SODIUM-CHANNEL DISTRIBUTION WITHIN THE RABBIT ATRIOVENTRICULAR NODE AS ANALYZED BY CONFOCAL MICROSCOPY

1. Paired 20 mu m thick sections of fresh frozen tissue taken from the frontal plane of the rabbit atrioventricular (AV) nodal region were p rocessed for histology and immunohistochemistry. Confocal microscopy w as used to image the distribution of sodium channels using IgG (R12) d eveloped against a highly conserved sequence in the interdomain 3-4 re gion of cloned sodium channels. 2. In ventricular and atrial cells, so dium channel immunofluorescence was localized to lateral membranes and T-tubules. In the open AV node, levels of sodium channel immunofluore scence in the transitional cell zone and in the lower nodal cell tract were comparable to that found in the atrial and ventricular myocardiu m. 3. In the enclosed AV node a gradation of sodium channel immunofluo rescence is present such that peripherally located circumferential tra nsitional cells display high levels of immunofluorescence, comparable to that of atrial and ventricular myocardium, while centrally located midnodal cells display decreased levels of or no immunofluorescence. 4 . In order to correlate the distribution of sodium channels with the d istribution of gap junctions, we used IgG directed against the carboxy l terminus of connexin43 (CT-360). Ventricular cell immunofluorescence was localized primarily to the intercalated disk region, while in the AV node, the pattern of distribution was found to be similar to that of sodium channels. 5. The reduced levels of and/or absence of immunof luorescence in the midnodal cell region indicates a paucity of sodium channel and connexin43 protein expression in this region of the AV nod e that would favour slow impulse conduction.