Morphological study of the atrioventricular conduction system and Purkinje fibers in yak.

We studied the morphology of the atrioventricular conduction system (AVCS) and Purkinje fibers of the yak. Light and transmission electron microscopy were used to study the histological features of AVCS. The distributional characteristics of the His-bundle, the left bundle branch (LBB), right bundle branch (RBB), and Purkinje fiber network of yak hearts were examined using gross dissection, ink injection, and ABS casting. The results showed that the atrioventricular node (AVN) of yak located in the right side of interatrial septum and had a flattened ovoid shape. The AVN of yak is composed of the slender, interweaving cells formed almost entirely of the transitional cells (T-cells). The His-bundle extended from the AVN, and split into left LBB and RBB at the crest of the interventricular septum. The LBB descended along the left side of interventricular septum. At approximately the upper 1/3 of the interventricular septum, the LBB typically divided into three branches. The RBB ran under the endocardium of the right side of interventricular septum, and extended to the base of septal papillary muscle, passed into the moderator band, crossed the right ventricular cavity to reach the base of anterior papillary muscle, and divided into four fascicles under the subendocardial layer. The Purkinje fibers in the ventricle formed a complex spatial network. The distributional and cellular component characteristics of the AVCS and Purkinje fibers ensured normal cardiac function.

The structure of the atrioventricular node in the heart of the female laying ostrich (Struthio camelus).

The electrical impulse for cardiac contraction is generated in the Sinoatrial node (SA node), subsequently spreads to the Atrioventricular node (AV node) and continues in the Atrioventricular bundle (AV bundle). The AV node may not always be present in different avian species and seems to differ in location and contents between species. In this study, the anatomy and histology of the AV node were studied five female adult ostriches (Struthio camelus). Routine paraffin sectioning and transmission electron microscopic method were performed. The study showed that in the ostrich, the AV node is located in the endocardium of the atrial surface of the right atrioventricular valve adjacent to the fibrous ring. The parenchyma of the AV node is formed by small specialized muscle fibres that are spread within a loose connective tissue network. The AV node is not covered by a connective tissue sheath and some arterioles are present. Nerve fibres are seen related to the node. Ultrastructurally, they stain lighter and contain fewer organized myofibrils than usual myocardial cells. The myofibril bundles run parallel to one another and have interspersed mitochondria, which display distinct cristae. The cells have a large euchromatic nucleus with a clear perinuclear area, and they connected by desmosomes. The ostrich is, thus, one of the birds that have the AV node, whose position varies from the other birds.

Temporal and spatial expression of collagens during murine atrioventricular heart valve development and maintenance.

Heart valve function is achieved by organization of matrix components including collagens, yet the distribution of collagens in valvular structures is not well defined. Therefore, we examined the temporal and spatial expression of select fibril-, network-, beaded filament-forming, and FACIT collagens in endocardial cushions, remodeling, maturing, and adult murine atrioventricular heart valves. Of the genes examined, col1a1, col2a1, and col3a1 transcripts are most highly expressed in endocardial cushions. Expression of col1a1, col1a2, col2a1, and col3a1 remain high, along with col12a1 in remodeling valves. Maturing neonate valves predominantly express col1a1, col1a2, col3a1, col5a2, col11a1, and col12a1 within defined proximal and distal regions. In adult valves, collagen protein distribution is highly compartmentalized, with ColI and ColXII observed on the ventricular surface and ColIII and ColVa1 detected throughout the leaflets. Together, these expression data identify patterning of collagen types in developing and maintained heart valves, which likely relate to valve structure and function.

Spectroscopic studies of the human heart conduction system ex vivo: implication for optical visualization.

Fluorescence excitation and emission spectra of the heart tissues specimens have been measured ex vivo with the aim of finding out the optical differences characteristic for the human heart conduction system (the His bundle) and ventricular myocardium. The optimal conditions enhancing the spectral differences between the His bundle and myocardium were found by recording the fluorescence signal in the range from 420 nm to 465 nm under the excitation at wavelengths starting from 320 nm to 370 nm. In addition, the spectral differences between the His bundle and the connective tissue, which is often present in the heart, could be displayed by comparing the ratios of fluorescence intensities being measured at above 460 nm under the preferred excitation of elastin and collagen. The left and right branches of the His bundle were visualized ex vivo in the interventricular septum of the human heart under illumination at 366 nm.

