Action potential transfer at the Purkinje-ventricular junction: role of transitional cells

Abstract

-At the Purkinje (P) -ventricular (V) junction a zone of "transitional (T)" cells is found. In the present study we investigated the role of these T cells in P-to-V conduction. Using the "model clamp" technique, an experimentally recorded rabbit P cell was coupled to a phase-2 Luo and Rudy (LR) model cell, which in turn was coupled to a strand of phase-2 LR model cells. In our experiments, the single LR model represents the T cell, while the strand of LR models represents subendocardial V cells. This approach enabled us to change selectively coupling conductance (G c ) between cells, presence of T cell, and relative size of cells. We demonstrated that: 1) a decrease of G c between P-T and T-V increases the delay of V activation, 2) the delay of V activation is importantly due to conduction between T and V cells, 3) there is a critical G c for successful conduction at the P-V junction, 4) the critical value of G c for conduction at the P-V junction is lower in presence (11.0±0.7 nS) than in absence (13.7±0.8 nS) of the T cell, and 5) enlargement of the T zone size hampers successful P-to-V conduction. Keywords -Heart, electrophysiology, gap junction, Purkinjeventricular junction, conduction I. INTRODUCTIONAt the junction between Purkinje (P) fibers and ventricular (V) myocardium a tiny zone is found populated with cells that are called "transitional (T)" [1,2]. The P fibers are connected via short thin branches to sheets of these T cells, which in turn are connected via short thin branches to the V myocardium [3]. This anatomic arrangement is thought to form a high-resistance barrier [3], which may benefit P-to-V conduction by shielding a relatively small P fiber from the electrical load imposed by a relatively large V mass [4]. In addition, it is thought to contribute to the discontinuous conduction at the P-V junction [4,5]. In the present study, we measured the effects of the presence of T cells on action potential transfer at the P-V junction. Using an extended version of the "model clamp" technique [6], we electrically coupled an isolated rabbit cardiac P cell by any desired value of "gap junctional conductance" to a strand of ventricular cells of the phase-2 Luo and Rudy (LR) model [7]. In between, we can add another LR model cell, representing in our experimental set-up the T cell. This approach allowed us to study selectively the importance of T cells in conduction at the P-V junction. II. METHODOLOGYCell preparation. Single P cells were isolated from rabbit hearts by enzymatic dissociation. Hearts were quickly removed from anaesthetized rabbits (1 ml/kg Hypnorm), mounted on a Langendorff perfusion apparatus, and perfused with the following solutions: 1) Tyrode's solution for 10 min; 2) nominally 'Ca 2+ -free' Tyrode's solution for 10 min; and 3) nominally 'Ca 2+ -free' Tyrode's solution with collagenase (59 U/L type B and 150 U/L type P; Boehringer) and 250 mg/L trypsin inhibitor (Boehringer) for 30 min. Subsequently, freerunning P fibers, free from ventricular tissue, were excised from both ven...

Model clamp: a computer tool to probe action potential transfer between cardiac cells

“…Today, with increased processor speeds, much shorter time steps are possible. At the same time, the number of model cells can be increased to simulate strands or sheets of cells [19][20][21]25].…”

“…We used the digital coupling clamp technique to further study the synchronization of sinoatrial node cells [23] and the facilitation of discontinuous action potential propagation between atrial cells by fast pacing [24]. Recently, as presented by Dr. Verkerk in another session at this meeting, we used our model clamp technique to study action potential transfer at the Purkinje-ventricular junction [25].…”