Increased plasma norepinephrine response to yohimbine in elderly men

Background. The effects of aging on sympathetic nervous system and adrenome dullary outflow were estimated by the measurement of plasma norepinephrine (NE) and epinephrine (EPI) responses to yohimbine and clonidine in healthy young and healthy older subjects. Methods. Yohimbine (0.65 mg/kg), clonidine (5 mu g/kg), and placebo were ad ministered on separate days in random order to 5 healthy older men (age 74 +/- 1 years) and 18 healthy young men (age 26 +/- 1 years). NE and EPI were measured by radioenzymatic assay in plasma samples obtained before and 30, 60, and 90 minutes after drug administration. Results. Plasma NE increases after yohimbine were greater in older men than in young men, but plasma NE decreases following clonidine did not differ b etween groups. Plasma NE and systolic blood pressure were higher in older m en than in young men at baseline but no longer differed 90 minutes after cl onidine. Plasma EPI increases after yohimbine and decreases after clonidine did not differ between groups. Conclusions. These results suggest increased sympathetic nervous system out flow in human aging that is not a function of reduced responsiveness of alp ha-2 adrenoreceptor-mediated feedback inhibition.