L-3,4-Dihydroxyphenylalanine (DOPA), a precursor of a neurotransmitter dopamine (DA), is synthetized by tyrosine hydroxylase in the cytoplasm of catecholaminergic neurons. We proposed that DOPA is a neurotransmitter. We previously reported that high K+ -evoked release of DOPA and DA from cultured PC12 cells, both of which were similarly decreased by deprivation of extracellular Ca2+. Using this system, we are attempting to elucidate the mechanism by which the DOPA release occurs. We found that bafilomycin inhibited the K+-evoked-release of DA, but not DOPA, while brefeldin A suppressed the release of DOPA but not DA, thereby suggesting the release of DOPA may occur through a secretion pathway distinct from that for DA in PC12 cells (JPS95). To further characterize the release of DOPA, we here examined the effects of several agents on the evoked release of DOPA and DA from cultured PC12 cells. Proline, valine, phenylalanine, tyrosine and alpha-methyl-p-tyrosine (10-5 M) showed no effect on DOPA and DA release. Among reagents tested, 3-iodo-tyrosine inhibited the release of DOPA without affecting that of DA. This finding indicates a higher sensitivity of the release of DOPA to 3-iodo-tyrosine than that of DA.
