Distribution and metabolism of norepinephrine, cyclic AMP and cyclic GMP in the atrioventricular conducting tissue of the bovine heart.

The distribution of norepinephrine (NE), cyclic AMP (cAMP) and cyclic GMP (cGMP) and the activities of related enzymes in the atrioventricular (A-V) conducting tissue of the bovine heart were examined. The concentration of NE in the atrium was about twice that in the ventricle. In the A-V conducting tissue, the concentration of NE was highest in the atrioventricular node (AVN) and lowest in the false tendon (FT), with intermediate levels in the bundle of His (HIS) and the right and left bundle branches (RLBB). The activity of monoamine oxidase (MAO) in the atrium was about 2.2 times that in the ventricle. In the A-V conducting tissue, the activity of MAO was highest in the HIS and lowest in the FT. The activity of catechol-o-methyltransferase (COMT) in the atrium and ventricle was similar, and that in the HIS was slightly, but not significantly, higher than that in other regions of the A-V conducting tissue. The concentration of cAMP in the ventricle was about twice that in the atrium. In the A-V conducting tissue, the concentration of cAMP was higher in the AVN and FT than in the HIS and RLBB. The distribution of adenylate cyclase (AC) was similar to that of NE. The phosphodiesterase (PDE) activity in the atrium and ventricle was similar. No significant difference was found in the level of PDE activity in different regions of the A-V conducting tissue. The concentration of cGMP was slightly, but not significantly, higher in the A-V conducting tissue than in the atrium or ventricle. In the A-V conducting tissue, the concentration of cGMP was highest in the FT and the concentrations in the HIS, RLBB and AVN were similar. These findings suggest that in the A-V conduction tissue, the regions that have the higher spontaneous pacemaker rates have higher NE content and AC activity, that is sensitivity to NE. Furthermore, the sensitivity for muscarinic cholinergic stimulation is higher in the conducting tissue (especially in the FT) than in the atrium and ventricle.

The atrioventricular node of the golden hamster (Mesocricetus auratus): a light and electron microscopic study including immunocytochemistry.

The atrioventricular (AV) node of the golden hamster is situated unusually high in the interatrial septum when compared to other species such as the rat. 2 main cell types, characterized by electron-lucent or electron-dense cytoplasm respectively, are found in the node; although both types contain numerous myofilaments these are irregularly arranged and sarcomeric banding is poor. A third variety comparising transitional cells, with features intermediate between the main nodal cells and general atrial myocardial cells, are found at the periphery of the node. Similar electron-lucent and electron-dense cells are also found in the bundle and the mean diameter of bundle cells increases as one passes from the node to the bundle bifurcation. In the node, specific heart granules (SHG) identified by ANP-28 immunoreactivity are found only in transitional cells and even here they are very sparse, unlike general atrial myocytes in which they are plentiful. Numerous nerve varicosities are present throughout the node and bundle and 5-hydroxydopamine (5-OHDA) labelling demonstrates that most of them have features of either noradrenergic or cholinergic terminals; a few non-cholinergic, non-adrenergic varicosities are also present.

Distribution of cardiac myosin isozymes in human conduction system. Immunohistochemical study using monoclonal antibodies.

To determine the presence and distribution of cardiac myosin isozymes in the human conduction system, we performed an immunohistochemical study using monoclonal antibodies CMA19 and HMC14, which are specific for myosin heavy chains of human atrial type (alpha-type) and ventricular type (beta-type), respectively. Serial frozen sections of human hearts were obtained from autopsy samples and examined by indirect immunofluorescence. Alpha-type was found in all myofibers of sinus node and atrio-ventricular node, and in 55.2 +/- 10.2% (mean +/- SD, n = 5) of the myofibers of ventricular conduction tissue, which consists of the bundle of His, bundle branches, and the Purkinje network. In contrast, beta-type was found in all myofibers of the atrio-ventricular node and ventricular conduction tissue, whereas almost all myofibers of the sinus node were unlabeled by HMC14. Although the number of ventricular myofibers labeled by CMA19 was small, the labeled myofibers were more numerous in the subepicardial region than in the subendocardial region. These findings show that the gene coding for alpha-type is expressed predominantly in specialized myocardium compared with the adjacent ordinary working myocardium.

Cyclic adenosine 3', 5'-monophosphate, adenyl cyclase and phosphodiesterase in the conduction system of bovine heart.

Cyclic adenosine 3', 5'-monophosphate (cyclic AMP) and adenyl cyclase and phosphodiesterase activities were determined in the specialized myocardial tissue of the conduction system of bovine heart and then compared with those in the ordinary myocardial tissue. The conduction system was comprised of the atrioventricular node (A-V node), the His bundle and the right and the left bundle branches (RBB and LBB). The content of cyclic AMP was higher in the ordinary myocardial tissue than in the specialized myocardial tissue. In the specialized myocardial tissue, its content was highest in the A-V node and lower in the His bundle than in the LBB and the difference between the contents in the RBB and the LBB was not significant. Adenyl cyclase activity as well as the content of cyclic AMP was higher in the ordinary myocardial tissue than in the specialized myocardial tissue. Its activity was higher in the A-V node than in the His bundle or the RBB, and the activities in the His bundle, the RBB and the LBB were similar. Phosphodiesterase activity was higher in the ordinary myocardial tissue than in the A-V node, and the activities in these 4 sections of the conduction system were similar.