Autonomic control and innervation of the atrioventricular junctional pacemaker

From Mice to Mainframes: Experimental Models for Investigation of the Intracardiac Nervous System

The intracardiac nervous system (IcNS), sometimes referred to as the "little brain" of the heart, is involved in modulating many aspects of cardiac physiology. In recent years our fundamental understanding of autonomic control of the heart has drastically improved, and the IcNS is increasingly being viewed as a therapeutic target in cardiovascular disease. However, investigations of the physiology and specific roles of intracardiac neurons within the neural circuitry mediating cardiac control has been hampered by an incomplete knowledge of the anatomical organisation of the IcNS. A more thorough understanding of the IcNS is hoped to promote the development of new, highly targeted therapies to modulate IcNS activity in cardiovascular disease. In this paper, we first provide an overview of IcNS anatomy and function derived from experiments in mammals. We then provide descriptions of alternate experimental models for investigation of the IcNS, focusing on a non-mammalian model (zebrafish), neuron-cardiomyocyte co-cultures, and computational models to demonstrate how the similarity of the relevant processes in each model can help to further our understanding of the IcNS in health and disease.

The Intrinsic Cardiac Nervous System and Its Role in Cardiac Pacemaking and Conduction

The cardiac autonomic nervous system (CANS) plays a key role for the regulation of cardiac activity with its dysregulation being involved in various heart diseases, such as cardiac arrhythmias. The CANS comprises the extrinsic and intrinsic innervation of the heart. The intrinsic cardiac nervous system (ICNS) includes the network of the intracardiac ganglia and interconnecting neurons. The cardiac ganglia contribute to the tight modulation of cardiac electrophysiology, working as a local hub integrating the inputs of the extrinsic innervation and the ICNS. A better understanding of the role of the ICNS for the modulation of the cardiac conduction system will be crucial for targeted therapies of various arrhythmias. We describe the embryonic development, anatomy, and physiology of the ICNS. By correlating the topography of the intracardiac neurons with what is known regarding their biophysical and neurochemical properties, we outline their physiological role in the control of pacemaker activity of the sinoatrial and atrioventricular nodes. We conclude by highlighting cardiac disorders with a putative involvement of the ICNS and outline open questions that need to be addressed in order to better understand the physiology and pathophysiology of the ICNS.

PR Interval and Junctional Zone

The atrioventricular junction is a pivotal component of the cardiac conduction system, a key electrical relay site between the atria and the ventricles. The sophisticated functions carried out by the atrioventricular junction are possible for the presence of a complex apparatus made of specialized anatomic structures, cells with specific ion-channel expression, a well-organized spatial distribution of intercellular junctions (connexins), cells with intrinsic automatism, and a rich autonomic innervation. This article reviews the main anatomic and electrophysiologic features of the atrioventricular junction, with a focus on cardiac preexcitation.

Zebrafish heart as a model to study the integrative autonomic control of pacemaker function

The cardiac pacemaker sets the heart's primary rate, with pacemaker discharge controlled by the autonomic nervous system through intracardiac ganglia. A fundamental issue in understanding the relationship between neural activity and cardiac chronotropy is the identification of neuronal populations that control pacemaker cells. To date, most studies of neurocardiac control have been done in mammalian species, where neurons are embedded in and distributed throughout the heart, so they are largely inaccessible for whole-organ, integrative studies. Here, we establish the isolated, innervated zebrafish heart as a novel alternative model for studies of autonomic control of heart rate. Stimulation of individual cardiac vagosympathetic nerve trunks evoked bradycardia (parasympathetic activation) and tachycardia (sympathetic activation). Simultaneous stimulation of both vagosympathetic nerve trunks evoked a summative effect. Effects of nerve stimulation were mimicked by direct application of cholinergic and adrenergic agents. Optical mapping of electrical activity confirmed the sinoatrial region as the site of origin of normal pacemaker activity and identified a secondary pacemaker in the atrioventricular region. Strong vagosympathetic nerve stimulation resulted in a shift in the origin of initial excitation from the sinoatrial pacemaker to the atrioventricular pacemaker. Putative pacemaker cells in the sinoatrial and atrioventricular regions expressed adrenergic β2 and cholinergic muscarinic type 2 receptors. Collectively, we have demonstrated that the zebrafish heart contains the accepted hallmarks of vertebrate cardiac control, establishing this preparation as a viable model for studies of integrative physiological control of cardiac function by intracardiac neurons.

