Molecular and pharmacological analysis of the noradrenaline transporter

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Noradrenaline transporters (NAT) located on plasma membranes of noradrenergic nerve terminals remove noradrenaline (NA) from the extracellular milieu via the Na^+/Cl^--dependent cotransport process and terminate the action of NA in the synapse. The pharmacological properties of NAT in adrenal medullary cells are similar to those of the NAT of central and peripheral noradrenergic neurons. NAT expressed in adrenal medullary cells provides a convenient model system to study the effects of drugs on this transporter.Treatment of cultured adrenal medullary cells with IFN-alpha caused a decrease in uptake [3H]NA by the cells in time (4-48h)- and concentration (300-IOOOU/ml)-dependent manners. Saturation analysis of [^3H]NA uptake showed that the inhibitory effect of lFN-alpha was due to a reduction in the maximal uptake velocity (Vimax) values without altering apparent Michaelis constant (Km) values. Scatchard analysis of [^3H]desipramine binding revealed that IFN-alpha decreased the maximal … More binding (Bmax) values without any change in the dissociation constant (Kd) values. These findings suggest that IFN-alpha suppresses the function of NAT by reducing the density of the transporter in cell membranes.Ketamine (1O-1OOOmuM) inhibited desipramine-sensitive uptake of [^3H]NA (IC_<50>=97muM). Saturation analysis showed that ketamine reduced Vmax of [^3H]NA uptake without changing Km, indicating a non-competitive inhibition. Other inhibitors of NAT, namely cocaine and desipramine, showed a competitive inhibition of [^3H]NA uptake while a derivative of ketamine, phencyclidine, showed a mixed type of inhibition. Scatchard analysis of [^3H]desipramine binding revealed that ketamine increased Kd without altering Bmax, indicating a competitive inhibition. In transfected Xenopus oocytes expressing the NAT, ketamine attenuated [^3H]NA uptake with a kinetic characteristic similar to that of cultured adrenal medullary cells. These findings are compatible with the idea that ketamine non-competitively inhibits the transport of NA by interacting with a site which partly overlaps the desipramine binding site on the NAT. Less