Periostin regulates atrioventricular valve maturation.

Cardiac valve leaflets develop from rudimentary structures termed endocardial cushions. These pre-valve tissues arise from a complex interplay of signals between the myocardium and endocardium whereby secreted cues induce the endothelial cells to transform into migratory mesenchyme through an endothelial to mesenchymal transformation (EMT). Even though much is currently known regarding the initial EMT process, the mechanisms by which these undifferentiated cushion mesenchymal tissues are remodeled "post-EMT" into mature fibrous valve leaflets remains one of the major, unsolved questions in heart development. Expression analyses, presented in this report, demonstrate that periostin, a component of the extracellular matrix, is predominantly expressed in post-EMT valve tissues and their supporting apparatus from embryonic to adult life. Analyses of periostin gene targeted mice demonstrate that it is within these regions that significant defects are observed. Periostin null mice exhibit atrial septal defects, structural abnormalities of the AV valves and their supporting tensile apparatus, and aberrant differentiation of AV cushion mesenchyme. Rescue experiments further demonstrate that periostin functions as a hierarchical molecular switch that can promote the differentiation of mesenchymal cells into a fibroblastic lineage while repressing their transformation into other mesodermal cell lineages (e.g. myocytes). This is the first report of an extracellular matrix protein directly regulating post-EMT AV valve differentiation, a process foundational and indispensable for the morphogenesis of a cushion into a leaflet.

Experimental analyses of the function of the proepicardium using a new microsurgical procedure to induce loss-of-proepicardial-function in chick embryos.

The proepicardium (PE) is a primarily extracardiac progenitor cell population that colonizes the embryonic heart and delivers the epicardium, the subepicardial and intramyocardial fibroblasts, and the coronary vessels. Recent data show that PE-derived cells additionally play important regulatory roles in myocardial development and possibly in the normal morphogenesis of the heart. Developmental Dynamics 233, 2005. Research on the latter topics profits from the fact that loss-of-PE-function can be experimentally induced in chick embryos. So far, two microsurgical techniques were used to produce such embryos: (1) blocking of PE cell transfer with pieces of the eggshell membrane, and (2) mechanical excision of PE. Both of these techniques, however, have their shortcomings. We have searched, therefore, for new techniques to eliminate the PE. Here, we show that loss-of-PE-function can be induced by photoablation of the PE. Chick embryos were treated in ovo by means of a window in the eggshell at Hamburger and Hamilton (HH) stage 16 (iday 3). The pericardial coelom was opened, and the PE was externally stained with a 1% solution of Rose Bengal by means of a micropipette. Photoactivation of the dye was accomplished by illumination of the operation field with visible light. Examination on postoperative day 1 (iday 4, HH stages 19/20) disclosed complete removal of PE in every experimental embryo. On iday 9 (HH stages 33/34), the survival rate of experimental embryos was 35.7% (15 of 42). Development of the PE-derivatives was compromised in the heart of every survivor. The abnormalities encompassed hydro- or hemopericardium, epicardium-free areas with aneurysmatic outward bulging of the ventricular wall, thin myocardium, defects of the coronary vasculature, and abnormal tissue bridges between the ventricles and the pericardial wall. Our results show that photoablation of the PE is a powerful technique to induce long-lasting loss-of-PE-function in chick embryos. We have additionally obtained new data that suggest that the embryonic epicardium may make important contributions to the passive mechanics of the developing heart.

The inferior right atrial isthmus: further architectural insights for current and coming ablation technologies.

BACKGROUND: Although linear ablation of the right atrial isthmus in patients with isthmus-dependent atrial flutter can be highly successful, recurrences and complications occur in some patients. Our study provides further morphological details for a better understanding of the structure of the isthmus. METHODS AND RESULTS: We examined the isthmic area in 30 heart specimens by dissection, histology, and scanning electron microscopy. This area was bordered anteriorly by the hinge of the tricuspid valve and posteriorly by the orifice of the inferior caval vein. With the heart in attitudinal orientation, we identified and measured the lengths of three levels of isthmus: paraseptal (24 +/- 4 mm), central (19 +/- 4 mm), and inferolateral (30 +/- 3 mm). Comparing the three levels, the central isthmus had the thinnest muscular wall and the paraseptal isthmus the thickest wall. At all three levels, the anterior part was consistently muscular whereas the posterior part was composed of mainly fibro-fatty tissue in 63% of hearts. The right coronary artery was less than 4 mm from the endocardial surface of the inferolateral isthmus in 47% of hearts. Inferior extensions of the atrioventricular node were present in the paraseptal isthmus in 10% of hearts, at 1-3 mm from the endocardial surface. CONCLUSIONS: The thinner wall and shorter length of the central isthmus together with its distance from the right coronary artery, and nonassociation with the atrioventricular node or its arterial supply, should make it the preferred site for linear radiofrequency ablation.

