Endogenous Organic Cations Research Articles

Investigations with Drugs and Pesticides Revealed New Species- and Substrate-Dependent Inhibition by Elacridar and Imazalil in Human and Mouse Organic Cation Transporter OCT2.

Multiple drugs are used to treat various indications as well as pesticides that are ingested unintentionally and enter the bloodstream. The residence time or bioavailability of these substances in circulation depends on several mechanisms, such as drug-drug interaction (DDI), drug-pesticide interaction, metabolizing enzymes and the hepatic and renal transport systems, involved in the elimination of the compounds from the body. One of these transporters is the Organic Cation Transporter 2 (OCT2) member of the solute carrier (SLC22) transporter family. OCT2 is highly expressed in the proximal tubule epithelial cells in human and mouse kidney, where it mediates the uptake of endogenous organic cations as well as numerous drugs and xenobiotics, and contributes to the first step of renal clearance. In this study, we examined OCT2 on two subjects: First, the transferability of data from mouse to human, since mice are initially examined in the development of new drugs to assess the renal excretion of organic cations. Second, to what extent the choice of substrate affects the properties of an inhibitor. For this purpose, the functional properties of hOCT2 and mOct2 were validated under the same experimental conditions with the known substrates metformin and 1-Methyl-4-phenylpyridinium iodide (MPP). While hOCT2 and mOct2 showed very low affinities for metformin with Km values of 3.9 mM and 3.5 mM, the affinity of hOCT2 and mOct2 for MPP (62 and 40 µM) was 64- and 89-fold higher, respectively. For our positive control inhibitor decynium22, we determined the following IC50 values for hOCT2 and mOct2: 2.2 and 2.6 µM for metformin uptake, and 16 and 6.9 µM for MPP uptake. A correlation analysis of the inhibitory effects of 13 drugs and 9 pesticides on hOCT2- and mOct2-mediated transport of metformin showed a correlation coefficient R2 of 0.88, indicating good interspecies correlation. Nevertheless, the bioenhancer elacridar and the fungicide imazalil showed species-dependent inhibitory potentials. Concentration-dependent inhibition of hOCT2- and mOct2-mediated metformin uptake by elacridar showed IC50 values of 20 µM and 1.9 µM and by imazalil 4.7 µM and 0.58 µM, respectively. In conclusion, although our data show comparable species-independent interactions for most compounds, there can be large species-specific differences in the interactions of individual compounds, which should be considered when extrapolating data from mice to humans. Furthermore, a comparison of the inhibitory potential of elacridar and imazalil on metformin uptake with that on MPP uptake reveals substrate-dependent differences in hOCT2 and mOct2 for both inhibitors. Therefore, it might be useful to test two different substrates in inhibition studies.

Renal Transporter-Mediated Drug-Biomarker Interactions of the Endogenous Substrates Creatinine and N1 -Methylnicotinamide: A PBPK Modeling Approach.

Endogenous biomarkers for transporter-mediated drug-drug interaction (DDI) predictions represent a promising approach to facilitate and improve conventional DDI investigations in clinical studies. This approach requires high sensitivity and specificity of biomarkers for the targets of interest (e.g., transport proteins), as well as rigorous characterization of their kinetics, which can be accomplished utilizing physiologically-based pharmacokinetic (PBPK) modeling. Therefore, the objective of this study was to develop PBPK models of the endogenous organic cation transporter (OCT)2 and multidrug and toxin extrusion protein (MATE)1 substrates creatinine and N1 -methylnicotinamide (NMN). Additionally, this study aimed to predict kinetic changes of the biomarkers during administration of the OCT2 and MATE1 perpetrator drugs trimethoprim, pyrimethamine, and cimetidine. Whole-body PBPK models of creatinine and NMN were developed utilizing studies investigating creatinine or NMN exogenous administration and endogenous synthesis. The newly developed models accurately describe and predict observed plasma concentration-time profiles and urinary excretion of both biomarkers. Subsequently, models were coupled to the previously built and evaluated perpetrator models of trimethoprim, pyrimethamine, and cimetidine for interaction predictions. Increased creatinine plasma concentrations and decreased urinary excretion during the drug-biomarker interactions with trimethoprim, pyrimethamine, and cimetidine were well-described. An additional inhibition of NMN synthesis by trimethoprim and pyrimethamine was hypothesized, improving NMN plasma and urine interaction predictions. To summarize, whole-body PBPK models of creatinine and NMN were built and evaluated to better assess creatinine and NMN kinetics while uncovering knowledge gaps for future research. The models can support investigations of renal transporter-mediated DDIs during drug development.

