Characteristics of tetraethylammonium transport in rabbit renal plasma-membrane vesicles.

Transport of [14C]tetraethylammonium (TEA), an organic cation, was studied in brush-border (BBMV) and basolateral (BLMV) membrane vesicles isolated from rabbit kidney cortex. In BBMV, the presence of an outwardly directed H+ gradient induced a marked stimulation of TEA uptake against its concentration gradient (overshoot phenomenon), whereas a valinomycin-induced inside-negative potential had no effect on TEA uptake. In BLMV, TEA uptake was significantly stimulated by the presence of an outwardly directed H+ gradient and by an inside-negative potential, but the effect of H+ gradient was absent when the vesicles were chemically 'voltage clamped'. In BBMV, internal H+ stimulated TEA uptake in a non-competitive manner by binding at a site with apparent pKa of 6.87. External H+ inhibited TEA uptake through a direct interaction with the putative H+/organic-cation exchanger at a site with apparent pKa of 6.78. Changing external pH while maintaining the pH gradient constant produced a result similar to that obtained by changing external pH alone. Increasing external H+ showed a mixed-type inhibition of TEA uptake. These results suggest that in the rabbit TEA transport across the basolateral membranes is driven by an inside-negative potential and that transport across the brush-border membrane is driven by a H+ gradient via an electroneutral H+/TEA antiport system.     

Cimetidine transport in rat renal brush border and basolateral membrane vesicles

The transport of cimetidine by rat renal brush border and basolateral membrane vesicles has been studied in relation to the transport system of organic cation. Cimetidine inhibited [3H]tetraethylammonium uptake by basolateral membrane vesicles in a dose dependent manner, and the degree of the inhibition was almost the same as that by unlabeled tetraethylammonium. In contrast, cimetidine inhibited the active transport of [3H]tetraethylammonium by brush border membrane vesicles more strongly than unlabeled tetraethylammonium did. In agreement with the transport mechanism of tetraethylammonium in brush border membranes, the presence of an H+ gradient ([H+]i greater than [H+]o) induced a marked stimulation of cimetidine uptake against its concentration gradient (overshoot phenomenon), and this concentrative uptake was inhibited by unlabeled tetraethylammonium. These results suggest that cimetidine can share common carrier transport systems with tetraethylammonium in renal brush border and basolateral membranes, and that cimetidine transport across brush border membranes is driven by an H+ gradient via an H+-organic cation antiport system.     

Evidence for an organic cation-proton antiport system in brush-border membranes isolated from the human term placenta

Uptake of guanidine, an endogenous organic cation, into brush-border membrane vesicles isolated from human term placentas was investigated. Initial uptake rates were manyfold greater in the presence of an outward-directed H+ gradient ([pH]o greater than [pH]i) than in the absence of a H+ gradient ([pH]o = [pH]i). Guanidine was transiently accumulated inside the vesicles against a concentration gradient in the presence of the H+ gradient. The H+ gradient-dependent stimulation of guanidine uptake was not due to a H+-diffusion potential because an ionophore (valinomycin or carbonylcyanide p-trifluoromethoxyphenylhydrazone)-induced inside-negative membrane potential failed to stimulate the uptake. In addition, uphill transport of guanidine could be demonstrated even in voltage-clamped membrane vesicles. The H+ gradient-dependent uptake of guanidine was inhibited by many exogenous as well as endogenous organic cations (cis-inhibition) but not by cationic amino acids. The presence of unlabeled guanidine inside the vesicles stimulated the uptake of labeled guanidine (trans-stimulation). These data provide evidence for the presence of an organic cation-proton antiporter in human placental brush-border membranes. Kinetic analysis of guanidine uptake demonstrated that the uptake occurred via two saturable, carrier-mediated transport systems, one being a high affinity, low capacity type and the other a low affinity, high capacity type. Studies on the effects of various cations on the organic cation-proton antiporter and the Na+-H+ exchanger revealed that these two transport systems are distinct.     

Evidence for tripeptide/H+ co-transport in rabbit renal brush-border membrane vesicles.

