Alanylglutamine dipeptide and growth hormone maintain PepT1-mediated transport in oxidatively stressed Caco-2 cells.

Reactive oxygen species (ROS) produced by gut mucosal cells during conditions such as inflammatory bowel disease (IBD) may impair mucosal repair and nutrient transport/absorptive function. Absorption of di- and tripeptides in the small intestine and colon is mediated by the H(+)-dependent transporter PepT1, but effects of oxidative stress on di- and tripeptide transport are unknown. We assessed whether exposure to hydrogen peroxide (H(2)O(2)) influences dipeptide transport in human colonic epithelial (Caco-2) cells. Uptake of [(14)C]glycylsarcosine (Gly-Sar) was used to evaluate PepT1-mediated dipeptide transport. Exposure to 1-5 mmol/L H(2)O(2) for 24 h caused a dose-dependent decrease in Gly-Sar transport, which was associated with decreased PepT1 transport velocity (V(max)). Treatment with alanylglutamine (Ala-Gln) or growth hormone (GH) did not alter Caco-2 Gly-Sar transport in the absence of H(2)O(2). However, both Ala-Gln and GH prevented the decrease in dipeptide transport observed with 1 mmol/L H(2)O(2) treatment. Ala-Gln, but not GH, maintained cellular glutathione and prevented the decrease in PepT1 protein expression. Thus, these agents should be further investigated as potential therapies to improve absorption of small peptides in disorders associated with oxidative injury to the gut mucosa.

Depressed patients have higher body temperature: 5-HT transporter long promoter region effects.

BACKGROUND: Depression has been associated with a decrease in intracellular serotonin (5-HT) reuptake through its transporter, SERT. The 5-HT transporter long promoter region (5-HTTLPR) deletion in the SERT gene has also been associated with a decrease in 5-HT reuptake. Conversely, increases in extracellular 5-HT have been associated with increased temperature. It has not been established, however, whether body temperature in depressed patients is different from controls. Here, we hypothesized that temperature would be increased in depressed patients as well as in those with the 5-HTTLPR deletion. METHODS: A strict oral temperature protocol employed single, cross-sectional, naturalistic time-of-day temperature measures in 125 subjects (46 normal controls, 79 outpatients with major depression). Controls and depressed patients were free of psychotropic medication and classified by the Structured Clinical Interview for Psychiatric Diagnoses. Eighty-one of the subjects (68 depressed, 13 normal) were additionally genotyped for 5-HTTLPR polymorphisms. RESULTS: Depressed patients had a significantly higher uncorrected body temperature (mean +/- SD 98.38 +/- 0.61 degrees F) than controls (mean +/- SD 98.13 +/- 0.59 degrees F; F = 4.8, p = 0.03). An age (F = 14.09, p < 0.001) and time-of-day (11.4, p = 0.001) correction revealed a more robust (F = 14.02, p < 0.001) difference between depressed patients (mean +/- SD 98.44 +/- 0.55 degrees F) and controls (mean +/- SD 98.02 +/- 0.56 degrees F). When normalized for age and circadian differences between subjects, random, outpatient oral temperatures had a sensitivity of 63% and a specificity of 76% in identifying the depressed subjects from the controls. Independent of depression, subjects with the 5-HTTLPR deletion (short SERT allele) were warmer (mean +/- SD 98.33 +/- 0.65 degrees F) than those lacking the short allele on either chromosome (mean +/- SD 97.91 +/- 0.69 degrees F; F = 7.0, p = 0.01). However, the genotype did not explain the temperature differences between controls and depressed patients. CONCLUSION: This is the first demonstration of an increased daytime body temperature in cases with major depression. Subjects with a corrected temperature above 98.3 degrees F were 2.6-fold more likely to be depressed. The results may strengthen the hypothesis of an inflammatory component of depression. In addition, the findings suggest a potential link between genetic differences in 5-HT transport and body temperature.

Antidepressant effects on kinase gene expression patterns in rat brain.

Multiple kinase pathways determine serotonin transporter (SERT) regulation. We hypothesized a decrease in kinase expression with chronic selective serotonin reuptake inhibitor (SSRI) administration necessary to regulate extracellular serotonin. We studied whole brain kinase mRNA expression on Affymetrix gene chips in rats treated with placebo 3 and 21 days, fluoxetine 3 and 21 days, and citalopram 21 days. Protein kinase C (PKC)-delta, PKC-gamma, stress-activated protein kinase, cAMP-dependent protein kinase beta isoform, Janus protein kinase, and phosphofructokinase M were all down regulated chronically with citalopram and fluoxetine, but not with acute fluoxetine. The results are consistent with homeostasis of SERT function through a decrease in PK expression.

