Conduction characteristics affected by histological features in the AV junction
The principal connexin in the working atrial and ventricular myocardium is Cx43, with medium conductance.2,4,6 The CN shows low expression levels of Cx43 in humans and animals.4,13 However, Cx40 expression (at both the mRNA and protein levels) is high in the CN and PB, where it is implicated in facilitating fast conduction in humans and rats.3,10 Cx45 is the predominant connexin expressed in the mammalian AVN.14 Thus, connexin expression could be used as a marker to recognize different conduction cell types. Moreover, connexins dominate electronic conduction, which provides approximately 30%of the conduction velocity. Another approximately 70% is dominated by ion channels.15 The RN that we describe in this study displayed the same pattern of connexin isoform expression as the CN and INE. Immunohistochemical labeling of the Nav1.5, Cav3.1 and HCN4 channels showed nearly equal expression of these ion channels in the CN, INE and RN (Figure 5D), indicating that these areas have highly uniform and conserved conduction properties. These data suggest that the RN and its connection with the CN may have the same conduction properties as the CN and INE, exhibiting relatively slow conduction.