Effects of ordinary meals on plasma concentrations of 3,4-dihydroxyphenylalanine, dopamine sulphate and 3,4-dihydroxyphenylacetic acid.

1. Plasma concentrations of 3,4-dihydroxyphenylalanine (DOPA), dopamine sulphate (DA-S), and 3,4-dihydroxyphenylacetic acid (DOPAC) in humans have been claimed to be indexes of sympathetic nervous activity, but the source and significance of plasma DOPA, DOPAC and DA-S have not been completely elucidated. 2. The effects of ordinary meals on plasma concentrations of total dopamine, mainly DA-S, DOPAC and DOPA were studied in seven healthy subjects. Venous blood was collected every hour for 25 h, while subjects were either fasting or received three meals at 9.00 hours, 13.00 hours and 18.00 hours. Catecholamines and metabolites were determined by reverse-phase HPLC with electrochemical detection. Neutral amino acids were measured by ion-exchange chromatography with photometric detection. 3. The food contained relatively little DOPA as compared with phenylalanine, tyrosine, isoleucine and tryptophan. The content of DA and DA-S varied considerably, with the greatest amount in the evening meal of open sandwiches. 4. Plasma DOPA decreased significantly after the meals at 13.00 hours and 18.00 hours, whereas concentrations of the other amino acids increased as expected. 5. Plasma DA-S increased significantly after meals and especially after the evening meal. Increments in DA-S above basal values after a meal were closely related to the content of DOPA+DA+DA-S in the meal. Plasma DOPAC increased significantly after the evening meal. 6. The decrease in plasma DOPA observed after a meal was probably due to uptake of DOPA by muscle tissue. Changes in plasma DA-S and DOPAC during this 25-h study reflected to a large extent the content of DOPA, DA and DA-S in the meals.