Sodium- and chloride-dependent, cocaine-sensitive, high-affinity binding of nisoxetine to the human placental norepinephrine transporter.

Maternal-facing brush border membranes prepared from normal term human placentas possess the norepinephrine transporter. We investigated the interaction of nisoxetine with the human norepinephrine transporter by examining the binding of this ligand to the placental brush border membranes. Scatchard analysis revealed that nisoxetine bound with high affinity to a single class of binding sites in the membranes (dissociation constant = 13.8 +/- 0.4 nM). This value obtained from equilibrium experiments matched the value (11.2 nM) which was calculated using the association and dissociation rate constants. The maximal binding capacity (Bmax) was 5.1 +/- 0.1 pmol/mg of protein. The binding exhibited an absolute requirement for Na+ as well as Cl-. Presence of these ions enhanced the binding affinity without affecting Bmax. Kinetic analyses revealed that the coupling ratio of Na+/nisoxetine was 2, whereas the coupling ratio of Cl-/nisoxetine was 1. The binding was most potently inhibited by the ligands of the norepinephrine transporter (desipramine and nomifensine). The ligands of the serotonin transporter (imipramine, paroxetine, and fluoxetine) showed intermediate inhibitory potencies, whereas the ligands of the dopamine transporter (bupropion and GBR 12909) were the least potent. Among the monoamines, dopamine was the most potent inhibitor, followed by norepinephrine and serotonin. Though both cocaine and its analog RTI-55 were powerful inhibitors of the binding, RTI-55 was approximately 150 times more effective than cocaine. The inhibition of binding by norepinephrine, cocaine, and RTI-55 was competitive. Uptake of norepinephrine measured in membrane vesicles was found to be inhibited by treatment of the vesicles with phenylarsine oxide, a reagent specific for vicinal dithiol groups.(ABSTRACT TRUNCATED AT 250 WORDS)