Secretion Of Organic Anions Research Articles

Intravenous Bilirubin, Dibromosulfophthalein, and Bromosulfophthalein Infusions Uncouple Biliary Phospholipid and Cholesterol Secretion from Bile Acid Secretion by Inhibiting Hepatic Phosphoglycoprotein-3 Activity in Pigs

Biliary secretion of organic anions disturbs the proportional relationship normally pertaining between biliary lipid and bile acid secretion. The mechanism causing this uncoupling of biliary lipid from bile acid secretion is incompletely understood. Impaired micellization of membrane lipids by bile due to biliary contamination with organic anions or lowering of biliary bile acid concentration by enhanced bile acid-independent bile flow has been proposed as causative factors. Recently, we found that hepatic phosphoglycoprotein 3 (pgp3) activity was reduced in pigs during bilirubin-induced uncoupling of biliary lipid from bile acid secretion. Pgp3 is the phosphatidylcholine flippase in the canalicular membrane sustaining biliary phospholipid secretion in pigs. This investigation was undertaken to examine whether bilirubin, dibromosulfophthalein, and bromosulfophthalein uncouple biliary lipid from bile acid secretion by the same mechanism. Hepatic bile was collected from 24 anesthetized pigs before and during infusion of 0.63 micromol x kg body wt(-1) x min(-1) intravenous bilirubin, dibromosulfophthalein, or bromosulfophthalein. Bile acid secretion was varied by intraportal cholic acid infusion. Hepatic pgp3 expression was measured by means of Western blot, using C219 antibody. Bilirubin > dibromosulfophthalein > bromosulfophthalein lowered biliary lipid secretion without altering hepatic pgp3 expression, increased bile acid-independent bile flow (bromosulfophthalein > dibromosulfophthalein > bilirubin), and enhanced the capacity of bile to micellize membrane lipids as assayed by means of erythrocyte lysis. Biliary bile acid concentration did not determine biliary lipid secretion. Bilirubin, dibromosulfophthalein, and bromosulfophthalein in bile uncouple biliary lipid from bile acid secretion by inhibiting hepatic pgp3 phosphatidylcholine flippase activity, putatively through diffusing into the pgp3 pore.

Drug secretion systems in renal tubular cells: functional models and molecular identity.

For the last several decades, functional characterization of renal drug handling has provided the conceptual framework of drug transport, especially drug secretion by the renal tubular cells. Functional models have been postulated for the two distinct groups of drugs with regard to their ionization characteristics at physiological pH (i.e., organic cations and organic anions). Organic cations are predicted to cross the basolateral membranes through a potential-sensitive uptake system along an inside-negative potential gradient, with secretion of organic cations into urine across apical membranes appearing to occur via a proton-organic cation antiport system. Organic anions are predicted to enter the cells against an electrical-potential gradient through basal membranes via an organic anion-dicarboxylic acid exchanger. Secretion of organic anions into urine across apical membranes is less well characterized, but the presence of an organic anion exchanger and a potential-driven transporter is hypothesized. Recent data obtained using molecular biology techniques have helped to elucidate molecular identity of each system postulated in these models. Novel drug transport proteins have also been discovered that were not part of the original organic cation/anion model, such as P-glycoprotein for hydrophobic neutral and cationic compounds. Moreover, the search for homologous genes of these transporters has led to the discovery of previously unknown transporter proteins, whose function has yet to be identified. Integral roles of these transport proteins in overall tubular handling of drugs remain to be determined.

Real-time assessment of alpha-ketoglutarate effect on organic anion secretion in perfused rabbit proximal tubules.