Optimal absorptive transport of the dipeptide glycylsarcosine is dependent on functional Na+/H+ exchange activity.

Optimal nutrient absorption across the intestinal epithelium is dependent on the co-ordinated activity of a number of membrane transporters. Di/tripeptide transport across the luminal membrane of the intestinal enterocyte is mediated by the H(+)-coupled di/tripeptide transporter hPepT1. hPepT1 function is dependent on the existence of a pH gradient (maintained, in part, by the action of the Na(+)/H(+) exchanger NHE3) across the apical membrane of the small intestinal epithelium. The physiological problem addressed here was to determine how two transporters (hPepT1 and NHE3), involved in nutrient absorption and pH(i) homeostasis, function co-operatively to maximise dipeptide absorption when both operate sub-optimally at typical mucosal surface pH values (pH 6.1-6.8). Functional hPepT1 activity in human intestinal epithelial (Caco-2) cell monolayers was determined by measurement of apical uptake and apical-to-basolateral transport of the dipeptide glycylsarcosine. The dependence of hPepT1 on NHE3 activity was measured (either after Na(+) removal or addition of the NHE3-selective inhibitor S1611) using both Caco-2 cell monolayers and hPepT1-expressing Xenopus laevis oocytes. Apical glycylsarcosine uptake in Caco-2 cell monolayers was modulated by apical pH, extracellular Na(+), incubation time and S1611. Uptake in hPepT1-expressing oocytes was independent of Na(+) or S1611. We conclude that functional NHE3 activity is required to allow optimal absorption of dipeptides across the human intestinal epithelium.

Cloning and functional characterization of a new subtype of the amino acid transport system N.

We have cloned a new subtype of the amino acid transport system N2 (SN2 or second subtype of system N) from rat brain. Rat SN2 consists of 471 amino acids and belongs to the recently identified glutamine transporter gene family that consists of system N and system A. Rat SN2 exhibits 63% identity with rat SN1. It also shows considerable sequence identity (50-56%) with the members of the amino acid transporter A subfamily. In the rat, SN2 mRNA is most abundant in the liver but is detectable in the brain, lung, stomach, kidney, testis, and spleen. When expressed in Xenopus laevis oocytes and in mammalian cells, rat SN2 mediates Na(+)-dependent transport of several neutral amino acids, including glycine, asparagine, alanine, serine, glutamine, and histidine. The transport process is electrogenic, Li(+) tolerant, and pH sensitive. The transport mechanism involves the influx of Na(+) and amino acids coupled to the efflux of H(+), resulting in intracellular alkalization. Proline, alpha-(methylamino)isobutyric acid, and anionic and cationic amino acids are not recognized by rat SN2.

Evidence for the transport of neutral as well as cationic amino acids by ATA3, a novel and liver-specific subtype of amino acid transport system A.

We report here on the cloning and functional characterization of the third subtype of amino acid transport system A, designated ATA3 (amino acid transporter A3), from a human liver cell line. This transporter consists of 547 amino acids and is structurally related to the members of the glutamine transporter family. The human ATA3 (hATA3) exhibits 88% identity in amino acid sequence with rat ATA3. The gene coding for hATA3 contains 16 exons and is located on human chromosome 12q13. It is expressed almost exclusively in the liver. hATA3 mediates the transport of neutral amino acids including alpha-(methylamino)isobutyric acid (MeAIB), the model substrate for system A, in a Na(+)-coupled manner and the transport of cationic amino acids in a Na(+)-independent manner. The affinity of hATA3 for cationic amino acids is higher than for neutral amino acids. The transport function of hATA3 is thus similar to that of system y(+)L. The ability of hATA3 to transport cationic amino acids with high affinity is unique among the members of the glutamine transporter family. hATA1 and hATA2, the other two known members of the system A subfamily, show little affinity toward cationic amino acids. hATA3 also differs from hATA1 and hATA2 in exhibiting low affinity for MeAIB. Since liver does not express any of the previously known high-affinity cationic amino acid transporters, ATA3 is likely to provide the major route for the uptake of arginine in this tissue.

Involvement of transporter recruitment as well as gene expression in the substrate-induced adaptive regulation of amino acid transport system A.