Angiotensin II may mediate apoptosis via AT1-receptors in the rat cardiac conduction system.

INTRODUCTION: Apoptosis and angiotensin II (Ang II) have been suggested as possible causes of arrhythmias. In addition, Ang II via Ang II type I (AT(1)-) receptors, has been demonstrated to induce cardiomyocyte apoptosis. The transgenic m(Ren-2)27 (TG) rat carries the additional Ren-2 gene, the expression of which results in an increase in cardiac Ang II, thus potentially affecting the cell growth/death equilibrium. In this study we have investigated the effect of Ang II, via AT(1)-receptors, on mediating apoptosis in a cardiac conduction system (SA node and AV nodes). MATERIALS AND METHODS: Heart sections from male two-day, one-week and two-week TG and Sprague-Dawley (SD) rats were stained with Masson Trichrome to localise the SA and AV nodes. The sections containing SA or AV nodes were processed for quantitation of apoptotic nuclei and AT(1)-receptors. RESULTS: The number of apoptotic nuclei/mm(2) in the SA and AV nodes were found to decrease from two days to two weeks in both the TG and the SD rats, and the number of apoptotic nuclei/mm(2) in the TG groups was significantly higher than that of the SD groups for all ages (p<0.05). The number of AT(1)-receptors/mm(2) in the SA node were found to decrease with increasing age, whereas the number of AT(1)-receptors/mm(2) in the AV node was increased in both TG and SD rats and the number of AT(1)-receptors/mm(2) in the three TG groups was significantly more than that of the three SD groups (p<0.05). DISCUSSION AND CONCLUSION: As a consequence of the additional renin gene in the TG rats, which results in the alteration of the local renin-angiotensin system, the numbers of AT(1)-receptors/mm(2) and apoptotic nuclei/mm(2) are increased. The number of apoptotic nuclei/mm(2) and AT(1)-receptors/mm(2) in the SA node decrease with maturation, whereas, the number of AT(1)-receptors in the AV node increase. Thus, there may be a correlation between Ang II and apoptosis in the SA node, which does not appear to be present in the AV node.

Fast pathway-His bundle connections in the rabbit heart.

OBJECTIVES: The incidence and the physiologic roles for direct fast pathway-His bundle connections were examined in 102 rabbit hearts. METHODS: Extracellular bipolar and intracellular microelectrode recordings were made from the superfused rabbit AV junction. RESULTS: In 13 of 27 preparations demonstrating anterior extensions of the fast pathway, the retrograde HA ERP and 2:1 block cycle length were shortened (128 +/- 12 and 145 +/- 5 msec, respectively) versus the remaining 89 preparations (178 +/- 15 and 185 +/- 10 msec, respectively, p < 0.01). The former values were similar to the ERP and 2:1 block cycle length of fast pathway transitional cells (128 +/- 23 and 141 +/- 4 msec, respectively), suggestive of a direct fast pathway-His bundle connection. A deflection recorded between the A and H potentials of the His bundle electrogram could be dissociated from both atrial and His bundle activation. Intracellular microelectrode recordings and light microscopy confirmed the deflection to be an accessory pathway consisting of an anterior extension of fast pathway transitional cells connecting the atrium and His bundle. Transection along the AV groove anterior to the compact AV node ( N = 5) increased the retrograde ERP and Wenckebach block cycle length by severing the AH connection, or transection of the penetrating bundle ( N = 4) produced antegrade AH block without altering rapid retrograde conduction. CONCLUSIONS: Fast pathway-His bundle connections were present in 13 of 102 rabbit hearts, providing an anatomic and physiologic basis for rapid retrograde VA conduction and a possible retrograde pathway for sustained AV nodal reentrant tachycardia.

[Ultrastructure of the heart conduction system in diphtheria]. / Ul'trastruktura provodiashchei sistemy serdtsa pri difterii.

