Extended atrial conduction system characterised by the expression of the HCN4 channel and connexin45.

OBJECTIVE: In the heart, there are multiple supraventricular pacemakers involved in normal pacemaking as well as arrhythmias and the objective was to determine the distribution of HCN4 (major isoform underlying the pacemaker current, I(f)) in the atria. METHODS: In the atria of the rat, the localisation of HCN4 and connexins was determined using immunohistochemistry, and electrical activity was recorded using extracellular electrodes. RESULTS: As expected, HCN4 and Cx45 (but not Cx43) were expressed in the sinoatrial node extending from the superior vena cava down the crista terminalis. The same pattern of expression of HCN4 and connexins was observed in a novel tract of nodal-like cells extending from the superior vena cava down the interatrial groove. Although the sinoatrial node was usually the leading pacemaker site, the novel tract of HCN4-expressing cells was capable of pacemaking and could act as the leading pacemaker site; there was evidence of a hierarchy of pacemakers. The same pattern of expression of HCN4 and connexins was also observed in the atrioventricular ring bundle (including the atrioventricular node) encircling the tricuspid valve, but not in the atrioventricular ring bundle encircling the mitral valve. HCN4 was not expressed in the pulmonary veins. CONCLUSIONS: The widespread distribution of HCN4 can explain the widespread location of the leading pacemaker site during sinus rhythm, the extensive region of tissue that has to be ablated to stop sinus rhythm, and the widespread distribution of ectopic foci responsible for atrial tachycardia.

Localization of Na+ channel isoforms at the atrioventricular junction and atrioventricular node in the rat.

BACKGROUND: The electrical activity of the atrioventricular node (AVN) is functionally heterogeneous, but how this relates to distinct cell types and the 3-dimensional structure of the AVN is unknown. To address this, we have studied the expression of Na(V)1.5 and other Na+ channel isoforms in the AVN. METHODS AND RESULTS: The rat AVN was identified by Masson's trichrome staining together with immunolabeling of marker proteins: connexin40, connexin43, desmoplakin, atrial natriuretic peptide, and hyperpolarization-activated and cyclic nucleotide-gated channel 4. Na+ channel expression was investigated with immunohistochemistry with isoform-specific Na+ channel antibodies. Na(V)1.1 was distributed in a similar manner to Na(V)1.5. Na(V)1.2 was not detected. Na(V)1.3 labeling was present in nerve fibers and cell bodies (but not myocytes) and was abundant in the penetrating atrioventricular (AV) bundle and the common bundle but was much less abundant in other regions. Na(V)1.5 labeling was abundant in the atrial and ventricular myocardium and the left bundle branch. Na(V)1.5 labeling was absent in the open node, penetrating AV bundle, AV ring bundle, and common bundle but present at a reduced level in the inferior nodal extension and transitional zone. Na(V)1.6 was not detected. CONCLUSIONS: Our findings provide molecular evidence of multiple electrophysiological cell types at the AV junction. Impaired AV conduction as a result of mutations in or loss of Na(V)1.5 must be the result of impaired conduction in the AVN inputs (inferior nodal extension and transitional zone) or output (bundle branches) rather than the AVN itself (open node and penetrating AV bundle).

Three-dimensional reconstruction of the rabbit atrioventricular conduction axis by combining histological, desmin, and connexin mapping data.

BACKGROUND: The 3D structure of the atrioventricular conduction axis incorporating detailed cellular and molecular composition, especially that relating to gap-junctional proteins, is still unclear, impeding mechanistic understanding of cardiac rhythmic disorders. METHODS AND RESULTS: A 3D model of the rabbit atrioventricular conduction axis was reconstructed by combining histological and immunofluorescence staining on serial sections. The exact cellular boundaries, especially those between transitional cells and atrial myocardium, were demarcated by a dense and irregular desmin-labeling pattern in conductive myocardium. The model demonstrates that the atrioventricular conduction axis is segregated into 2 connecting compartments, 1 predominantly expressing connexin45 (compact node and transitional cells) and the other predominantly coexpressing connexin43 and connexin45 (His bundle, lower nodal cells, and posterior nodal extension). The transitional zone shows unique features of spatial complexity, including a bridging bilayer structure (a deep transitional zone connecting with a superficial atrial-transitional overlay) and asymmetrical continuity (wider atrial-transitional interfaces and shorter atrial-axial distances in the hisian portion than in the ostial portion). In the latter compartment, the His bundle, lower nodal cells, and posterior nodal extension form a continual axis and longitudinal transitional-axial interface. CONCLUSIONS: Key findings of the present study are the demonstration of a distinct anatomical border between transitional and atrial cells, connection between transitional cells and both lower nodal cells and posterior nodal extension, and distinctive connexin expression patterns in different compartments of the rabbit atrioventricular conduction axis. These features, synthesized in a novel 3D model, provide a structural framework for the interpretation of nodal function.