We investigated the molecular mechanism involved in the adaptive regulation of the amino acid transport system A, a process in which amino acid starvation induces the transport activity. These studies were done with rat C6 glioma cells. System A activity in these cells is mediated exclusively by the system A subtype, amino acid transporter A2 (ATA2). The other two known system A subtypes, ATA1 and ATA3, are not expressed in these cells. Exposure of these cells to an amino acid-free medium induces system A activity. This process consists of an acute phase and a chronic phase. Laser-scanning confocal microscopic immunolocalization of ATA2 reveals that the acute phase is associated with recruitment of preformed ATA2 from an intracellular pool to the plasma membrane. In contrast, the chronic phase is associated with an induction of ata2 gene expression as evidenced from the increase in the steady-state levels of ATA2 mRNA, restoration of the intracellular pool of ATA2 protein, and blockade of the induction by cycloheximide and actinomycin D. The increase in system A activity induced by amino acid starvation is blocked specifically by system A substrates, including the non-metabolizable alpha-(methylamino)isobutyric acid.

Growth factors regulation of rabbit sodium-dependent neutral amino acid transporter ATB0 and oligopeptide transporter 1 mRNAs expression after enteretomy.

BACKGROUND: Sucessful intestinal adaptation after massive enterectomy is dependent on increased efficiency of nutrient transport. However, midgut resection (MGR) in rabbits induces an initial decrease in sodium-dependent brush border neutral amino acid transport, whereas parenteral epidermal growth factor (EGF) and growth hormone (GH) reverse this downregulation. We investigated intestinal amino acid transporter B0 (ATB0) and oligopeptide transporter 1 (PEPT 1) mRNA expression after resection and in response to EGF and/or GH. METHODS: Rabbits underwent anesthesia alone (control) or proximal, midgut, and distal resections. Full-thickness intestine was harvested from all groups on postoperative day (POD) 7, and on POD 14 from control and MGR rabbits. A second group of MGR rabbits received EGF and/or GH for 7 days, beginning 7 days after resection. ATB0 and PEPT 1 mRNA levels were determined by Northern blot analysis. RESULTS: In control animals, ileal ATB0 mRNA abundance was three times higher than jejunal mRNA, whereas PEPT 1 mRNA expression was similar. By 7 and 14 days after MGR, jejunal ATB0 mRNA abundance was decreased by 50% vs control jejunum. A 50% decrease in jejunal PEPT 1 message was delayed until 14 days after MGR. Treatment with EGF plus GH did not alter ATB0 mRNA expression but doubled PEPT 1 mRNA in the jejunum. CONCLUSION: The site of resection, time postresection, and growth factors treatment differentially influence ATB0 and PEPT 1 mRNA expression. Enhanced sodium-dependent brush border neutral amino acid transport with GH plus EGF administration is independent of increased ATB0 mRNA expression in rabbit small intestine after enterectomy.

Na+ - and Cl- -coupled active transport of nitric oxide synthase inhibitors via amino acid transport system B(0,+).

Nitric oxide synthase (NOS) inhibitors have therapeutic potential in the management of numerous conditions in which NO overproduction plays a critical role. Identification of transport systems in the intestine that can mediate the uptake of NOS inhibitors is important to assess the oral bioavailability and therapeutic efficacy of these potential drugs. Here, we have cloned the Na+ - and Cl- -coupled amino acid transport system B(0,+) (ATB(0,+)) from the mouse colon and investigated its ability to transport NOS inhibitors. When expressed in mammalian cells, ATB(0,+) can transport a variety of zwitterionic and cationic amino acids in a Na+ - and Cl- -coupled manner. Each of the NOS inhibitors tested compete with glycine for uptake through this transport system. Furthermore, using a tritiated analog of the NOS inhibitor N(G)-nitro-L-arginine, we showed that Na+ - and Cl- -coupled transport occurs via ATB(0,+). We then studied transport of a wide variety of NOS inhibitors in Xenopus laevis oocytes expressing the cloned ATB(0,+) and found that ATB(0,+) can transport a broad range of zwitterionic or cationic NOS inhibitors. These data represent the first identification of an ion gradient-driven transport system for NOS inhibitors in the intestinal tract.

Na+- and Cl--coupled active transport of carnitine by the amino acid transporter ATB(0,+) from mouse colon expressed in HRPE cells and Xenopus oocytes.

