Refractoriness of the sheep superior vena cava myocardial sleeve

The cardiomyocytes in the superior vena cava (SVC) myocardial sleeve have distinct action potentials and ionic current profiles, but the refractoriness of these cells has not been reported. Using standard intracellular microelectrode techniques, we demonstrated in sheep that the effective refractory period (ERP) of the cardiomyocytes in the SVC (114.7 +/- 6.5 ms) is shorter than that in the inferior vena cava (IVC) (166.7 +/- 6.2 ms), right atrial free wall (RAFW) (201.0 +/- 6.0 ms) and right atrial appendage (RAA) (203.1 +/- 5.8 ms) (P < 0.05). The right atrial cardiomyocyte ERP was heterogeneously shortened by acetylcholine, a muscarinic type 2 receptor (M(2)R) agonist. After perfusion with 15 microM acetylcholine, the shortest ERP occurred in the SVC (the ERP in the SVC, IVC, RAFW and RAA was 53.6 +/- 2.7, 98.9 +/- 2.2, 121.8 +/- 6.0 and 109.7 +/- 5.1 ms, respectively; P < 0.05). Carbachol (1 microM), another M(2)R agonist, produced a similar effect as acetylcholine. Furthermore, we used methoctramine, a M(2)R blocker, 4-DAMP, a muscarinic type 3 receptor (M(3)R) blocker, and tropicamide, a muscarinic type 4 receptor (M(4)R) blocker to inhibit the acetylcholine-induced ERP shortening of SVC cardiomyocytes, and found that the 50% inhibitory concentration for methoctramine, 4-DAMP and tropicamide was 5.91, 45.72 and 80.34 nM, respectively. Therefore, we conclude that the sheep SVC myocardial sleeve is a unique electrophysiological region of the right atrium with the shortest ERP both under physiological condition and under cholinergic agonist stimulation. M(2)R might play a major role in the response of the SVC myocardial sleeve to parasympathetic nerve tone. The association between the distinct refractoriness in SVC and atrial fibrillation originating from the region deserves further investigation.

Differential expression of connexin43 gap junctions in cardiomyocytes isolated from canine thoracic veins

We investigated the phenotypic features of cardiomyocytes, including the gap junctions, in the myocardial sleeve of thoracic veins. Single cardiomyocytes, isolated from the canine pulmonary veins (PV) and superior vena cava (SVC) using digestive enzymes, were examined by immunoconfocal microscopy using antisera against connexin43 (Cx43), Cx40, and other cell markers. The results showed that isolated cardiomyocytes displayed rod shapes of various sizes, ranging from <50 microm to >200 microm in length, and all the cells expressed alpha-actinin and vinculin. Gap junctions made of various amounts of Cx43 and Cx40 were found at the cell borders. These two connexins were extensively co-localized. Comparison between the thoracic veins showed that cells of the SVC contained more Cx43 gap junctions (total Cx43 gap junctions area per cell surface area, 4.0 +/- 0.2% vs 1.5 +/- 0.2%; p<0.01). In addition, for single-nucleus cells, those from the PV were longer (103.7 +/- 3.6 vs 85.0 +/- 3.1 microm; p<0.01) but narrower (14.4 +/- 0.5 vs 16.9 +/- 0.9 microm; p<0.01). In conclusion, canine thoracic veins contain cardiomyocytes with differences in shape and gap junctions, suggesting that the electrical conduction properties may be different between the thoracic veins.

Preliminary study of external interatrial muscle fascicles

The atria are highly complex multidimensional structures composed of a heterogeneous branching network of subendocardial muscular bundles. The relief of the inner part of the right atrium includes the crista terminalis as well as multiple pectinate muscles that bridge the thinner atrial free walls and appendages. However, a handful of studies have focused attention on the role of the naturally occurring complexities of the atrial subendocardial muscle structures in the mechanisms of cardiac arrhythmias. In accordance with the facts mentioned above, it was decided to examine the morphology and topography of the external interatrial junctions and related structures in order to define the possible anatomical basis of impulse propagation in focal atrial fibrillation. Research was conducted on material consisting of 15 human hearts of both sexes (female--6, male--9) from 18 to 82 years of age. In addition we were concerned, on the basis of the history and electrocardiograph tracings, that none of the patients had shown focal and non-focal type of atrial fibrillation. The classic macroscopic methods of anatomical evaluation were used. The walls of the atria were prepared via a stereoscopic microscope, the pericardium and fatty tissue were eliminated from the surface of the atria, visualising muscle fibres linking both of the atria, and the beginnings and the endpoints of fascicles in the right and left atrium were estimated. The structure, large muscle bundle, was present in all examined hearts. The muscle fascicle was descending from the anterior wall of the right atrium just below the orifice of the superior vena cava. The fascicle, running towards the left atrium, divided into two branches, one of which joined with the superior fascicle from the posterior wall and created one running above the interatrial septum and infiltrating into the wall of the left atrium on its superior surface between the superior pulmonary veins. The other branch of the anterior fascicle was running across the anterior wall of the atria and it penetrated into the left atrium muscle in the region of the inferior pole of the left auricle outlet. On the posterior wall of the atria three types of interatrial fascicles were distinguished: unifascicular, bifascicular and trifascicular. The bifascicular type was the most frequent configuration (9 cases--60.0%), in 5 cases it was trifascicular (33.3%) and finally the unifascicular configuration was observed in just 1 heart (6.7%). On the basis of our study we can conclude that the external interatrial fascicles are the constant structure of the heart, although they may have a variable morphology. Those structures could be responsible for physiological conduction between the atria and may play an important role in patients with atrial fibrillation.

