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No Evidence of Impaired Functional Sympatholysis in Young Obese Adults

In healthy individuals, sympathetic vasoconstriction is blunted in exercising muscles, contributing to the matching of blood flow with metabolic demand. This “functional sympatholysis” may be impaired in young obese adults due to greater sympathetic activation and reduced local vasodilatory capacity of both small and large arteries, but this remains poorly understood. We tested the hypothesis that functional sympatholysis is impaired in obese adults compared with lean counterparts. Participants were 13 lean (female=6; 26±1 yrs; 22.4±0.5 kg/m2) and 14 obese adults (female=7; 27±1 yrs; 32.6±0.6 kg/m2). Lower body negative pressure (LBNP −20 mmHg, 2 min), to evoke reflex‐mediated increases in sympathetic nerve activity, was performed at rest and during steady‐state dynamic handgrip exercise (HG, 30% maximal voluntary contraction; 50% duty cycle). Ultrasonography [brachial diameter, forearm blood flow (FBF), forearm vascular conductance (FVC)], and beat‐to‐beat cardiovascular variables [mean arterial pressure (MAP), heart rate (HR), and cardiac index (CI)] were obtained. FBF and FVC were normalized to lean forearm mass, and CI was cardiac output indexed to body surface area. No group differences were found at rest or during handgrip before LBNP for any variable. LBNP evoked similar decreases in FBF (−18.6±3.0% lean vs. −17.8±3.1% obese) and FVC (−16.6±3.1% lean vs. −14.9±3.1% obese) in the resting forearms of the lean and obese adults (P>0.05 for group). During HG, reductions in FBF (−1.7±3.8% lean vs. −2.0±2.3% obese) and FVC (1.2±4.3% lean vs. 0.1±2.5% obese) to LBNP were blunted in the exercising forearms of both groups (P>0.05 for group). In both lean and obese subjects, LBNP decreased MAP and CI at rest and during HG, but decreased brachial artery diameter and increased HR only during HG (all; P<0.05 for time effect). Our preliminary data suggests that functional sympatholysis is not impaired in young obese adults without comorbidities.