Substrate specificity and mechanism of the intestinal clonidine uptake by Caco-2 cells

Purpose. This study was performed to characterize the Substrate specificity and mechanism of the intestinal clonidine transport. Methods. Uptake of [H-3]clonidine into Caco-2 cells was investigated. Interaction with drug was studied in competition assays. Results. Uptake of [H-3]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 (K-t) and 16.6 +/- 1.8 nmol/2 min per mg of protein (V-max) 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 [H-3]clonidine uptake with K-i 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.