Structure-function relationship in the sinus and atrioventricular nodes

Recently published optical mapping studies of larger mammals, including humans, have identified functionally discrete sinoatrial exit pathways of activation. This is in line with earlier mapping studies of the dog and the human but in contrast with findings in the mouse and the rabbit, wherein a propagation wave front pattern of activation has been described. It underpins the complex three-dimensional (3D) organization of the cardiac pacemaking and conduction system in larger species, wherein sinoatrial and atrioventricular nodal physiologies both demonstrate identifiable activation pathways, which coincide with anatomic landmarks and histologic architecture, so that in addition to muscle fiber orientation and cell coupling, these intrinsic factors act to determine excitation pathways. This complex 3D organization increases the effect of source-to-sink mismatch both by greater variability in the space constant of tissue and by the 3D projection of this effect in all directions. Mathematical modeling provides a means to study these interactions, and newer models should incorporate these additional factors and their effect into the 3D structure of large mammal physiology.

Anatomic localization and autonomic modulation of atrioventricular junctional rhythm in failing human hearts

BACKGROUND

The structure-function relationship in the atrioventricular junction (AVJ) of various animal species has been investigated in detail; however, less is known about the human AVJ. In this study, we performed high-resolution optical mapping of the human AVJ (n = 6) to define its pacemaker properties and response to autonomic stimulation.

METHODS AND RESULTS

Isolated, coronary-perfused AVJ preparations from failing human hearts (n = 6, 53 ± 6 years) were optically mapped using the near-infrared, voltage-sensitive dye, di-4-ANBDQBS, with isoproterenol (1 μmol/L) and acetylcholine (1 μmol/L). An algorithm detecting multiple components of optical action potentials was used to reconstruct multilayered intramural AVJ activation and to identify specialized slow and fast conduction pathways (SP and FP). The anatomic origin and propagation of pacemaker activity was verified by histology. Spontaneous AVJ rhythms of 29 ± 11 bpm (n = 6) originated in the nodal-His region (n = 3) and/or the proximal His bundle (n = 4). Isoproterenol accelerated the AVJ rhythm to 69 ± 12 bpm (n = 5); shifted the leading pacemaker to the transitional cell regions near the FP and SP (n = 4) and/or coronary sinus (n = 2); and triggered reentrant arrhythmias (n = 2). Acetylcholine (n = 4) decreased the AVJ rhythm to 18 ± 4 bpm; slowed FP/SP conduction leading to block between the AVJ and atrium; and shifted the pacemaker to either the transitional cell region or the nodal-His region (bifocal activation).

CONCLUSIONS

We have demonstrated that the AVJ pacemaker in failing human hearts is located in the nodal-His region or His bundle regions and can be modified with autonomic stimulation. Moreover, we found that both the FP and SP are involved in anterograde and retrograde conduction.

Immunohistochemical characterization of the intrinsic cardiac neural plexus in whole-mount mouse heart preparations

BACKGROUND

The intrinsic neural plexus of the mouse heart has not been adequately investigated despite the extensive use of this species in experimental cardiology.

OBJECTIVE

The purpose of this study was to determine the distribution of cholinergic, adrenergic, and sensory neural components in whole-mount mouse heart preparations using double immunohistochemical labeling.

METHODS/RESULTS

Intrinsic neurons were concentrated within 19 ± 3 ganglia (n = 20 mice) of varying size, scattered on the medial side of the inferior caval (caudal) vein on the right atrium and close to the pulmonary veins on the left atrium. Of a total of 1,082 ± 160 neurons, most somata (83%) were choline acetyltransferase (ChAT) immunoreactive, whereas 4% were tyrosine hydroxylase (TH) immunoreactive; 14% of ganglionic cells were biphenotypic for ChAT and TH. The most intense ChAT staining was observed in axonal varicosities. ChAT was evident in nerve fibers interconnecting intrinsic ganglia. Both ChAT and TH immunoreactivity were abundant within the nerves accessing the heart. However, epicardial TH-immunoreactive nerve fibers were predominant on the dorsal and ventral left atrium, whereas most ChAT-positive axons proceeded on the heart base toward the large intrinsic ganglia and on the epicardium of the root of the right cranial vein. Substance P-positive and calcitonin gene-related peptide-immunoreactive nerve fibers were abundant on the epicardium and within ganglia adjacent to the heart hilum. Small intensely fluorescent cells were grouped into clusters of 3 to 8 and were dispersed within large ganglia or separately on the atrial and ventricular walls.

CONCLUSION

Although some nerves and neuronal bundles of the mouse epicardial plexus are mixed, most express either adrenergic or cholinergic markers. Therefore, selective stimulation and/or ablation of the functionally distinct intrinsic neural pathways should allow the study of specific effects on cardiac function.

