Circulation, Vol 78, 49-59, Copyright © 1988 by American Heart Association
U Scherrer, SF Vissing and RG Victor
Although previous studies in both animals and humans have suggested that
cardiac baroreceptors modulate reflex sympathetic vasoconstriction during
exercise, more recent studies in conscious animals have not supported this
view. To further test this concept in humans, we measured sympathetic nerve
discharge with intraneural microelectrodes while we used static handgrip to
activate the exercise pressor reflex and nonhypotensive lower-body negative
pressure (LBNP) to selectively unload the cardiac baroreflex. In nine
healthy subjects, we measured blood pressure, heart rate, central venous
pressure, and muscle sympathetic nerve activity (MSNA) from the peroneal
nerve (resting leg) during 2 minutes of static handgrip at 20% and 30% of
maximal voluntary contraction (MVC) alone and in combination with LBNP at
-5 mm Hg. Handgrip alone (exercise reflex) at 20% and 30% MVC caused graded
increases in MSNA. LBNP alone (cardiac reflex) did not alter blood pressure
or heart rate but decreased central venous pressure by 2.5 +/- 0.1 mm Hg
(mean +/- SEM, p less than 0.05) and increased MSNA by 92 +/- 22% over the
control value. Most important, handgrip performed during LBNP (interaction
of reflexes) caused increases in MSNA that were comparable with the
increases during handgrip alone: +114 +/- 32% versus +175 +/- 89% at 20%
MVC and +328 +/- 101% versus +431 +/- 110% at 30% MVC (handgrip plus LBNP
vs. handgrip alone, p greater than 0.1). Pressor and heart rate responses
to handgrip also were unaffected by LBNP. In five additional experiments,
comparable findings were obtained when the LBNP was superimposed on
handgrip rather than handgrip being superimposed on LBNP. In conclusion,
this study provides direct evidence in humans that nonhypotensive LBNP does
not augment muscle sympathetic outflow during static handgrip and
challenges the concept of an important interaction between cardiac
baroreceptor and exercise pressor reflexes during this form of exercise.
ARTICLES
Effects of lower-body negative pressure on sympathetic nerve responses to static exercise in humans. Microneurographic evidence against cardiac baroreflex modulation of the exercise pressor reflex
Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas 75235-9034.
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