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

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Abstract
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.