The extent of the specialized atrioventricular ring tissues.

BACKGROUND: The so-called specialized tissues within the heart are the sinus node, the atrioventricular conduction system, and the Purkinje network. Further structures with the characteristics of specialized tissue are also found within the atrioventricular junction, although they are less well described. OBJECTIVE: The purpose of this study was to demonstrate the location and extent of these atrioventricular ring specialized tissues, showing their relationship with the normal atrioventricular conduction system. METHODS: We identified the tissues using histology combined with immunohistochemical labeling with connexin43 (Cx43), the major gap junction in heart, and HCN4, the major isoform of the funny channel. RESULTS: We observed rings of specialized tissue mainly in hearts from rats, mice, and guinea pigs, negative for Cx43 but positive for HCN4. Each ring takes its origin from an inferior extension of the atrioventricular node. The rightward ring runs around the vestibule of the tricuspid valve, whereas the leftward ring encircles the mitral valve. On returning toward the atrial septum, the tricuspid ring crosses over the penetrating part of the atrioventricular conduction system, reuniting with the mitral ring to form a superiorly located retroaortic node. The atrioventricular conduction system itself continues beyond the origin of the right and left bundle branches, forming an aortic ring that ascends toward the retroaortic node but fails to make contact because of the intervening area of aortic-to-mitral valvar fibrous continuity. CONCLUSION: Rings of conduction tissue take their origin from inferior extensions of the atrioventricular node, passing rightward and leftward to encircle the orifices of the tricuspid and mitral valves and reuniting to form an extensive retroaortic node. Thus, a ring with morphologic features justifying a definition of specialized conduction tissue surrounds the atrioventricular junctions, although its function has yet to be established.

Ionic mechanisms underlying region-specific remodeling of rabbit atrial action potentials caused by intermittent burst stimulation.

BACKGROUND: Pulmonary veins (PVs) and the coronary sinus (CS) play pivotal roles in triggering some episodes of atrial fibrillation. In isolated rabbit right or left atrial preparations, a 3-hour intermittent burst pacing protocol shortens action potential duration (APD) in CS and PV, but not in sinus node (SN) and left Bachmann bundle (BB) regions. OBJECTIVE: The purpose of this study was to use patch clamp techniques to study the rapidly inactivating (I(to)) and sustained (I(sus)) K(+) currents as well as Ca(2+) currents (I(Ca)) in cells dispersed from intermittent burst pacing and sham PV, BB, CS, and SN regions to determine whether changes in these currents contributed to APD shortening. METHODS: Real-time polymerase chain reaction was performed for transient outward K(+) and Ca(2+) channel subunit mRNAs to determine if intermittent burst pacing affected expression levels. RESULTS: I(to) densities were unaffected by intermittent burst pacing in PV and Bachmann bundle cells. mRNA levels of K(V)4.3, K(V)4.2, K(V)1.4, and KChIP2 subunits of I(to) in both regions were stable. In CS cells, I(to) densities in intermittent burst pacing were greater than in sham (P <.05), but there were no parallel mRNA changes. I(Ca) density of PV cells was reduced from 14.27 +/- 2.08 pA/pF (at -5 mV) in sham to 7.52 +/- 1.65 pA/pF in intermittent burst pacing PV cells (P <.05) due to a significant shift in voltage dependence of activation. These results were seen in the absence of mRNA changes in alpha(1C) and alpha(1D) Ca(2+) channel subunits. In contrast, intermittent burst pacing had no effect on Ca(2+) current densities and kinetics of CS cells, but decreased alpha(1)C and alpha(1)D mRNA levels. CONCLUSION: There is region-specific remodeling of I(to) and I(Ca) by intermittent burst pacing protocols in rabbit atrium. Increased I(to) in CS cells could account for the APD shortening observed with intermittent burst pacing, whereas an intermittent burst pacing-induced shift in voltage dependence of activation may contribute to APD shortening in PV cells.

His-Purkinje lineages and development.

The heartbeat is initiated and coordinated by a multi-component set of specialized muscle tissues collectively referred to as the pacemaking and conduction system. Over the last few years, impetus has gathered into unravelling the cellular and molecular processes that regulate differentiation and integration of this essential cardiac network. One focus of our collective work has been the developmental history of cells comprising His-Purkinje tissues of the conduction system. This interest in part arose from studies of the expression of connexins in periarterial Purkinje fibres of the chick heart. Using lineage-tracing strategies, including those based on replication-defective retroviruses and adenoviruses, it has been shown that conduction cells are derived from multipotent, cardiomyogenic progenitors in the tubular heart. Moreover, heterogeneity within myocardial clones has indicated that the elaboration of the conduction system in the chick embryo occurs by progressive, localized recruitment from within this pool of cardiomyogenic cells. Cell birth dating has revealed that inductive conscription of cells to central elements of the conduction system (e.g. the His bundle) precedes recruitment to the peripheral components of the network (i.e. subendocardial and periarterial Purkinje fibres). Birth dating studies in rodents suggest an analogous recruitment process is occurring in this species. In addition to summarizing earlier work, this chapter provides information on ongoing studies of cell-cell signalling and transcriptional mechanisms that may regulate the development of His-Purkinje tissues.

