AJP - Heart and Circulatory Physiology, Vol 243, Issue 2 219-H225, Copyright © 1982 by American Physiological Society
Fade of cardiac responses during tonic vagal stimulation
P. Martin, M. N. Levy and Y. Matsuda
We applied trains of stimuli to the vagosympathetic trunks of anesthetized
dogs and studied the time courses of the resultant chronotropic and
inotropic responses. These responses were maximum soon after the onset of
the test stimulus train but then declined over the next 1-5 min despite
continued stimulation. The fade ratio was defined as the magnitude of this
decline divided by the magnitude of the maximum response. For both
inotropic and chronotropic responses, maximum increased with stimulation
frequency, but the fade ratio did not change. In some experiments,
conditioning stimulus trains of variable duration were applied before a
standard rest period, after which the test stimulus train was applied. The
longer the conditioning period, the lower was the subsequent fade ratio of
the inotropic responses to the test stimulation train. In other
experiments, a conditioning train of 2 min was applied, and then variable
rest periods were interposed before the test train was applied. The longer
the rest period, the greater were the subsequent maximum and fade ratios of
the inotropic responses to the test stimulus train. These results indicate
that some factor persists well after the cardiac responses to a given
stimulus, and this factor affects the next response to an identical vagal
stimulation. The chronotropic responses faded about three times faster than
the inotropic responses. Thus different mechanisms may account for the fade
of the inotropic and chronotropic responses.