L-Phe-L-Pro-L-Ala is a tripeptide which is hydrolysable almost exclusively by dipeptidyl peptidase IV in rabbit renal brush-border membrane vesicles. In order to delineate the mechanism of the transport of an intact tripeptide across the brush-border membrane, we studied the characteristics of the uptake of [3H]Phe-Pro-Ala in membrane vesicles in which the activity of dipeptidylpeptidase IV was completely inhibited by treatment with di-isopropyl fluorophosphate. In these vesicles, uptake of radiolabel from the tripeptide was found to be Na(+)-independent, but was greatly stimulated by an inwardly directed H+ gradient. The H(+)-gradient-dependent radiolabel uptake appeared to be an active process, because the time course of uptake exhibited an overshoot phenomenon. The process was also electrogenic, being stimulated by an inside-negative membrane potential. Under the uptake-measurement conditions there was no detectable hydrolysis of [3H]Phe-Pro-Ala in the incubation medium when di-isopropyl fluorophosphate-treated membrane vesicles were used. Analysis of intravesicular contents revealed that the radiolabel inside the vesicles was predominantly (greater than 90%) in the form of intact tripeptide. These data indicate that the uptake of radiolabel from [3H]Phe-Pro-Ala in the presence of an inwardly directed H+ gradient represents almost exclusively uptake of intact tripeptide. Uphill transport of the tripeptide was also demonstrable in the presence of an inwardly directed Na+ or K+ gradient, but only if nigericin was added to the medium. Under these conditions, nigericin, an ionophore for Na+, K+ and H+, was expected to generate a transmembrane H+ gradient. Uptake of Phe-Pro-Ala in the presence of a H+ gradient was inhibited by di- and tri-peptides, but not by free amino acids. It is concluded that tripeptide/H+ co-transport is the mechanism of Phe-Pro-Ala uptake in rabbit renal brush-border membrane vesicles.     

Biotin uptake mechanisms in brush-border and basolateral membrane vesicles isolated from rabbit kidney cortex

Biotin transport was studied using brush-border and basolateral membrane vesicles isolated from rabbit kidney cortex. An inwardly directed Na+ gradient stimulated biotin uptake into brush-border membrane vesicles and a transient accumulation of the anion against its concentration gradient was observed. In contrast, uptake of biotin by basolateral membrane vesicles was found to be Na+-gradient insensitive. Generation of a negative intravesicular potential by valinomycin-induced K+ diffusion potentials or by the presence of Na+ salts of anions of different permeabilities enhanced biotin uptake by brush-border membrane vesicles, suggesting an electrogenic mechanism. The Na+ gradient-dependent uptake of biotin into brush-border membrane vesicles was saturable with an apparent Km of 28 microM. The Na+-dependent uptake of tracer biotin was significantly inhibited by 50 microM biotin, and thioctic acid but not by 50 microM L-lactate, D-glucose, or succinate. Finally, the existence in both types of membrane vesicles of a H+/biotin- cotransport system could not be demonstrated. These results are consistent with a model for biotin reabsorption in which the Na+/biotin- cotransporter in luminal membranes provides the driving force for uphill transport of this vitamin.     

Dicarboxylate transport in renal basolateral and brush-border membrane vesicles.

Characteristics of succinate transport were determined in basolateral and brush-border membrane vesicles (BLMV and BBMV, respectively) isolated in parallel from rabbit renal cortex. The uptake of succinate was markedly stimulated by the imposition of an inwardly directed Na+ gradient, showing an "overshoot" phenomenon in both membrane preparations. The stimulation of succinate uptake by an inwardly directed Na+ gradient was not significantly affected by pH clamp or inhibition of Na(+)-H+ exchange. The Na(+)-dependent and -independent succinate uptakes were not stimulated by an outwardly directed pH gradient. The Na dependence of succinate uptake exhibited sigmoidal kinetics, with Hill coefficients of 2.17 and 2.38 in BLMV and BBMV, respectively. The Na(+)-dependent succinate uptake by BLMV and BBMV was stimulated by a valinomycin-induced inside-negative potential. The Na(+)-dependent succinate uptake by BLMV and BBMV followed a simple Michaelis-Menten kinetics, with an apparent Km of 22.20 +/- 4.08 and 71.52 +/- 0.14 microM and a Vmax of 39.0 +/- 3.72 and 70.20 +/- 0.96 nmol/(mg.min), respectively. The substrate specificity and the inhibitor sensitivity of the succinate transport system appeared to be very similar in both membranes. These results indicate that both the renal brush-border and basolateral membranes possess the Na(+)-dependent dicarboxylate transport system with very similar properties but with different substrate affinity and transport capacity.     