The Importance of Tubular Function in Chronic Kidney Disease.

Glomerular filtration rate (GFR) and proteinuria-albuminuria are the renal functional parameters currently used to evaluate chronic kidney disease (CKD) severity. However, tubular secretion is another important renal functional parameter to be taken into account since proximal tubule (PT) secretion, in particular, is a crucial renal mechanism for endogenous organic cations, anions and drug elimination. The residual diuresis is a relevant survival predictor in patients on dialysis, since their urine is produced by the glomerular and tubular functions. It has been hypothesized that drugs which up-regulate some renal tubular transporters could contribute to uremic toxin excretion, and nephroprevention. However, if tubular transporters' down-regulation observed in CKD patients and experimental models is a PT adaptation to avoid intracellular accumulation and damage from uremic toxins, consequently the increase of toxin removal by inducing tubular transporters' up-regulation could be deleterious to the kidney. Therefore, a deeper understanding of this phenomenon is currently needed. In conclusion, tubular function has an important role for endogenous organic cations, anions and drug excretion in CKD patients, and a deeper understanding of its multiple mechanisms could provide new therapeutic alternatives in this population.

Cationic uremic toxins affect human renal proximal tubule cell functioning through interaction with the organic cation transporter

Several organic cations, such as guanidino compounds and polyamines, have been found to accumulate in plasma of patients with kidney failure due to inadequate renal clearance. Here, we studied the interaction of cationic uremic toxins with renal organic cation transport in a conditionally immortalized human proximal tubule epithelial cell line (ciPTEC). Transporter activity was measured and validated in cell suspensions by studying uptake of the fluorescent substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium-iodide (ASP(+)). Subsequently, the inhibitory potencies of the cationic uremic toxins, cadaverine, putrescine, spermine and spermidine (polyamines), acrolein (polyamine breakdown product), guanidine, and methylguanidine (guanidino compounds) were determined. Concentration-dependent inhibition of ASP(+) uptake by TPA, cimetidine, quinidine, and metformin confirmed functional endogenous organic cation transporter 2 (OCT2) expression in ciPTEC. All uremic toxins tested inhibited ASP(+) uptake, of which acrolein required the lowest concentration to provoke a half-maximal inhibition (IC50 = 44 ± 2 μM). A Dixon plot was constructed for acrolein using three independent inhibition curves with 10, 20, or 30 μM ASP(+), which demonstrated competitive or mixed type of interaction (K i = 93 ± 16 μM). Exposing the cells to a mixture of cationic uremic toxins resulted in a more potent and biphasic inhibitory response curve, indicating complex interactions between the toxins and ASP(+) uptake. In conclusion, ciPTEC proves a suitable model to study cationic xenobiotic interactions. Inhibition of cellular uptake transport was demonstrated for several uremic toxins, which might indicate a possible role in kidney disease progression during uremia.

Differential changes in functional activity of organic cation transporters in rats with uranyl nitrate-induced acute renal failure

We studied the impact of experimental kidney failure on the pharmacokinetics of a model organic cation and investigated the underlying mechanism(s) of the organic cation transporters. The systemic pharmacokinetics and tissue distribution of triethylmethylammonium (TEMA), a model organic cation, were characterized after intravenous doses of 0.3-30 μmol/kg in rats with or without uranyl nitrate-induced acute renal failure (UN-ARF). To study the effect of endogenous substrates in plasma from UN-ARF rats on organic cation transport, rOCT- or rOCT2-dependent uptake of tetraethylammonium (TEA) was studied in rOCT1-transfected or rOCT2-transfected LLC-PK1 cells, respectively. As a result, the AUC for TEMA was increased, probably because of decreased total clearance, and the tissue-to-plasma concentration ratio (T/P ratio) of TEMA was unchanged in the liver but decreased significantly in the kidneys of UN-ARF rats. In vitro, the uptake of TEA was decreased significantly by adding UN-ARF plasma, compared with control plasma, in rOCT2-overexpressing LLC-PK1 cells, but not in rOCT1-overexpressing LLC-PK1 cells. These observations suggest that the induction of UN-ARF leads to an accumulation of endogenous organic cation(s), probably rOCT2 substrate(s), in the plasma, thereby affecting the TEMA pharmacokinetics and distribution to the kidneys in rats.

