Effects of administration of dopamine and L-DOPA to dogs on their plasma level of dopamine sulfate.

The effect of intravenous administration of dopamine (DA) or L-3,4-dihydroxyphenylalanine (L-DOPA), its immediate precursor, on the level of DA sulfate in dog plasma was examined, to clarify the source and physiological significance of DA sulfate which is present at high level in the plasma. After DA administration, the plasma level of free DA increased markedly, but the level of DA sulfate did not change. However, after administration of L-DOPA, the levels of both free DA and DA sulfate increased greatly. After a single injection of L-DOPA, increase in the level of free DA was transient, but that of DA sulfate persisted for a long time. These results suggest that some of the DA sulfate in dog plasma is formed from circulating L-DOPA, not from circulating DA, and that formation of DA conjugate may play a role in regulating the plasma level of free DA.