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Increased thermogenic responsiveness to intravenous β‐adrenergic stimulation in habitually exercising humans is not related to skeletal muscle β 2 ‐adrenergic receptor density
Author(s) -
Stob Nicole R.,
Seals Douglas R.,
Jensen Jørgen,
Van Baak Marleen A.,
Steig Amy J.,
Lindstrom Rachel C.,
Bikman Benjamin T.,
Bell Christopher
Publication year - 2007
Publication title -
experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0958-0670
DOI - 10.1113/expphysiol.2007.038174
Subject(s) - adrenergic , stimulation , endocrinology , skeletal muscle , medicine , adrenergic receptor , receptor
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.