Dopamine and its metabolites in human tears

Dopamine is known to stimulate ion transport in the corneal epithelium. Since epithelium is in contact with tears, we searched for dopamine and its metabolites in tears. Twenty normal subjects participated in this study. Twenty microliters of tears were collected in capillary tubes after trigeminal stimulation and immediately analyzed using high-performance liquid chromatography with electrochemical detection. Dopamine and two of its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were identified and measured. The mean concentration of dopamine was 8.9 +/- 5.1 ng/ml; DOPAC, 2.2 +/- 1.6 ng/ml; and HVA, 0.55 +/- 0.4 ng/ml. These results permit us to suggest that dopamine is deaminated by monoamine oxidase, then methylated by catechol-O-methyltransferase and that catabolism occurs in the lacrimal gland and perhaps in corneal epithelium. Finally, part of the dopamine involved in ion transport may come from tears.