1. ATB(0,+) is an amino acid transporter energized by transmembrane gradients of Na+ and Cl(-) and membrane potential. We cloned this transporter from mouse colon and expressed the clone functionally in mammalian (human retinal pigment epithelial, HRPE) cells and Xenopus laevis oocytes to investigate the interaction of carnitine and its acyl esters with the transporter. 2. When expressed in mammalian cells, the cloned ATB(0,+) was able to transport carnitine, propionylcarnitine and acetylcarnitine. The transport process was Na(+) and Cl(-) dependent and inhibitable by the amino acid substrates of the transporter. The Michaelis constant for carnitine was 0.83 +/- 0.08 mM and the Hill coefficient for Na(+) activation was 1.6 +/- 0.1. 3. When expressed in Xenopus laevis oocytes, the cloned ATB(0,+) was able to induce inward currents in the presence of carnitine and propionylcarnitine under voltage-clamped conditions. There was no detectable current in the presence of acetylcarnitine. Carnitine-induced currents were obligatorily dependent on the presence of Na(+) and Cl(-). The currents were saturable with carnitine and the Michaelis constant was 1.8 +/- 0.4 mM. The analysis of Na(+)- and Cl(-)-activation kinetics revealed that 2 Na(+) and 1 Cl(-) were involved in the transport of carnitine via the transporter. 4. These studies describe the identification of a novel function for the amino acid transporter ATB(0,+). Since this transporter is expressed in the intestinal tract, lung and mammary gland, it is likely to play a significant role in the handling of carnitine in these tissues. 5. A Na(+)-dependent transport system for carnitine has already been described. This transporter, known as OCTN2 (novel organic cation transporter 2), is expressed in most tissues and transports carnitine with high affinity. It is energized, however, only by a Na(+) gradient and membrane potential. In contrast, ATB(0,+) is a low-affinity transporter for carnitine, but exhibits much higher concentrative capacity than OCTN2 because of its energization by transmembrane gradients of Na(+) and Cl(-) as well as by membrane potential.

Structure, function, and tissue expression pattern of human SN2, a subtype of the amino acid transport system N.

We have cloned a new subtype of the amino acid transport system N from a human liver cell line. This transporter, designated SN2, consists of 472 amino acids and exhibits 62% identity with human SN1 at the level of amino acid sequence. SN2-specific transcripts are expressed predominantly in the stomach, brain, liver, lung, and intestinal tract. The sizes of the transcripts vary in different tissues, indicating tissue-specific alternative splicing of the SN2 mRNA. In contrast, SN1 is expressed primarily in the brain and liver and there is no evidence for the presence of multiple transcripts of varying size for SN1. When expressed in mammalian cells, the cloned human SN2 mediates Na(+)-coupled transport of system N-specific amino acid substrates (glutamine, asparagine, and histidine). In addition, SN2 also transports serine, alanine, and glycine. Anionic amino acids, cationic amino acids, imino acids, and N-alkylated amino acids are not recognized as substrates by human SN2. The SN2-mediated transport process is Li(+)-tolerant and highly pH-dependent. The Michaelis-Menten constant for histidine uptake via human SN2 is 0.6 +/- 0.1 mM. The gene coding for SN2 is located on human chromosome Xp11.23. Successful cloning of SN2 provides the first molecular evidence for the existence of subtypes within the amino acid transport system N in mammalian tissues.

Structure and function of ATA3, a new subtype of amino acid transport system A, primarily expressed in the liver and skeletal muscle.

To date, two different transporters that are capable of transporting alpha-(methylamino)isobutyric acid, the specific substrate for amino acid transport system A, have been cloned. These two transporters are known as ATA1 and ATA2. We have cloned a third transporter that is able to transport the system A-specific substrate. This new transporter, cloned from rat skeletal muscle and designated rATA3, consists of 547 amino acids and has a high degree of homology to rat ATA1 (47% identity) and rat ATA2 (57% identity). rATA3 mRNA is present only in the liver and skeletal muscle. When expressed in Xenopus laevis oocytes, rATA3 mediates the transport of alpha-[(14)C](methylamino)isobutyric acid and [(3)H]alanine. With the two-microelectrode voltage clamp technique, we have shown that exposure of rATA3-expressing oocytes to neutral, short-chain aliphatic amino acids induces inward currents. The amino acid-induced current is Na(+)-dependent and pH-dependent. Analysis of the currents with alanine as the substrate has shown that the K(0. 5) for alanine (i.e., concentration of the amino acid yielding half-maximal current) is 4.2+/-0.1 mM and that the Na(+):alanine stoichiometry is 1:1.

cDNA structure, genomic organization, and promoter analysis of the mouse intestinal peptide transporter PEPT1.