To determine the quantitative roles of the basolateral and luminal Na(+)-dicarboxylate (Na-DC) cotransporters in establishing and maintaining the alpha-ketoglutarate (alphaKG) gradient required for renal tubular secretion of organic anions, we measured net steady-state transepithelial secretion of fluorescein (FL) in real time in isolated, perfused S2 segments of rabbit renal proximal tubules. Net "basal" FL secretion in the absence of exogenous alphaKG had a K(t) of approximately 4 microM and a maximal transepithelial secretion rate (J(max)) of approximately 380 fmol. min(-1). mm(-1) (where K(t) is the FL concentration that produces one-half the J(max)). It could be almost completely inhibited by basolateral p-aminohippurate (PAH). Selective inhibition of the basolateral Na-DC cotransporter indicated that recycling via this transporter of alphaKG that had been exchanged for FL supports approximately 25% of the "basal" FL secretion. Physiological alphaKG concentrations of 10 microM in the bath or 50 microM in the perfusate stimulated net secretion of FL by approximately 30 or approximately 20%, respectively. These data indicate that the basolateral Na-DC cotransporter supports approximately 42% of the net FL secretion. The luminal and basolateral effects of physiological concentrations of alphaKG were additive, indicating that the combined function of the luminal and basolateral Na-DC cotransporters can support approximately 50% of the net FL secretion. This apparently occurs by their establishing and maintaining approximately 50% of the outwardly directed alphaKG gradient that is responsible for driving basolateral FL/alphaKG exchange. The remaining approximately 50% would be maintained by metabolic production of alphaKG in the cells.

Is Pediatric Labeling Really Necessary?

Labeling refers to the label on the drug container and all printed materials, including the package insert, that accompanies the product. Labeling of a drug indicates that there is substantial evidence from adequate and well controlled clinical trials for the safe and effective use of that drug. Labeling provides important information on clinical pharmacology, indications and usage, contraindications, precautions, adverse effects, dosage, and administration. Unfortunately for children, most drug labeling contains the precautionary disclaimer, because safety and efficacy in children have not been established. The availability of safe and effective drugs has been directly responsible for the improvement in health over the past 50 years. Children essentially have been excluded from the benefit of the many therapeutic advances that have marked pharmaceutic drug development. The failure to include children in clinical trials during drug development leads to delay in implementing potentially effective treatment. Most US food and Drug Administration-approved drugs lack approval for use in all children or are restricted to certain pediatric age groups, primarily older children.1 Only a few of the new drugs released in this country each year are approved for use in children. This lack of information on the safe and effective use of drugs in the most vulnerable patients, infants and neonates, is of greatest concern. Only five of the 80 most frequently used drugs have been approved for use in this population. Another problem is that most drugs are not available in suitable pediatric dosage forms. They are not available in appropriate dosage sizes, lack liquid formulation, and taste peculiar to the child, making compliance difficult. Pharmacies extemporaneously prepare many drugs in liquid dosage forms for use in children. These dosage forms are not sufficiently tested to determine stability, efficacy, or expiration dating. For solid dosage forms, parents often must divide adult tablets …

Recent developments in teleost renal transport

The teleost renal tubule continues to be a valuable tool with which to investigate epithelial membrane transport mechanisms and factors that control them. Several newer aspects of fish renal transport are reviewed, and hypothetical models incorporating these findings are proposed. Fluorescence microscopy has revealed a new Na-independent organic anion (OA) transport process and new information about the classical Na-dependent transporter. With the latter, OA enters the cytosol in exchange for a-ketoglutarate (αKG) at the basolateral membrane (BLM). Larger molecular weight OAs enter independently of (αKG and exit to the lumen on a brush-border membrane (BBM) ATP-dependent transporter. The Na-dependent OA transporter is inhibited by activation of protein kinase C (PKC). A primary messenger is dopamine whereas adrenergic stimulation increases OA secretion. Avid sulfate secretion by the proximal tubule involves two anion exchangers in series. Recent studies implicate carbonic anhydrase (CA) in facilitation of secretion through dehydroxylation of bicarbonate entering from the lumen in exchange for cellular sulfate. Hydroxyl ions resulting from this catalysis exchange for interstitial sulfate. Factors that activate PKC stimulate phosphate (Pi) secretion; those that activate protein kinase A stimulate Pi reabsorption. The flounder Na-coupled Pi transporter has been cloned (NaPi-II), kinetically characterized, and localized mainly to the BLM of proximal segment II (PII) and BBM of collecting duct. Morphology and ion distribution studies strongly implicate aggregation of Mg in cytosolic vesicles as a step in Mg secretion. These were found in PII and certain collecting duct cells. J. Exp. Zool. 283:653–661, 1999. © 1999 Wiley-Liss, Inc.

