Electrotonic inhibition and summation of impulse conduction in mammalian Purkinje fibers

Models of electrotonically mediated transmission were created by superfusion of the central fiber segment of a three-compartment Purkinje fiber preparation with either an "ion-free" or "ischemic" solution or by localized application of pressure. The frequency-dependent impairment of impulse conduction across such inexcitable gaps was found to be the result of influences exerted not only by impulses transmitted across the area of block but also by impulses blocked at the proximal border of the inexcitable zone. The electrotonic image of a nonconducted response was observed to exert an important inhibitory effect on the electronically mediated transmission of a subsequent impulse, thus causing block or delay. This phenomenon, which we have termed electrotonic inhibition, shows both time and voltage dependence. The related phenomenon of electrotonic summation describes the facilitation of conduction that occurs when two subthreshold potentials occur close enough in time to fuse. These data provide a demonstration of Wedensky inhibition in heart tissues and point to electrotonic inhibition as a mechanism of concealed conduction. Electrotonically mediated phenomena may explain both temporal and spatial inhibition and summation previously described in nodal or depressed tissues and ascribed to partial active invasion of intermediary tissue.