We describe in this report the cDNA structure, functional characteristics, genomic organization, and promoter analysis of the mouse H(+)-coupled low-affinity peptide transporter PEPT1. The mouse PEPT1 cDNA cloned from a kidney cDNA library is approximately 3.1 kb long and encodes a protein of 709 amino acids. When expressed heterologously in mammalian cells and in Xenopus laevis oocytes, mouse PEPT1 mediates H(+)-coupled electrogenic transport of the dipeptide glycylsarcosine. The mouse pept1 gene, cloned from a genomic DNA library in bacterial artificial chromosome, is approximately 38 kb long and consists of 23 exons and 22 introns. 5'-Rapid amplification of cDNA ends with poly(A)(+) RNA from mouse intestine has identified the transcription start site that lies 31 bp upstream of the translation start site. The promoter region upstream of the transcription start site does not contain the TATA box but possesses three GC boxes which are the binding sites for the transcription activator SP1. Functional analysis of the promoter region using the luciferase reporter assay in Caco-2 cells (a human intestinal cell line that express PEPT1 constitutively) and five different 5'-deletion fragments of the promoter has shown that essential promoter/enhancer elements are present within 1140 bp upstream of the transcription start site.

Transport of N-acetylaspartate by the Na(+)-dependent high-affinity dicarboxylate transporter NaDC3 and its relevance to the expression of the transporter in the brain.

N-Acetylaspartate is a highly specific marker for neurons and is present at high concentrations in the central nervous system. It is not present at detectable levels anywhere else in the body other than brain. Glial cells express a high-affinity transporter for N-acetylaspartate, but the molecular identity of the transporter has not been established. The transport of N-acetylaspartate into glial cells is obligatory for its intracellular hydrolysis, a process intimately involved in myelination. N-Acetylaspartate is a dicarboxylate structurally related to succinate. We investigated in the present study the ability of NaDC3, a Na(+)-coupled high-affinity dicarboxylate transporter, to transport N-acetylaspartate. The cloned rat and human NaDC3s were found to transport N-acetylaspartate in a Na(+)-coupled manner in two different heterologous expression systems. The Michaelis-Menten constant for N-acetylaspartate was approximately 60 microM for rat NaDC3 and approximately 250 microM for human NaDC3. The transport process was electrogenic and the Na(+):N-acetylaspartate stoichiometry was 3:1. The functional expression of NaDC3 in the brain was demonstrated by in situ hybridization and reverse transcription-polymerase chain reaction as well as by isolation of a full-length functional NaDC3 from a rat brain cDNA library. In addition, the expression of a Na(+)-coupled high-affinity dicarboxylate transporter and the interaction of the transporter with N-acetylaspartate were demonstrable in rat primary astrocyte cultures. These studies establish NaDC3 as the transporter responsible for the Na(+)-coupled transport of N-acetylaspartate in the brain. This transporter is likely to be an essential component in the metabolic role of N-acetylaspartate in the process of myelination.

Structure, function, and regional distribution of the organic cation transporter OCT3 in the kidney.

We examined in this study the expression of the potential-sensitive organic cation transporter OCT3 in the kidney. A functionally active OCT3 was cloned from a mouse kidney cDNA library. The cloned transporter was found to be capable of mediating potential-dependent transport of a variety of organic cations including tetraethylammonium. This function was confirmed in two different heterologous expression systems involving mammalian cells and Xenopus laevis oocytes. We have also isolated the mouse OCT3 gene and deduced its structure and organization. The OCT3 gene consists of 11 exons and 10 introns. In situ hybridization studies in the mouse kidney have shown that OCT3 mRNA is expressed primarily in the cortex. The expression is evident in the proximal and distal convoluted tubules. The expression of OCT3 in human kidney was confirmed by RT-PCR. We have also cloned OCT3 from human placenta and human kidney. Human OCT3 exhibits 86% identity with mouse OCT3 in amino acid sequence. Human OCT3 was found to transport tetraethylammonium and a variety of other organic cations. The transport process was electrogenic. We conclude that OCT3 is expressed in mammalian kidney and that it plays an important role in the renal clearance of cationic drugs.

Primary structure, functional characteristics and tissue expression pattern of human ATA2, a subtype of amino acid transport system A.