Nitric oxide-induced regulation of renal organic cation transport after renal ischemia-reperfusion injury

Renal organic cation transporters are downregulated by nitric oxide (NO) in rat endotoxemia. NO generated by inducible NO synthase (iNOS) is substantially increased in the renal cortex after renal ischemia-reperfusion (I/R) injury. Therefore, we investigated the effects of iNOS-specific NO inhibition on the expression of the organic cation transporters rOct1 and rOct2 (Slc22a1 and Slc22a2, respectively) after I/R injury both in vivo and in vitro. In vivo, N(6)-(1-iminoethyl)-L-lysine (L-NIL) completely inhibited NO generation after I/R injury. Moreover, L-NIL abolished the ischemia-induced downregulation of rOct1 and rOct2 as determined by qPCR and Western blotting. Functional evidence was obtained by measuring the fractional excretion (FE) of the endogenous organic cation serotonin. Concordant with the expression of the rate-limiting organic cation transporter, the FE of serotonin decreased after I/R injury and was totally abolished by L-NIL. In vitro, ischemia downregulated both rOct1 and rOct2, which were also abolished by L-NIL; the same was true for the uptake of the organic cation MPP. We showed that renal I/R injury downregulates rOct1 and rOct2, which is most probably mediated via NO. In principle, this may be an autocrine effect of proximal tubular epithelial cells. We conclude that rOct1, or rOct1 and rOct2 limit the rate of the renal excretion of serotonin.

Functional Analysis of Human Organic Cation Transporter OCT3 (SLC22A3) Polymorphisms

We analyzed the functional properties of five single nucleotide polymorphisms (SNPs) in organic cation transporter OCT3 gene (SLC22A3) resulting in the amino acid changes with a transient expression system. Three SNPs (A116S, T400I, and A439V) exhibited reduced uptake of both [ 3H]histamine and [3H]MPP+, although their protein expressions were detected in the plasma membrane of transfected cells. This study suggests that the OCT3 variants will contribute to interindividual variations leading to the differences in cationic drug disposition as well as certain disease processes such as hypertension, allergic diseases, and neuropsychiatric diseases by the clearance of endogenous organic cations such as biogenic amines.

The transport of organic cations in the small intestine: Current knowledge and emerging concepts

A wide variety of drugs and endogenous bioactive amines are organic cations (OCs). Approximately 40% of all conventional drugs on the market are OCs. Thus, the transport of xenobiotics or endogenous OCs in the body has been a subject of considerable interest, since the discovery and cloning of a family of OC transporters, referred to as organic cation transporter (OCTs), and a new subfamily of OCTs, OCTNs, leading to the functional characterization of these transporters in various systems including oocytes and some cell lines. Organic cation transporters are critical in drug absorption, targeting, and disposition of a drug. In this review, the recent advances in the characterization of organic cation transporters and their distribution in the small intestine are discussed. The results of the in vitro transport studies of various OCs in the small intestine using techniques such as isolated brush-border membrane vesicles, Ussing chamber systems and Caco-2 cells are discussed, and in vivo knock-out animal studies are summarized. Such information is essential for predicting pharmacokinetics and pharmacodynamics and in the design and development of new cationic drugs. An understanding of the mechanisms that control the intestinal transport of OCs will clearly aid achieving desirable clinical outcomes.

Regulation of organic cation transport

Transport of organic cations (OC) is important for the recycling of endogenous OC and also a necessary step for detoxification of exogenous OC in the body. Even though the identification and characterisation of numerous OC transporters in recent years has allowed the elucidation of molecular mechanisms underlying OC transport, elucidation of the regulation of this transport is just beginning. This review summarises the general properties of OC transport and then analyses the literature on the regulation of these processes. Studies on short- and long-term regulation of OC transport are considered separately. Important aspects of short-term regulation have been clarified and the regulatory pathways of several OC transporters have been characterised. Short-term regulation appears to be transporter subtype-, tissue- and species-dependent and to involve transporter phosphorylation. Transporter phosphorylation may alter the affinity for substrates or/and expression on the plasma membrane. Even though several studies have shown long-term regulation of OC transport, the pathophysiological meaning of these changes are not well understood. In this case, regulation seems to be subtype-, tissue- and gender-specific. Further research is necessary to clarify this important issue of regulation of OC transport.