Cx43 and dual-pathway electrophysiology of the atrioventricular node and atrioventricular nodal reentry.

Fluorescent imaging has revealed that posterior nodal extensions provide the anatomical substrate for the dual-pathway electrophysiology of the atrioventricular (AV) node during normal conduction and reentry. The reentry can be intranodal, or as well as the posterior nodal extensions, it can involve an endocardial layer of atrial/atrial-nodal (A/AN) cells as part of the AV nodal reentry (AVNR) circuit. Using fluorescent imaging with a voltage-sensitive dye and immunolabeling of Cx43, we mapped the electrical activity and structural substrate in 3 types of AVNR induced by premature atrial stimulation in 8 rabbit hearts. In 6 cases, the AVNR pathway involved (1) a fast pathway (FP), (2) the A/AN layer, and (3) a slow pathway (SP). In 4 cases, reentry took the path (1) SP, (2) A/AN layer, and (3) FP. In 2 cases, reentry was intranodal, propagating between the 2 posterior nodal extensions. Immunolabeling revealed that the FP and SP are formed by Cx43-expressing bundles surrounded by tissue without Cx43. Cx43-expressing posterior nodal extensions are the substrate of AVNR during both intranodal and extranodal reentry.

An atrioventricular canal domain defined by cardiac troponin I transgene expression in the embryonic myocardium.

During early cardiac development the atrial myocardium is continuous with the ventricular myocardium throughout the atrioventricular canal. The atrioventricular canal undergoes complex remodelling involving septation, formation of atrioventricular valves and insulation between atria and ventricles except at the level of the atrioventricular node. Understanding of these processes has been hampered by the lack of markers specific for this heart region. We have generated transgenic mice expressing beta-galactosidase under the control of the cardiac troponin I gene that show transgene expression mainly confined to the atrioventricular canal myocardium during early embryonic development. With further development beta-galactosidase positive cells are observed in the atrioventricular node and in the lower rim of both right and left atria, supporting the view that atrioventricular canal myocardium contributes to the atrioventricular node and is in part incorporated into the lower rim of the atria. These results identify the atrioventricular canal myocardium as a distinct transcriptional domain.

Ontogeny of Ro hYRNAs in human heart.

The Ro complex is composed of three ribonucleoproteins of 60, 54, and 52 kDa. The Ro60 is associated with the 1-5 hYRNAs. Autoantibodies against Ro are found in sera from lupus patients. Lupus frequently presents in female patients during childbearing years and transplacental passage of maternal anti-Ro/La autoantibodies has been implicated in triggering neonatal lupus and congenital heart block (CHB). Since expression of Ro ribonucleoproteins occurs before the onset of CHB, epitope spreading of Ro hYRNAs could arise during cardiac ontogeny, thus maternal autoantibodies would damage a previously developed AV node. To explore the cardiac ontogeny of Ro hYRNAs, embryonic and adult tissues were obtained from legal autopsies. In situ hybridization using oligonucleotides for hY1, hY3, hY4, hY5, and Ro60 was performed on sections of cardiac tissue; their corresponding cDNAs were amplified by RT-PCR. The principal results were as follows: expression of hY4/5RNAs was greater in fetal tissues from 8-12 weeks of development. The subcellular distribution of hYRNAs in embryonic tissues was nucleocytoplasmic, whereas in the adult heart it was cytoplasmic. In conclusion, hYRNAs expression occurs during early cardiac development.

Connexin45 (alpha 6) expression delineates an extended conduction system in the embryonic and mature rodent heart.

