DISTRIBUTION OF ATRIAL AND NODAL CELLS WITHIN THE RABBIT SINOATRIAL NODE - MODELS OF SINOATRIAL TRANSITION

Background-In the sinoatrial node (SAN) the course of the action poten tial gradually changes from the primary pacemaker region toward the at rium. It is not known whether this gradient results from different int rinsic characteristics of the nodal cells, from an increasing electrot onic interaction with the atrium, or from both. Therefore we have char acterized the immunohistochemical, morphological, and electrophysiolog ical correlates of this functional gradient. Methods and Results-The d istribution of rabbit nodal myocytes in the SAN has been studied by im munohistochemistry. After cell isolation, the electrophysiological cha racteristics of different nodal cell types were measured. (1) The stai ning pattern of a neurofilament protein coincides with the electrophys iologically mapped pacemaker region in the SAN. (2) Enzymatic digestio n of the SAN reveals three morphologically different nodal cell types and one atrial type. Of each nodal cell type, neurofilament-positive a s well as neurofilament-negative myocytes are found. Atrial cells are all neurofilament-negative. (3) In contrast to previous findings, we o bserved atrial cells in the very center of the SAN. The relative numbe r of atrial cells gradually increases from the central pacemaker area toward the atrium. (4) Differences in electrophysiological characteris tics between individual nodal cells are not associated with difference s in cell type. Conclusions-(1) The expression of neurofilaments can b e used to delineate the nodal area in the intact SAN but is not suffic iently sensitive for characterizing all individual isolated nodal cell s. (2) A fundamentally different organization of the SAN is presented: The gradual increase in density of atrial cells from the dominant are a toward the crista terminalis in the SAN causes a gradual increase of atrial electrotonic influence that may be an important cause of the g radual transition of the nodal to the atrial type of action potential.