CN cells.

The maximum rate of rise of the action potential (dV/dtmax) was also different between these cells.44 The expression profile of sodium and calcium ion channels underlie this phenomenon. Specifically, in AM and VM, which have higher Nav1.5 expression levels and INa as the major depolarising current, a much higher dV/dtmax (80–100 V/s) was recorded. In contrast, in AVN cells with very low Nav1.5 and high Cav3.1 expression, ICa,L is the major depolarising current. This results in a small dV/dtmax (4–6 V/s) and gives the AVN cells their characteristic slow AP upstroke. TC cells had an intermediate dV/dtmax (22 V/s), possibly due to a mix of both types of currents.

Finally, AVN cells also had significantly shorter AP durations relative to AM and VM (113 ms relative to 155 or 215 ms, respectively).44 Phase 2 or the plateau phase was not very pronounced in these APs. AP duration heterogeneity was possibly due to the delayed rectifier potassium channel distribution, specifically hERG. However, TC and PB cells had AP durations closer to that of AM.