We previously demonstrated that alpha 6 (Cx45), one of the three connexins of the mammalian myocardium, is preferentially expressed in the peripheral portion of the ventricular conduction system in rats and mice. Here we report that alpha 6 is also prominently immunolocalized in the atrioventricular node and His bundle of these species. The distribution of immunolocalized alpha 6 reveals that the node and bundle form part of an extended central conductive network circumscribing the AV and outflow junctional regions of the fetal, and less continuously, the adult heart. Of the three cardiac connexins, alpha 6 is the isoform most continuously expressed by conduction tissues, and may thus account for the recently reported viability of the alpha 5 (Cx40) knockout mouse. It is concluded that alpha 6 expression is a defining feature of the heterogenous tissues comprising the atrioventricular conduction system of the rodent heart.

Development of the cardiac conduction tissue in human embryos using HNK-1 antigen expression: possible relevance for understanding of abnormal atrial automaticity.

BACKGROUND: Abnormal atrial automaticity in young patients with structurally normal hearts is often located around the pulmonary veins and in sinus venosus-related parts of the right atrium. We hypothesize that these ectopic pacemaker sites correspond to areas of embryonic myocardium with an early phenotypic differentiation, as indicated by differences in antigen expression during normal cardiac development. METHODS AND RESULTS: In human embryos ranging in age from 42 to 54 days of gestation, the development of the cardiac conduction system was studied with the use of HNK-1 immunohistochemistry. HNK-1 stains the developing atrioventricular conduction system, ie, the bundle branches, His bundle, right atrioventricular ring, and retroaortic ring. In addition, the myocardium around the common pulmonary vein showed transient HNK-1 antigen expression. In the right atrium, 3 HNK-1-positive connections were demonstrated between the sinoatrial node and the right atrioventricular ring. An anterior tract through the septum spurium connects the sinoatrial node with the anterior right atrioventricular ring, and 2 posterior tracts connect the sinoatrial node with the posterior right atrioventricular ring through the right venous valve (future crista terminalis) and sinus septum, encircling the coronary sinus. The medioposterior part of the right atrioventricular ring connected to the His bundle and the medioanterior part form 2 node-like structures. CONCLUSIONS: In patients with abnormal atrial automaticity, the distribution of left and right atrial pacemaker foci correspond to areas of the embryonic myocardium that temporarily express the HNK-1 antigen.

Congenital cystic tumors of the atrio-ventricular node: successful demonstration by an abbreviated dissection of the conduction system.

Pathologists are dissuaded from the pathological study of the conduction system by the high cost and complexity of a traditional complete study. However, a simplified, low-cost approach can produce concrete information when performed in carefully selected cases of atrioventricular block, as demonstrated in the two cases of congenital cystic tumor of the atrioventricular node described in this report.

Histological evidence for cell proliferation activity in cystic tumor (endodermal heterotopia) of the atrioventricular node.

Cystic tumor (endodermal heterotopia) of the atrioventricular (AV) node in a 95-year-old female is described. Electrocardiograms showed complete AV blockage with a narrow QRS morphology resulting from the long-standing first degree of AV blockage since age 61. A cardiac pacemaker was implanted at age 83. The patient died of pneumonia at age 95. Autopsy demonstrated a normal heart weight (320 g), normal coronary artery and normal myocardium. Histological examination of the conduction system revealed a marked fibrotic sinoatrial (SA) node and a cystic lesion in the lower right atrium, including the approaches to the AV node. The bundle of His and its branches were completely intact. Cellular nests and cystically dilated tubules were scattered within a loose tissue stroma, which measured 18 x 12 x 5 mm. There were several mitotic figures among the tubules, as well as nuclear fragments similar to apoptotic bodies and exfoliation of the tumor cells. Immunohistochemical studies demonstrated a positive reaction for epithelial membrane antigen, carcinoembryonic antigen, CA19-9, cytokeratin and secretory component in the tubular cells, and a negative reaction for thrombomodulin. Endocrine cells among the tubules were positive only for calcitonin and serotonin. Ultrastructurally, sparse microvilli and desmosome-like structures between the tumor cells were observed. These findings support that this lesion originates from the endoderm. Moreover, it is likely that progression in the degree of AV block may be explained by the cell proliferation activity of the tumor.

Spatial distribution of nerve processes and beta-adrenoreceptors in the rat atrioventricular node.