Recent developments in teleost renal transport

The teleost renal tubule continues to be a valuable tool with which to investigate epithelial membrane transport mechanisms and factors that control them. Several newer aspects of fish renal transport are reviewed, and hypothetical models incorporating these findings are proposed. Fluorescence microscopy has revealed a new Na-independent organic anion (OA) transport process and new information about the classical Na-dependent transporter. With the latter, OA enters the cytosol in exchange for a-ketoglutarate (αKG) at the basolateral membrane (BLM). Larger molecular weight OAs enter independently of (αKG and exit to the lumen on a brush-border membrane (BBM) ATP-dependent transporter. The Na-dependent OA transporter is inhibited by activation of protein kinase C (PKC). A primary messenger is dopamine whereas adrenergic stimulation increases OA secretion. Avid sulfate secretion by the proximal tubule involves two anion exchangers in series. Recent studies implicate carbonic anhydrase (CA) in facilitation of secretion through dehydroxylation of bicarbonate entering from the lumen in exchange for cellular sulfate. Hydroxyl ions resulting from this catalysis exchange for interstitial sulfate. Factors that activate PKC stimulate phosphate (Pi) secretion; those that activate protein kinase A stimulate Pi reabsorption. The flounder Na-coupled Pi transporter has been cloned (NaPi-II), kinetically characterized, and localized mainly to the BLM of proximal segment II (PII) and BBM of collecting duct. Morphology and ion distribution studies strongly implicate aggregation of Mg in cytosolic vesicles as a step in Mg secretion. These were found in PII and certain collecting duct cells. J. Exp. Zool. 283:653–661, 1999. © 1999 Wiley-Liss, Inc.

Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Kidney proximal tubule cells take up Krebs cycle intermediates for metabolic purposes and for secretion of organic anions through dicarboxylate/organic anion exchange. Alteration in reabsorption of citrate is closely related to renal stone formation. The presence of distinct types of sodium-coupled dicarboxylate transporters has been postulated on either side of the polarized epithelial membrane in the kidney proximal tubule. Using a PCR-based approach, we isolated a novel member of the sodium-dependent dicarboxylate/sulfate transporter called SDCT2. SDCT2 is a 600-amino acid residue protein that has 47-48% amino acid identity to SDCT1 and NaDC-1, previously identified in kidney and intestine. Northern analysis gave a single band of 3.3 kb for SDCT2 in kidney, liver, and brain. In situ hybridization revealed that SDCT2 is prominently expressed in kidney proximal tubule S3 segments and in perivenous hepatocytes, consistent with the sites of high-affinity dicarboxylate transport identified based on vesicle studies. A signal was also detected in the meningeal layers of the brain. SDCT2 expressed in Xenopus oocytes mediated sodium-dependent transport of di- and tricarboxylates with substrate preference for succinate rather than citrate, but excluding monocarboxylates. SDCT2, unlike SDCT1, displayed a unique pH dependence for succinate transport (optimal pH 7.5-8.5) and showed a high affinity for dimethylsuccinate, two features characteristic of basolateral transport. These data help to interpret the mechanisms of renal citrate transport, their alteration in pathophysiological conditions, and their role in the elimination of organic anions and therapeutic drugs.

A nucleoside-sensitive organic cation transporter in opossum kidney cells.

