Fructose suppresses uric acid excretion to the intestinal lumen as a result of the induction of oxidative stress by NADPH oxidase activation

BackgroundA high intake of fructose increases the risk for hyperuricemia. It has been reported that long-term fructose consumption suppressed renal uric acid excretion and increased serum uric acid level. However, the effect of single administration of fructose on excretion of uric acid has not been clarified.MethodsWe used male Wistar rats, which were orally administered fructose (5 g/kg). Those rats were used in each experiment at 12 h after administration.ResultsSingle administration of fructose suppressed the function of ileal uric acid excretion and had no effect on the function of renal uric acid excretion. Breast cancer resistance protein (BCRP) predominantly contributes to intestinal excretion of uric acid as an active homodimer. Single administration of fructose decreased BCRP homodimer level in the ileum. Moreover, diphenyleneiodonium (DPI), an inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox), recovered the suppression of the function of ileal uric acid excretion and the Bcrp homodimer level in the ileum of rats that received single administration of fructose.ConclusionsSingle administration of fructose decreases in BCRP homodimer level, resulting in the suppression the function of ileal uric acid excretion. The suppression of the function of ileal uric acid excretion by single administration of fructose is caused by the activation of Nox. The results of our study provide a new insight into the mechanism of fructose-induced hyperuricemia.     

ABCG2 Dysfunction Increases Serum Uric Acid by Decreased Intestinal Urate Excretion

ATP-binding cassette transporter G2 (ABCG2), also known as breast cancer resistance protein (BCRP), is identified as a high-capacity urate exporter and its dysfunction has an association with serum uric acid (SUA) levels and gout/hyperuricemia risk. However, pathophysiologically important pathway(s) responsible for the ABCG2-mediated urate excretion were unknown. In this study, we investigated how ABCG2 dysfunction affected the urate excretion pathways. First, we revealed that mouse Abcg2 mediates urate transport using the membrane vesicle system. The export process by mouse Abcg2 was ATP-dependent and not saturable under the physiological concentration of urate. Then, we characterized the excretion of urate into urine, bile, and intestinal lumen using in vivo mouse model. SUA of Abcg2-knockout mice was significantly higher than that of control mice. Under this condition, the renal urate excretion was increased in Abcg2-knockout mice, whereas the urate excretion from the intestine was decreased to less than a half. Biliary urate excretion showed no significant difference regardless of Abcg2 genotype. From these results, we estimated the relative contribution of each pathway to total urate excretion; in wild-type mice, the renal excretion pathway contributes approximately two-thirds, the intestinal excretion pathway contributes one-third of the total urate excretion, and the urate excretion into bile is minor. Decreased intestinal excretion could account for the increased SUA of Abcg2-knockout mice. Thus, ABCG2 is suggested to have an important role in extra-renal urate excretion, especially in intestinal excretion. Accordingly, increased SUA in patients with ABCG2 dysfunction could be explained by the decreased excretion of urate from the intestine.     

Multidrug transporter ABCG2/breast cancer resistance protein secretes riboflavin (vitamin B2) into milk

The multidrug transporter breast cancer resistance protein (BCRP/ABCG2) is strongly induced in the mammary gland during pregnancy and lactation. We here demonstrate that BCRP is responsible for pumping riboflavin (vitamin B(2)) into milk, thus supplying the young with this important nutrient. In Bcrp1(-/-) mice, milk secretion of riboflavin was reduced >60-fold compared to that in wild-type mice. Yet, under laboratory conditions, Bcrp1(-/-) pups showed no riboflavin deficiency due to concomitant milk secretion of its cofactor flavin adenine dinucleotide, which was not affected. Thus, two independent secretion mechanisms supply vitamin B(2) equivalents to milk. BCRP is the first active riboflavin efflux transporter identified in mammals and the first transporter shown to concentrate a vitamin into milk. BCRP activity elsewhere in the body protects against xenotoxins by reducing their absorption and mediating their excretion. Indeed, Bcrp1 activity increased excretion of riboflavin into the intestine and decreased its systemic availability in adult mice. Surprisingly, the paradoxical dual utilization of BCRP as a xenotoxin and a riboflavin pump is evolutionarily conserved among mammals as diverse as mice and humans. This study establishes the principle that an ABC transporter can transport a vitamin into milk and raises the possibility that other vitamins and nutrients are likewise secreted into milk by ABC transporters.     