Atrioventricular (AV) nodal conduction time is known to be modulated by the autonomic nervous system. The presence of numerous parasympathetic and sympathetic nerve fibres in association with conduction tissue in the heart is well authenticated. In this study, confocal microscopy was used to image the distribution of antibodies directed against the general neuronal marker PGP 9.5, tyrosine hydroxylase (TH), vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP) and beta1 and beta2-adrenoreceptors. Serial 12 microm sections of fresh frozen tissue taken from the frontal plane of the rat atrioventricular node, His bundle and bundle branches were processed for histology, acetylcholinesterase (AChE) activity and immunohistochemistry. It was found that the AV and ventricular conduction systems were more densely innervated than the atrial and ventricular myocardium as revealed by PGP 9.5 immunoreactivity. Furthermore, the transitional cell region was more densely innervated than the midnodal cell region, while spatial distribution of total innervation was uniform throughout all AV nodal regions. AChE-reactive nerve processes were found throughout the AV and ventricular conduction systems, the spatial distribution of which was nonuniform exhibiting a paucity of AChE-reactive nerve processes in the central midnodal cell region and a preponderance in the circumferential transitional cell region. TH-immunoreactivity was uniformly distributed throughout the AV and ventricular conduction systems including the central midnodal and circumferential transitional cell regions. Beta1-adrenoreceptors were found throughout the AV and ventricular conduction systems with a preponderance in the circumferential transitional cell region. Beta2-adrenoreceptors were localised predominantly in AV and ventricular conduction systems with a paucity of expression in the circumferential transitional cell region. These results demonstrate that the overall uniform distribution of total nerve processes is comprised of nonuniformly distributed subpopulations of parasympathetic and sympathetic nerve processes. The observation that the midnodal cell region exhibits a differential spatial pattern of parasympathetic and sympathetic innervation suggests multiple sites for modulation of impulse conduction within this region. Moreover, the localisation of beta2-ARs in the AV conduction system, with an absence of expression in the circumferential transitional cell layer, suggests that subtype-specific pharmacological agents may have distinct effects upon AV nodal conduction.

Reduced cardiac conduction velocity and predisposition to arrhythmias in connexin40-deficient mice.

Intercellular channels of gap junctions are formed in vertebrates by the protein family of connexins and allow direct exchange of ions, metabolites and second messenger molecules between apposed cells (reviewed in [1-3]). In the mouse, connexin40 (Cx40) protein has been detected in endothelial cells of lung and heart and in certain heart muscle cells: atrial myocytes, cells of the atrial ventricular (AV) node and cells of the conductive myocardium, which conducts impulses from the AV node to ventricular myocyctes [3]. We have generated mice homozygous for targeted disruption of the Cx40 gene (Cx40-/-mice). The electrocardiograph (ECG) parameters of Cx40-/- mice were very prolonged compared to those of wild type (Cx40+/+) mice, indicating that Cx40-/- mice have lower atrial and ventricular conduction velocities. For 6 out of 31 Cx40-/- animals, different types of atrium-derived abnormalities in cardiac rhythm were recorded, whereas continuous sinus rhythm was observed for the 26 Cx40+/+ and 30 Cx40+/- mice tested. The expression levels of other connexins expressed in heart (Cx37, Cx43 and Cx45) were the same in Cx40-/- and Cx40+/+ mice. Our results demonstrate the function of Cx40 in the regulation and coordination of heart contraction and show that cardiac arrhythmogenesis can not only be caused by defects in the ion channels primarily involved in cellular excitation but also by defects in intercellular communication through gap junction channels. As the distribution of Cx40 protein is similar in mouse and human hearts, further functional analysis of Cx40 should yield relevant insights into arrhythmogenesis in human patients.

The common 4977 base pair deletion of mitochondrial DNA preferentially accumulates in the cardiac conduction system of patients with Kearns-Sayre syndrome.

Previous studies of cytochrome c oxidase (complex IV of the respiratory chain) in the heart of a 26-year-old man with longstanding Kearns-Sayre syndrome and fatal congestive cardiomyopathy had revealed the presence of randomly distributed enzyme-deficient cardiomyocytes, both in the contractile and the conducting myocardium. In the present study, the conduction system of the heart was screened for the occurrence of the common 4977 base pair deletion (8, 482-13, 459) of mitochondrial DNA (mtDNA) in formalin-fixed, paraffin-embedded tissue and compared with the contractile myocardium. Polymerase chain reaction analysis revealed that in the sinus node, the atrioventricular node, and the bundle branches, 35 to 40% of total mtDNA molecules harbored the common deletion. In contrast, in the contractile myocardium, 10 to 20% of total mtDNA was deleted (P = .05). These results demonstrate that in Kearns-Sayre syndrome, the conduction system of the heart preferentially accumulates the common deletion. This finding might help to explain the high prevalence of cardiac dysrhythmias in this syndrome.

