Increased thermogenic responsiveness to intravenous β‐adrenergic stimulation in habitually exercising humans is not related to skeletal muscle β₂‐adrenergic receptor density

Abstract

Habitually exercising adults demonstrate greater thermogenic responsiveness to β-adrenergic receptor (β-AR) stimulation compared with their sedentary peers, but the molecular mechanisms involved are unknown. To determine the possible role of increased β-AR density, we studied 32 healthy adults: 17 habitual aerobic exercisers (age 45 ± 5 years, 11 males) and 15 sedentary (49 ± 5 years, 7 males). Maximal oxygen uptake (43.7 ± 2.5 versus 31.6 ± 2.9 ml kg −1 min −1 , P = 0.002, mean ± S.E.M.) and vastus lateralis muscle maximal citrate synthase activity (1.70 ± 0.36 versus 0.58 ± 0.11 μmol min −1 g −1 , P = 0.008) were higher in the habitually exercising subjects. Resting energy expenditure (EE) adjusted for fat-free mass (FFM) was similar in the habitually exercising (5903 ± 280 kJ day −1 ) and sedentary adults (6054 ± 289 kJ day −1 , P = 0.43). The percentage increase in EE (ΔEE%; indirect calorimetry, ventilated hood) above resting EE in response to β-AR stimulation (intravenous isoproterenol at 6, 12 and 24 ng (kg FFM) −1 min −1 ) was greater (7.1 ± 1.2, 13.7 ± 1.0, 20.7 ± 1.3 versus 5.9 ± 0.9, 9.9 ± 1.4, 15.9 ± 1.70%, respectively, P = 0.04), and the dose of isoproterenol required to increase EE by 10% above resting EE was lower (8.2 ± 1.5 versus 17.1 ± 4.1 ng (kg FFM) −1 min −1 , P = 0.03) in the habitually exercising adults. In contrast, vastus lateralis muscle β 2 -AR density was similar in the habitually exercising and sedentary subjects (7.46 ± 0.29 versus 7.44 ± 0.60 fmol (mg dry weight muscle) −1 , P = 0.98), and was not related to ΔEE% (r = 0.02, P = 0.94) or to the isoproterenol dose required to increase EE by 10% above resting EE (r = −0.06, P = 0.76). These findings indicate that increased β 2 -AR density is not a mechanism contributing to the greater thermogenic responsiveness to β-AR stimulation in adult humans who regularly perform aerobic exercise.

Systemic β-Adrenergic Stimulation of Thermogenesis Is Not Accompanied by Brown Adipose Tissue Activity in Humans

“…The increase of ∼20% in energy expenditure during ISO was comparable to that during previous studies with similar dosages (22–24). Contrary to our expectations, however, ISO did not activate BAT in 9 of 10 subjects.…”

No Effect of Short-Term Green Tea Extract Supplementation on Metabolism at Rest or During Exercise in the Fed State

“…Previous studies suggest a sedentary lifestyle reduces sensitivity of β-adrenergic receptors (Bell et al, 2001;Stob et al, 2007). If GTC potentially improve this sensitivity, it might explain why resting studies, using sedentary participants, observe more consistent improvements in EE then exercise studies, which generally use trained or recreationally active subjects.…”

Acute β-adrenergic stimulation does not alter mitochondrial protein synthesis or markers of mitochondrial biogenesis in adult men

“…Subjects received a continuous intravenous infusion of isoproterenol (ISO; 12 ng ⅐ kg fat free mass Ϫ1 ⅐ min Ϫ1 ) or an isovolumetric infusion of saline (CON) over 60 min using a Harvard syringe pump (Harvard Apparatus, Holliston, MA) as performed previously by our group (2,4,52,53). During the infusion period, beat-by-beat heart rate was monitored continuously via electrocardiogram, while blood pressure was determined every 10 min using an automated sphygmomanometer.…”