Electrophysiological Characterization of the Polyspecific Organic Cation Transporter Plasma Membrane Monoamine Transporter

Abstract

ABSTRACT:Plasma membrane monoamine transporter (PMAT) is a polyspecific organic cation (OC) transporter that transports a variety of endogenous biogenic amines and xenobiotic cations. Previous radiotracer uptake studies showed that PMAT-mediated OC transport is sensitive to changes in membrane potential and extracellular pH, but the precise role of membrane potential and protons on PMAT-mediated OC transport is unknown. Here, we characterized the electrophysiological properties of PMAT in Xenopus laevis oocytes using a two-microelectrode voltage-clamp approach. PMAT-mediated histamine uptake is associated with inward currents under voltage-clamp conditions, and the currents increased in magnitude as the holding membrane potential became more negative. A similar effect was also observed for another cation, nicotine. Substrate-induced currents were largely independent of Na ؉ but showed strong dependence on membrane potential and pH of the perfusate. Detailed kinetic analysis of histamine uptake revealed that the energizing effect of membrane potentials on PMAT transport is mainly due to an augmentation of I max with little effect on K 0.5 . At most holding membrane potentials, I max at pH 6.0 is approximately 3-to 4-fold higher than that at pH 7.5, whereas K 0.5 is not dependent on pH. Together, these data unequivocally demonstrate PMAT as an electrogenic transporter and establish the physiological inside-negative membrane potential as a driving force for PMAT-mediated OC transport. The important role of membrane potential and pH in modulating the transport activity of PMAT toward OCs suggests that the in vivo activity of PMAT could be regulated by pathophysiological processes that alter physiological pH or membrane potential.

Impaired Monoamine and Organic Cation Uptake in Choroid Plexus in Mice with Targeted Disruption of the Plasma Membrane Monoamine Transporter (Slc29a4) Gene

“…The membrane localization of PMAT in CP cells corroborates a role of this transporter in the uptake of substrate from the CSF into CP epithelial cells, the first step of OC efflux at the BCSFB. Recent electrophysiological work from our laboratory unequivocally demonstrated that PMAT functions as an electrogenic transporter and that PMAT-mediated OC uptake is associated with inwardly directed currents that are strongly dependent on membrane potential and pH of the perfusate (37). Therefore, the physiological inside-negative membrane potential in CP epithelial cells would serve as a driving force for OC accumulation against a substrate concentration gradient.…”

Coexistence of Passive and Proton Antiporter-Mediated Processes in Nicotine Transport at the Mouse Blood–Brain Barrier

“…Previous studies have demonstrated that nicotine inhibits OCT, such as OCT1-3 (SLC22A1-3) and OCTN1-2, in vitro (8,26) and may be a substrate of PMAT and MATE1 (27,28). There is conflicting evidence for the expression and function of Oct1 and/or Oct2 at the BBB in vitro (29,30).…”

Polyspecific organic cation transporters and their impact on drug intracellular levels and pharmacodynamics

“…This allows the OCTs to accumulate a substrate with intracellular concentrations much higher (up to 10 fold) than its extracellular concentration [24]. Similar to the OCTs, PMAT also functions as an electrogenic transporter that utilizes the physiological inside-negative membrane potential as a driving force [25]. The transport activity of PMAT can be further stimulated by an acidic pH [25,26].…”

“…Similar to the OCTs, PMAT also functions as an electrogenic transporter that utilizes the physiological inside-negative membrane potential as a driving force [25]. The transport activity of PMAT can be further stimulated by an acidic pH [25,26]. MATEs, however, are proton/organic cation exchangers [16].…”