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      2.3.1 Substrate and Inhibitor Selectivity

      2.3.2 Regulation

Schematic illustration of (a) alternating-access transport model for translocation of substrates by organic cation and zwitterion transporters. (b) Types of transport mechanisms for organic cation and zwitterion transporters.

      2.3.3 Animal Models

      In vivo studies in mice in which individual transporters have been removed genetically (knockout (KO) mice) provide valuable insights in potential physiologic and biomedical functions of OCTs. However, species differences between OCTs of humans and rodents impose limitations on the ability to apply conclusions obtained from the mouse experiments to humans. Oct1 knockout mice are viable and fertile, with no obvious physiological abnormalities when compared with their wild‐type littermates, suggesting that Oct1 has minimal impact on normal physiology [24]. However, at the biochemical level, disruption of OCT1 in mice affects both lipid and glucose metabolism by reducing the hepatic uptake of thiamine [2]. Additionally, disruption of Oct1 in mice has significant impact on the disposition of organic cations. For example, when administered the prototypical organic cation, TEA, Oct1 −/− mice show significantly reduced uptake of TEA into the liver. In accordance with reduced hepatic uptake of TEA, biliary excretion is lower in Oct1 −/− mice [24]. Additionally, direct intestinal excretion of TEA is reduced by approximately 50%. In addition to TEA, Oct1 −/− mice have similar decreases in hepatic uptake of other OCT1 substrates (i.e., MPP+ and meta‐iodobenzylguanidine (MIBG)) [24].

Transporter Model substrates Substrates Model inhibitors Inhibitors
OCT1 MPP+, TEA, ASP+, metformin Endogenous: serotonin, acylcarnitines, choline, acetylcholine, creatinine, agmatine, thiamine Exogenous: acyclovir, quinidine, quinine, thiamine, sumatriptan, ondansetron, morphine Quinidine, verapamil Exogenous: atropine, abacavir, tenofovir, zidovudine, spironolactone, ondansetron, quinine, midazolam
OCT2 TEA, ASP+, MPP+, NBD‐MTMA, metformin Endogenous: creatinine, choline, serotonin, dopamine, histamine Exogenous: amphetamine, cisplatin, cimetidine, phenformin Quinidine, cimetidine Endogenous: testosterone Exogenous: doxepin, zolpidem, ritonavir, imipramine, tramadol, tacrine, olanzapine
OCT3 MPP+, ASP+, metformin Endogenous: creatinine, agmatine, dopamine, progesterone, testosterone Exogenous: atropine, prazosin, cimetidine, verapamil, nicotine

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