Tramadol Inhibits Norepinephrine Transporter Function at Desipramine-Binding Sites in Cultured Bovine Adrenal Medullary Cells

Tramadol is a widely used analgesic, but its mode of action is not well understood. To study the effects of tramadol on norepinephrine transporter (NET) function, we assayed the effect of tramadol on [3H]-norepinephrine ([3H]-NE) uptake and [3H]-desipramine binding to plasma membranes isolated from bovine adrenal medulla. We then characterized [14C]-tramadol binding in cultured bovine adrenal medullary cells. Tramadol inhibited the desipramine-sensitive uptake of [3H]-NE by the cells in a concentration-dependent manner (50% inhibitory concentration = 21.5 ± 6.0 μM). Saturation analysis revealed that tramadol increased the apparent Michaelis constant of [3H]-NE uptake without changing the maximal velocity, indicating that inhibition occurred via competition for the NET (inhibition constant, Ki = 13.7 μM). Tramadol inhibited the specific binding of [3H]-desipramine to plasma membranes. Scatchard analysis of [3H]-desipramine binding revealed that tramadol increased the apparent dissociation constant (Kd) for binding without altering maximal binding, indicating competitive inhibition (Ki = 11.2 μM). The binding of [14C]-tramadol to the cells was specific and saturable, with a Kd of 18.1 ± 2.4 μM. These findings indicate that tramadol competitively inhibits NET function at desipramine-binding sites.