Proton gradient-dependent transport of glycine in rabbit renal brush-border membrane vesicles

This study describes evidence for the existence of a H+/glycine symport system in rabbit renal brush-border membrane vesicles. An inward proton gradient stimulates glycine transport across the brush-border membrane, and this H+-driven glycine uptake is attenuated by the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone. It is a positive rheogenic process, i.e. the H+-dependent glycine uptake is further enhanced by an intravesicular negative potential. Glycine uptake is stimulated to a lesser degree by an inward Na+ gradient. H+-dependent glycine uptake is inhibited by sarcosine (69%), an analog amino acid, imino acids (proline 81%, hydroxy proline 67%), and beta-alanine (31%), but not by neutral (L-leucine) or basic (L-lysine) amino acids. The results demonstrate that H+ glycine co-transport system in rabbit renal brush-border membrane vesicles is a carrier-mediated electrogenic process and that transport is shared by imino acids and partially by beta-alanine.     

Evidence for tripeptide-proton symport in renal brush border membrane vesicles. Studies in a novel rat strain with a genetic absence of dipeptidyl peptidase IV

We have investigated the transport characteristics of L-phenylalanyl-L-prolyl-L-alanine in renal brush-border membrane vesicles isolated from Japan Fisher 344 rats. This particular rat strain genetically lacks dipeptidyl peptidase IV. Owing to the absence of this enzyme, the tripeptide was found to be completely resistant to hydrolysis by the renal brush-border membrane vesicles. Uptake of the tripeptide into these membrane vesicles in the presence of an inwardly directed Na+ gradient was slightly greater than in the presence of a K+ gradient, but there was no evidence for active transport. On the contrary, uptake was very rapid in the presence of an inside-alkaline transmembrane pH gradient, and accumulation of the tripeptide inside the vesicles against a concentration gradient could be demonstrated under these conditions. The uptake was drastically reduced by dissipation of the pH gradient. The uptake was stimulated by an inside-negative membrane potential and inhibited by an inside-positive membrane potential. Moreover, the uptake was greater in voltage-clamped membrane vesicles than in control vesicles. Many di- and tripeptides inhibited this pH gradient-stimulated uptake of Phe-Pro-Ala. The apparent dissociation constant for the tripeptide was 48 microM. High performance liquid chromatography analysis of the intravesicular content at the peak of the overshoot revealed that the tripeptide was transported across the membrane almost entirely in the intact form. These data provide the first direct evidence for the presence of an electrogenic tripeptide-proton symport in renal brush-border membranes.     

Proton-coupled transport of glycylglycine in rabbit renal brush-border membrane vesicles

Transport of glycylglycine into rabbit renal brush-border membrane vesicles was found to be Na+-independent, H+ gradient-dependent and electrogenic. Marked overshoot uptake of the dipeptide was observed when an inward-directed proton gradient and inside-negative potential difference were imposed simultaneously across the vesicular membranes. Saturable depolarization of vesicular membranes could be demonstrated with glycylglycine by use of a fluorescent cyanine dye, di-S-C3(5). The results indicate that glycylglycine is contransported with H+ across the membranes.     

H+ coupled uphill transport of aminocephalosporins via the dipeptide transport system in rabbit intestinal brush-border membranes

The transport characteristics of aminocephalosporin antibiotics, possessing an alpha-amino group and a carboxyl group, in brush-border membranes isolated from rabbit small intestine have been studied by a rapid filtration technique. The uptake of cephradine by brush-border membrane vesicles was stimulated by the countertransport effect of dipeptides, which indicates the existence of a common carrier transport system. An inward H+ gradient ([pH]i = 7.5 to 8.4, [pH]o = 6.0) stimulated cephradine uptake against a concentration gradient (overshoot phenomenon), and this stimulation was reduced when the H+ gradient was subjected to rapid dissipation by the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone, a protonophore. A valinomycin-induced K+ diffusion potential (interior-negative) stimulated H+ gradient-dependent cephradine uptake without altering the equilibrium value. The uptake of other aminocephalosporins (cefadroxil, cefaclor, cephalexin) was also stimulated in the presence of an inward H+ gradient, while the uptake of cephalosporins without the alpha-amino group (cefazolin, cefotiam) was not changed in the presence or absence of the H+ gradient. These results suggest that the transport of aminocephalosporins can be driven actively by an inward H+ gradient via the dipeptide transport system in the intestinal brush-border membranes, and that the process results in the transfer of a positive charge.     