Electrophysiology and arrhythmogenic activity of single cardiomyocytes from canine superior vena cava

BACKGROUND

The superior vena cava (SVC) has been proved to be a focal point in the initiation of paroxysmal atrial fibrillation. The autonomic nervous system plays an important role in the genesis of atrial fibrillation. However, the arrhythmogenic potentials of SVC and its responses to autonomic agents are not clear. The purpose of this study was to isolate single SVC cardiomyocytes and to investigate their electrophysiological characteristics, as well as the direct effects of autonomic agents.

METHODS AND RESULTS

Canine SVC cardiomyocytes were isolated by perfusion with digestive enzymes. The action potentials and ionic currents were investigated in single SVC cardiomyocytes using the whole-cell clamp technique. Dissociation of the SVC yielded rod-shaped single cardiomyocytes with (n=74, 51%) or without (n=71, 49%) pacemaker activities. There were similar densities of inward Ca2+, delayed rectifier K+, transient inward, inward rectifier K+, and pacemaker currents between SVC cardiomyocytes with and without pacemaker activity. SVC cardiomyocytes with pacemaker activity have, however, greater transient outward currents than those without pacemaker activity. In SVC cardiomyocytes, acetylcholine (5.5 micromol/L) abolished the spontaneous activities, but isoproterenol (10 nmol/L), atropine (10 micromol/L), and phenylephrine (10 micromol/L) accelerated the spontaneous activity and induced the occurrences of early or delayed afterdepolarizations.

CONCLUSIONS

These findings suggest that SVC cardiomyocytes have distinct action potentials and ionic current profiles that may be responsible for the arrhythmogenic activity of the SVC.

Correlation of endothelial vimentin content with hemodynamic parameters

In mammalian species, vimentin is the sole intermediate filament protein of endothelial cells lining the chambers of the heart and the inner surface of large blood vessels. Obvious quantitative differences in the vimentin-like immunoreactivity of endothelial cells observed in different vascular segments led us to undertake a systematic survey on the endothelial content of vimentin throughout the heart chambers, the vena cava, the pulmonary trunk, and the aorta of the pig. Immunostaining and immunoblotting showed that vimentin in endothelial cells of cardiovascular segments exposed to high shear stress and blood pressure (pulmonary trunk, aorta, left ventricle) is approximately 2- to -3-fold higher than in endothelial cells exposed to lower levels of hemodynamic stress (vena cava, left and right atria, right ventricle). Throughout the aorta, an approximately 1.5-fold increase in the vimentin contents was observed in a proximal to distal direction. The total endothelial amount of vimentin was determined to be 1.2% (inferior vena cava) and 2-3.5% (aorta) of total cellular protein. These data support the notion that the endothelial vimentin cytoskeleton can adapt to different hemodynamic loads, indicating that vimentin might help endothelial cells to withstand the mechanical forces exerted by blood flow and blood pressure.

Small intestinal submucosa as a superior vena cava graft in the dog

Autogenous spiral vein grafts and ePTFE have been used for reconstruction of the superior vena cava with moderate success. We tested autogenous small intestine submucosa as a superior vena cava interpositional graft in nine dogs. All dogs received aspirin and warfarin sodium for the first 8 weeks after surgery. Graft patency was evaluated by serial venography. One dog died from excessive anticoagulation. Eight dogs were sacrificed at periodic intervals until 72 weeks after surgery. Patent grafts had no evidence of thrombosis, aneurysm, or stenosis. The grafts consisted of dense, organized collagenous connective tissue with a complete endothelial cell layer on the luminal surface. Two dogs are alive at 28 and 34 months after surgery. Graft patency was 89% (eight of nine grafts). We conclude that autogenous small intestine submucosa can be used as a superior vena cava graft in the dog and is worthy of further investigations.

Determination of areal densities of blood vessel wall components in histological sections by means of image analysis

A technique is described which allows quantification of specifically stained, intermingled vessel wall components in paraffin-embedded tissue. The dissociation of elastic, muscular and connective tissue and the determination of their areal densities in the superior vena cava and in the ascending aorta of the dog, was performed by automated image analysis. The percentages of elastic+connective tissue, determined by this technique, correlated significantly with the data derived from biochemical measurements. It was further found that the structure of the vena cava wall was a function of the distance from the right atrium.

[Cytofluorimetric study of small, intensely fluorescing cells in the atria of rats following a single injection of guanethidine]

The fluorescence intensity of small high-fluorescent (SHF) cells of rat auricles was measured after the treatment by the modified Falk method in health and 3,14 and 72 hours after a single injection of guanethidine. No significant changes in the fluorescence intensity were found on analysis of the entire pool of SHF auricular cells and of individual SHF cells, while SHF cells of the clasters of the superior caval vein and interauricular septum manifest counterphasic deviations from the basal line of fluorescence, with these deviations being most remarkable 14 hours after the injection. The changes in the fluorescence intensity of SHF cells, which were seen within the periods of time corresponding with the minimum noradrenaline content in the heart allow the conclusion that SHF cell function is in good agreement with the heart noradrenaline level. The line of changes in the catecholamine content of SHF cells in the areas under consideration is likely to attest to the regional principle of the SHF cell pool organization in auricular tissue.

Membrane characteristics of cultured endothelial cells: identification of gap junction

Membrane characteristics of cultured endothelial cells derived from the superior vena cava of the rabbit were studied in freeze-fracture replicas. Gap junctions were identified, but tight junctions were not seen. In addition to pinocytic vesicles, fenestrations of plasma membrane which are not normally seen in the endothelium of vena cava in situ were found. This finding may be explained by the absence of platelets in the culture medium and is consistent with the current concept of endothelial-platelet interaction.