A Reevaluation of Substrate Specificity of the Rat Cation Transporter rOCT1

Abstract

The substrate specificity of the previously cloned rat cation transporter rOCT1, which is expressed in kidney, liver, and small intestine, was reevaluated. rOCT1 is the first member of a new protein family comprising electrogenic and polyspecific cation transporters that transport hydrophilic cations like tetraethylammonium, choline, and monoamine neurotransmitters. Previous electrical measurements suggested that cations like quinine, quinidine, and cyanine 863, which have been classified as type 2 cations in the liver, are also transported by rOCT1, since they may induce inward currents in rOCT1 expressing Xenopus oocytes (Busch, A. E., Quester, S., Ulzheimer, J. C., Waldegger, S., Gorboulev, V., Arndt, P., Lang, F., and Koepsell, H. (1996) J. Biol. Chem. 271, 32599 -32604). Tracer flux measurements with oocytes and with stably transfected human embryonic kidney cells showed that [3 H]quinine and [ 3 H]quinidine are not transported by rOCT1. The voltage dependence observed for the quinine-or quinidineinduced inward currents in rOCT1-expressing oocytes, and tracer efflux measurements indicate that the inward currents by type 2 cations are generated by the inhibition of electrogenic efflux of transported type 1 cations. Therefore, rOCT1 cannot contribute to transport of type 2 cations in the liver and the hepatic transporter for type 2 cations remains to be identified.

Regulation of human organic cation transporter hOCT2 by PKA, PI3K, and calmodulin-dependent kinases

“…Next, we investigated the substrate specificity of hOCT2 by studying inhibition of ASP ϩ uptake by known substrates of organic cation transport, like TEA ϩ , TPA ϩ , and cimetidine, and by the inhibitor of organic cation transporters, quinine (23). TPA ϩ , TEA ϩ , and cimetidine inhibited ASP ϩ uptake concentration dependently, with EC 50 values of 2.7, 35, and 36 M, respectively (Fig.…”

Individual PKC-Phosphorylation Sites in Organic Cation Transporter 1 Determine Substrate Selectivity and Transport Regulation

“…The transport mediated by rOCT1 has been characterized as electrogenic, independent of Na ϩ and pH, and bidirectional by radioactive tracer flux measurements with oocytes from Xenopus laevis expressing rOCT1 (17). rOCT1 can translocate organic cations like TEA ϩ and choline (18), cathecolamines (19), and nucleosides like 2Ј-deoxytubercidin (20), whereas cations like tetrapentylammonium (TPA ϩ ) and cyanine 863 are nontransported inhibitors of the transporter (21). For this transport protein 12 transmembrane domains, a large hydrophilic extracellular loop between the first and the second predicted transmembrane domain and an intracellular localization of the Nand C-termini have been described (22).…”

Interaction of Organic Cations with a Newly Identified Plasma Membrane Monoamine Transporter

“…Classic substrates of the OCTs include MPP ϩ , TEA, guanidine, choline (Busch et al, 1996;Gorboulev et al, 1997;Grundemann et al, 1999;Kekuda et al, 1998;Sweet et al, 2001), the biogenic amines, such as histamine, and the monoamine neurotransmitters (Breidert et al, 1998;Busch et al, 1998;Grundemann et al, 1998;Jonker and Schinkel, 2004). Bulkier and more hydrophobic cations (i.e., the type II cations) are often potent inhibitors, but not substrates, for the OCTs (Nagel et al, 1997). So far, three OCT isoforms, OCT1, OCT2, and OCT3, which share high sequence similarities and a common 12-transmembrane domain structure, have been identified and characterized from human and other mammalian species (Dresser et al, 2001;Wright and Dantzler, 2004).…”