Insertion of gap junction channels. Propagation of the repolarization of cell A5 was produced when sufficient gap-junction (g.j) channels were inserted between the cardiac muscle cells and smooth muscle cells. A: Record obtained when 10,000 gj-channels were inserted (equivalent to a g.j. resistance (Rgj) of 1.0 MΩ). Note that the rising phase of the APs from all 5 cells were superimposed, indicating that they all fired simultaneously. Also note that repolarization propagated in a retrograde direction down the 5-cell chain. B: 1,000 gj-channels inserted (Rgj of 10 MΩ). Again, the rising phase of the APs of the 5 cells were nearly superimposed. C: 100 gj-channels (Rgj of 100 MΩ). With less coupling, the rising phase of the APs of the 5 cells were separated in time. The velocity of propagated repolarization (θr) was further slowed. D: Rgj = 1.0 MΩ(10,000 channels). The rising phase of the APs from all 5 cells were superimposed. Retrograde propagation of repolarization was very fast. E: Rgj = 10 MΩ(1000 channels). The rising phase of the 5 APs were still superimposed, but now the retrograde propagation velocity was slowed. F: Rgjj = 100 MΩ(100 channels). The rising phase of the 5 APs are now separated, indicating velocity of antegrade propagation (θa) was slowed. Velocity of retrograde propagation (θr) was slow.