IMMUNOHISTOCHEMICAL DELINEATION OF THE CONDUCTION SYSTEM .1. THE SINOATRIAL NODE

We have raised a mouse monoclonal antibody that reacts specifically wi th the myocytes of the sinoatrial node of the bovine heart. By use of this antibody (445-6E10) and antibodies against the gap junction prote in connexin43, the periphery of the sinoatrial node and the distributi on of gap junctions in the nodal region were studied. The reaction pat terns of 445-6E10 and anti-connexin43 are exactly complementary; ie, c onnexin43 was not detected in the nodal myocytes but was clearly prese nt in the atrial myocytes. Both reaction patterns demonstrate that nod al myocytes and atrial myocytes can unambiguously be distinguished by their characteristic molecular phenotype. The transitional nodal myocy tes at the periphery of the node that have intermediate morphological and electrophysiological characteristics could now clearly be defined as nodal by our immunohistochemical criteria. The center of the node i s surrounded by a region of interdigitating nodal and atrial bundles. Nodal bundles, coming from the center of the node, penetrate the atria l myocardium aligned at atrial bundles, forming histological connectio ns between nodal and atrial myocytes at regular distances. This interd igitating arrangement of bundles of connexin43-negative nodal and conn exin43-positive atrial myocytes is also found in the human and rat hea rt. We hypothesize that the architecture of the periphery of the node is important to prevent silencing of the pacemaking nodal myocytes by the atrium while ensuring a sufficient source loading of the nodal myo cytes.