[Changes in plasma norepinephrine levels to stepwise tilting--comparisons between the young and aged]

Area of Science:

• Physiology
• Gerontology
• Cardiovascular Regulation

Background:

• Autonomic nervous system function changes with age.
• Norepinephrine is a key indicator of sympathetic nervous system activity.
• Understanding age-related differences in autonomic responses is crucial for geriatric health.

Purpose of the Study:

• To compare plasma norepinephrine levels during stepwise head-up tilting between young and aged normal volunteers.
• To investigate the relationship between plasma norepinephrine response and muscle sympathetic nerve activity (MSNA) in different age groups.
• To determine if age-related differences in autonomic function are more evident in plasma norepinephrine or MSNA.

Main Methods:

• Plasma norepinephrine levels were measured in young (22-26 years) and aged (66-81 years) volunteers.
• Participants underwent stepwise head-up tilting (15°, 30°, 45°, 80°) with blood sampling at each stage.
• Norepinephrine changes were correlated with tilting angles and compared with concurrent MSNA data.

Main Results:

• Plasma norepinephrine increased with tilting angle in both groups, showing a positive linear correlation.
• Aged individuals had higher absolute norepinephrine levels at 0°, 15°, and 30° tilts.
• The relative increase in norepinephrine was lower in the aged at 45° and 80° tilts compared to the young.
• Differences in norepinephrine response between age groups were less distinct than observed differences in MSNA.

Conclusions:

• Aging alters plasma norepinephrine responses to orthostatic stress.
• While baseline norepinephrine is higher in the aged, their sympathetic response rate to tilting is blunted.
• Muscle sympathetic nerve activity provides a more sensitive marker of age-related autonomic dysfunction than plasma norepinephrine levels.