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 β2-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 β2-AR density is not a mechanism contributing to the greater thermogenic responsiveness to β-AR stimulation in adult humans who regularly perform aerobic exercise. © 2007 The Authors.