An Official Journal of the American Heart Association BRIEF REVIEWS Intercellular Communication in Cardiac Muscle

THE heart muscle was conceptualized as an anatomical syncytium for many years (Heidenhain, 1901; Godlewsky, 1902). With the development of electron microscopy, it was proved that cardiac fibers are composed of isolated cells separated by specialized zones—the so-called intercalated discs (Sjostrand and Andersson, 1954). The discs comprise three junctional specializations: (1) the macula adhaerens (desmosome), (2) the fascia adhaerens, and (3) the gap junction or nexus (van Breeman, 1953; Moore and Ruska, 1957; Sjostrand et al., 1958). At the level of desmosomes, the apposing plasma membranes are separated by a gap of 250-300 A and a fibrillar material is seen inside the adjacent cells (Muir, 1965; Fawcett, 1966). At the nexus, the cell membranes are in intimate contact (Dewey and Barr, 1964; Goodenough and Revel, 1970), and in its central area, several structures are seen bridging the gap with a periodicity of 100 A (Fig. 1). Freeze-cleaving studies indicate that the nexus is characterized by particles closely packed in a hexagonal array (Kreutziger, 1968; Bullivant, 1969; Goodenough and Revel, 1970; McNutt and Weinstein, 1970; Peracxhia, 1973, 1980). These particles are usually 70-80 A in diameter showing dots (20-25 A) in the central region (Gilula, 1978). Hydrophilic channels connecting the cytoplasm of adjacent cells seem to pass through the center of the nexus subunits (Loewenstein, 1966; Furshpan and Potter, 1968; Bennett and Dunham, 1970; De Mello, 1980a—see Fig. 2).