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Updated: Aug 14, 2026

Methods to Study Epithelial Transport Protein Function and Expression in Native Intestine and Caco-2 Cells Grown in 3D
Published on: March 16, 2017
Substrate specificity and mechanism of the intestinal clonidine uptake by Caco-2 cells
Wiebke Fischer1, Linda Metzner, Kathrin Hoffmann
1Membrane Transport Group, Biozentrum, Martin Luther University Halle-Wittenberg, Weinbergweg 22, D-06120, Halle, Germany.
Purpose:
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.
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