Renal secretion of organic cations and anions are pleiotropic, active processes in mammals. Some nucleosides such as deoxyadenosine (dAdo), 2-chlorodeoxyadenosine, and azidothymidine are secreted by human and rodent kidneys. Previous work (J. A. Nelson, J. F. Kuttesch, Jr., and B. H. Herbert. Biochemical Pharmacology 32: 2323-2327, 1983) indicated a role for the classic organic cation transporter (OCT) in the secretion of the dAdo analog, 2'-deoxytubercidin, by mouse kidney. Using [14C]tetraethylammonium bromide ([14C]TEA) as a substrate, we tested several renal cell lines for a nucleoside-sensitive OCT. American opossum kidney proximal tubule cells (OK) express a cimetidine-sensitive and metabolic-dependent ability to efflux TEA. Other classic OCT inhibitors and several nucleosides also inhibit TEA efflux by these cells in a manner reflecting structural specificity for the carrier. Inhibition of OCT by nucleosides is not a universal feature of OCTs, since TEA transport mediated by cloned rat kidney OCT2 in the Xenopus laevis oocyte system was not inhibited by the same nucleosides. In conclusion, OK cells appear to possess an OCT that may also transport some nucleosides by a novel carrier.

Cloning of the human kidney PAH transporter: narrow substrate specificity and regulation by protein kinase C.

Conserved from fish to mammals, renal proximal tubule organic anion secretion plays an important role in drug and xenobiotic elimination. Studies with the model substrate p-aminohippurate (PAH) have suggested that a basolateral PAH/alpha-ketoglutarate exchanger imports diverse organic substrates into the proximal tubule prior to apical secretion. cDNAs encoding PAH transporters have been cloned recently from rat and flounder. Here we report the cloning of a highly similar human PAH transporter (hPAHT) from human kidney. By Northern blot analysis and EST database searching, hPAHT mRNA was detected in kidney and brain. PCR-based monochromosomal somatic cell hybrid mapping placed the hPAHT gene on chromosome 11. When expressed transiently in vitro, hPAHT catalyzed time-dependent and saturable [3H]PAH uptake (Km of approximately 5 microM). Preincubation with unlabeled alpha-ketoglutaric or with glutaric acid stimulated tracer PAH uptake, and preincubation with unlabeled PAH stimulated tracer alpha-ketoglutarate uptake, results that are consistent with PAH/alpha-ketoglutarate exchange. Several structurally diverse organic anions cis-inhibited PAH uptake. Like rat OAT1 organic anion transporter, hPAHT was inhibited by furosemide, indomethacin, probenecid, and alpha-ketoglutarate. Unlike OAT1, hPAHT was not inhibited by prostaglandins or methotrexate (MTX). Moreover, tracer PGE2 and MTX were not transported, indicating that the substrate specificity for transport by hPAHT is not broad. PAH uptake was inhibited by phorbol 12-myristate 13-acetate (PMA) in a dose- and time-dependent fashion, but not by the inactive 4alpha-phorbol-12,13 didecanoate. PMA-induced inhibition was blocked by staurosporine. Thus the protein kinase C-mediated inhibition of basolateral organic anion entry previously reported in intact tubules is likely due, at least in part, to direct modulation of the PAH/alpha-ketoglutarate exchanger.

In liver transplantation, T tube bile represents total bile flow: physiological and scintigraphic studies on biliary secretion of organic anions.