Immunohistochemical expression of atrial natriuretic peptide (ANP) in the conducting system and internodal atrial myocardium of human hearts.

Expression and distribution of atrial natriuretic peptide (ANP) were studied immunohistochemically in the conducting system and internodal atrial myocardium of 5 adult human hearts. Myocytes from the sinus node and compact atrioventricular node were usually ANP-negative; only a very few cells exhibited ANP immunoreactivity. These ANP-positive myocytes were small and did not appear to be trapped working atrial myocytes which are larger than nodal cells. The transitional cell zones of the sinus node and the atrioventricular node were composed of bundles of ANP-positive myocytes, intermingled with non-reactive myocytes. The internodal atrial myocardium exhibited a comparable intensity of myocyte staining in each case examined. Thus, morphologically distinct connecting pathways between the sinus node and the atrioventricular node with regard to myocyte ANP immunoreactivity could not be demonstrated, reinforcing the notion that they actually do not exist. The penetrating bundle, branching bundle and bundle branches were usually composed of ANP-negative myocytes although some ANP-positive myocytes were observed in the branching bundle and bundle branches in 4 cases. Myocytes from the ventricular conducting tissue presenting ANP immunoreactivity have been designated Purkinje fibers and have been found in several mammalian species.

Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart.

The delayed rectifier potassium current (IK) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics of IK in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at -40 mV and 5 microM external nifedipine was used to block L-type calcium current (ICa,L). Significant IK was observed with pulses to potentials more positive than -30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation of IK occurred at -4.9 and -4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significant IK tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100-120 ms in both cell types. Pulses of longer duration did not activate further IK and this suggested that only the rapid component of IK, called IKr, was present in either cell type. Moreover, IK tails after pulses to all potentials were blocked completely by E-4031, a selective blocker of IKr. The reversal potential of IK varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of -81, -56 and -32 mV, respectively, which are close to values predicted if the IK channel were highly selective for K. The time constants for deactivation of IK in ventricle and AVN on return to -40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation of IK in AVN cells was a distinguishing feature and suggests that there may be differences in the IKr channel protein between ventricular and AVN cells.

Metaiodobenzylguanidine as an index of atrioventricular nodal adrenergic activity.

UNLABELLED: Despite its importance, little is known about the uptake, storage and release of catecholamines in the atrioventricular (AV) node and His bundle. Previous in vitro studies have been limited by metabolism of norepinephrine. Metaiodobenzylguanidine (MIBG) shares many transport properties with norepinephrine and is considered a functional marker of adrenergic activity. METHODS: We used [125I]MIBG +/- 99mTc-sestamibi (99mTc-MIBI) and [123I]MIBG +/- 201TI] to evaluate regional differences in adrenergic activity between cardiac conductive and contractile elements in rats. Histological localization of the AV node and His bundle was performed using stains for acetylcholinesterase. RESULTS: Densitometric evaluation of autoradiographs, obtained from 20-mu thick sections of hearts from rats injected with either [125I]MIBG +/- 99mTc-MIBI (n = 4) and [123I]MIBG +/- 201TI (n = 6), revealed that there was approximately 30% more MIBG uptake in the AV node and His bundle compared to atrial or ventricular muscle (p < 0.05). Color-coded functional maps, generated by computer to simultaneously display 123I or [125I]MIBG and perfusion markers, revealed that the heterogeneous distribution of MIBG was independent of myocardial blood flow. CONCLUSION: When used as a selective functional marker of adrenergic activity in the cardiac conduction system, 123I- or [125I]MIBG autoradiography demonstrates increased adrenergic activity in the AV node and His bundle compared with the left ventricle. MIBG imaging provides a new research technique to probe in vivo modulation of AV nodal and His bundle sympathetic activity.

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.

Alpha-B crystallin in the normal human myocardium and cardiac conducting system.

