Variability of plasma catecholamine levels: age, duration of posture and time of day.

1. In order to define factors which influence plasma catecholamine levels, and which might be controlled in the interests of reproducability and comparability, plasma noradrenaline plus adrenaline (NA + A) was measured repeatedly in normal subjects by a double isotope technique. 2. Age and posture were important determinants for plasma NA + A levels, whereas time of day was less important. 3. Levels were higher in old men aged 70--92 years than in young men aged 19--28 years, measured after 30 min or 9 h (overnight) recumbency, or after 5 min standing. 4. Duration of recumbency and of upright posture were both important. Recumbent levels were lower after 9 h (overnight) than after 30 min (mid-morning). Upright posture caused an increase in levels within 2 min. Levels peaked between 5 and 20 min, were lower after 12 h than after 20 min, but upright levels were always higher than levels after 9 h recumbency. 5. During continuous recumbency, levels were lowest at 24.00 h, and increased thereafter. During a second hospitalisation, levels at 09.00 h were again higher than levels at 06.00 h after overnight recumbency. 6. Variability of levels obtained by repeated sampling was lower while sitting, during a normal upright day, than after 30 min recumbency. Upright legels may be a better index of cardiovascular sympathetic activity.