The present study was performed to clarify the recovery of hepatocellular uptake and the biliary secretion of bile acids during the first 14 days after orthotopic liver transplantation (OLT) and to determine the fraction of bile flow appearing outside through the T tube and entering the duodenum. Therefore, we determined primary and secondary bile acids in bile samples obtained from the T tube at day 5 after OLT, while the T tube was permanently open, and at days 10 and 14 after OLT, i.e., 4 and 9 days after closure of the T tube, respectively, thus restoring enterohepatic bile acid circulation. In addition, we performed hepatobiliary scintigraphy using technetium 99m-labeled [2,4,6 trimethyl-3-bromo]imino-diacetic acid (technetium 99m-BRIDA) in 12 patients between days 4 and 17 after OLT. Chromatographic analyses of biliary bile acids showed no secondary bile acids during the first 5 days after OLT, as opposed to 10 and 14 days after OLT when enterohepatic circulation was restored. Eleven patients with an uncomplicated postoperative course after OLT showed a significantly reduced hepatic uptake and biliary secretion of 99mTc-BRIDA during the first days after OLT with progressive recovery. One patient with an acute allograft rejection episode showed almost no uptake and only minimal secretion. The bile fraction appearing outside through the inserted T tube represented 94.6% +/- 6.2% of the injected 99mTc-BRIDA. We conclude that OLT results in markedly impaired hepatocellular uptake and biliary secretion of organic anions. Simultaneously, bile acid synthesis is significantly reduced, which, in addition, diminishes bile secretion of the graft. We show that T tube bile is a valid tool for bile physiological studies in patients in whom transplantation was successfully performed.

Recent advances in molecular mechanisms of nephrotoxicity

Numerous drugs and endogenous compounds are efficiently excreted from the renal proximal tubule via two carrier-mediated pathways, that are organic anion and organic cation transport systems. Since most nephrotoxicants are taken up into renal target cells for further actions, these transport systems seem to be an early event for nephrotoxicity. Recent advances in nephrotoxicity are molecular cloning of several transporters related to important toxic compounds in the kidney. An organic cation transporter 1 (OCT1) was cloned in 1994. On the other hand, we recently isolated a complementary DNA that encodes an organic anion transporter 1 (OAT1) as an anion/dicarboxylate exchanger of the basolateral membrane of proximal tubule. Transepithelial secretion of organic anion consists of an influx of anionic substrates into the cell through the basolateral membrane and their efflux to the urine across the apical membrane. OAT1 displays a remarkably wide substrate specificity, including endogenous substrates, a variety of drugs with different structures and natural toxins. We further isolated homologs of OAT series such as liver-specific OAT2 and kidney-, liver- and brain-expressing OAT3. Because the amino acid sequence of OAT1 shows 38% identity to OCT1, a newly defined `multispecific organic ion transporter superfamily' will provide potential tools to assess mechanisms of many nephrotoxicants including drugs and xenobiotics, and contribute also in understanding more precisely nephrotoxic mechanisms of chemicals.

Hypothetical framework for enhanced renal tubular secretion of drugs in cystic fibrosis

Several clinical studies demonstrate reduced serum concentrations of renally excreted drugs in patients with cystic fibrosis (CF). To explain this phenomenon, we propose a model supporting increased proximal tubular secretion of certain drugs in individuals with CF. We hypothesize that the chloride channel located on the apical surface of renal proximal tubular cells and controlled by the cystic fibrosis transmembrane conductance regulator (CFTR) operates suboptimally in CF patients, and that the abnormal CFTR decreases Cl − reabsorption, resulting in an increased concentration of CI − in the tubular lumen. We postulate that, in an effort to maintain homeostasis, luminal CI − moves intracellularly in exchange for organic anions. The result of stimulating this anion exchanger is an increased rate of organic anion secretion by the renal tubule. Hence, due to enhanced tubular secretion, individuals with CF demonstrate increased tubular clearance of organic anion drugs, resulting in lower steady state serum concentrations.

