Does The Bilateral Comparison Of Vascular Conductance During One-handed Exercise Reflect Local Sympatholysis?

Sympathetic vasoconstriction is locally reduced in the active limb during exercise (i.e., functional sympatholysis) to supply blood to the contracting muscle. In general, functional sympatholysis is assessed as the difference between the change in vascular conductance (VC) caused by sympathetic activation at rest and during exercise in the active side of the limb (local effect limited to the active muscle). However, sympathetic activation such as the cold pressure test (CPT) increases cardiac output (CO) and increases VC in both active and inactive limbs (not local effect). Conversely, a comparison of VC changes in active and inactive limbs during exercise could be a more reflective indicator of local sympatholysis, but whether it is relevant to traditional methods when changes in systemic circulation are small remained unknown. PURPOSE: The aim of this study was to determine whether the difference in VC between active and inactive limbs correlates with traditional functional sympatholysis, using lower body negative pressure (LBNP), a sympathetic stimulation that causes small changes in systemic circulation. METHODS: Twelve adults (6 M/6F, 22 ± 8 yrs) underwent sympathetic stimulation with LBNP (−20 mmHg) at rest and while performing dynamic one-handed handgrip exercise (10% and 15% of the maximum voluntary contraction). Brachial artery diameter and velocity in active and inactive limbs were simultaneously measured using Doppler ultrasound. Blood pressure and CO were continuously measured using finger plethysmography. Percent change in VC by LBNP (%ΔVC) was calculated following two METHODS: 1) %ΔVCexercise-rest, (traditional methods) and 2) differences in %ΔVC during exercise between the active and inactive limbs (%ΔVCactive-inactive). RESULTS: Mean arterial pressure remained unaltered by LBNP (P = 0.14). CO had a slight but significant decrease via LBNP (P < 0.01). A positive correlation was observed between the %ΔVCexercise-rest and the %ΔVCactive-inactive (P < 0.01, r = 0.61). CONCLUSIONS: Our results suggest that the reduced VC reduction by LBNP during exercise between active and inactive limbs demonstrates local functional sympatholysis. This method may be useful in assessing functional sympatholysis when using sympathetic activation to elicit a systemic circulatory response such as CPT. JSPS KAKENHI Grant Number 21K11290K