In vitro and in vivo interaction of moxidectin with BCRP/ABCG2

The study characterizes the interaction between BCRP/ABCG2 and moxidectin by means of cellular transport, and pharmacokinetic studies in Bcrp1 (−/−) and wild-type mice. Milbemycin moxidectin ([³H]-moxidectin) was tested for its ability to be transported across MCDK-II epithelial monolayer cultures transfected with BCRP. In a second approach, accumulation assays by BCRP-expressing Xenopus laevis oocytes were carried out. Finally, pharmacokinetic studies were performed in order to establish the role of the transporter in milk secretion and tissue distribution. The efflux was negligible in polarized cells but moxidectin was efficiently transported in BCRP-expressing X. laevis oocytes. The transport was blocked by an acridone derivative, a novel BCRP inhibitor. Moxidectin secretion into breast milk was decreased in Bcrp1-knockout mice and the milk to plasma ratio was 2-fold higher in wild-type mice after i.v. administration. Drug accumulation in intestinal content, bile, and intestine was higher in wild-type mice but the plasma concentration was not different.Moxidectin is identified as a BCRP substrate since its Bcrp1-mediated secretion into breast milk and the involvement of Bcrp1 in intestinal and bile secretion has been demonstrated. This interaction has pharmacokinetic and toxicological consequences. The most important toxicological consequences of the interaction between BCRP and moxidectin may be related with the presence of drug residues in milk.     

Synthesis and in vivo evaluation of [11C]tariquidar,a positron emission tomography radiotracer based on a third-generation P-glycoprotein inhibitor

The aim of this study was to develop a positron emission tomography (PET) tracer based on the dual P-glycoprotein (P-gp) breast cancer resistance protein (BCRP) inhibitor tariquidar (1) to study the interaction of 1 with P-gp and BCRP in the blood–brain barrier (BBB) in vivo. O-Desmethyl-1 was synthesized and reacted with [¹¹C]methyl triflate to afford [¹¹C]-1. Small-animal PET imaging of [¹¹C]-1 was performed in naïve rats, before and after administration of unlabeled 1 (15 mg/kg, n = 3) or the dual P-gp/BCRP inhibitor elacridar (5 mg/kg, n = 2), as well as in wild-type, Mdr1a/b^(?/?), Bcrp1^(?/?) and Mdr1a/b^(?/?)Bcrp1^(?/?) mice (n = 3). In vitro autoradiography was performed with [¹¹C]-1 using brain sections of all four mouse types, with and without co-incubation with unlabeled 1 or elacridar (1 μM). In PET experiments in rats, administration of unlabeled 1 or elacridar increased brain activity uptake by a factor of 3–4, whereas blood activity levels remained unchanged. In Mdr1a/b^(?/?), Bcrp1^(?/?) and Mdr1a/b^(?/?)Bcrp1^(?/?) mice, brain-to-blood ratios of activity at 25 min after tracer injection were 3.4, 1.8 and 14.5 times higher, respectively, as compared to wild-type animals. Autoradiography showed approximately 50% less [¹¹C]-1 binding in transporter knockout mice compared to wild-type mice and significant displacement by unlabeled elacridar in wild-type and Mdr1a/b^(?/?) mouse brains. Our data suggest that [¹¹C]-1 interacts specifically with P-gp and BCRP in the BBB. However, further investigations are needed to assess if [¹¹C]-1 behaves in vivo as a transported or a non-transported inhibitor.     