Cellular and molecular mechanisms of renal tubular secretion of organic anions and cations

A wide variety of endogenous organic ions and xenobiotics are secreted into the urine via organic anion and cation transport systems, expressed in brush-border and basolateral membranes of renal tubular cells. Using membrane vesicles isolated from the kidney, cultured renal epithelial cells, isolated renal tubules, and slices of renal cortex, extensive studies have been done regarding the mechanisms of renal tubular secretion of organic ions. Basolateral entry of organic anions is mediated by the organic anion/dicarboxylate exchange system, whereas apical extrusion of organic anions from epithelial cells is mediated by an anion exchanger and/or by membrane potential-sensitive transport systems. Studies using membrane vesicles have made clear the fact that the basolateral transport of organic cations is stimulated by inside-negative membrane potential, whereas the transport of organic cations in brush-border membranes is achieved by a proton gradient. Trasport studies using cultured renal epithelial cells have shown other aspects of organic ion transport, such as regulatory mechanisms for transcellular transport of orgnaic anions and cations. The recent development of molecular techniques has greatly advanced our understanding of the molecular aspects of various transport processes. In 1994, a cDNA clone encoding the prototype organic cation transporter was isolated from rat kidney. Within the last 3 years, several organic anion and cation transporters in the kidney have been identified by different cloning techniques. In this review, we describe the mechanisms mediating renal tubular secretion of organic anions and cations, including recent topics in this area.

Cyclic AMP stimulates sorting of the canalicular organic anion transporter (Mrp2/cMoat) to the apical domain in hepatocyte couplets.

The canalicular membrane of rat hepatocytes contains an ATP-dependent multispecific organic anion transporter, also named multidrug resistance protein 2, that is responsible for the biliary secretion of several amphiphilic organic anions. This transport function is markedly diminished in mutant rats that lack the transport protein. To assess the role of vesicle traffic in the regulation of canalicular organic anion transport, we have examined the redistribution of the transporter to the canalicular membrane and the effect of cAMP on this process in isolated hepatocyte couplets, which retain secretory polarity. The partial disruption of cell-cell contact, due to the isolation procedure, leaves the couplet with both remnant apical membranes, as a source of apical proteins, and an intact apical domain and lumen, to which these proteins are targeted. The changes in distribution of the transporter were correlated to the apical excretion of a fluorescent substrate, glutathione-methylfluorescein. The data obtained in this study show that the transport protein, endocytosed from apical membrane remnants, first is redistributed along the basolateral plasma membrane. Then it is transcytosed to the remaining apical pole in a microtubule-dependent fashion, followed by the fusion of transporter-containing vesicles with the apical membrane. The cAMP analog dibutyrylcAMP stimulates all three steps, resulting in increased apically located transport protein, glutathione-methylfluorescein transport activity and apical membrane circumference. These findings indicate that the organic anion transport capacity of the apical membrane in hepatocyte couplets is regulated by cAMP-stimulated sorting of the multidrug resistance protein 2 to the apical membrane. The relevance of this phenomenon for the intact liver is discussed.

Influence of Sex Differences on the Renal Secretion of Organic Anions

Influence of Sex Differences on the Renal Secretion of Organic Anions

Efficient in vitro vectorial transport of a fluorescent conjugated bile acid analogue by polarized hepatic hybrid WIF-B and WIF-B9 cells.

Efficient transport of bile acids, a typical characteristic of hepatocytes, is partially lost in most hepatoma cell lines and in normal hepatocytes after some days in culture. We have tested whether the polarized rat hepatoma-human fibroblast hybrid WIF (hybrids between W138 and Fao cells) cells previously obtained by our group were able to perform vectorial transport of the fluorescent bile acid derivative cholylglycylamidofluorescein (CGamF) towards the bile canaliculi (BC). Four different WIF clones were analyzed. All were well polarized, as shown by the formation of spherical and even tubular BC-like structures and by the restricted localization at the BC, visualized by immunofluorescence, of the apical membrane marker HA4, a possible bile acid carrier. WIF-B and its subclone WIF-B9 were found to accumulate CGamF in 65% to 75% of their BC. This transport was time, temperature, and partly sodium dependent and was inhibited by coincubation with the parental natural bile salt cholylglycine. Dinitrophenyl glutathione, a substrate of the canalicular multispecific organic anion transporter, did not inhibit CGamF canalicular secretion, whereas it greatly impaired the canalicular secretion of a non-bile acid organic anion, fluorescein, generated intracellularly from fluorescein diacetate. Confocal microscopy confirmed the presence of CGamF in the cytoplasm, supporting a transcellular route from medium to BC. In contrast, two other polarized clones exhibited a poor ability (WIF 12-6) or no ability (WIF12-1 TGdelta) to vectorially transport CGamE In conclusion, WIF-B and WIF-B9 exhibit not only structural but also functional polarity, at least as far as vectorial organic anion transport is concerned.

