Simultaneous measurement of monoamines, their metabolites and precursor amino acids in plasma and urine from healthy subjects by HPLC/multielectrode electrochemistry

The circadian changes in the concentrations of monoamines and their metabolites in plasma [tyrosine (Tyr), 3-methoxydopa (3-OMD), dopamine (DA), norepinephrine (NE), epinephrine (EN), homovanilic acid (HVA), vanillylmandelic acid (VMA), 3-methoxy-4-hydroxyphenylglycol (MHPG), tryptophan (Trp), 5-hydroxyindoleacetic acid (5-HIAA)] obtained at (05:00 AM and 13:00 PM) and their total contents in 24-hour urine from 15 healthy subjects were simultaneously determined using an HPLC/multi-electrode electrochemical detection. Their precursor amino acids, Tyr and Trp, and dopamine β-hydroxylase (DBH) activity, which is mainly secreted from the peripheral catecholamine systems of the sympathetic nerves and adrenal medulla into the blood and shows circadian variations (minimum at 05:00 AM and maximum at 13:00 PM) were also determined. Many monoamine parameters, especially NE and EN, showed higher values in plasma at 13:00 PM, whereas the levels of HVA, VMA and 5-HIAA tended to be low at 13:00 PM. Significant correlations were observed between each monoamine.