Hydrogen ion-coupled transport of D-glucose by phlorizin-sensitive sugar carrier in intestinal brush-border membranes

In rabbit intestinal brush-border membrane vesicles, Na+-independent D-glucose uptake in the presence of an inside-negative transmembrane potential was found to be stimulated by an imposed pH gradient. Na+-independent, pH-dependent and phlorizin-sensitive D-glucose-evoked potentials could be recorded from isolated toad intestine. The obtained data suggest that phlorizin-sensitive D-glucose carriers of intestinal brush-border membrane can interact with H+ when Na+ is absent.     

A proton gradient is the driving force for uphill transport of lactate in human placental brush-border membrane vesicles

The characteristics of lactate transport in brush-border membrane vesicles isolated from normal human full-term placentas were investigated. Lactate transport in these vesicles was Na+-independent, but was greatly stimulated when the extravesicular pH was made acidic. In the presence of an inwardly directed H+ gradient ([H+]o greater than [H+]i), transient uphill transport of lactate could be demonstrated. This H+ gradient-dependent stimulation was not a result of a H+ diffusion potential. Transport of lactate in the presence of the H+ gradient was not inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid or by furosemide, ruling out the participation of an anion exchanger in placental lactate transport. Many monocarboxylates strongly interacted with the lactate transport system, whereas, with the single exception of succinate, dicarboxylates did not. The monocarboxylates pyruvate and lactate, but not the dicarboxylate succinate, when present inside the vesicles, were able to exert a trans-stimulatory effect on the uptake of radiolabeled lactate. Kinetic analyses provided evidence for a single transport system with a Kt of 4.1 +/- 0.4 mM for lactate and a Vmax of 54.2 +/- 9.9 nmol/mg of protein/30 s. Pyruvate inhibited lactate transport competitively, by reducing the affinity of the system for lactate without altering the maximal velocity. It is concluded that human placental brush-border membranes possess a transport system specific for lactate and other monocarboxylates and that this transport system is Na+-independent and is energized by an inwardly directed H+ gradient. Lactate-H+ symport rather than lactate-OH- antiport appears to be the mechanism of the H+ gradient-dependent lactate transport in these membranes.     

Distinct transport activity of tetraethylammonium from L-carnitine in rat renal brush-border membranes

We investigated the contribution of the Na(+)/L-carnitine cotransporter in the transport of tetraethylammonium (TEA) by rat renal brush-border membrane vesicles. The transient uphill transport of L-carnitine was observed in the presence of a Na(+) gradient. The uptake of L-carnitine was of high affinity (K(m)=21 microM) and pH dependent. Various compounds such as TEA, cephaloridine, and p-chloromercuribenzene sulfonate (PCMBS) had potent inhibitory effects for L-carnitine uptake. Therefore, we confirmed the Na(+)/L-carnitine cotransport activity in rat renal brush-border membranes. Levofloxacin and PCMBS showed different inhibitory effects for TEA and L-carnitine uptake. The presence of an outward H(+) gradient induced a marked stimulation of TEA uptake, whereas it induced no stimulation of L-carnitine uptake. Furthermore, unlabeled TEA preloaded in the vesicles markedly enhanced [14C]TEA uptake, but unlabeled L-carnitine did not stimulate [14C]TEA uptake. These results suggest that transport of TEA across brush-border membranes is independent of the Na(+)/L-carnitine cotransport activity, and organic cation secretion across brush-border membranes is predominantly mediated by the H(+)/organic cation antiporter.     

