title: The plasma membrane of leading pacemaker cells in the rabbit sinus node. A qualitative and quantitative ultrastructural analysis.
abstract: We studied the fine structure of the plasma membrane of electrophysiologically identified leading pacemaker cells from the rabbit sinus node, using both ultrathin sections of fixed tissue and replicas of freeze-cleaved material. We found that differences exist between sinus node and working myocardial membranes, but these are only quantitative. The caveolae or sarcolemmal invaginations are present in very large numbers; they increase the surface area of the plasma membrane by about 100%. The small macular nexuses that are present represent 0.2% of the membrane surface area. Nexuses are therefore about 10 times less numerous in leading sinus node cells than in working myocardium cells. A single equivalent electrical representation of the sinus node shows, nevertheless, that an appreciable electrical coupling may be expected.
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000648:ultrastructure; D005260:Female; D007365:Intercellular Junctions / Q000648:ultrastructure; D008297:Male; D011817:Rabbits; D012849:Sinoatrial Node / Q000166:cytology / Q000648:ultrastructure*