br Conflict of interest statement br Acknowledgements We are

Conflict of interest statement
Acknowledgements We are very grateful to Maura Agate for her excellent editorial assistance. The work reported in this article was supported largely by the Italian MIUR.
Introduction Atenolol is a selective β1-adrenergic receptor blocker that is orally effective in the treatment of hypertension, angina pectoris, and cardiac arrhythmias [1–3]. This drug, of which the calpain inhibitor 1 is rather moderate with the bioavailability of about 50% [4], has generally been assumed to be absorbed by simple diffusion in the intestine [5]. However, based on recent findings of significant reductions in its absorption by simultaneous administration with apple juice [6] and with orange juice [7], it may be needed to assume that carrier-mediated uptake transport, which could be inhibited by some constituents of those fruit juices, is involved in its absorption. It may be possible that the disposition of this hydrophilic drug, which little undergoes metabolism and is eliminated almost exclusively (90%) by excretion into the urine in unchanged form after absorption [8], is regulated by membrane transporters. Organic anion transporting polypeptide 1A2 (OATP1A2/SLCO1A2) and OATP2B1/SLCO2B1, which have recently been suggested to be involved in the intestinal uptake of various anionic drugs [9], can also be inhibited by such plant components, and it could lead to decreases in the absorption of their substrate drugs [10,11]. It should be of interest to note that some β1-adrenergic receptor blockers structurally analogous to atenolol have been reported to be transported by OATP1A2 and OATP2B1 [12,13], even though this class of drugs is cationic. Particularly, talinolol is known as an OATP1A2 substrate, and its absorption is also inhibited by flavonoids [12]. Thus, OATP1A2 and OATP2B1 may also contribute to the absorption of atenolol, but another possibility could not be excluded that some other transporters for cationic compounds may participate in that process. We, here, describe our successful attempt to identify human OCT1/SLC22A1 as an atenolol transporter, which may be involved in intestinal atenolol absorption. OCT1 is known to be typically expressed in the basolateral membrane of hepatocytes and contribute to extrusion of various cationic compounds including endogenous compounds and drugs, such as tetraethylammonium (TEA), 1-methyl-4-phenylpyrimidine (MPP+), acetylcholine and metformin [14–16]. Diphenhydramine, atropine and desipramine are, on the other hand, known as its typical inhibitors that are not transported. Recently, its presence and potential role in drug transport has also been suggested in the intestine in the brush border and basolateral membranes.
Materials and methods
Results
Discussion We here have described the novel function of OCT1 as an atenolol transporter. OCT1 is well known to be involved in the elimination of various organic cationic compounds in the liver [18]. Therefore, OCT1 may be also in operation for atenolol uptake in the liver. However, the role of this process in the elimination of atenolol is unclear, because atenolol is known to be almost exclusively excreted into the urine from the kidney [4,8]. Recently, OCT1 is reported to be localized to the apical membrane in polarized Caco-2 cells, which are widely used as a model of the human small intestinal epithelial cell [20,26]. However, the localization of OCT1 in the epithelial cells in the human tissue remains inconclusive, because it has also been suggested that OCT1 plays a role in exporting its substrates across the basolateral membrane in the intestine as well as liver and kidney [18,27]. Taken all these together, OCT1 may contribute to the transport of cationic compounds at both the apical and basolateral membranes in enterocytes. Fruit juices have been known to inhibit enzymes involved in metabolism and transporters involved in drug absorption in the intestine [28–30]. Some components of grapefruit juice have been shown to be potent inhibitors of cytochrome P450 (CYP) 3A4, one of the major drug metabolism enzymes [31]. Furthermore, OATPs, which could be involved in the absorption of various anionic drugs in the intestine, has also been suggested to be inhibited by constituents of fruit juices [10–13]. It was previously reported that the plasma concentrations of atenolol are significantly decreased by co-administration of apple juice and orange juice [6,7]. According to pharmacokinetic analysis, Cmax and AUC were significantly reduced by 68% and 81%, respectively, by apple juice and by 49% and 40%, respectively, by orange juice. However, the half life of its elimination was not changed [6,7]. This suggested that apple and orange juices only affected the absorption process, but not the elimination process. Our present study showed that OCT1-mediated atenolol transport can be inhibited by various flavonoids, including those in apple and orange juices. In addition, inhibition by phloretin and quercetin, which are the major components of apple juice [22,32], were found to be noncompetitive. This would be a finding to support the suggestion that OCT1 participates in atenolol absorption in the small intestine. Although OATP2B1, which is also known to be inhibited by fruit juices, has been suggested to be involved in the absorption of some β1-adrenergic receptor blockers [13], its genetic variation is reported not to affect the disposition of atenolol [6] and, furthermore, we could not detect transport activity of OATP1A2 and OATP2B1 for atenolol (Fig. 1).