Evaluation Of Functional Sympatholysis Occurring Within Contracting Skeletal Muscle Microvasculature In Humans

Abstract

The ability of contracting skeletal muscle to blunt sympathetic vasoconstriction, termed functional sympatholysis, is thought to contribute to supplying adequate blood to the exercising muscle despite increased sympathetic tone. However, previous studies in humans primarily investigated vascular responses of exercising limb as a whole that reflects not only responses of contracting skeletal muscles but also responses of inactive tissues such as skin, adipose and bones. PURPOSE: We aimed to more directly investigate the functional sympatholysis occurring within exercising skeletal muscle in humans. METHODS: In 26 healthy male volunteers, we examined the effects of acute sympathoexcitation by 90-s forehead cooling at rest and during dynamic handgrip exercise at 10% and 30% of maximum voluntary contraction (10%Ex and 30%Ex), respectively. The handgrip exercises were continued 3 min and forehead cooling was applied at latter half of the exercise period. The subjects also performed handgrip exercises without forehead cooling as control conditions. We employed near-infrared diffuse correlation spectroscopy, an emerging optical technique for noninvasive measurement of deep tissue hemodynamics, to continuously measure the microcirculatory blood flow index (BFI) within the flexor digitorum superficialis muscle, the muscle primarily responsible for handgrip. RESULTS: The acute sympathoexcitation induced significant decrease in vascular conductance (BFI / mean arterial pressure) at rest (baseline vs. forehead cooling: 1.00 ± 0 vs. 0.75 ± 0.03 AU, p < 0.05) and during 10%Ex (control vs. forehead cooling: 1.66 ± 0.08 vs. 1.32 ± 0.10 AU, p < 0.05), but not during 30%Ex (control vs. forehead cooling: 4.00 ± 0.56 vs. 3.66 ± 0.50 AU, p > 0.05). In addition, the percentage reduction in vascular conductance by forehead cooling was significantly decreased during 30%Ex compared to rest (-25.2 ± 2.5 vs. -4.9 ± 5.1 %, p < 0.05). CONCLUSIONS: Our study demonstrated the functional sympatholysis occurring within the contracting skeletal muscle microvasculature in humans. Furthermore, our results revealed the clear intensity-dependent response such that mild exercise hardly interferes with sympathetic vasoconstriction, whereas moderate exercise substantially attenuates it.