Contribution of I(K,ADO) to the negative dromotropic effect of adenosine.

OBJECTIVE: Despite the pathophysiological and therapeutic significance of the negative dromotropic effect of adenosine, its underlying ionic mechanism, and specifically the role of the adenosine-activated K(+) current (I(K,ADO)) is not experimentally defined. Therefore, we studied the contribution of I(K,ADO) to the negative dromotropic effect of adenosine. METHODS: Effects of adenosine on single atrioventricular nodal and left atrial myocytes from rabbits were studied using the whole cell configuration of the patch clamp technique. Complementary experiments were done in rabbit and guinea pig isolated hearts instrumented to measure the atrium-to-His bundle interval. RESULTS: In contrast to its effect in atrial myocytes, Ba(2+) selectively and completely blocked I(K,ADO) at membrane potentials from -70 to 0 mV in atrioventricular nodal myocytes and abolished the adenosine-induced leftward shift of the reversal membrane potential. Ba(2+) alone did not significantly prolong the A-H interval, but markedly attenuated the A-H interval prolongation caused by adenosine. In guinea pig heart, EC(50) values ( pD(2) +/- SEM) for adenosine-induced atrium-to-His bundle interval prolongation were 3.3 micromol/L (5.48 +/- 0.04) and 13.2 micromol/L (4.88 +/- 0.05, P < 0.001) in the absence and presence of Ba(2+), respectively. Despite species-dependent differences in sensitivities to adenosine (guinea pig > rabbit), the relative contribution of adenosine-activated K(+) current to the atrium-to-His bundle interval prolongation was nearly identical. In guinea pig hearts it ranged from 37.8 % (P = 0.013) to 72.5 % (P < 0.001) at 2 to 6 micromol/L adenosine, respectively. CONCLUSION: I(K,ADO) contributes significantly to the negative dromotropic effect of adenosine, but predominantly at relatively high concentrations of the nucleoside.

3-D characterization of ganglion cells of the terminal nerve plexus of rat atrioventricular junction.

Three-dimensional (3-D) morphology of neurons of the terminal nerve plexus of the atrioventricular junction was examined in a scanning electron microscope. Distributions of different cell types encountered as well as their relations to different structures of the atrioventricular specialized tissue were also studied. Most neurons were found disseminated in a thin connective tissue layer separating different segments of the atrioventricular conductive tissue from the interventricular septum. Sometimes, they formed small pluricellular ganglia (up to 5 neurons) but, frequently, they occurred isolated in the terminal ramifications of the intramural nerve plexus of specialized tissue. Some intranodal neurons could also be identified. According to their 3-D morphology, nerve cells of the perinodal ganglionated plexus could be divided into three categories: (1) Large unipolar neurons were scattered throughout the atrioventricular junction. Their long and thin axonal projections were often directed towards the interventricular septum. (2) Large pseudounipolar or bipolar neurons were located at a few specific loci, namely all along the bundle of His and its bifurcation into the right and left bundle branches. Frequently, they occurred solitary and immersed amongst strands of surrounding muscle cells. Only occasional synaptic impacts could be identified on the surface of neuronal bodies of these bipolar neurons. On the other hand, their dendritic varicosities were richly innervated. Due to their irregular shape, intimate association with muscular elements and their topographical superposition with occasional spindle-like structures, these nerve cells recall prospective sensory neurons involved in integration of mechanical and neural stimuli to the heart. (3) Small multipolar interneurons could be identified in the retronodal ganglion and within right and left bundle branches. The present description of morphological heterogeneity of intramural nerve cells agrees with recent morphological and functional classifications of autonomic neurons and supports the idea that, at the level of the atrioventricular junction, a self-governed neuronal network may be operating.

Morphological classification of atrial muscle in the atrioventricular junctional area--3-dimensional reconstruction of serial sections of the human heart.