The alpha crystallins are major protein components of the ocular lens and show both structural and functional homology with the family of small heat shock proteins. alpha B crystallin is also present in various extra-lenticular tissues, with a high concentration in cardiac muscle. In this study, the myocardium and conducting system from 15 adult and 25 fetal and infant hearts were examined by immunohistochemistry using a previously characterized antiserum to alpha B crystallin. Contractile myocardium showed moderate staining, with particular localization to Z bands and intercalated discs. Fibres of the sino-atrial and atrio-ventricular nodes and His bundle showed less intense staining than contractile fibres, whereas fibres of the left and right bundle branches showed more intense staining. This distribution is similar to that previously demonstrated for the intermediate filament desmin. This observation, together with currently available evidence, suggests that cardiac alpha B crystallin may play a role in protecting the cytoskeleton during cell stress. For practical purposes, immunostaining with alpha B crystallin greatly facilitates the identification of cardiac conducting fibres.

Autoradiographic localization and quantitation of beta 1- and beta 2-adrenoceptors in the human atrioventricular conducting system: a comparison of patients with idiopathic dilated cardiomyopathy and ischemic heart disease.

The density and distribution of beta 1- and beta 2-adrenoceptors in the atrioventricular conducting system and interatrial and interventricular septa from human hearts with idiopathic dilated cardiomyopathy and ischemic heart disease was determined by quantitative autoradiography using (-)[125I]cyanopindolol and the selective beta 1-adrenoceptor antagonist CGP 20712A and the selective beta 2-adrenoceptor antagonist ICI 118,551. Both beta 1- and beta 2-adrenoceptors were present in the atrioventricular node, bundle of His, interatrial and interventricular septa. No differences in the density or proportions of beta 1- and beta 2-adrenoceptors in the atrioventricular node, bundle of His, interatrial septum and interventricular septum were observed between hearts with idiopathic dilated cardiomyopathy or ischemic heart disease (P > 0.05) so further analysis did not distinguish between the two aetiologies. The density of beta 1-adrenoceptors was lower in the bundle of His (5.0 +/- 1.7 fmol/mg protein) than in the atrioventricular node (22.2 +/- 5.7 fmol/mg protein, P < 0.05), the interatrial septum (29.6 +/- 4.5 fmol/mg protein, P < 0.001) and interventricular septum (24.9 +/- 5.2 fmol/mg protein, P < 0.005, n = 8 for all values). The atrioventricular node, interatrial and interventricular septa had similar densities of beta 1-adrenoceptors (P = 0.60, ANOVA). The distribution of beta 2-adrenoceptors in the atrioventricular node (21.5 +/- 4.1 fmol/mg protein), bundle of His (12.9 +/- 2.6 fmol/mg protein) and atrial (16.7 +/- 2.3 fmol/mg protein) and septal myocardium (13.8 +/- 2.5 fmol/mg protein, n = 8 for all values) was uniform (P = 0.18, ANOVA). The percentage of beta 1- and beta 2-adrenoceptors in the atrioventricular node, bundle of His, interatrial and interventricular septa was uneven (P < 0.001, ANOVA). There was a higher proportion of beta 2-adrenoceptors in the bundle of His (72 +/- 6%) than in the atrioventricular node (51 +/- 3%, P < 0.01), interatrial septum (36 +/- 1%, P < 0.001) and interventricular septum (36 +/- 1%, P < 0.001).

[Angiotensin II receptors in the heart conduction tissue]. / I recettori dell'angiotensina II nel tessuto di conduzione cardiaco.

Angiotensin II exerts a positive chronotropic effect on mammalian heart. This effect can be mediated either by activation of the sympathetic nervous system or binding to specific receptors located in cardiac conduction tissue. Recently, two angiotensin II receptor subtypes have been identified in rat tissues. In the present study, we have investigated tissue distribution of angiotensin II receptor subtypes in conduction tissue obtained from rat hearts, employing an in situ autoradiographic technique. Receptor subtypes were identified by use of the peptide antagonists losartan (AT1) and PD123,177 (AT2). Angiotensin II binding was homogeneously distributed in right and left ventricle, and interventricular septum, with each subtype accounting for approximately 50% of the specific binding. Receptor density in the conduction tissue, identified by acetylcholinesterase histochemistry, was quantitated by counting the silver grains on tissue sections which, following incubation, were dipped in photographic emulsion and developed. In both sinoatrial and atrioventricular node AT1 and AT2 receptors were homogeneously distributed. Receptor density in sinoatrial node was comparable to myocardium, whereas it was significantly higher (p < 0.02) in atrioventricular node. In conclusion, both AT1 and AT2 receptors are homogeneously distributed in cardiac conduction tissue. Binding density is higher in the atrioventricular node than in the other structures examined in our study, suggesting a possible role of angiotensin II in the atrioventricular conduction.