Inducible recombination in the cardiac conduction system of minK: CreERT² BAC transgenic mice.

Inducible Cre recombination is a powerful technology that allows for spatial and temporal modulation of gene expression in vivo. Diseases of the cardiac conduction system (CCS) pose a significant clinical burden but are not currently well understood at the molecular level. To enable inducible recombination in the murine CCS, we created a minK:CreERT(2) bacterial artificial chromosome (BAC) transgenic mouse line. Cre activity is present after tamoxifen administration in the atrioventricular (AV) node, AV bundle, and bundle branches of adult transgenic mice. We anticipate that by enabling inducible recombination specifically in the AV node, bundle, and bundle branches, minK:CreERT(2) BAC transgenic mice will prove useful in advancing our understanding of CCS disease and function.

[Enzyme activity in the normal atrioventricular node and in the bundle of His in young children]. / Aktivity enzymu v normálním atrioventrikulárním uzlu a v Hisove svazku u malých detí.

The authors examined by histochemical methods the atrioventricular node and the bundle of His in five infants who died from unnatural deaths and in a four-and-half-year-old child who died immediately following haemorrhagic infarction of the gut. Both structures differed, similarly as in adult age, from working muscle by a lower oxidation capacity, higher activity of the pentose-phosphate shunt and a high acetylcholine esterase activity.

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.

Development changes in the human cardiac isomyosin distribution: an immunohistochemical study using monoclonal antibodies.

With monoclonal antibodies (Mab) specific for myosin heavy chain (MHC) isozymes, we have investigated the isomyosin content of atrial, ventricular and conductive fibers of 19 human fetuses (ranging from 14-36 weeks of gestation) and 3 newborns (2 days-2 weeks). In addition, the conduction system of 2 human adult hearts was studied. The fetal atrium is composed mostly of alpha-MHC during the first 23 weeks of gestation. beta-MHC is already expressed as traces at 14 weeks of gestation, and its expression increases progressively until birth, resulting in a great augmentation in beta-MHC. During this course, beta-MHC always predominates in certain areas (the crista terminalis and the interatrial septum) but not in other areas (the auricles). Preceding birth, the fetal ventricle is composed mostly of beta-MHC. From 14 weeks of gestation to birth, alpha-MHC is expressed in very rare fibers. Then, after birth, a large number of fibers simultaneously synthesize alpha-MHC. The AV node and His bundle system were labelled with anti-alpha and anti-beta Mab in fetal, newborn, and adult hearts with a double gradient of distribution: spatial (a higher proportion of alpha-containing fibers in the AV node than in the distal portion of the bundle of branches) and temporal (a higher proportion of alpha-containing fibers at a given point in fetal development than in the adult heart). One of the twenty-five hearts studied had an isomyosin distribution pattern not accorded to its age. Interestingly, it was clinically diagnosed as having idiopathic hypertrophic cardiomyopathy.

An enzyme histochemical study of human sinus node, coronary sinus, and mitral valve muscle.

An enzyme histochemical study of the sinoatrial node, the coronary sinus, and the atrial muscle extending into the anterior mitral valve was performed on human hearts. Investigation of the activity and localization of the structurally bound enzymes was performed by conventional histochemical techniques. Determination of the activity of nonstructurally or weakly structurally bound enzymes was performed by histochemical techniques in which leakage of enzymes during the incubation period was reduced by the application of semipermeable membranes. The sinoatrial node is characterized by a high degree of anaerobic enzyme capacity and a relatively low degree of aerobic enzyme capacity. The discriminatory nature of these reactions allows examination of the structure of the sinoatrial node and its approaches. The presence of transitional cells was confirmed; isolated clusters of nodal cells were found in the atrial myocardium around the sinoatrial node, but no evidence of specialized tissue forming the beginning of an internodal pathway was found by this technique. The specific histochemical reactions that characterize the sinoatrial node also occur in the atrial muscle, extending into the anterior mitral valve, the anterior wall of the coronary sinus, and the atrial tissue near the orifice of the coronary sinus. These observations seem to corroborate the hypothesis that arrhythmic ectopic foci can arise in these regions.

[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.

Phosphorylase activity as a histochemical marker of specialized tissue of heart.

Phosphorylase activity has been investigated histochemically under controlled conditions in the specialized tissue and the general myocardium of goat heart using glucose-1-phosphate as the substrate. It has been observed that the phosphorylase content of the nodal and conducting tissue is very high as compared to the general myocardium. The phosphorylase content of the cardiac neural elements is also high. Since the connective tissue which surrounds the nodal and conducting tissue has almost no phosphorylase activity, the nodal and conducting tissue is sharply demarcated by the histochemical reaction for phosphorylase. This reaction can, therefore, be used as a specific and expedient method for the localization of specialized tissue as an exploratory and confirmatory test.