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This Article Full Text Full Text (PDF) All Versions of this Article: 92/5/823    most recent expphysiol.2007.038174v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Stob, N. R. Articles by Bell, C. Search for Related Content PubMed PubMed Citation Articles by Stob, N. R. Articles by Bell, C.

¹ Department of Integrative Physiology, University of Colorado at Boulder, Boulder, CO 80309-0354, USA ² Department of Physiology, National Institute of Occupational Health, N-0033 Oslo, Norway ³ Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht (NUTRIM), Maastricht University, Maastricht, The Netherlands ⁴ Department of Health and Exercise Science, Colorado State University, Fort Collins, CO 80523-1582, USA

Habitually exercising adults demonstrate greater thermogenic responsiveness to ß-adrenergic receptor (ß-AR) stimulation compared with their sedentary peers, but the molecular mechanisms involved are unknown. To determine the possible role of increased ß-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 (EE%; indirect calorimetry, ventilated hood) above resting EE in response to ß-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 ß₂-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 EE% (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 ß₂-AR density is not a mechanism contributing to the greater thermogenic responsiveness to ß-AR stimulation in adult humans who regularly perform aerobic exercise.

(Received 18 April 2007; accepted after revision 14 June 2007; first published online 15 June 2007)
Corresponding author C. Bell: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO 80523-1582, USA. Email: cbell{at}cahs.colostate.edu