Calcium dynamics and the mechanisms of atrioventricular junctional rhythm

OBJECTIVES

The purpose of this study was to test the hypothesis that rhythmic spontaneous sarcoplasmic reticulum calcium (Ca) release (the "Ca clock") plays an important role in atrioventricular junction (AVJ) automaticity.

BACKGROUND

The AVJ is a primary backup pacemaker to the sinoatrial node. The mechanisms of acceleration of AVJ intrinsic rate during sympathetic stimulation are unclear.

METHODS

We simultaneously mapped transmembrane potential and intracellular Ca in Langendorff-perfused canine AVJ preparations that did not contain sinoatrial node (n = 10).

RESULTS

Baseline AVJ rate was 37.5 +/- 4.0 beats/min. The wavefront from leading pacemaker site propagated first through the slow pathway, then the fast pathway and atria. There was no late diastolic Ca elevation (LDCAE) at baseline. Isoproterenol up to 3 micromol/l increased heart rate to 100 +/- 6.8 beats/min, concomitant with the appearance of LDCAE that preceded the phase 0 of action potential by 97.3 +/- 35.2 ms and preceded the onset of late diastolic depolarization by 23.5 +/- 3.5 ms. Caffeine also produced LDCAE and AVJ acceleration. The maximal slope of LDCAE and diastolic depolarization always colocalized with the leading pacemaker sites. Ryanodine markedly slowed the rate of spontaneous AVJ rhythm. Isoproterenol did not induce LDCAE in the presence of ryanodine. The I(f) blocker ZD 7288 did not prevent LDCAE or AVJ acceleration induced by isoproterenol (n = 2).

CONCLUSIONS

Isoproterenol and caffeine induced LDCAE and accelerated intrinsic AVJ rhythm. Consistent colocalization of the maximum LDCAE and the leading pacemaker sites indicates that the Ca clock is important to the intrinsic AVJ rate acceleration during sympathetic stimulation.

Anatomy and electrophysiology of the human AV node

The atrioventricular node (AVN) has mystified generations of investigators over the last century and continues today to be at the epicenter of debates among anatomists, experimentalists, and electrophysiologists. Over the years, discrepancies have remained in regard to correlating components of AVN structure to function, as evidenced by studies from microelectrodes, optical mapping, and the electrophysiology laboratory. Historically, the AVN has been defined by classical histological methods; however, with recent advances in molecular biology techniques, a more precise characterization of structure is becoming attainable. Distinct molecular compartments are becoming apparent based on connexin staining and genotyping, providing new insight into previously characterized functional aspects of the AVN and its surrounding structures. Advances in optical mapping have provided a unique opportunity for correlating structure and function--unmasking properties of the native AVN pacemaker and providing further insight into basic mechanisms involved in AV conduction. Additionally, procurement of explanted human hearts have provided a unique opportunity to further characterize the human AVN structurally and functionally with both molecular biology techniques and optical mapping. With the elucidation of basic elements of both structure and function via molecular investigation and optical mapping, new opportunities are becoming apparent in utilizing the unique properties of the AVN for pursuing novel clinical applications relevant to clinical electrophysiology.

Connexin 43 expression delineates two discrete pathways in the human atrioventricular junction

Gap junction expression has been studied in the atrioventricular junction (AVJ) of many species, however, their distribution in the human AVJ is unknown. The AVJ expression of the gap junction protein connexin 43 (Cx43) is species dependent; therefore we investigated its distribution in the human AVJ. Using Masson trichrome histology, we reconstructed the AVJ of three normal human hearts and one with dilated cardiomyopathy in three dimensions. Cx43 was immunolabeled with vimentin and alpha-actinin to determine the cellular origin of Cx43 and was quantified in the following structures: interatrial septum (IAS), His bundle, compact node (CN), lower nodal bundle (LNB), leftward and rightward nodal extensions (LE and RE), and inferior, endocardial, and left-sided transitional cells. Histology revealed two nodal extensions in three of four hearts. Cx43 was found in the myocytes, but not fibroblasts, of the AVJ. LE and CN Cx43 was lower than the IAS (P < 0.05) and the RE, LNB, and His all expressed Cx43 similarly, with approximately half of IAS expression (RE: 44 +/- 36%; LNB: 50 +/- 26%; His: 48 +/- 12%, P = NS compared with IAS). Cx43 levels in transitional cells were similar to the IAS (P = not significant). Cx43 was found in myocytes of the human AVJ, and its expression pattern delineates two separate continuous structures: one consists of the LE and CN with little Cx43, and the other consists of the His, LNB, and RE expressing approximately half the Cx43 of the IAS. The differential Cx43 expression may provide each structure with unique conduction properties, contributing to arrhythmias arising from the AVJ.