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1 Department of Pharmaceutics, University of Washington, Seattle, Washington, United States
2 Seattle, Washington, United States; Department of Pharmaceutics, University of Washington, Seattle, Washington, United States
* To whom correspondence should be addressed. E-mail: jowang{at}u.washington.edu.
Plasma membrane monoamine transporter (PMAT) is a novel membrane transporter recently cloned and characterized in our laboratory. We previously demonstrated that PMAT functions as a polyspecific organic cation transporter and efficiently transports many organic cations such as monoamine neurotransmitters and 1-methyl-4-phenylpyridinium (MPP+). In this study, we explored the role of PMAT in the renal handling of organic cations. Using a polyclonal antibody directed towards the N-terminal 66 residues of human PMAT, we showed that the PMAT protein (~ 55 kDa) is expressed in the human kidney, and is primarily targeted to the apical membranes when expressed in polarized Madin-Darby canine kidney (MDCK) cells. Using MDCK cells stably expressing human PMAT, we showed that PMAT-mediated MPP+ is strongly dependent on extracellular pH. Lowering extracellular pH from 7.4 to 6.6 greatly stimulated PMAT-mediated MPP+ uptake, whereas elevating extracellular pH to 8.2 abolished transporter activity. Kinetic analysis revealed that the apparent Vmax at pH 6.6 is about 4-fold higher than that at pH 7.4, whereas the apparent Km values were not statistically different at these two conditions. Under acidic conditions (pH 6.6), the proton ionophore, FCCP, drastically reduced PMAT-mediated MPP+ uptake, suggesting that the stimulatory effect of proton may be due to transporter coupling with a proton gradient. Taken together, our data suggest that PMAT is expressed on the apical membranes of renal epithelial cells and may utilize luminal proton gradient to drive organic cation reabsorption in the kidney.
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