The production mechanism of amniotic fluid dopamine in rats.

The production of dopamine in amniotic fluid was studied using Wistar rats. The dopamine concentrations in the amniotic fluid increased significantly between each day of gestation from day 18 to day 21, and a marked increase was observed during the last two days. The concentrations of dopamine in the fetal plasma, however, did not change with the gestational age. There was a significant increase in the fetal plasma L-dopa, which is the precursor of dopamine, without any changes in maternal plasma L-dopa levels during late gestation. Dopa decarboxylase (DDC) activity was also measured in the fetal rat kidney from gestation day 17 to day 21. DDC activity, which was low on day 17, increased with advanced gestation. DDC activity on day 21 was 0.68 nmol/mg tissue/h, about 13-fold higher than on day 17. These results suggest that fetal plasma L-dopa may be converted to dopamine by fetal kidney DDC, and that the dopamine is voided into the amniotic cavity in fetal urine.