Cytoarchitectonics and ultrastructure of the node and conducting myocardiocytes of the conducting system of the heart and surrounding blood capillaries and nervous fibers are described. In diphtheria, metabolic and destructive changes were found in the node myocytes, conducting myocardiocytes and surrounding structures. The most noticeable changes were seen in the left pedicle of the atrioventricular fascicle and later others components of the conducting heart system. These changes are more pronounced in clear cells of conducting myocardiocytes as compared to the node myocytes.

The elusive extracellular AV nodal potential: studies from the canine heart, ex vivo.

Various forms of extracellular recordings from the AV node (AVN) have been reported. However, lack of consistent validation have precluded the use of such recordings in experimental and clinical studies. In 14 Langendorff perfused dog hearts, the triangle of Koch (TOK) was exposed and an octapolar electrode catheter (2 mm rings, 2 mm spacing) was inserted under the endocardium so that the bipolar pairs recorded electrograms from the apex to the base of the TOK. All recording were filtered between 0.05 and 250 Hz, except for a His bundle (Hb) recording (30-250 Hz) made from another bipolar electrode catheter placed in the aortic root. Transmembrane action potentials (AP) were recorded close to the sites of extracellular electrograms. Pin electrodes at the periphery of the bath were arranged to register two ECG leads from the volume conductor. During recovery of electrical activity 11 of 14 preparations developed a junctional rhythm that initially manifested only an AV nodal extracellular and corresponding intracellular AV nodal potentials followed gradually by conduction to the Hb and ventricles but no retrograde atrial activation; 3 preparations initially produced Hb rhythms based on extracellular and transmembrane AP recordings from the AVN and Hb. The amplitude and duration of the AVN extracellular potentials (average: 97 +/- 26 microV and 92 +/- 25 msec, respectively) during AVN rhythms, significantly differed from those during atrial pacing (262 +/- 185 microV and 78 +/- 26 msec, p < 0.05). Histologic sections of the sites underlying the electrodes recording AVN potentials showed AVN tissue throughout. We conclude that extracellular AV nodal potentials are independent waveforms with specific qualitative and quantitative characteristics that distinguish them from adjacent atrial, transitional, Hb or ventricular potentials.

[Changes of the heart conduction system after exposure to fluorouracil]. / Izmeneniia provodiashchei sistemy serdtsa pod vliianiem ftoruratsila.

Using 20 male rats the electron microscopic study of conducting system of the heart was performed to assess the effect of fluorouracil--an antineoplastic drug belonging to antimetabolite group. A daily dose of 15 mg/kg of fluorouracil was injected intraperitoneally for 5 days; three courses of injections with the intervals of three weeks were performed. Toxic effect of fluorouracil is demonstrated after the first course of injections and is revealed as a predominant injury of pacemaker cells in sinoatrial and atrioventricular nodes. Latent pacemaker myocytes and Purkinje fibers undergo "calcium injury" during the second and the third courses of fluorouracil injections. The irreversible damage of contractile and specialized myocytes results in fibrosis which develops predominantly in the areas of atrioventricular bundle and its branches.

Ultrastructural changes in the sinuatrial and atrioventricular nodes of the heart of the monkey (Macaca fascicularis) after bilateral vagotomy.

This study describes the ultrastructural changes in the sinu-atrial (SA) and atrio-ventricular (AV) nodes of the monkey (Macaca fascicularis) after bilateral mid-cervical vagotomy at 1, 3, 5, 7 and 14 days post-operations. The changes were similar in both types of nodal cells. The most obvious feature of the degenerating nodal cells was the swollen mitochondria with disrupted cristae. Other changes include increased granular sarcoplasmic reticulum, increased glycogen particles, vacuolation of mitochondria and increased lysosomal activity. Axonal profiles in the vicinity of the nodal cells showed swelling and vacuolation. Cardiac neurons also showed some changes such as distended granular endoplasmic reticulum, increased accumulation of glycogen particles and increased lipofuscin granules. Macrophages and Schwann cells were the main scavengers in removing the degenerated nodal cells and axonal profiles. In the case of affected cardiac neurons, satellite cells seemed to act as main scavenger cells. It is postulated that the nodal cells are dependent on the incoming fibres of the vagus nerve for their survival. By an understanding of the ultrastructural changes in the nodal cells after bilateral vagotomy, it may help in developing new strategies to explore in depth of the conducting system of the heart.

