The relation of plasma norepinephrine and epinephrine levels over time in humans

Plasma norepinephrine (NE) and epinephrine (E) levels were measured at 15 min intervals in 5 male and 7 female, healthy recumbent humans over periods of 4 h (n = 9), or 8 h (n = 3). Levels of both plasma catecholamines (CA) fluctuated both tonically and phasically over time. There was a pronounced downward ("tonic") trend in the levels of the 4 h subjects who had slept at home but no consistent trend was seen in the 8 h subjects who slept in the test bed overnight. The average (+/- S.E.) of median plasma NE levels was significantly lower (231.9 +/- 15.0 pg/ml) in the 4 h group than the 8 h group (356.9 +/- 6.7 pg/ml) probably due to the younger age of the 4 h group and the weakly positive correlation of NE levels with age. Plasma E levels showed no correlation with age and were not significantly different in the 4 h (111.6 +/- 27.0 pg/ml) or 8 h (98.2 +/- 13.7 pg/ml) groups. NE and E levels correlated well in 9 out of 12 subjects when the tonic trend was considered but levels correlated in only 6 out of 12 subjects when the tonic trend was removed and only phasic variations considered. Cardiovascular parameters showed inconsistent correlations to plasma CA in the 8 h subjects although plasma NE correlated positively with mean arterial blood pressure in all 3. These data suggest that resting plasma levels of both CA vary both tonically and phasically and correlate well in only some individuals, particularly when considered on a short-term, phasic basis.