Synthesis and High Affinity Uptake of Serotonin and Dopamine by Human Y79 Retinoblastoma Cells

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Abstract
Human Y79 retinoblastoma cells are capable of synthesizing the putative retinal neurotransmitters dopamine and serotonin. Separation of the catecholamines and indolamines by high performance liquid chromatography combined with electrochemical detection showed that the cells readily convert tyrosine to 3,4‐dihydroxyphenylalanine (DOPA) and, to a lesser extent, dopamine. When DOPA was added, a large quantity of dopamine was produced, as well as norepinephrine, epinephrine, and 3,4‐dihydroxyphenylacetic acid. Exogenous tryptophan added to the cells was partially converted to 5‐hydroxytryptophan and serotonin. A larger quantity of serotonin was produced when 5‐hydroxytryptophan was added. Y79 cells have a high and low‐affinity uptake system for dopamine and serotonin. The K′m and V′max for the high‐affinity uptake of dopamine and serotonin are 2.34 ± 0.64 and 3.63 ± 1.15 μM and 4.77 ± 1.12 and 3.20 ± 1.20 pmol min‐1 mg protein‐1, respectively. These kinetic parameters are similar to those reported for other retinal preparations where dopamine and serotonin have been suggested to function as neurotransmitters. Tyrosine and tryptophan, the physiologic precursors of dopamine and serotonin, respectively, and phenylalanine are also taken up by high and low‐affinity transport systems. The kinetic parameters for their high‐affinity uptake systems are all very similar, suggesting that they may be taken up by the same transporter. These studies show that a tumor cell line derived from the human retina synthesizes dopamine and serotonin and has high affinity uptake systems for these compounds and their pre cursors. This is consistent with the possibility that dopamine and serotonin may function as neurotransmitters in the human retina and that the Y79 retinoblastoma may be a suitable cellular model system in which to study their transport and metabolic properties.