Gap junctions are composed of aggregations of membrane channels, called connexons, joined with similar connexons in adjacent cells to form intercellular pathways for the diffusion of ions and small molecules (Caspar et al., 1977; Makowski et al., 1977). The resulting intercellular communication is unique in that adjacent cells exchange cytoplasmic molecules directly, with no secretion into the extracellular space (Bennett, 1966; Loewenstein, 1966). Due to the large size of the intercellular channels formed by connexon pairs, the exchange of molecules between cells is nonspecific, and includes the entire pool of ions and small metabolites in each cell (Gilula, Reeves & Steinbach, 1972; Pitts & Simms, 1977; Simpson, Rose & Loewenstein, 1977; Goodenough, Dick & Lyons, 1980). This form of intercellular communication is ideally suited for the role of intercellular buffering of cytoplasmic ions (Corsaro & Migeon, 1977; Ledbetter & Lubin, 1979), synchronization of cellular behavior, such as the coordinated contraction of myocardial cells (Barr, Dewey & Berger, 1965) and the cell-to-cell coordination of metachronal waves (Moss & Tamm, 1987; Sanderson, Chow & Dirksen, 1988). Involvement of gap junction-mediated intercellular communication has also been suggested for growth control and embryonic differentiation (Loewenstein, 1966; Furshpan & Potter, 1968; Warner, Guthrie & Gilula, 1984; Loewenstein & Azarnia, 1988). Due to the sharing of low molecular weight substrate pools, gap junctions will also function to suppress the deleterious effects of somatic cell mutation in a variety of enzymes (Subak-Sharpe, Burk & Pitts, 1969; Cox et al., 1970).