Integrated Effects of Sympathetic Vasoconstriction and Local Vasodilation in Human Skeletal Muscle and Skin Microvasculature

Abstract

Both sympathetic vasoconstriction and locally-mediated vasodilation play important roles in regulation of blood flow to skeletal muscle and skin during exercise. However, the integrated effects of these two vascular regulatory mechanisms in microcirculation are unclear. PURPOSE: We aimed to investigate integration of sympathetic vasoconstriction and local vasodilation in the skeletal muscle and skin microvasculature in humans. METHODS: In 39 healthy volunteers, we measured blood flow index of flexor carpi radialis muscle using diffuse correlation spectroscopy and monitored skin blood flow at the proximal site by laser-Doppler flowmetry, simultaneously. We examined the effects of acute sympathoexcitation by forehead cooling on relatively weak or robust vasodilatory responses during post-occlusive reactive hyperemia (PORH) induced by 70 s or 10 min arterial occlusion at upper arm, respectively. To increase sympathetic tone during PORH, forehead cooling was begun 60 s before the occlusion release and ended 60 s after the release. RESULTS: The acute sympathoexcitation diminished the peak and duration of vasodilation in both skeletal muscle and skin in 70 s occlusion trials [peak skeletal muscle vascular conductance (MVC): 5.6 ± 0.4 vs. 4.1 ± 0.3, peak cutaneous vascular conductance (CVC): 3.4 ± 0.2 vs. 2.9 ± 0.2 AU, 50 % decay time of MVC: 19 ± 0.8 vs. 13 ± 0.6, 50 % decay time of CVC: 17 ± 1.0 vs. 14 ± 1.0 s, p < 0.05]. The sympathetic inhibition of vasodilation was blunted under robust vasodilatory stimuli produced by 10 min occlusion. This blunt of the sympathetic inhibition was greater in skeletal muscle than in skin, especially in those initial and peak vasodilation. Indeed, the sympathoexcitation reduced the peak vasodilation only in skin (peak MVC: 10.2 ± 0.9 vs. 9.7 ± 0.9, p > 0.05, peak CVC: 7.7 ± 0.5 vs. 6.5 ± 0.5 AU, p < 0.05) while it accelerated the initial vasodilation selectively in skeletal muscle (area of MVC during first 15 s of PORH: 75.2 ± 6.5 vs. 83.0 ± 6.9, p < 0.05, area of CVC: 51.6 ± 2.7 vs. 51.7 ± 3.4 AUs, p > 0.05). CONCLUSION: We conclude that, in humans, the integration of sympathetic vasoconstriction and local vasodilation has different effects in skeletal muscle and skin. Such different effects would be importantly involved in selective control of perfusion in microcirculation of different tissues.