STRUCTURAL DETERMINANTS OF SLOW CONDUCTION IN THE CANINE SINUS NODE

Structure of Sinus Node Gap Junctions, Introduction: To elucidate the role of tissue structure as a determinant of the unique conduction pro perties of the sinus node, we compared the spatial distribution of int ercellular connections at gap junctions in the sinus node to the more rapidly conducting crista terminalis and left ventricle, which have be en studied previously. Methods and Results: Samples of four canine sin us nodes were prepared for electron microscopy, The total number and s patial orientation of neighboring myocytes connected by ultrastructura lly identified intercalated disks and, gap junctions to nine randomly selected index cells were determined by sequentially examining subseri al sections, Sinus node cells were sparsely interconnected compared to the extent of interconnections observed previously in other tissues, A typical sinus node cell aas connected to only 4.8 +/- 0.7 neighbors compared with 11.3 +/- 2.2 cells in the left ventricle and 6.4 +/- 1.7 cells in the crista terminaIis, Sinus node interconnections occurred at small intercalated disks that usually connected cells in partial si de-to-side and end-to-end juxtaposition, In contrast, left ventricular myocytes are interconnected at large intercalated disks that adjoin m any cells in pure side-to-side and end-to-end orientations, Crista ter minalis myocytes are connected primarily in end-to-end fashion. The ag gregate gap junction profile length per unit myocyte area mas 26.5 tim es greater in the left ventricle and 5.0 times greater in the crista t erminalis than in the sinus node. Conclusion: Sinus node myocytes exhi bit small, sparsely distributed gap junctions that interconnect cells in complex patterns of lateral and terminal apposition, These structur al features are consistent with the unique conduction properties of th e sinus node.