AJP - Heart and Circulatory Physiology, Vol 241, Issue 6 850-H856, Copyright © 1981 by American Physiological Society
Changes in vagal phasic chronotropic responses with sympathetic stimulation in the dog
S. L. Stuesse, D. W. Wallick, H. Zieske and M. N. Levy
Sympathetic stimulation both shortens the cardiac cycle and potentiates the
cardiac response to vagal stimulation. In the present study the effects of
sympathetic stimulation on the chronotropic responses of the heart to brief
bursts of vagal stimulation were determined in open-chest anesthetized
dogs. The sinoatrial nodal pacemaker cells demonstrate a paradoxical
response to repetitive bursts of vagal stimuli over a certain portion of
the cardiac cycle. That is, the cardiac cycle length does not increase but
actually decreases as the vagal stimulation frequency is raised. Background
levels of sympathetic stimulation do not significantly alter the range over
which this "paradoxical" response occurs. Sympathetic stimulation decreases
the cardiac chronotropic response to short bursts of vagal stimuli
regardless of the time in the cardiac cycle that the stimulus is given;
however, it does not decrease the time from the minimum vagal chronotropic
response to the subsequent atrial depolarization although the total cardiac
cycle is shortened. Since sympathetic stimulation shifts the overall
temporal relationship between vagal stimulation and pacemaker response,
small changes in sympathetic tone may greatly alter the cardiac response to
phasic vagal stimulation if the vagal stimulus is given at certain times in
the cardiac cycle.