ABCG2 Dysfunction Increases the Risk of Renal Overload Hyperuricemia

ATP-binding cassette transporter, sub-family G, member 2 (ABCG2/BCRP) is identified as a high-capacity urate exporter, and its dysfunction has an association with serum uric acid levels and gout/hyperuricemia risk. Generally, hyperuricemia has been classified into urate “overproduction type,” “underexcretion type,” and “combined type” based on only renal urate excretion, without considering an extra-renal pathway such as gut excretion. In this study, we investigated the effects of ABCG2 dysfunction on human urate handling and the mechanism of hyperuricemia.

Clinical parameters for urate handling including urinary urate excretion (UUE) were examined in 644 Japanese male outpatients with hyperuricemia. The severity of their ABCG2 dysfunction was estimated by genotype combination of two common ABCG2 variants, nonfunctional Q126X (rs72552713) and half-functional Q141K (rs2231142).

Contrary to the general understanding that ABCG2 dysfunction leads to decreased renal urate excretion, UUE was significantly increased by ABCG2 dysfunction (P = 3.60 × 10^?10). Mild, moderate, and severe ABCG2 dysfunctions significantly raised the risk of “overproduction” hyperuricemia including overproduction type and combined type, conferring risk ratios of 1.36, 1.66, and 2.35, respectively.

The present results suggest that common dysfunctional variants of ABCG2 decrease extra-renal urate excretion including gut excretion and cause hyperuricemia. Thus, “overproduction type” in the current concept of hyperuricemia should be renamed “renal overload type,” which is caused by two different mechanisms, “extra-renal urate underexcretion” and genuine “urate overproduction.”

Our new concept will lead to a more accurate diagnosis and more effective therapeutic strategy for hyperuricemia and gout.     

ABCG2 expression and uric acid metabolism of the intestine in hyperuricemia model rat

Abstract

To elucidate roles of the intestine in uric acid (UA) metabolism, we examined ABCG2 expression, tissue UA content and xanthine oxidoreductase (XOR) activity in different intestinal segments. Male SD rats were assigned to control group or oxonic acid-induced hyperuricemia (HUA) group. In control rats, ABCG2 was present both in villi and crypts in each segment. Tissue UA content and XOR activity were relatively high in duodenum and jejunum. However, in HUA rats, tissue UA content was significantly elevated in the ileum, whereas it remained unaltered in other segments. Moreover, ABCG2 expression in the HUA group was upregulated both in the villi and crypts of the ileum. These data indicate that the ileum may play an important role in the extra-renal UA excretion.     

Hyperuricemia enhances intracellular urate accumulation via down-regulation of cell-surface BCRP/ABCG2 expression in vascular endothelial cells

Hyperuricemia has been recognized as an independent risk factor for cardiovascular disease. Urate stimulates NADPH oxidase and induces production of reactive oxygen species (ROS); consequently, intracellular urate accumulation can induce oxidative stress leading to endothelial dysfunction. Here, we studied the mechanism involved, using human umbilical vascular endothelial cells (HUVEC) as a model. Pretreatment with 15 mg/dL unlabeled uric acid (corresponding to hyperuricemia) resulted in increased uptake of [¹⁴C]uric acid at steady-state by HUVEC, whereas pretreatment with 5 mg/dL uric acid (in the normal serum concentration range) did not. However, the initial uptake rate of [¹⁴C]uric acid was not affected by uric acid at either concentration. These results suggest that efflux transport of uric acid is decreased under hyperuricemic conditions. We observed a concomitant decrease of phosphorylated endothelial nitric oxide synthase. Plasma membrane expression of breast cancer resistance protein (BCRP), a uric acid efflux transporter, was decreased under hyperuricemia, though the total cellular expression of BCRP remained constant. Uric acid did not affect expression of another uric acid efflux transporter, multidrug resistance associated protein 4 (MRP4). Moreover, phosphorylation of Akt, which regulates plasma membrane localization of BCRP, was decreased. These uric acid-induced changes of BCRP and Akt were reversed in the presence of the antioxidant N-acetylcysteine. These results suggest that in hyperuricemia, uric acid-induced ROS generation inhibits Akt phosphorylation, causing a decrease in plasma membrane localization of BCRP, and the resulting decrease of BCRP-mediated efflux leads to increased uric acid accumulation and dysregulation of endothelial function.     