To trace anatomical structures that might be associated with the dual atrioventricular (AV) nodal pathway and to investigate the morphologic characteristics of the cells that form these pathways, we examined serial sections of the AV junctional area with a light microscope and reconstructed them 3-dimensionally with a computer. Twelve hearts were obtained at autopsy from patients who had not shown AV conduction disturbances or supraventricular tachycardia before death. The method of Lev et al was used to prepare serial sections. Fascicles of atrial muscle contiguous with the AV node were examined with a light microscope and were classified into 3 groups, on the basis of morphologic characteristics and myocyte diameter. A computer was used to reconstruct 3-dimensionally the course of the fascicles and surrounding structures. At the border of the AV node and bundle of His relatively large myocytes extended directly into the AV bundle from the anterosuperior interatrial septum. Morphologically, the course was considered to be consistent with the fast pathway. In contrast, small cells that entered the AV node from the inferoposterior interatrial septum resembled sinus node cells with few myofibrils and a winding shape. These cells extended from the coronary sinus ostium to the tricuspid valve annulus and are thought to make up the slow pathway.

A quantitative assessment of innervation in the conduction system of the calf heart.

BACKGROUND: The aim of the present investigation was to determine the relative distribution of autonomic and sensory nerves in the cardiac conduction tissues of calves. METHODS: A quantitative immunohistochemical and histochemical technique was adopted. RESULTS: Immunoreactivity to the general neuronal marker protein gene product 9.5 (PGP 9.5) demonstrated that all regions of the conduction system possessed a higher relative density of total nerves when compared with the surrounding myocardial tissues. Unlike myocardial innervation, the conduction system did not display an atrial-to-ventricular gradient in nerve density. PGP 9.5-immunoreactive nerve trunks and varicose nerve fibres were more numerous in the transitional atrioventricular node and the penetrating atrioventricular bundle than in either the sinus node, compact atrioventricular node, or bundle branches. The Purkinje network of the ventricular conduction tissues possessed a rich supply of PGP 9.5-immunoreactive nerve trunks and varicose nerve fibres. Acetylcholinesterase (AChE)-positive nerves were the main subtype identified in the sinus and atrioventricular nodes and in the ventricular conduction tissues, representing 50-80% of the area occupied by PGP 9.5-immunoreactive nerves. The compact atrioventricular node possessed AChE-positive and tyrosine hydroxylase (TH)-immunoreactive nerves in similar proportions (45%), although, in general, TH-immunoreactive nerves had a lower relative nerve density than AChE-positive nerves. Neuropeptide Y (NPY)-immunoreactive nerves represented the main peptide-containing subpopulation and occurred throughout the conduction system, displaying a similar pattern of distribution and relative density to those demonstrating TH immunoreactivity. Nerve fibres immunoreactive for somatostatin, vasoactive intestinal polypeptide, substance P, and calcitonin gene-related peptide formed relatively minor subpopulations. CONCLUSIONS: The general innervation of the bovine conduction tissues exhibits significant regional variation. Throughout all regions of the conduction system, AChE-positive nerve represented the dominant subtype when compared with TH- and NPY-immunoreactive nerves. The distribution and relative density of nerve subtypes in the tissues of the bovine conduction system are similar to those observed in man, whereas differences were observed in other regions, such as the atrioventricular bundle and bundle branches. This finding must be considered by those making interspecies comparisons.

Histochemistry of the atrioventricular conducting system during postnatal development.

The atrioventricular conduction system of 27 infants coming to autopsy was examined by histochemical methods. Twenty one were sudden infant deaths; six were explained deaths. A decrease of oxidoreductases and hydrolases activities was found in clusters of conducting cells protruding from the atrioventricular (AV) node and His bundle left into the collagen of the septum. These findings reflect regressive changes connected with reduction of the AV conducting system during postnatal development.

[DNA synthesis in myocytes of the heart conduction system in myocardial infarction induced in young rats]. / Sintez DNK v miotsitakh provodiashchei sistemy serdtsa pri infarkte miokarda, indutsirovannom u molodykh krys.

The myocardial infarction was induced in 13- and 21-day-old rats. The extent of the DNA synthesis activation in myocytes of conduction system compartments compared with atrial and ventricular myocytes was evaluated by cumulative indices of labeled nuclei after 30-times-repeated 2H-thymidine injections at 12h intervals. Labeling indices in atrial myocytes are two to six times higher (0.01 < P < 0.001) in rats with infarction than in control. On the contrary, in conduction system compartments there are no statistically significant differences between control and experimental groups (P > 0.05). An activation of DNA synthesis can occur in myocytes of His bundle in some animals.

Leu-7 immunoreactivity in human and rat embryonic hearts, with special reference to the development of the conduction tissue.

