Study of diurnal fluctuations of plasma methoxyamines in healthy volunteers

We studied the diurnal fluctuations of plasma concentrations of methoxyamines (metanephrine and normetanephrine) and of their parent amines (epinephrine and norepinephrine) in normotensive subjects.

Serial blood sampling at 09.00 h, 11.00 h, 12.00 h, 14.00 h, 16.00 h, 18.00 h and 20.00 h in 28 healthy volunteers at rest. Determination of plasma concentrations of free catecholamines and total methoxyamines (free and sulpho-conjugates) was carried out by high-performance liquid chromatography with electrochemical detection.

Mean (+/- SD) plasma concentrations of total metanephrine (MN) and normetanephrine (NMN) were 4.31 +/- 1.73 nmol/l (range: 0.96-9.3 nmol/l) and 8.13 +/- 2.54 nmol/l (range: 3.14-17.0 nmol/l), respectively. The NMN/MN ratio ranged between 0.8 and 7.8 (mean +/- SD 2.1 +/- 1.0). Mean plasma concentrations of free epinephrine and norepinephrine were 0.21 +/- 0.12 nmol/l (range: 0.06-1.39 nmol/l) and 1.61 +/- 0.62 nmol/l (range: 0.47-4.01 nmol/l), respectively. Despite marked intraindividual fluctuations, mean methoxyamine and catecholamine levels remained constant over the entire duration of the experiment.

The absence of fluctuations of plasma levels of total methoxyamines suggests that their measurement could be carried out at any time within the diurnal time frame. Further investigations, however, remain necessary to validate these findings in patients with hypertension and/or pheochromocytoma, and to explain the ever important intraindividual variation in plasma concentrations of methoxyamines and of their parent compounds.