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 microM). 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, K(i) = 13.7 microM). 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 (K(d)) for binding without altering maximal binding, indicating competitive inhibition (K(i) = 11.2 microM). The binding of [14C]-tramadol to the cells was specific and saturable, with a K(d) of 18.1 +/- 2.4 microM. These findings indicate that tramadol competitively inhibits NET function at desipramine-binding sites.


CONCLUSIONS
Tramadol competitively inhibits norepinephrine transporter function at desipramine-binding sites in the adrenal medullary cells and probably the noradrenergic neurons of the descending inhibitory system.
Sagata
Kenichiro
K
Department of Anesthesiology, University of Occupational and Environmental Health, School of Medicine, Kitakyushu, Japan.
Minami
Kouichiro
K
Yanagihara
Nobuyuki
N
Shiraishi
Munehiro
M
Toyohira
Yumiko
Y
Ueno
Susumu
S
Shigematsu
Akio
A
D000313:Adrenal Medulla / Q000378:metabolism*; D000701:Analgesics, Opioid / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D003891:Desipramine / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism*; D050484:Norepinephrine Plasma Membrane Transport Proteins; D011955:Receptors, Drug / Q000378:metabolism*; D027981:Symporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014147:Tramadol / Q000378:metabolism / Q000494:pharmacology*
D000701:Analgesics, Opioid; D002352:Carrier Proteins; D009292:Narcotic Antagonists; D050484:Norepinephrine Plasma Membrane Transport Proteins; D011955:Receptors, Drug; D027981:Symporters; C036079:desipramine receptor; D009270:Naloxone; D014147:Tramadol; D003891:Desipramine; D009638:Norepinephrine