The distribution pattern of Leu-7 (HNK-1) in developing human embryonic hearts and rat hearts was studied by immunohistochemistry. Human and rat embryos at Streeter's stages XIII approximately XX and fetus stage I were used. Leu-7, which is absent in the newborn rat heart, is expressed transiently in the embryo and fetus I stages. The earliest embryonic heart shows two incomplete circular structures with immunoreactivity in the myocardium along the primitive atrioventricular cushion and bulboventricular canal. These two structures become localized topographically in the definitive atrioventricular node and atrioventricular bundle after rearrangement and partial disappearance during embryonic development. At Streeter's stages XVIII approximately XX, Leu-7 immunoreactivity appears to localize topographically in almost all the pathways of the conduction system, although some discontinuities are observed in the atrioventricular junction and atrial internodal tracts. Thereafter, immunoreactivity decreases gradually and differentially by site and stage. The precise nature of Leu-7 immunoreactive cells, that is, whether or not they are neurogenic or myogenic, is not revealed by this study. The present observations are discussed in connection with the hypothesis that specialized ring tissue is the primordium of the conduction system.

[Quantitative analysis of tissue and cellular components of the specialized internodal conductive pathways of the heart]. / Kolichestvennyi analiz tkanevykh i kletochnykh komponentov spetsializirovannykh provodiashchikh mezhuzlovykh putei serdtsa.

A light-optic and electron-microscopic study of internodal specialized pathways in the rat interatrial cardiac septum is reported. The topography of conductive pathways is described. Morphometric investigation yielded comparative quantitative characteristics of tissue and cellular composition of the working and conductive septal myocardium. A single type of myocytes was demonstrated in specialized conductive pathways. The findings were compared with those obtained in other species. Principles of identification of specialized conductive pathways are discussed.

Glycogen-containing lysosomes and glycogen loss in the cardiocytes of embryonic and neonatal mice.

In the mouse embryo the transformation of mesenchymal cells into cardiac myocytes was seen to be indicated by the appearance of glycogen and myosin filaments. In the early force-producing muscle cells, but particularly in the specialized cells destined to become part of the A-V conduction system, the glycogen became abundant. With maturation of the embryo and differentiation of the cardiocytes, the glycogen content decreased in both the force-producing and "conducting" muscle fibers. In late fetal life, and spectacularly after parturition, the loss of glycogen content in the specialized cells was accompanied by the appearance of glycogen-containing lysosomes. Changes in the structure of such lysosomes indicated ongoing dissolution and uncommon use of metabolizable glycogen.

[Cells of the conduction pathways of the human bundles of His. Histoenzymologic and ultrastructural study]. / Les cellules des voies de conduction dans le faisceau de His humain. Etude histoenzymologique et ultrastructurale.

A human His bundle was studied two hours after death by histoenzymological techniques and electron microscopy. The pathway had a much higher cholinesterase activity than the working myocardium, due to its richness in nerve endings: this was confirmed by electron microscopy which also distinguished "common" contractile cells (working cells) from P type and "intermediary" cells; those were by far the most common, presenting an ultrastructure identical to that of the Purkinje cells, classically described in the bundle branches alone. These findings and the unique longitudinal architectural organisation of the His bundle, confirm the studies of JAMES and may explain the rapidity of conduction in this structure.

Probit analysis of the atrioventricular (AV) junctional tissues of the ferret heart.

Probit frequency analysis, a graphic method for determining whether a population is normally distributed, skewed, or multinodal, was used to determine whether P cells are present in different regions of the AV junction in the ferret heart. This analysis indicated that at least 95% of the cells of the transitional zone, superficial AV node, deep AV node, and distal AV bundle of the ferret heart are morphologically homogeneous. In the proximal AV bundle a large cell population is found in addition to the AV bundle cells. The probit analysis was also used to characterize the shape of the cells of each region of the AV junction further. AV nodal cells are not as elongated as the atrial muscle cells and AV bundle cells. These nodal cells also do not branch as extensively as the AV bundle cells.

[Ultrastructure of the cardiomyocytes of the pedicles of the bundle of His in the rat heart]. / Ul'trastruktura kardiomiotsitov nozhek puchka Gisa v serdtse krysy.

Light optic and electron microscopic investigation on composition and geometry of His' bundle branching in the upper part of the intraventricular septum was made. Branching of both peduncles of the atrioventricular (AV) bundle were demonstrated to be represented by a single cell type, its ultrastructure was described. Possibility to apply different ultrastructural criteria in order to determine this (III) type of cells in the conducting system of the heart was discussed.