Age-related changes in sympathetic activity: biochemical measurements and target organ responses.

The aim of this study was to evaluate using several parameters the effect of aging on sympathetic nervous system activity. We measured heart rate, skin conductance level and free and total plasma catecholamines in 60 normal subjects aged 20-80 years, divided into three age groups, in basal supine conditions and in response to standing and to hyperventilation. Basal heart rate was similar in all subjects and failed to correlate with age. Skin conductance level decreased with increasing age and correlated negatively and significantly with age. In response to standing and to hyperventilation, heart rate and skin conductance level increased in all groups but the values attained became lower in older subjects. Basal free and total norepinephrine (NE) and total epinephrine (E) correlated positively and significantly with age. In response to standing, free NE determined at the peak heart rate response, increased significantly only in young subjects. In response to hyperventilation free and total NE increased in all subjects. In conclusion, our study supports and extends previous findings that sympathetic nervous system activity is altered in aging. Moreover, these data also show that biochemical measurements of sympathetic nerve activity do not reflect the same alterations as those involving target organ responses.