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A reevaluation of substrate specificity of the rat cation transporter rOCT1
1Max-Planck-Insitiut für Biophysik, Kennedy-Allee 70, D-60596 Frankfurt, Germany. anat010@rzbox.uni-wuerzburg.de
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 [3H]quinine and [3H]quinidine are not transported by rOCT1. The voltage dependence observed for the quinine- or quinidine-induced 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.
Insights
The rat cation transporter rOCT1 does not transport type 2 cations like quinine. Inward currents observed were due to inhibition of other cation transport, not direct transport by rOCT1.
Area of Science:
- Molecular biology
- Physiology
- Biochemistry
Background:
- The rat cation transporter rOCT1 (organic cation transporter 1) is expressed in the kidney, liver, and small intestine.
- rOCT1 is the first identified member of a protein family that transports hydrophilic cations.
- Previous studies suggested rOCT1 transported type 2 cations, such as quinine and quinidine.
Purpose of the Study:
- To reevaluate the substrate specificity of the rat cation transporter rOCT1.
- To determine if rOCT1 transports type 2 cations previously suggested by electrical measurements.
Main Methods:
- Tracer flux measurements were performed using oocytes and stably transfected human embryonic kidney cells.
- Electrical measurements, including voltage dependence, were analyzed.
- Tracer efflux measurements were conducted.
Main Results:
- Tracer flux assays demonstrated that [3H]quinine and [3H]quinidine are not transported by rOCT1.
- Observed inward currents induced by type 2 cations in rOCT1-expressing oocytes were not due to direct transport.
- These currents result from the inhibition of electrogenic efflux of transported type 1 cations.
Conclusions:
- rOCT1 does not transport type 2 cations, including quinine and quinidine.
- The previously observed electrical effects were indirect, caused by inhibition of other transporter activities.
- The specific hepatic transporter responsible for type 2 cation transport remains unidentified.