Characteristics of tripeptide transport in human jejunal brush-border membrane vesicles

These studies are aimed at characterizing the transport of the tripeptide, glycylglycyl-L-proline (GlyGlyPro) across human jejunal brush-border membrane vesicles. GlyGlyPro (0.65 mM) was hydrolyzed by brush-border membrane vesicles with the extent of hydrolysis per mg protein being 23% at 0.5 min, 57% at 1 min and complete hydrolysis at 60 min. Treatment of the membrane vesicles with gel-complexed papain (to remove membrane peptidases) resulted in minimal hydrolysis of GlyGlyPro up to 10 min of incubation. Measurement of GlyGlyPro influx with papain-treated vesicles in the presence of increasing medium osmolarity showed that uptake occurred into an osmotically reactive intravesicular space. Transport of GlyGlyPro with normal and papain-treated membrane vesicles was similar in the presence of an inward Na+ or K+ gradient. No overshoot phenomenon was observed in the presence of an inward proton gradient (extravesicular pH 5.5; intravesicular pH 7.5). An interior negative membrane potential induced by a K+ diffusion potential in the presence of valinomycin stimulated the uptake of the peptide. The effect of increasing concentrations on initial rates of GlyGlyPro uptake revealed the presence of a saturable component as well as a diffusional component. Preloading the membrane vesicles with 20 mM glycylsarcosylsarcosine stimulated uptake by 4-fold. Uptake of GlyGlyPro was inhibited greater than 50% by dipeptides and tripeptides and less than 15% by free amino acids. These results indicate that GlyGlyPro uptake in jejunal brush-border membrane vesicles is not energized by a Na+ or proton gradient and that transport occurs by carrier-mediated and diffusional processes.     

Kinetics of D-glucose transport across the intestinal brush-border membrane of the cat

1. D-glucose transport across the intestinal brush-border membrane of the cat, a carnivorous animal, was investigated using isolated brush-border membrane vesicles (BBMV). Kinetic experiments were performed under zero-trans conditions (initial [Na+]in and [Gluc]in = O) with the transmembrane electrical potential difference clamped to zero. 2. D-glucose uptake by the BBMV was strongly stimulated by an inwardly directed Na+-gradient. Uptake under Na+-free conditions seemed to occur by simple diffusion. 3. The apparent kinetic constants (Vmax, Km) of Na+-dependent D-glucose transport were computed by forcing initial uptake rates at 0.002-10.0 mmol/l D-glucose to either a Michaelis-Menten type equation with a single or with two carrier-mediated components. 4. Best fit of the experimental data was obtained with the two-component model indicating the existence of two Na+-dependent carrier-mediated mechanisms. System 1 and system 2 differ with respect to the transport velocity as well as the substrate affinity constants with Vmax being 2.5-fold and Km being 5-fold higher for system 1 compared with system 2.     

Role of chloride on carrier-mediated transport of p-aminohippurate in rat renal basolateral membrane vesicles

Effect of inorganic anions on p-amino[3H]hippurate transport in renal basolateral membranes has been studied using the vesicles preloaded with unlabeled p-aminohippurate (countertransport condition). The uptake of p-amino[3H]hippurate was stimulated by the outward gradient of unlabeled p-aminohippurate and the labeled substrate was accumulated into the vesicles against its concentration gradient in the presence of Cl-. The substitution of SCN- and SO4(2-) for Cl- in both sides of the vesicles depressed the initial rate and the overshoot magnitude of p-amino[3H]hippurate uptake. These results suggest that Cl- may play an important role for the carrier-mediated transport system of organic anion in renal basolateral membranes.     

Carrier-mediated transport system for cephalexin in human placental brush-border membrane vesicles

The uptake of cephalosporin antibiotics, cephalexin, was studied with brush-border microvillous plasma membrane vesicles prepared and purified from human full-term placental syncytiotrophoblasts. The uptake of cephalexin by the membrane vesicles was not stimulated in the presence of an Na+ gradient from the outside to the inside of the vesicles, whereas alpha-(methylamino)isobutyrate uptake into the vesicles of the same preparation was stimulated by an Na+ gradient. The equilibrium level of cephalexin uptake decreased with increasing osmolarity of the medium, which indicates that cephalexin is transported into the membrane vesicles. When cephalexin concentrations were varied, the initial rate of uptake obeyed Michaelis-Menten kinetics with Km and Vmax values of 2.29 mM and 2.98 nmol/mg of protein per 60 s, respectively. The uptake of cephalexin was inhibited by structural analogues and sulfhydryl modifying reagents. These results indicate the existence of a carrier-mediated transport system for cephalexin in the human placental brush-border membranes.     

A proton gradient, not a sodium gradient, is the driving force for active transport of lactate in rabbit intestinal brush-border membrane vesicles.