Ultrastructural study of the effects of 6-hydroxydopamine on the sinuatrial and atrioventricular nodes of the heart of the monkey (Macaca fascicularis).

The present study examined the ultrastructural changes in the sinuatrial and atrioventricular nodes of the heart of the monkey (Macaca fascicularis) after 6-hydroxydopamine (6-OHDA) at survival times of 1, 3, 5, 7 and 14 days. Changes ranged from dissolution of the cytoplasm to amorphous appearance and darkening of the nodal cells. Initially, the ultrastructural changes were quite similar in both sinuatrial (SA) and atrioventricular (AV) nodal cells but in the later stage, especially at fourteen days, affected SA nodal cells showed empty-looking appearance while affected AV nodal cells displayed a darkened appearance. The cardiac neurons also showed ultrastructural changes such as diffuse accumulation of glycogen particles, distended cisternae of the rough endoplasmic reticulum and increased lipofuscin granules. Some of the vacuolated axonal profiles containing large dense-cored vesicles were in close association with the somata of the cardiac neurons. There were also changes in the non-neuronal cells such as darkening and vacuolation of the cells capping the neurons. Macrophages and Schwann cells were activated to engulf the degenerating nodal cells and axonal profiles. The ultrastructural changes in the nodal cells and the cardiac neurons reflect a disturbance in the cell metabolism presumably brought about by the impairment of the sympathetic nervous system.

[Electron microscopy of the nervous apparatus of the heart sinoauricular zone in patients with idiopathic syndrome of long Q-T interval]. / Elektronno-mikroskopicheskoe issledovanie nervnogo apparata sinoaurikuliarnoi oblasti serdtsa bol'nykh s idiopaticheskim sindromom udlinennogo intervala Q-T.

All nerve components of the sinus node (SN), adjacent working myocardium and nervous ganglions of 12 patients, from 9 to 50 years old, with an idiopathic syndrome of a long Q-T interval were studied electron-microscopically. Quantitative evaluation of the myocardium innervation density in this heart region was performed. Destructive changes of the myelinated and non-myelinated nervous fibers and gliocytes in the conductive myocardium of SN and working myocardium as well as in the neurocytes of the adjacent nervous ganglions were observed. Possible causes of the ultrastructural pathology in the heart sinoauricular region are discussed.

Ultrastructure of the atrioventricular junctional area in the heart of Molossus molossus Pallas 1766 (Chiroptera: Molossidae).

The atrioventricular junctional area (AVJA) consists of a group of structures that connects the atrial and ventricular myocardium. Five hearts of an insect-eating bat were studied in light and transmission electron microscopy. In M. molossus, the AVJA consists in a mass of muscle fibers intermingled with variable amount of connective tissue and blood vessels surrounded by the adjacent myocardium and the attachment of the right atrioventricular and aortic valves in the fibrous skeleton. In light microscopy, conducting cells of the AV node and bundle can be distinguished from working cells: smaller size, paler staining reaction and the presence of e sheath of connective tissue surrounding each cell (largely composition by type I collagen fibers). Three cell types are observed in the AVJA. Nodal cells are irregular with few cytoplasmic organelles and several slender sarcolemmal modifications. Myofibrils are sparse and not clearly observable. Transitional cells are spindle-shaped and grouped together into bundles. The cytoplasm, poor in glycogen, has scarce electron-density and myofibrils organized into sarcomeres. Caveolae is observed randomly distributed at the periphery of the cell. The AV bundle cells are elongated with clusters of myofibrils organized in the periphery and a glycogen free area around the nucleus. Ventricular cells are bigger than the atrial ones and show well-developed myofibrils in alternated rows with mitochondria. Lipid droplets are seen near mitochondria and glycogen granules. Intercalated discs and T-tubules are found in working cells but not in conducting ones. The fibrous skeleton has collagen fibers intercalated with fibroblasts.

Sodium channel distribution within the rabbit atrioventricular node as analysed by confocal microscopy.