Impaired activity of the bile canalicular organic anion transporter (Mrp2/cmoat) is not the main cause of ethinylestradiol-induced cholestasis in the rat.

To test the hypothesis that impaired activity of the bile canalicular organic anion transporting system mrp2 (cmoat) is a key event in the etiology of 17alpha-ethinylestradiol (EE)-induced intrahepatic cholestasis in rats, EE (5 mg/kg subcutaneously daily) was administered to male normal Wistar (NW) and mrp2-deficient Groningen Yellow/Transport-deficient Wistar (GY/TR-) rats. Elevated plasma bilirubin levels in GY/TR- rats increased upon EE-treatment from 65 +/- 8.4 micromol/L to 183 +/- 22.7 micromol/L within 3 days, whereas bilirubin levels remained unaffected in NW rats. Biliary bilirubin secretion was 1.5-fold increased in NW rats but remained unaltered in GY/TR- rats. Plasma bile salt concentrations remained unchanged in both strains, although hepatic levels of the sinusoidal Na+-taurocholate cotransporting protein (ntcp) were markedly reduced. Biliary secretion of endogenous bile salt was not affected in either strain. A clear reduction of mrp2 levels in liver plasma membranes of NW rats was found after 3 days of treatment. The bile salt-independent fraction of bile flow (BSIF) was reduced from 2.6 to 2.0 microL/min/100 g body weight in NW rats with a concomitant 62% reduction of biliary glutathione secretion. The absence of mrp2 and biliary glutathione in GY/TR- rats did not prevent induction of EE-cholestasis; a similar absolute reduction of BSIF, i.e., from 1.1 to 0.6 microL/min/100 g bodyweight, was found in these animals. EE treatment caused a reduction of the maximal biliary secretory rate (S(RM)) of the mrp2 substrate, dibromosulphthalein (DBSP), from 1,040 to 695 nmol/min/100 g body weight (-38%) in NW rats and from 615 to 327 nmol/min/100 g body weight (-46%) in GY/TR- rats. These results demonstrate that inhibition of mrp2 activity and/or biliary glutathione secretion is not the main cause of EE-induced cholestasis in rats. The data indicate that alternative pathways exist for the biliary secretion of bilirubin and related organic anions that are also affected by EE.

Modulation of liver canalicular transport processes by the tyrosine-kinase inhibitor genistein: Implications of genistein metabolism in the rat

Rat liver cells express the multispecific organic anion transporter (cmoat, cmrp, mrp2) and P-glycoprotein (Pgp) in their canalicular membranes, proteins that are homologous to the multidrug-resistance related protein (MRP) and multidrug resistance (MDR) gene products in multidrug resistant tumor cells. We tested whether genistein, a modulator of drug resistance in tumor cells, affects biliary secretion of substrates of canalicular multispecific organic anion transporter (cmoat) (glucuronides of bilirubin and rhodamine, glutathione conjugate of bromsulphthalein) and of P-glycoprotein (Pgp) (rhodamine), respectively. Using the isolated perfused rat liver of control Wistar rats (TR+) and of a mutant strain (TR-) that expresses Pgp but not cmoat, we show that genistein effectively inhibits the secretion of anionic substrates of cmoat in Wistar rats but stimulates secretion of cationic rhodamine in TR-rats. Genistein is subject to glucuronidation and sulfatation and secretion of genistein and its metabolites stimulates bile flow in Wistar rats, but secretion is nearly absent in TR-rats. Because genistein and its metabolites are substrates for cmoat, inhibition of anion secretion by genistein is partially explained by competition for this transporter. Genistein is also a substrate of uridindiphosphate (UDP)-glucuronyltransferase isoenzyme(s). Inhibition of glucuronidation reduces the availability of bilirubin and rhodamine glucuronates for transport via cmoat, but unconjugated cationic rhodamine becomes available for transport via Pgp at an increased cellular concentration. Daidzein, a genistein analogue with no effect on protein tyrosine kinase (PTK) shows Similar effects on secretion of organic anions and cations supporting the conclusion that genistein affects transport in liver mainly through competition with other substrates at the sites of glucuronidation and transport via cmoat. (Hepatology 1997 Dec;26(6):1467-76)

