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-adrenergically mediated thermogenesis
in men
Department of Human Biology, Maastricht University, 6200 MD Maastricht, The Netherlands
The age-dependent
alterations in 
-adrenergically mediated thermogenesis were
investigated in 11 young (mean ± SE age: 21.9 ± 0.5 yr) and 9 older (52.9 ± 2.1 yr) men during intravenous infusion of the
nonselective -agonist isoprenaline (Iso). The older men had higher
basal plasma norepinephrine (327.7 ± 35.8 vs. 159.0 ± 18.2 pg/ml, P < 0.001) and epinephrine
(75.1 ± 18.1 vs. 29.1 ± 5.3 pg/ml,
P < 0.05) concentrations than the
young. The -adrenergically mediated thermogenesis was diminished in
the older men, as reflected by the significantly higher plasma Iso
concentration needed to increase resting energy expenditure by 15%
(236.1 ± 51.0 vs. 107.6 ± 11.4 pg/ml,
P < 0.05). Additionally, both dose
(39.4 ± 6.6 vs. 19.1 ± 1.5 ng · kg fat-free
mass
1 · min1,
P < 0.01) and plasma concentration
(332.2 ± 59.1 vs. 119.3 ± 14.0 pg/ml,
P < 0.01) of Iso needed
to increase resting heart rate by 25 beats/min were higher in older
than in younger subjects, suggesting that the age-related decline in
-adrenergic sensitivity is a generalized defect not related to a
specific tissue or response. In conclusion, aging is associated with a
diminished -adrenergically mediated thermogenesis. This blunted
thermogenic response may contribute to a positive energy balance and
thus promote increased fat storage and obesity.
energy expenditure; chronotropic response; age; sympathetic nervous system; isoprenaline
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