title: Substrate specificity and mechanism of the intestinal clonidine uptake by Caco-2 cells.
abstract: OBJECTIVE
  This study was performed to characterize the substrate specificity and mechanism of the intestinal clonidine transport.
  
  
  METHODS
  Uptake of [3H]clonidine into Caco-2 cells was investigated. Interaction with drugs was studied in competition assays.
  
  
  RESULTS
  Uptake of [3H]clonidine was linear for up to 2 min, Na+-independent, and insensitive to changes in membrane potential, but strongly H+-dependent. The uptake rate of clonidine was saturable with kinetic parameters of 0.5+/-0.1 mM (Kt) and 16.6+/-1.8 nmol/2 min per mg of protein (Vmax) at an outside pH of 7.5. Many drugs such as clonidine, guanabenz, methamphetamine, imipramine, clomipramine, nortriptyline, quinine, xylazine, ephedrine, and diphenhydramine strongly inhibited the [3H]clonidine uptake with Ki values between 0.15 and 1 mM.
  
  
  CONCLUSIONS
  Clonidine is transported by a carrier-mediated process. Substrate specificity and mechanism are very similar to the transport described in blood-brain barrier endothelial cells. The transport characteristics do not correspond to carriers for organic cations of the SLC22 family or the choline transporters CHT1 and CLT1. The system might be identical to the H+/tertiary amine antiporter. It interacts with a large number of both hydrophilic and lipophilic cationic drugs, and also, interestingly, with opiates.
authors.affiliation: Membrane Transport Group, Biozentrum, Martin Luther University Halle-Wittenberg, Weinbergweg 22, D-06120, Halle, Germany.
authors.forename: Wiebke
authors.initials: W
authors.lastname: Fischer
authors.forename: Linda
authors.initials: L
authors.lastname: Metzner
authors.forename: Kathrin
authors.initials: K
authors.lastname: Hoffmann
authors.forename: Reinhard H H
authors.initials: RH
authors.lastname: Neubert
authors.forename: Matthias
authors.initials: M
authors.lastname: Brandsch
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000493:pharmacokinetics*; D000643:Ammonium Chloride / Q000494:pharmacology; D001812:Blood-Brain Barrier / Q000187:drug effects; D018938:Caco-2 Cells; D003000:Clonidine / Q000493:pharmacokinetics*; D004347:Drug Interactions; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007408:Intestinal Absorption / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D004364:Pharmaceutical Preparations / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D000316:Adrenergic alpha-Agonists; D004364:Pharmaceutical Preparations; D000643:Ammonium Chloride; D003000:Clonidine