Modulation of liver canalicular transport processes by the tyrosine-kinase inhibitor genistein: implications of genistein metabolism in the rat.

Rat liver cells express the multispecific organic anion transporter (cmoat, cmrp, mrp2) and P-glycoprotein (Pgp) in their canalicular membranes, proteins that are homologous to the multidrug-resistance related protein (MRP) and multidrug resistance (MDR) gene products in multidrug resistant tumor cells. We tested whether genistein, a modulator of drug resistance in tumor cells, affects biliary secretion of substrates of canalicular multispecific organic anion transporter (cmoat) (glucuronides of bilirubin and rhodamine, glutathione conjugate of bromsulphthalein) and of P-glycoprotein (Pgp) (rhodamine), respectively. Using the isolated perfused rat liver of control Wistar rats (TR+) and of a mutant strain (TR-) that expresses Pgp but not cmoat, we show that genistein effectively inhibits the secretion of anionic substrates of cmoat in Wistar rats but stimulates secretion of cationic rhodamine in TR- rats. Genistein is subject to glucuronidation and sulfatation and secretion of genistein and its metabolites stimulates bile flow in Wistar rats, but secretion is nearly absent in TR- rats. Because genistein and its metabolites are substrates for cmoat, inhibition of anion secretion by genistein is partially explained by competition for this transporter. Genistein is also a substrate of uridindiphosphate (UDP)-glucuronyltransferase isoenzyme(s). Inhibition of glucuronidation reduces the availability of bilirubin and rhodamine glucuronates for transport via cmoat, but unconjugated cationic rhodamine becomes available for transport via Pgp at an increased cellular concentration. Daidzein, a genistein analogue with no effect on protein tyrosine kinase (PTK) shows Similar effects on secretion of organic anions and cations supporting the conclusion that genistein affects transport in liver mainly through competition with other substrates at the sites of glucuronidation and transport via cmoat.

Dual pathways for organic anion secretion in renal proximal tubule.

Transport on the "classical" organic anion system in renal proximal tubule is specific, active, Na-dependent, and ouabain sensitive. Here we review recent studies using intact teleost proximal tubules and laser scanning confocal microscopy which show that the secretion of large organic anions, such as, fluorescein-methotrexate (FL-MTX, Mw 923 Da) is handled by a separate and distinct organic anion transport system. In contrast to the classical system, FL-MTX uptake into cells and secretion into the tubular lumen was ouabain insensitive and largely Na-independent. KCN did not affect cellular uptake but abolished secretion into the lumen. PAH and probenecid, potent inhibitors of transport on the classical system, were weak inhibitors of FL-MTX transport. Uptake and secretion of FL-MTX were inhibited by micromolar concentrations of other organic anions (MTX, folate, bromocresol green, bromosulfonphthalein). FL-MTX secretion into the lumen was inhibited by leukotriene C4 and cyclosporine A, neither of which affected transport of the model substrate for the classical system, fluorescein. Thus, FL-MTX secretion is specific, but largely Na-independent and ouabain-insensitive. Both the basolateral and luminal steps in FL-MTX transport differ from those associated with fluorescein and P-aminohippurate secretion.