Expression of a Cocaine-Sensitive Norepinephrine Transporter in the Human Placental Syncytiotrophoblast

Maternal-facing brush border membrane vesicles isolated from normal term human placentas were found to accumulate norepinephrine in a concentrative manner in the presence of an inwardly directed NaCl gradient. Both Na⁺ and Cl⁻ were obligatory for maximal uptake. The NaCl-dependent norepinephrine uptake was further stimulated by the presence of K⁺ or an acidic pH in the intravesicular medium. The uptake process was electrogenic, being stimulated by an inside-negative membrane potential, and this characteristic was observed in the absence as well as in the presence of K⁺ inside the vesicles. Kinetic analyses revealed that one Na⁺ and one Cl⁻ were involved per transport of one norepinephrine molecule. The apparent Michaelis-Menten constant for norepinephrine was 104 ± 5 nM. The uptake process exhibited higher affinity for dopamine than for norepinephrine but had low affinity for serotonin and histamine. The uptake of norepinephrine was inhibited very effectively by nomifensine, desipramine, imipramine, and cocaine, but much less effectively by bupropion and GBR 12909. Northern blot analysis with the cDNA of the human (SK-N-SH cell) norepinephrine transporter as the probe revealed that the human placenta contained two mRNAs, 5.8 and 3.6 kb in size, which hybridized to the probe. The JAR human placental choriocarcinoma cells were found unable to accumulate norepinephrine in a NaCl-dependent manner. These cells were also found not to contain mRNAs which hybridized to the norepinephrine cDNA probe in northern blot. It is concluded that the human placental syncytiotrophoblast expresses a cocaine-sensitive norepinephrine transporter and that these findings may be directly relevant and important to the clinical complications of maternal cocaine abuse during pregnancy.