Overdrive suppression of conduction at the canine Purkinje-muscle junction

We have shown previously that overdrive suppression of conduction in depolarized His-Purkinje tissue requires conduction asymmetry. In this study we examined whether overdrive suppression of conduction can occur at the Purkinje-muscle junction, where natural asymmetry of conduction exists. Canine Purkinje-muscle preparations were superfused with hyperkalemic Tyrode's solution (KCl 8 to 12 mM), and action potentials were recorded from Purkinje, junctional, and muscle cells. Initially, the Purkinje fiber was paced at the shortest cycle length at which 1:1 anterograde Purkinje-muscle conduction occurred. The papillary muscle then was paced for 10 to 50 beats at shorter cycle lengths during which, because of conduction asymmetry at the Purkinje-muscle junction, 1:1 retrograde muscle-Purkinje conduction also occurred. After overdrive papillary muscle pacing, Purkinje fiber pacing at the same cycle length that previously resulted in 1:1 conduction now produced transient Purkinje-muscle conduction block (overdrive suppression of conduction). The degree and duration of overdrive suppression of conduction were proportional to the rate and duration of overdrive pacing. After overdrive pacing, Purkinje cell action potential amplitude and Vmax recovered within 300 msec, yet conduction block persisted for up to 7 sec. In contrast, excitability in papillary muscle cells near the Purkinje-muscle junction increased continuously after overdrive pacing. These data suggest that rapid activation of Purkinje cells during overdrive pacing was not required for overdrive suppression of conduction and that restoration of conduction after overdrive pacing was determined primarily by recovery of excitability in papillary muscle cells. Transient Purkinje-muscle conduction block after periods of rapid ventricular rates might account for overdrive-induced conduction disturbances normally attributed to bundle branch block.