Variation in activity of monoamine metabolizing enzymes in rat liver during pregnancy.

1. Catechol-0-methyltransferase (COMT) and monoamine oxidase (MAO) activities in rat liver were measured during pregnancy, parturition and postpartum. Compared with activity in non-pregnant controls, both enzymes showed a significant decrease in activity which was most pronounced at day 18. 2. The metabolism of intravenously infused [3H]-adrenaline to [3H]-metanephrine and to [3H]-acidic metabolites was also significantly depressed during pregnancy but had returned to control values by the 21st day. 3. The effects of reserpine and/or nialamide on hepatic COMT and MAO were studied in control and 20-day-pregnant rats. Their action on COMT activity differed in the two groups. MAO was inhibited to a similar extent in these groups whether the drugs were given separately or in combination. 4. It seems possible that the changes in endocrine function which occur during pregnancy are responsible for the observed alterations in enzyme activity.