We report here on the primary structure and functional characteristics of the protein responsible for the system A amino acid transport activity that is known to be expressed in most human tissues. This transporter, designated ATA2 for amino acid transporter A2, was cloned from the human hepatoma cell line HepG2. Human ATA2 (hATA2) consists of 506 amino acids and exhibits a high degree of homology to rat ATA2. hATA2-specific mRNA is ubiquitously expressed in human tissues. When expressed in mammalian cells, hATA2 mediates Na+-dependent transport of alpha-(methylamino)isobutyric acid, a specific model substrate for system A. The transporter is specific for neutral amino acids. It is pH-sensitive and Li+-intolerant. The Na+:amino acid stoichiometry is 1:1.

Cloning and functional expression of ATA1, a subtype of amino acid transporter A, from human placenta.

This report describes the primary structure and functional characteristics of human ATA1, a subtype of the amino acid transport system A. The human ATA1 cDNA was isolated from a placental cDNA library. The cDNA codes for a protein of 487 amino acids with 11 putative transmembrane domains. The transporter mRNA ( approximately 9.0 kb) is expressed most prominently in the placenta and heart, but detectable level of expression is evident in other tissues including the brain. When expressed heterologously in mammalian cells, the cloned transporter mediates Na(+)-coupled transport of the system A-specific model substrate alpha-(methylamino)isobutyric acid. The transport process is saturable with a Michaelis-Menten constant of 0. 89 +/- 0.12 mM. The Na(+):amino acid stoichiometry is 1:1 as deduced from the Na(+)-activation kinetics. The transporter is specific for small short-chain neutral amino acids. The gene for the transporter is located on human chromosome 12.

Transport of valganciclovir, a ganciclovir prodrug, via peptide transporters PEPT1 and PEPT2.

In clinical trials, valganciclovir, the valyl ester of ganciclovir, has been shown to enhance the bioavailability of ganciclovir when taken orally by patients with cytomegalovirus infection. We investigated the role of the intestinal peptide transporter PEPT1 in this process by comparing the interaction of ganciclovir and valganciclovir with the transporter in different experimental systems. We also studied the interaction of these two compounds with the renal peptide transporter PEPT2. In cell culture model systems using Caco-2 cells for PEPT1 and SKPT cells for PEPT2, valganciclovir inhibited glycylsarcosine transport mediated by PEPT1 and PEPT2 with K(i) values (inhibition constant) of 1.68+/-0.30 and 0.043+/- 0.005 mM, respectively. The inhibition by valganciclovir was competitive in both cases. Ganciclovir did not interact with either transporter. Similar studies done with cloned PEPT1 and PEPT2 in heterologous expression systems yielded comparable results. The transport of valganciclovir via PEPT1 was investigated directly in PEPT1-expressing Xenopus laevis oocytes with an electrophysiological approach. Valganciclovir, but not ganciclovir, induced inward currents in PEPT1-expressing oocytes. These results demonstrate that the increased bioavailability of valganciclovir is related to its recognition as a substrate by the intestinal peptide transporter PEPT1. This prodrug is also recognized by the renal peptide transporter PEPT2 with high affinity.

Structural and functional characteristics and tissue distribution pattern of rat OCTN1, an organic cation transporter, cloned from placenta.

This report describes the structure, function, and tissue distribution pattern of rat OCTN1 (novel organic cation transporter 1). The rat OCTN1 cDNA was isolated from a rat placental cDNA library. The cDNA is 2258 bp long and codes for a protein of 553 amino acids. Its amino acid sequence bears high homology to human OCTN1 (85% identity) and rat OCTN2 (74% identity). When expressed heterologously in mammalian cells, rat OCTN1 mediates Na(+)-independent and pH-dependent transport of the prototypical organic cation tetraethylammonium. The transporter interacts with a variety of structurally diverse organic cations such as desipramine, dimethylamiloride, cimetidine, procainamide, and verapamil. Carnitine, a zwitterion, interacts with rat OCTN1 with a very low affinity. However, the transport of carnitine via rat OCTN1 is not evident in the presence or absence of Na(+). We conclude that rat OCTN1 is a multispecific organic cation transporter. OCTN1-specific mRNA transcripts are present in a wide variety of tissues in the rat, principally in the liver, intestine, kidney, brain, heart and placenta. In situ hybridization shows the distribution pattern of the transcripts in the brain (cerebellum, hippocampus and cortex), kidney (cortex and medulla with relatively more abundance in the cortical-medullary junction), heart (myocardium and valves) and placenta (labyrinthine zone).