Short-term sympathoadrenal inhibition augments the thermogenic response to β-adrenergic receptor stimulation

Abstract

Sedentary behavior is associated with an attenuated thermogenic response to β-adrenergic receptor (β-AR) stimulation, an important regulator of energy expenditure (EE) in humans. Chronic stimulation of β-ARs, via heightened activity of the sympathoadrenal system, leads to diminished β-AR function. We have investigated the hypothesis that the thermogenic response of sedentary adults to β-AR stimulation will be increased during short-term sympathoadrenal inhibition. Using a randomly ordered, repeated measures study design, resting EE (REE; indirect calorimetry, ventilated hood technique) and the % increase in EE above REE (%ΔEE) during acute i.v. isoproterenol administration (nonselective β-AR agonist; 6, 12, and 24 ng/kg fat-free mass per min)were determined in 16 sedentary adults (nine females and seven males, 25±1 years, body mass index: 26.1±0.9 kg/m2, maximal oxygen uptake: 40±2 ml/kg per min (mean±S.E.M.)) in the basal state and on the 6th day of transdermal clonidine administration (centrally acting α2-AR agonist; 0.2 mg/day). Relative to baseline, clonidine inhibited sympathoadrenal activity, as evidenced by decreased plasma norepinephrine concentration (1.04±0.13 vs 0.34±0.03 nmol/l; P<0.001), skeletal muscle sympathetic nerve activity (22.5±3.8 vs 8.5 ±1.9 bursts/min; P=0.003), and resting heart rate (63±2 vs 49±1 beats/min; P<0.001). Sympathoadrenal inhibition decreased REE (6510±243 vs 5857±218 kJ/day; P<0.001), increased respiratory exchange ratio (0.84±0.01 vs 0.86±0.01; P=0.03), and augmented the thermogenic response to β-AR stimulation (%ΔEE: 11±2, 16±2, and 24±2 vs 14±1, 20±2, and 31±2; P=0.04). These data demonstrate that in sedentary humans, short-term inhibition of sympathoadrenal activity increases the thermogenic response to β-AR stimulation, an important determinant of EE and hence energy balance. © 2010 Society for Endocrinology.