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Departments of Physiology and Internal Medicine, Harry S. Moss Heart Center, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9034
Because arterial baroreceptor and skeletal
muscle receptor afferents project to cardiovascular regions in the
lower brain stem such as the nucleus tractus solitarii (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 three levels of carotid sinus pressure 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
-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 beats/min 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 beats/min
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 (i.e., facilitation or inhibition) between the baroreflex
and the exercise pressor reflex is dependent on 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.
skeletal muscle receptor afferent stimulation; nucleus tractus solitarii; ventrolateral medulla; cardiovascular-related neurons; cardiovascular pressor reflexes; exercise
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