Institution: | aMembrane Transport Group, Biozentrum, Martin-Luther-University Halle-Wittenberg, Weinbergweg 22, D-06120 Halle, Germany bInstitute of Pharmacy, Faculty of Sciences I, Martin-Luther-University Halle-Wittenberg, Halle, Germany |
Abstract: | After transport across several epithelial barriers including the blood–brain barrier, clonidine interacts with 2-adrenergic receptors and imidazoline binding sites in the brain. We hypothesized that neuronal cells take up clonidine thereby removing the drug from the extracellular fluid compartment. Uptake of 3H]clonidine into SH-SY5Y neuroblastoma cells was linear for up to 1 min, unaffected by inside directed Na+ or Cl? gradients but strongly inhibited by an outside pH of 6.0. The cells accumulated 3H]clonidine 50–70-fold uphill against a concentration gradient. Unlabeled clonidine, guanabenz, imipramine, diphenhydramine, maprotiline, quinine and the endogenous monoamine phenylethylamine (2 mM) strongly inhibited the 3H]clonidine uptake by 60–95%. Tetraethylammonium, choline and N-methyl-4-phenylpyridinium had no effect. The accumulation at pH 7.5 was saturable with an apparent Michaelis–Menten constant (Kt) of 0.7 mM. We conclude that SH-SY5Y cells not only bind clonidine to extracellular receptors but also take up the drug rapidly by a specific and concentrative mechanism. |