Atrial fibrillation (AF) is the most common cardiac arrhythmia. AF is thought to be triggered by ectopic beats, originating primarily in the myocardial sleeves surrounding the pulmonary veins (PVs). The mechanisms underlying these cardiac arrhythmias remain unclear. To investigate this, frozen sections of heart and lung tissue from adult rats without arrhythmia were obtained in different planes, stained with Masson's trichrome, and immunolabeled for connexin 43 (Cx43), caveolin-3 (Cav3), hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4), Nav1.5, Kir2.1, and the calcium handling proteins sarcoplasmic/endoplasmic reticulum calcium-ATPase 2a (SERCA2a) and ryanodine receptor 2 (RyR2). Transverse sections offered the best view of the majority of the PVs in the tissue samples. Cx43 was observed to be expressed throughout the atria, excluding the sinoatrial and atrioventricular nodes, and in the myocardial sleeves of the PVs. In contrast, HCN4 was only expressed in the sinoatrial and atrioventricular nodes. The immunodensity of Cav3, Nav1.5, Kir2.1, SERCA2a and RyR2 in the PVs imaged was similar to that in atria. The results suggest that in the absence of arrhythmia, the investigated molecular properties of the ion channels of rat PV cardiomyocytes resemble those of the working myocardium. This indicates that ectopic beats originating in the myocardial sleeves of the PVs occur only under pathological conditions.
Keywords: atrial fibrillation; connexin 43; ectopic beat; ion channels; pulmonary vein.