Radiosynthesis and in vivo evaluation of 1-[18F]fluoroelacridar as a positron emission tomography tracer for P-glycoprotein and breast cancer resistance protein

Aim of this study was to label the potent dual P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) inhibitor elacridar (1) with (18)F to provide a positron emission tomography (PET) radiotracer to visualize Pgp and BCRP. A series of new 1- and 2-halogen- and nitro-substituted derivatives of 1 (4a-e) was synthesized as precursor molecules and reference compounds for radiolabelling and shown to display comparable in vitro potency to 1 in increasing rhodamine 123 accumulation in a cell line overexpressing human Pgp (MDCKII-MDR1). 1-[(18)F]fluoroelacridar ([(18)F]4b) was synthesized in a decay-corrected radiochemical yield of 1.7±0.9% by a 1-step no-carrier added nucleophilic aromatic (18)F-substitution of 1-nitro precursor 4c. Small-animal PET imaging of [(18)F]4b was performed in na?ve rats, before and after administration of unlabelled 1 (5 mg/kg, n=3), as well as in wild-type and Mdr1a/b((-/-))Bcrp1((-/-)) mice (n=3). In PET experiments in rats, administration of unlabelled 1 increased brain activity uptake by a factor of 9.5 (p=0.0002, 2-tailed Student's t-test), whereas blood activity levels remained unchanged. In Mdr1a/b((-/-))Bcrp1((-/-)) mice, the mean brain-to-blood ratio of activity at 60 min after tracer injection was 7.6 times higher as compared to wild-type animals (p=0.0002). HPLC analysis of rat brain tissue extracts collected at 40 min after injection of [(18)F]4b revealed that 93±7% of total radioactivity in brain was in the form of unchanged [(18)F]4b. In conclusion, the in vivo behavior of [(18)F]4b was found to be similar to previously described [(11)C]1 suggesting transport of [(18)F]4b by Pgp and/or BCRP at the rodent BBB. However, low radiochemical yields and a significant degree of in vivo defluorination will limit the utility of [(18)F]4b as a PET tracer.     

Localization of breast cancer resistance protein (Bcrp) in endocrine organs and inhibition of its transport activity by steroid hormones

Breast cancer resistance protein (BCRP) is known for its protective function against the toxic effects of exogenous compounds. In addition to this, a role in the transport of endogenous compounds has been described. Since BCRP in the plasma membrane was shown to be regulated by sex steroids, we investigated the presence and possible role of BCRP in steroid hormone-producing organs. Therefore, the presence and localization of Bcrp was investigated in endocrine organs of wild-type mice. Furthermore, the interaction of various steroid hormones with human BCRP activity was studied. Quantitative PCR revealed Bcrp mRNA in the pituitary and adrenal glands, pancreas, ovary, testis and adipose tissue. Immunohistochemistry revealed the presence of Bcrp in the cortex of the adrenal gland and in plasma membranes of adipocytes. In the pituitary gland, pancreas, ovary and testis, Bcrp was mainly located in the capillaries. The interaction between BCRP and 12 steroid hormones was studied using membrane vesicles of HEK293-BCRP cells. Estradiol, testosterone, progesterone and androstenedione inhibited BCRP-mediated uptake of (3)H-estrone sulphate (E(1)S) most potently, with calculated inhibitory constant (Ki) values of 5.0?±?0.2, 36?±?14, 14.7?±?1.3 and 217?±?13?μM, respectively. BCRP function was attenuated non-competitively, which implies an allosteric inhibition of BCRP-mediated E(1)S transport by these steroids. In conclusion, localization of Bcrp in endocrine organs together with the efficient allosteric inhibition of the efflux pump by steroid hormones are suggestive for a role for BCRP in steroid hormone regulation.     

