Binding of cocaine-like radioligands to the dopamine transporter at 37 degrees C: effect of Na+ and substrates.
Although dopamine (DA) translocation by the DA transporter (DAT) requires Na+, a role for Na+ in the DA recognition step in the translocation cycle has been questioned. Thus, when binding techniques were used to indirectly measure the affinity of DA for DAT via its potency in inhibiting cocaine analog binding, no stimulation of DA binding was observed when assay temperature was at or below room temperature. The present work describes the use of 3H-labeled cocaine analogs for assays at 37 degrees C. When there is sufficient Na+ in the medium (> or =25 mM), [3H]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([3H]CIT) is an excellent radioligand to label human DAT with high affinity in membrane preparations of HEK-293 cells expressing the transporter. However, at 0 and 5 mM of Na+, appreciable binding of [3H]CIT occurs to proteins other than DAT, hampering accurate assessment of DAT-associated binding. No such problems occur with the binding of the 4-fluoro homolog of [3H]CIT, [3H]CFT at 37 degrees C, and this radioligand can be used at low [Na+], provided enough protein is present in the assay. The application of these assays show that, in contrast to the strong Na+ dependency of the binding of CFT, the substrates DA, D-amphetamine, p-tyramine, and DL-octopamine are not stimulated by Na+. This demonstrates that lack of Na+ stimulation of binding of substrates, including DA to DAT, in membrane preparations at room temperature is not caused by the reduced fluidity of the frozen state of the hydrocarbon membrane interior at this temperature as compared with the liquid-expanded state at 37 degrees C.
Wang
Lijuan C
LC
Department of Biomedical and Therapeutic Sciences, University of Illinois College of Medicine, PO Box 1649, Peoria, IL 61656-1649, USA.
Cui
Xiao-Nan
XN
Chen
Nianhang
N
Reith
Maarten E A
ME
D001667:Binding, Competitive / Q000187:drug effects / Q000502:physiology; D002460:Cell Line; D003042:Cocaine / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000493:pharmacokinetics; D004298:Dopamine / Q000378:metabolism*; D050483:Dopamine Plasma Membrane Transport Proteins; D018765:Dopamine Uptake Inhibitors / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004622:Embryo, Mammalian / Q000166:cytology; D006801:Humans; D007668:Kidney / Q000166:cytology; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins / Q000378:metabolism*; D009419:Nerve Tissue Proteins; D009662:Nortropanes / Q000493:pharmacokinetics; D011869:Radioligand Assay / Q000379:methods*; D012964:Sodium / Q000494:pharmacology*; D013696:Temperature; D014316:Tritium / Q000493:pharmacokinetics
C103655:2-carbomethoxy-3-(4-chlorophenyl)-8-(3-fluoropropyl)nortropane; D050483:Dopamine Plasma Membrane Transport Proteins; D018765:Dopamine Uptake Inhibitors; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D009662:Nortropanes; C493214:SLC6A3 protein, human; D014316:Tritium; C021279:(1R-(exo,exo))-3-(4-fluorophenyl)-8-methyl-8- azabicyclo(3.2.1)octane-2-carboxylic acid, methyl ester; D012964:Sodium; D003042:Cocaine; D004298:Dopamine