Effect of P-glycoprotein modulators on the human extraneuronal monoamine transporter

The aim of this work was to investigate the effect of P-glycoprotein modulators on human extraneuronal monoamine transporter (EMT)-mediated transport. The experiments were performed using a cell line from human embryonic kidney (HEK293 cells) stably transfected with pcDNA3hEMT (293 hEMT), or with pcDNA3 alone (293 control). Of the P-glycoprotein modulators tested, rhodamine123, verapamil and daunomycin concentration-dependently inhibited EMT-mediated uptake of [ 3H]1-methyl-4-phenylpyridinium ([ 3H]MPP +). The corresponding IC 50's were found to be 3.6, 37 and 130 μM, respectively. By contrast, vinblastine, digitoxin and cyclosporine A were devoid of effect. The endogenous organic cation tyramine, but not choline, inhibited EMT-mediated transport (IC 50 of 468 μM). Moreover, l-arginine and l-histidine (up to 1 mM) did not affect [ 3H]MPP + uptake. Finally, MPP + and tyramine trans-stimulated [ 3H]MPP + uptake, but rhodamine123 had no effect, and verapamil and daunomycin trans-inhibited [ 3H]MPP + uptake. In conclusion, this study shows that several cationic modulators of P-glycoprotein inhibit EMT-mediated transport. As a consequence, the interaction of P-glycoprotein modulators with EMT must be taken into account, and the consequences of this interaction must not be forgotten when using such drugs in vivo.

Uptake properties of lamivudine (3TC) by a continuous renal epithelial cell line

The purpose of this study was to characterize the renal uptake properties of the cytidine analog and antiretroviral agent 3TC. The uptake of radiolabelled 3TC was measured at 37 degrees C in a continuous porcine renal epithelial cell line (i.e., LLC-PK1 cells) grown as a monolayer on an impermeable support. 3TC (5 microM) uptake (37 degrees C) by the monolayer cells was saturable (Km = 1.2 +/- 0.2 mM) but not significantly altered by various dideoxynucleoside analog drugs, nucleosides, and nucleoside transport inhibitors, suggesting that a nucleoside transporter is not involved in 3TC uptake. A number of endogenous organic cation probes and inhibitors significantly reduced 3TC uptake by the monolayer cells. Quinine, trimethoprim (TMP), and tetraethylammonium (TEA) inhibited 3TC uptake in a dose dependent manner with IC50 values of 0.6 mM, 0.63 mM, and 1.9 mM, respectively. In turn, the uptake of the typical organic cation substrate TEA was inhibited by high concentrations of 3TC. An outwardly directed proton gradient significantly increased the uptake of 3TC by the monolayer cells, suggesting the involvement of a proton exchange process. Conversely, in the presence of monensin, a Na+/H+ ionophore, the uptake of 3TC was significantly reduced. These results suggest that the uptake of 3TC by a cultured renal epithelium may be mediated by an organic cation-proton exchanger. The observed clinical interaction between 3TC and trimethoprim may be explained by competition for a common renal organic cation tubular transporter.

Uptake properties of lamivudine (3TC) by a continuous renal epithelial cell line

The purpose of this study was to characterize the renal uptake properties of the cytidine analog and antiretroviral agent 3TC. The uptake of radiolabelled 3TC was measured at 37 degrees C in a continuous porcine renal epithelial cell line (i.e., LLC-PK1 cells) grown as a monolayer on an impermeable support. 3TC (5 microM) uptake (37 degrees C) by the monolayer cells was saturable (Km = 1.2 +/- 0.2 mM) but not significantly altered by various dideoxynucleoside analog drugs, nucleosides, and nucleoside transport inhibitors, suggesting that a nucleoside transporter is not involved in 3TC uptake. A number of endogenous organic cation probes and inhibitors significantly reduced 3TC uptake by the monolayer cells. Quinine, trimethoprim (TMP), and tetraethylammonium (TEA) inhibited 3TC uptake in a dose dependent manner with IC50 values of 0.6 mM, 0.63 mM, and 1.9 mM, respectively. In turn, the uptake of the typical organic cation substrate TEA was inhibited by high concentrations of 3TC. An outwardly directed proton gradient significantly increased the uptake of 3TC by the monolayer cells, suggesting the involvement of a proton exchange process. Conversely, in the presence of monensin, a Na+/H+ ionophore, the uptake of 3TC was significantly reduced. These results suggest that the uptake of 3TC by a cultured renal epithelium may be mediated by an organic cation-proton exchanger. The observed clinical interaction between 3TC and trimethoprim may be explained by competition for a common renal organic cation tubular transporter.

