Monoamine oxidase activity in the term human placenta.

Using a radioenzymatic assay, placental monoamine oxidase (MAO) activity was measured at term after delivery in normal and high-risk pregnancies where decreases in placental blood flow previously were shown. MAO activity in placentas of healthy controls after spontaneous labor was similar to that after elective cesarean section not in labor (mean +/- SE, 133 +/- 18 versus 100 +/- 15 nmol/min/mg protein, respectively). Compared to controls, there was a significant reduction in placental MAO activity in high-risk pregnancies (chronic hypertension, toxemia, and diabetes mellitus), 71 +/- 14, 69 +/- 22, and 69 +/- 7, respectively (P less than 0.05). These differences also were maintained when data were expressed per total placental weight. Effects of antihypertensive drugs on MAO activity in healthy placental tissue were assessed. In homogenates, both hydralazine and magnesium sulfate reduced enzyme activity, while in explants this was not observed. The effects of certain metabolites (which are elevated in plasma of diabetic patients) on healthy homogenates also were studied. Only butyrate reduced enzyme activity. In conclusion, placental MAO activity in vitro is low in term high-risk pregnancies. This may reduce local metabolic inactivation of catecholamines and serotonin and consequently lead to a decrease in blood flow. Such a direct relationship must be confirmed in further studies.