AJP - Heart and Circulatory Physiology, Vol 251, Issue 4 710-H715, Copyright © 1986 by American Physiological Society
Suppressive effect of acetylcholine on action potentials of canine paranodal fibers
W. W. Tse
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.
