The effect of age on the beta-adrenergic lipolytic response in healthy humans

We evaluated the effect of age on the lipolytic response to intravenous infusion of isoproterenol in 12 elderly (age, > 60 years) and 12 young (age, 21 to 34 years) volunteers to examine if there is alteration in innervated beta-adrenergic responsiveness in tissues other than the heart. Lipolysis was evaluated by measuring the plasma concentrations of free fatty acids and glycerol. We also measured the plasma concentration of isoproterenol during infusion so that we could calculate comparable plasma isoproterenol concentrations to lipolytic responses in the two age groups. Our data show that, at equivalent infusion rates of isoproterenol, the two age groups achieved equivalent isoproterenol concentrations. The elderly had a higher concentration of free fatty acids but equivalent concentrations of glycerol as compared with the young subjects at equivalent isoproterenol plasma concentrations. However, our data were complicated by the fact that at the higher infusion rates of isoproterenol, the elderly showed a greater sympathetic stimulation than the young subjects as measured by plasma norepinephrine concentrations. Nonetheless, our data could not show that the elderly subjects were more resistant to beta-adrenergic receptor-mediated stimulated lipolysis. Thus innervated beta-adrenergic receptor hyporesponsiveness caused by aging may not necessarily extend to all organ systems.