Microelectrode Demonstration of Wedensky Facilitation in Canine Cardiac Purkinje Fibers

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Abstract
Microelectrode techniques were used to examine changes in excitability of canine Purkinje fibers at sites distal to complete conduction block. Immediately distal to a site where impulse propagation failed, it was possible to record subthreshold depolarizations. Intracellular stimulation of the Purkinje cells exhibiting these subthreshold responses with constant-current pulses of subthreshold intensity enabled full-amplitude, propagated action potentials to develop when the subthreshold stimuli were timed to occur during the slow upstroke of the subthreshold depolarization. This period of a decreased current threshold requirement for stimulation observed during the first 40-50 msec of the subthreshold depolarization was followed by a period during which suprathreshold stimuli were necessary to evoke propagated responses. Propagated action potentials which interrupted the subthreshold response during its repolarization phase demonstrated reduced maximal rising velocities. We concluded that summation of appropriately timed subthreshold stimuli could induce full-amplitude, regenerative, all-or-none responses and that the changes in excitability of subthreshold responses were not solely voltage dependent. Sodium inactivation appeared to be responsible for the depressed rate of action potentials that interrupted the subthreshold response during its repolarization phase and might participate in the coincident depressed excitability.

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