Interaction between carotid baroreflex and exercise pressor reflex
depends on level of baroreceptor afferent input.
Potts, Jeffrey T., and Jianhua Li.
Departments of Physiology and Internal Medicine, Harry S. Moss
Heart Center, University of Texas Southwestern Medical Center, 5323
Harry Hines Blvd. Dallas, TX 75235-9034
APStracts 5:0086H, 1998.
Since arterial baroreceptor and skeletal muscle receptor afferents
project to cardiovascular regions in the lower brainstem, such as the
nucleus tractus solitarius (NTS), it is likely that the level of
baroreceptor afferent input will modify the excitatory cardiovascular
responses evoked by contraction-sensitive skeletal muscle afferents.
The purpose of this study was to determine the effect of carotid
sinus baroreceptor afferent input (CSA) on reflex heart rate (HR) and
mean arterial pressure (MAP) responses evoked by activation of
skeletal muscle receptor afferents (SMA). CSA input was servo
-controlled at 3 levels of carotid sinus pressure (Low, Mid, Hi) using
the isolated carotid sinus preparation, and SMA input was varied by
induced muscle contraction (L7-S1 ventral root stimulation) or
passive muscle stretch. Experiments were performed in [alpha]
-chloralose-anesthetized and vagotomized dogs (n=9). When CSA input
was low (106+/-35 mmHg), electrically-induced muscle contraction
increased HR and MAP (30+/-8 bpm and 42+/-12 mmHg, respectively,
P<0.05). However, when CSA input was high (221+/-9 mmHg), the
reflex changes in HR and MAP during muscle contraction were
attenuated (6+/-4 bpm and 18+/-4 mmHg, respectively, P<0.05).
Similarly, the sympathoexcitatory responses evoked by passive muscle
stretch were attenuated in a baroreceptor-dependent manner. These
results suggest that changing CSA input from low (106 mmHg) to high
(221 mmHg) shifts the interaction from facilitation to inhibition.
Therefore, it is concluded that the nature of the interaction (ie.
facilitation or inhibition) between the baroreflex and the exercise
pressor reflex is dependent upon the level of baroreceptor input.
Moreover, our findings substantiate early studies showing that the
level of afferent input from arterial baroreceptors is a powerful
modulator of sympathoexcitation evoked by mechanically and
metabolically-sensitive skeletal muscle receptors.
Received 9 December 1997; accepted in final form 18 February
1998.
APS Manuscript Number H1124-7.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1998 The American Physiological Society.
Published in APStracts on 9 March 1998