Abstract
The properties of the atrio-ventricular (AV) node in the neonate heart and its role in unique pediatric cardiac arrhythmias such as Junctional Ectopic Tachycardia (JET) are poorly understood. This is due in large part to the dearth of information on the structure and physiology of the AV node in the immature myocardium. Sino-atrial nodal cells (SANCs), AV nodal tissues and myocytes (AVNCs) were isolated from neonatal (10 day old) rabbits, and the histological, immunohistological and electrophysiological properties were characterized. Masson's trichrome histological staining clearly delineated the structures within the AV nodal area including the inferior nodal extension (INE), compact node (CN) and the His bundle (HB) region. AV tissue sections and AVNC were immunolabeled with antibodies against neurofilament 160 (NF160), connexin 43 (Cx43), hyperpolarization-activated, cyclic nucleotide modulated channel (HCN4), sodium/calcium exchanger (NCX), ryanodine receptor (RyR), sarcoplasmic/endoplasmic reticulum Ca2+ pump (SERCA) and phospholamban (PLB). Triple labeled AVNC co-expressed NF160, SERCA2a and PLB with the latter two proteins showing a high degree of colocalization. The majority (59%) of NF160 positive AVNC also expressed HCN4, however, this figure was significantly higher in SANC (88%). Spontaneous action potentials recorded from single neonatal AVNCs were uniformly atrio-nodal (AN)-type. Hyperpolarization-activated funny current (If) was observed in 55% (11 out of 21 cells) of AVNCs consistent with the immunocytochemistry. This represents the first report on the detailed electrophysiological and immunohistochemical analysis of the AV node in neonates and lays the groundwork for a better understanding of heart rate regulation and unique arrhythmias in the neonate heart.
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