1. Paired 20 microns thick sections of fresh frozen tissue taken from the frontal plane of the rabbit atrioventricular (AV) nodal region were processed for histology and immunohistochemistry. Confocal microscopy was used to image the distribution of sodium channels using IgG (R12) developed against a highly conserved sequence in the interdomain 3-4 region of cloned sodium channels. 2. In ventricular and atrial cells, sodium channel immunofluorescence was localized to lateral membranes and T-tubules. In the open AV node, levels of sodium channel immunofluorescence in the transitional cell zone and in the lower nodal cell tract were comparable to that found in the atrial and ventricular myocardium. 3. In the enclosed AV node a gradation of sodium channel immunofluorescence is present such that peripherally located circumferential transitional cells display high levels of immunofluorescence, comparable to that of atrial and ventricular myocardium, while centrally located midnodal cells display decreased levels of or no immunofluorescence. 4. In order to correlate the distribution of sodium channels with the distribution of gap junctions, we used IgG directed against the carboxyl terminus of connexin43 (CT-360). Ventricular cell immunofluorescence was localized primarily to the intercalated disk region, while in the AV node, the pattern of distribution was found to be similar to that of sodium channels. 5. The reduced levels of and/or absence of immunofluorescence in the midnodal cell region indicates a paucity of sodium channel and connexin43 protein expression in this region of the AV node that would favour slow impulse conduction.

Angiotensin II receptor expression in the conduction system and arterial duct of neonatal and adult rat hearts.

Paralleling the classic circulating system, recent evidence has demonstrated the presence of a cardiac renin-angiotensin system, as well as the synthesis of angiotensin II in the heart. Two receptors for angiotensin II have been identified and classified as AT1 and AT2. The proportions of these receptor subtypes vary with the tissues, species and stage of development. From the results of other studies, it might be generalized that the expression of angiotensin II receptors and the proportion of AT2 receptor subtype are much higher in fetal and neonatal tissues than in the same tissues from an adult. The aim of this study was to specifically evaluate the AT1/AT2 ratio in the neonatal and adult conduction systems of rat hearts by means of quantitative autoradiogrphy. In the neonatal hearts, angiotensin II binding sites were highly concentrated in the vasculature, arterial duct, and conduction system, whereas their concentrations were barely detectable in the myocardium. Incubation with selective angiotensin II receptor ligands (losartan and CGP 42112) revealed that AT2 was the major subtype in vasculature (86 +/- 3%) and conduction system (73 +/- 4%). In the adult conduction system, the total expression of angiotensin II receptors was greatly reduced meanwhile the AT1 receptors represented the major proportion of the binding sites (80 +/- 3%). Our results demonstrated that the pattern of angiotensin II receptor expression in the conduction system of the rat heart is developmentally regulated. We suggest, as others have already, that the renin-angiotensin system plays a role during the early stage of cardiac development.

Immunoelectron microscopic localization of HNK-1 in the embryonic rat heart.

To confirm the role of HNK-1 in conduction tissue, the ultrastructural localization of monoclonal antibody HNK-1 was analyzed in developing rat hearts at embryonal day 14.5 by immunoelectron microscopic labeling procedures with post-embedding immunogold staining. Tissue sections in different planes containing the sino-atrial (SA) node, atrio-ventricular (AV) node and His bundle were used to demonstrate HNK-1. Immunogold labeling was detected on the cell surfaces and in the extracellular matrices of cells that had features common to conduction tissue cells. Non-specialized contractile myocytes were not labeled by this antibody. Furthermore, immunogold labeling was more prominent in wide intracellular spaces than in narrow intercellular spaces, and rarely observed in cell-cell contact regions. The cell surfaces and extracellular matrices of mesenchymal cells in the endocardial cushion, which contacts the His bundle, were also positive, suggesting the involvement of tract formation to the AV node. These findings may indicate that HNK-1 plays an important role in cell-cell adhesion processes both temporally and spatially in the developing conduction tissue. It was concluded, therefore, that HNK-1 is a suitable marker of the embryonic heart conduction system and might be useful in analyzing anomalous conduction systems, as in congenital heart disease.

Nerve fibres containing neuropeptide Y in the atrioventricular valves of Japanese monkey and rat; a light and electron microscopic study.

Dense distribution of varicose fibres containing neuropeptide Y-like immunoreactivity (NPY-LI) was found in the atrioventricular valves of the Japanese monkey, and moderately in the rat. The immunoelectron microscopy using immunogolds resulted in the localization of NPY-LI within the dense-cored vesicles which existed with the small clear vesicles in the unmyelinated axons near the endocardium. These NPY-LI-containing fibres may participate in regulation of vasomotor role or other functions of the atrioventricular valves.