Molecular pharmacology of organic cation transporters in kidney.

The homeostasis of endogenous organic cations, such as choline and N P-methylnicotinamide, monoamine neurotransmitters, cationic drugs, such as cimetidine, morphine, quinine and amantadine, and of cationic xenobiotics is controlled by reabsorption and excretion in the small intestine, by metabolic conversion and excretion in the liver, and by reabsorption and excretion in the kidney [1–6]. In the kidney organic cations may be ultrafiltrated in the glomeruli and reabsorbed or secreted in renal tubules. Hydrophilic cations are readily ultrafiltrated, however, more hydrophobic cations are bound to plasma membrane proteins that may not permeate the filtration barrier. Secretion and reabsorption of organic cations have been described in renal proximal tubules but may also occur in distal tubules or collecting ducts [7–11]. These processes have been functionally characterized by measurements using intact kidneys or tissue slices [10, 12–14], by microperfusion experiments using proximal tubules [7, 9, 15], by uptake measurements using isolated tubules or cells [8, 11, 16, 17], and by uptake measurements using membrane vesicles from proximal tubules [18–28]. In luminal and basolateral membranes of proximal tubules different transport processes have been demonstrated, although all the involved transport systems may not have been identified by these measurements because a variety of different polyspecific organic cation transporters with overlapping substrate specificity are expressed [5]. After the cloning of the first organic cation transporter rOCT1 from a rat kidney library [29], an increasing number of homologous cation transporters have been identified and it has been shown that the renal anion transporters also belong to this new protein family. The functional characterization of the expressed transporters and their immunohistochemical localization in the kidney has begun [30–35]. This review describes the molecular structure of renal organic cation transporters and homologous gene products. The transport properties and renal localization of the cloned cation transporters are summarized and their presumed role in renal cation handling of endogenous and exogenous cations is discussed.

Altered Clearance of N-1 Methylnicotinamide Associated With the Use of Low Doses of Cyclosporine

To improve the monitoring of patients on low doses of cyclosporine there is a need for new tests of tubular function. N-1 methylnicotinamide (NMM) is an endogenous organic cation that is secreted by the proximal tubule and its clearance can be measured. In 27 patients with psoriasis, serial measurements of NMN clearance, plasma aldosterone, plasma chloride, bicarbonate, and magnesium were compared with changes in the radionuclide measurement of glomerular filtration rate and renal blood flow before, during, and after a 3-month course of low-dose cyclosporine (< 5 mg/kg/d). N-1 methylnicotinamide clearance decreased significantly with cyclosporine only (2.5 mg/kg/d, n = 10, P < 0.01). Recovery of NMN clearance lagged behind that of glomerular filtration rate and renal blood flow. Serum magnesium decreased significantly on cyclosporine (2.5 mg/kg, n = 10, P < 0.01; 5 mg/kg, n = 9, P < 0.0001). In the whole group, plasma potassium increased significantly (n = 27, P < 0.02) and plasma aldosterone was inappropriately low. Low doses of cyclosporine in psoriasis cause a reduced clearance of NMN, hypomagnesemia, and a variable hyperchloremic acidosis. Nifedipine may alter these biochemical variables without necessarily improving renal hemodynamics. The delayed recovery of NMN clearance in comparison with renal haemodynamic measurements following cyclosporine therapy suggests that this noninvasive test of tubular function may be a marker of persisting cyclosporine nephrotoxicity and that it should be evaluated further.

Organic cation transport by rat liver plasma membrane vesicles: studies with tetraethylammonium.

