Pacing-induced automaticity in sheep Purkinje fibers

The induction and decay of pacing-induced automaticity were studied in 15 sheep Purkinje fiber preparations superfused with modified Tyrode's solution containing norepinephrine, 2.5 x 10(-7) M. All preparations were quiescent prior to pacing. Spontaneous automaticity could be induced in each preparation provided a sufficient number of pacing stimuli were applied at a fast enough rate. Once the minimum pacing requirements for induction of automaticity were exceeded, the number, total duration, and fastest rate of the induced beats were proportional to the number and rate of the pacing stimuli up to a maximum which could not be exceeded. Consecutive trains of stimuli were additive in inducing automaticity, provided that the pauses between them were short enough to preclude the time-dependent return of automatic properties to their pre-pacing level. Prolonged sequence of fixed pacing trains resulted in stable degrees of automaticity which depended on the length of the pause between them. These observations permit a semiquantitative description of induced automatic behavior and help establish a model that may be useful in future studies.