An inward-directed H+ gradient markedly stimulated lactate uptake in rabbit intestinal brush-border membrane vesicles, and uphill transport against a concentration gradient could be demonstrated under these conditions. Uptake of lactate was many-fold greater in the presence of a H+ gradient than in the presence of a Na+ gradient. Moreover, there was no evidence for uphill transport of lactate in the presence of a Na+ gradient. The H+-gradient-dependent stimulation of lactate uptake was not due to the effect of a H+-diffusion potential. The uptake process in the presence of a H+ gradient was saturable [Kt (concn. giving half-maximal transport) for lactate 12.7 +/- 4.5 mM] and was inhibited by many monocarboxylates. It is concluded that a H+ gradient, not a Na+ gradient, is the driving force for active transport of lactate in rabbit intestinal brush-border membrane vesicles.     

Transport of procainamide via H(+)/tertiary amine antiport system in rabbit intestinal brush-border membrane

Transport characteristics of procainamide in the brush-border membrane isolated from rabbit small intestine were studied by a rapid-filtration technique. Procainamide uptake by brush-border membrane vesicles was stimulated by an outward H(+) gradient (pH(in) = 6.0, pH(out) = 7.5) against a concentration gradient (overshoot phenomenon), and this stimulation was reduced when the H(+) gradient was subjected to rapid dissipation by the presence of a protonophore, FCCP. An outward H(+) gradient-dependent procainamide uptake was not caused by H(+) diffusion potential. The initial uptake of procainamide was inhibited by other tertiary amines with N-dimethyl or N-diethyl moieties in their structures, such as triethylamine, dimethylaminoethyl chloride, and diphenhydramine, but not by tetraethylammonium and thiamine. Furthermore, procainamide uptake was stimulated by preloading the vesicles with these tertiary amines (trans-stimulation effect), indicating the existence of a specific transport system for tertiary amines. These findings indicate that procainamide transport in the intestinal brush-border membrane is mediated by the H(+)/tertiary amine antiport system that recognizes N-dimethyl or N-diethyl moieties in the structures of tertiary amines.     

Specific interaction of 5-(N-methyl-N-isobutyl)amiloride with the organic cation-proton antiporter in human placental brush-border membrane vesicles. Transport and binding.

The interaction of 5-(N-methyl-N-isobutyl)amiloride (MIBA) with brush-border membrane vesicles isolated from normal human term placentas was investigated using two parameters: binding and transport. The binding of MIBA to placental membranes was specific and temperature- and pH-dependent, and the apparent dissociation constant (Kd) for the process was 58 +/- 2 microM. The binding was inhibited by other amiloride analogs and also by clonidine and cimetidine with a rank order potency: MIBA > benzamil > dimethylamiloride > amiloride > clonidine > cimetidine. These compounds also inhibited Na(+)-H+ exchanger activity in these membrane vesicles, but with a different order of potency: dimethylamiloride > MIBA > amiloride > benzamil > cimetidine > clonidine. The membrane vesicles were also able to transport MIBA into the intravesicular space, and the transport was stimulated many-fold by the presence of an outwardly directed H+ gradient across the membrane. The H+ gradient was the driving force for uphill accumulation of MIBA inside the vesicles. The transport process was electrically silent. The transport of MIBA was inhibited by other amiloride analogs and by clonidine and cimetidine, and the order of potency was the same as the order with which these compounds inhibited the binding of MIBA. The Michaelis-Menten constant (Kt) for the transport process was 46 +/- 2 microM. The binding as well as the transport were also inhibited by Na+ and Li+. Interestingly, tetraethylammonium and N1-methylnicotinamide, two of the commonly used substrates in organic cation transport studies, failed to inhibit the binding and transport of MIBA. Furthermore, although the outwardly directed H+ gradient-dependent uphill transport of tetraethylammonium could be demonstrated in renal brush-border membrane vesicles, there was no evidence for the presence of a transport system for this prototypical organic cation in placental brush-border membrane vesicles. It is concluded that the human placental brush-border membranes possess an organic cation-proton antiporter which accepts MIBA as a substrate, the low affinity binding site for MIBA observed in these membranes represents this antiporter, and that the placental organic cation-proton antiporter is distinct from the widely studied renal organic cation-proton antiporter.