Received April 18, 2007
Revised May 23, 2007
Accepted after revision June 14, 2007
Autonomic Neuroscience [200]
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Increased Thermogenic Responsiveness to Intravenous Beta-Adrenergic Stimulation in Habitual Exercisers Is Not Related To Skeletal Muscle Beta2-Adrenergic Receptor Density
Nicole R Stob 1,
Douglas R Seals 1,
Jørgen Jensen 2,
Marleen A van Baak 3,
Amy J Steig 1,
Rachel C Lindstrom 1,
Benjamin T Bikman 1,
Christopher Bell 4*
1 University of Colorado at Boulder
2 National Institute of Occupational Health
3 Maastricht University
4 Colorado State University
* To whom correspondence should be addressed. E-mail: cbell{at}cahs.colostate.edu.
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Abstract |
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Habitually exercising adults demonstrate greater thermogenic responsiveness to beta-adrenergic receptor (
-AR) stimulation compared with their sedentary peers, but the molecular mechanisms involved are unknown. To determine the possible role of increased -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 vs. 31.6±2.9 ml kg-1 min-1, P=0.002, mean±SE) and vastus lateralis muscle maximal citrate synthase activity (1.70±0.36 vs. 0.58±0.11µmol min-1 g-1, P=0.008) were higher in the habitually exercising subjects. Resting energy expenditure (EE) adjusted for fat-free mass was similar in the habitually exercising (5903±280 kJ day-1) and sedentary (6054±289 kJ day-1) adults (P=0.43). The % increase in EE (
EE%; indirect calorimetry, ventilated hood) above resting EE in response to -AR stimulation (intravenous isoproterenol: 6, 12 and 24 ng kg fat-free mass-1 min-1) was greater (7.1±1.2, 13.7±1.0, 20.7±1.3 vs. 5.9±0.9, 9.9±1.4, 15.9±1.70%, P=0.04), and the dose of isoproterenol required to increase EE 10% above resting EE was lower (8.2±1.5 vs. 17.1±4.1 ng kg FFM-1 min-1, P=0.03) in the habitually exercising adults. In contrast, vastus lateralis muscle 2-AR density was similar in the habitually exercising and sedentary subjects (Bmax: 7.46±0.29 vs. 7.44±0.60 fmol mg dw muscle-1, P=0.98), and was not related to EE% (r=0.02, P=0.94) or the isoproterenol dose required to increase EE 10% above resting EE (r=-0.06, P=0.76). These findings indicate that increased 2-AR density is not a mechanism contributing to the greater thermogenic responsiveness to -AR stimulation in adult humans who regularly perform aerobic exercise.
Key Words:
Adrenergic, Isoprenaline, Sympathetic nervous system
