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1 School of Physical and Health Education and Department of Physiology, Queen's University, Kingston, Ontario K7L 3N6; and 2 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
The purpose of this study was to
test the hypothesis that sympathetic vasoconstriction is rapidly
blunted at the onset of forearm exercise. Nine healthy subjects
performed 5 min of moderate dynamic forearm handgrip exercise during
60 mmHg lower body negative pressure (LBNP) vs. without
(control). Beat-by-beat forearm blood flow (Doppler
ultrasound), arterial blood pressure (finger photoplethysmograph), and
heart rate were collected. LBNP elevated resting heart rate by ~45%.
Mean arterial blood pressure was not significantly changed (P = 0.196), but diastolic blood pressure was elevated
by ~10% and pulse pressure was reduced by ~20%. At rest, there
was a 30% reduction in forearm vascular conductance (FVC) during LBNP
(P = 0.004). The initial rapid increase in FVC with
exercise onset reached a plateau between 10 and 20 s of 126.6 ± 4.1 ml · min
1 · 100 mmHg1 in control vs. only 101.6 ± 4.1 ml · min1 · 100 mmHg1 in LBNP (main effect of condition,
P = 0.003). This difference was quickly abolished
during the second, slower phase of adaptation in forearm vascular tone
to steady state. These data are consistent with a rapid onset of
functional sympatholysis, in which local substances released with the
onset of muscle contractions impair sympathetic neural vasoconstrictor effectiveness.
muscle blood flow; Doppler ultrasound; lower body negative presssure
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