Influence of sympathetic vasoconstriction on contraction‐ and mechanically‐induced rapid vasodilation in humans

Abstract

Brief muscle contractions elicit a rapid vasodilation that appears due, in part, to mechanical effects on the vasculature. We tested the hypothesis that sympathetic tone modulates contraction‐ and mechanically‐induced rapid vasodilation in humans. We measured forearm blood flow (FBF; Doppler ultrasound) and calculated the vascular conductance (FVC) responses to a range of single, 1‐sec isometric contractions (10–75% maximum voluntary contraction; MVC) and mechanical compressions (50–300 mmHg) of the forearm. Contractions and compressions were performed during supine control conditions and during 10 degrees head‐down tilt (HDT; sympathetic inhibition; n = 10), and compressions were also performed during control and during −20 mmHg lower body negative pressure (LBNP; sympathetic activation; n = 10). HDT increased and LBNP decreased resting FVC by ∼20%, respectively. The immediate (1st cardiac cycle post contraction) vasodilator responses at 10 and 25% MVC were greater during HDT vs control, as were the total hyperemic responses to all contraction intensities (P<0.05). In contrast, alterations in sympathetic tone via HDT or LBNP did not influence the vasodilator responses to forearm compressions (all P>0.05). We conclude that sympathetic tone restrains contraction‐induced rapid vasodilation, but does not influence mechanically‐induced vasodilation in humans.Supported by AG‐022337