title: Increased thermogenic responsiveness to intravenous beta-adrenergic stimulation in habitually exercising humans is not related to skeletal muscle beta2-adrenergic receptor density.
abstract: Habitually exercising adults demonstrate greater thermogenic responsiveness to beta-adrenergic receptor (beta-AR) stimulation compared with their sedentary peers, but the molecular mechanisms involved are unknown. To determine the possible role of increased beta-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 micromol 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 (DeltaEE%; indirect calorimetry, ventilated hood) above resting EE in response to beta-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 beta(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 DeltaEE% (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 beta(2)-AR density is not a mechanism contributing to the greater thermogenic responsiveness to beta-AR stimulation in adult humans who regularly perform aerobic exercise.
authors.affiliation: Department of Integrative Physiology, University of Colorado at Boulder, Boulder, CO 80309-0354, USA.
authors.forename: Nicole R
authors.initials: NR
authors.lastname: Stob
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Seals
authors.forename: Jensen
authors.initials: J
authors.lastname: Jørgen
authors.forename: Marleen A
authors.initials: MA
authors.lastname: van Baak
authors.forename: Amy J
authors.initials: AJ
authors.lastname: Steig
authors.forename: Rachel C
authors.initials: RC
authors.lastname: Lindstrom
authors.forename: Benjamin T
authors.initials: BT
authors.lastname: Bikman
authors.forename: Christopher
authors.initials: C
authors.lastname: Bell
mesh_terms: D000318:Adrenergic beta-Agonists / Q000008:administration & dosage*; D000328:Adult; D001823:Body Composition; D001833:Body Temperature Regulation / Q000187:drug effects / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D007275:Injections, Intravenous; D007545:Isoproterenol / Q000008:administration & dosage*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism; D011659:Pulmonary Gas Exchange / Q000502:physiology; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*
chemical_list: D000318:Adrenergic beta-Agonists; D018343:Receptors, Adrenergic, beta-2; D007545:Isoproterenol