The canine atrioventricular (AV) junction comprises three major tissues: paranodal fibers (PNF), AV node (AVN), and His bundle (HB). In the present study, dissection-exposed, in vitro canine AV junctional preparations were used. The object of the study was to determine whether the PNF or AVN was more sensitive to the suppressive effect of acetylcholine (ACh). In five experiments these tissues were stimulated antegradely and retrogradely, and their action potentials were recorded simultaneously under the influence of ACh (0.5 micrograms/ml). Results indicated the PNF were more sensitive to the suppressive effect of ACh than were the AVN. In another group of 13 experiments, the effects of ACh at 0.05-0.3 micrograms/ml on rate of rise of phase 0 of action potentials (Vmax), peak potential, resting membrane potential, and action potential duration of the PNF were determined. Results indicated that ACh exerted a strong suppressive effect on Vmax and amplitude of the action potentials and had little effect on the resting membrane potential and action potential duration of the PNF. In 10 of 13 preparations, ACh also suppressed the response of PNF, resulting in generation of one action potential to every two stimuli. In conclusion, these findings suggest that PNF could be the tissue responsible for vagal-induced AV conduction block.
