Abstract
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 approximately 45%. Mean arterial blood pressure was not significantly changed (P = 0.196), but diastolic blood pressure was elevated by approximately 10% and pulse pressure was reduced by approximately 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 mmHg(-1) in control vs. only 101.6 +/- 4.1 ml. min(-1). 100 mmHg(-1) 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.
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