Conduction of the cardiac impulse. 3. Characteristics of very slow conduction.

The excitability of short segments (5-7 mm) of bundles of canine Purkinje fibers was depressed by exposure to 15-18 mM K(+), to 15-18 mM K(+) plus 5 x 10(-6) epinephrine or norepinephrine, to low K(+), and to low Na(+). The depressed segment was in the center chamber of a three-chamber bath; the ends of the bundle were exposed to normal Tyrode solution. Each method of depression resulted in slow and probably decremental conduction with an effective conduction velocity in the middle chamber of about 0.05 m/sec, or one-way block, or two-way block with summation of the graded responses in the depressed region. The action potential in the depressed segment (the slow response) differs from the normal action potential in its response to applied stimuli. A second active depolarization can be evoked by cathodal stimulation during much of the slow response. The response in the depressed segment is graded. The response of depressed fibers may depend on excitatory events similar to those responsible for the slow component of the cardiac action potential. It is suggested that the slow response can propagate, at least decrementally, in fibers in which the rapid, Na(+)-dependent upstroke is absent, and can cause reentrant excitation by so doing.