EFFECT OF AGING ON BETA-ADRENERGICALLY MEDIATED THERMOGENESIS IN MEN

The age-dependent alterations in beta-adrenergically mediated thermoge nesis 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 beta-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 pgiml, P < 0.05) concentrations than the young. The beta-adrenergically mediate d thermogenesis was diminished in the older men, as reflected by the s ignificantly higher plasma Iso concentration needed to increase restin g 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.k g fat-free mass(-1) min(-1), P < 0.01) and plasma concentration (332.2 +/- 59.1 vs. 119.3 +/- 14.0 pg/ml, P < 0.01) of Iso needed to increas e resting heart rate by 25 beats/min were higher in older than in youn ger subjects, suggesting that the age-related decline in beta-adrenerg ic sensitivity is a generalized defect not related to a specific tissu e or response. In conclusion, aging is associated with a diminished be ta-adrenergically mediated thermogenesis. This blunted thermogenic res ponse may contribute to a positive energy balance and thus promote inc reased fat storage and obesity.