Recently, an organic cation:H+ antiport was selectively identified on the sinusoidal domain of rat liver with the use of the endogenous organic cation N1-methylnicotinamide (NMN). Absence of NMN+:H+ exchange on canalicular membrane suggested that this transport process was primarily involved in organic cation uptake, leaving the mechanism(s) for organic cation secretion into bile unknown. To further define hepatic organic cation transport, we examined the characteristics of tetraethylammonium (TEA) transport in basolateral (blLPM) and canalicular (cLPM) rat liver plasma membrane vesicles. In cLPM vesicles, under voltage-clamped conditions, an outwardly directed H+ gradient stimulated [14C]TEA uptake compared with [14C]TEA uptake under pH-equilibrated conditions, consistent with electroneutral TEA:H+ exchange. The proton ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone had no effect on [14C]TEA uptake, demonstrating that pH-dependent [14C]TEA uptake was not the result of a H+ diffusion potential. In the absence of a pH gradient, the intravesicular presence of TEA trans-stimulated uptake of [14C]TEA. Procainamide ethobromide (PAEB), vecuronium, and tributylmethylammonium (TBuMA), organic cations selectively excreted in bile, cis-inhibited pH-dependent TEA uptake. In contrast, in blLPM vesicles, no pH gradient-dependent [14C]TEA uptake was demonstrated. Instead, basolateral [14C]TEA uptake was significantly stimulated by a valinomycin-induced intravesicular-negative K+ diffusion potential. Basolateral [14C]TEA uptake was also cis-inhibited by PAEB, vecuronium, and TBuMA, but not by NMN. Conversely, PAEB, vecuronium, and TBuMA had no effect on basolateral pH-dependent [3H]NMN uptake. These findings suggest that organic cation transport, with TEA as a model quaternary amine, across the canalicular membrane is driven by an electroneutral organic cation:H+ exchange and that the transport of certain organic cations across the basolateral membrane is via a carrier-mediated system stimulated by an inside-negative membrane potential.

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.

Estimation of renal secretory function for organic cations by endogenous N1-methylnicotinamide in rats with experimental renal failure.

To assess whether the secretory clearance of N1-methylnicotinamide (NMN), an endogenous organic cation, represents renal tubular secretion of the organic cation, the relationship between the secretory clearance of NMN, CLscn(NMN), and that of tetraethylammonium bromide (TEA), CLscn(TEA), was examined in normal and experimental renal failure (ERF) rats. TEA was selected as a representative organic cation secreted by the kidney. ERF was induced by glycerol, folate, salicylate, uranium, and gentamicin, substances which have been demonstrated to produce specific damage to the kidney by pathophysiological studies. Glomerular filtration rate (GFR), CLscn(NMN), and CLscn(TEA) decreased significantly in most of ERF rats, while blood urea nitrogen (BUN) increased significantly in all ERF rats. There was a statistically significant correlation (r = 0.952, p less than 0.001) between the endogenous CLscn(NMN) and CLscn(TEA) in both the normal and ERF rats. Correlation analysis revealed that CLscn(NMN) was superior to GFR in the degree of relationship to CLscn(TEA), but BUN could not be used as an index for the secretion of NMN or TEA. Although the plasma concentration of NMN in most of the ERF rats was much higher than that in the normal rats, it affected neither the urinary clearance of NMN itself nor the excretion of TEA. From these findings, we propose that CLscn(NMN) can be used as an index to assess renal tubular function for the secretion of organic cations that are excreted by both filtration and secretion without reabsorption.

Renal tubule transport of organic cations.

Transcellular active tubular transport of organic cations occurs in the proximal renal tubule in the direction of excretion. These organic cations may be primary, secondary, tertiary, or quaternary amines. Endogenous neurohumors such as choline and catecholamines and drugs such as morphine and tetraethylammonium are representative transportable organic cations. Competitive inhibition for transport is found among organic cations. Organic anions, however, do not compete for this transport. Organic cations used as drugs may interact with the transport of endogenous organic cations to alter the excretion patterns of both. Bidirectional active tubule transport can be demonstrated for choline and is accompanied by simultaneous renal metabolism of choline. Studies using vesicles prepared from luminal and antiluminal membranes of renal cortex suggest that organic cation transport occurs in both vesicle preparations. However, only the luminal vesicles showed the characteristics of carrier-mediated uphill transport.