Transfer and metabolism of norepinephrine studied from maternal-to-fetal and fetal-to-maternal sides in the in vitro perfused human placental lobe.

An in vitro system for the dual perfusion of an isolated lobe of human placenta was used to study transfer and metabolism of 3H-labeled norepinephrine at two different concentrations. At 39 mumol/L, the transfer of total radioactivity from the maternal to the fetal side was 11.60% +/- 0.60% and significantly higher (p less than 0.05) than that in the opposite direction (6.50% +/- 0.37%). In both directions almost 50% of tritium was transferred as norepinephrine while the rest was metabolized and appeared on the other side as normetanephrine, dihydroxymandelic acid, and vanillylmandelic acid. When a reduced concentration (870 nmol/L) was infused into either the maternal or the fetal compartment, transfer and metabolism were similar to those determined before. Analysis of the tissue showed 50% of total radioactivity as normetanephrine, dihydroxymandelic acid, and vanillylmandelic acid and 17% as conjugates. Inclusion of the monoamine oxidase inhibitor clorgyline in the perfusate significantly reduced fetal-to-maternal transfer but not transfer in the reverse direction. In addition, inhibition of placental monoamine oxidase resulted in a considerable reduction in dihydroxymandelic acid and vanillylmandelic acid while the normetanephrine fraction increased. Norepinephrine significantly increased glucose utilization and lactate production. A possible physiologic role of norepinephrine transferred from the fetal into the maternal circulation during labor is discussed.