The multidrug transporter MATE1 sequesters OCs within an intracellular compartment that has no influence on OC secretion in renal proximal tubules

Secretion of organic cations (OCs) across renal proximal tubules (RPTs) involves basolateral OC transporter (OCT)2-mediated uptake from the blood followed by apical multidrug and toxin extruder (MATE)1/2- mediated efflux into the tubule filtrate. Whereas OCT2 supports electrogenic OC uniport, MATE is an OC/H⁺ exchanger. As assessed by epifluorescence microscopy, cultured Chinese hamster ovary (CHO) cells that stably expressed human MATE1 accumulated the fluorescent OC N,N,N-trimethyl-2-[methyl(7- nitrobenzo[c][l,2,5]oxadiazol-4-yl)amino]ethanaminium (NBDMTMA) in the cytoplasm and in a smaller, punctate compartment; accumulation in human OCT2-expressing cells was largely restricted to the cytoplasm. A second intracellular compartment was also evident in the multicompartmental kinetics of efflux of the prototypic OC [³H]1-methyl-4-phenylpyridinium (MPP) from MATE1-expressing CHO cells. Punctate accumulation of NBD-MTMA was markedly reduced by coexposure of MATE1-expressing cells with 5 μM bafilomycin (BAF), an inhibitor of V-type H⁺-ATPase, and accumulation of [³H]MPP and [³H]NBD-MTMA was reduced by >0% by coexposure with 5 μM BAF. BAF had no effect on the initial rate of MATE1-mediated uptake of NBD-MTMA, suggesting that the influence of BAF was a secondary effect involving inhibition of V-type H⁺-ATPase. The accumulation of [³H]MPP by isolated single nonperfused rabbit RPTs was also reduced > 30% by coexposure to 5 μM BAF, suggesting that the native expression in RPTs of MATE protein within endosomes can increase steady-state OC accumulation. However, the rate of [³H]MPP secretion by isolated single perfused rabbit RPTs was not affected by 5 μM BAF, suggesting that vesicles loaded with OCs⁺ are not likely to recycle into the apical plasma membrane at a rate sufficient to provide a parallel pathway for OC secretion.