Abnormal automaticity in canine Purkinje fibers focally subjected to low external concentrations of calcium.

The false tendon was isolated from the dog heart and mounted under a partition plate in the center of a chamber which was filled with Tyrode's solution. Two microelectrodes were positioned closely on either side of the plate and inserted into the false tendon. The partition did not alter repolarization of the action potential in normal Tyrode's solution. When EDTA was added to one side of the preparation, a low-plateau potential appeared after repolarization on the other side, which contained normal Tyrone's solution. This change occurred concurrently with a prolongation of the action potential in the calcium-free Tyrode's solution. The low-plateau potential was a reflection of the electrotonic potential from the nearby cells whose action potentials were prolonged. Repetitive firing of the action potential was frequently observed when abrupt differences in repolarization were recorded at close interelectrode distances. These results suggest that differences in repolarization of closely apposed cells can initiate arrhythmias.