Histochemical studies of uric acid in some insects. 3. Excretion of uric acid by the malpighian tubules in Calliphora vicina and Rhodnius prolixus

In adult Calliphora uric acid is excreted throughout the Malpighian tubules. Histochemical preparations for the light microscope show uric acid passing through the cells and forming crystalline spheres in immediate contact with the microvilli. Uric acid appears to be synthesized and discharged into the haemolymph by the fat body cells. In Rhodnius there is no visible uric acid in the cells or lumen of the upper segment of the tubule (two-thirds of the total length of the tubule) apart from occasional deposits in the basal lamina. All uric acid excretion depends on the lower segment. Electron micrographs after argentaffin staining show high concentration of uric acid in the cytoplasm below the basal lamina (which also contains uric acid deposits). Uric acid is visible throughout the cell, particularly aroand the mitochondria; it is absent from the infolded plasma membrane and from all vacuoles. At the lumen there is a concentrated deposit of uric acid immediately beyond the plasma membrane. The uric acid particles unite with particles of unstained matrix material to form crystalline spheres. The fat body shows active synthesis of uric acid which is discharged by the cells into the intercellular channels and so to the basal lamina through which it passes into the haemolymph. As judged by histochemical preparations the haemolymph contains a high concentration of uric acid, very variable in different sites. Likewise large variations in uric acid secretion occur in different parts of the fat body.     

几种天然产物对高尿酸血症大鼠血清尿酸水平的影响初探

目的:探讨几种天然产物对高尿酸血症大鼠血清尿酸水平及尿酸排泄的影响.方法:对wistar大鼠灌胃氧嗪酸钾和酵母膏,制作高尿酸血症大鼠动物模型.灌胃给药褐藻糖胶、柠檬酸钾和东哥阿里提取物,2周后采血并进行代谢实验,检测血清尿酸、尿素氮,24小时尿液体积、pH值、尿酸浓度及总量,分析三种活性物质对机体尿酸水平、尿酸排泄、肾脏功能的影响.结果:三种物质均可显著降低高尿酸血症模型大鼠的血清尿酸水平,其中东哥阿里提取物组的24小时排泄尿酸总量较模型组显著降低,褐藻糖胶对实验大鼠的血清尿素氮水平升高有抑制作用.结论:三种活性物质对高尿酸血症大鼠血清尿酸浓度有降低作用,其中褐藻糖胶对肾脏功能有保护作用,从而保证尿酸的顺利排泄,而东哥阿里在降低血尿酸水平的同时,24小时尿液中排泄的尿酸总量也显著低于模型对照组,其机制可能与抑制尿酸生成有关.     

Influence of breast cancer resistance protein (Abcg2) and p-glycoprotein (Abcb1a) on the transport of imatinib mesylate (Gleevec) across the mouse blood-brain barrier

Imatinib, a protein tyrosine kinase inhibitor, may prevent the growth of glioblastoma cells. Unfortunately, its brain distribution is restricted by p-glycoprotein (p-gp or multidrug resistance protein Mdr1a), and probably by breast cancer resistance protein (Bcrp1), two efflux pumps expressed at the blood-brain barrier (BBB). We have used in situ brain perfusion to investigate the mechanisms of imatinib transport across the mouse BBB. The brain uptake of imatinib in wild-type mice was limited by saturable efflux processes. The inhibition of p-gp, by valspodar and zosuquidar, increased imatinib uptake (2.5-fold), as did the deficiency of p-gp in Mdr1a/1b(-/-) mice (5.5-fold). Perfusing imatinib with the p-gp/Bcrp1 inhibitor, elacridar, enhanced the brain uptake of imatinib in wild-type (4.1-fold) and Mdr1a/1b(-/-) mice (1.2-fold). However, the brain uptake of imatinib was similar in wild-type and Bcrp1(-/-) mice when it was perfused at a non-saturating concentration. The brain uptake of CGP74588, an active metabolite of imatinib, was low. It was increased by perfusion with elacridar (twofold), but not with valspodar and zosuquidar. CGP74588 uptake was 1.5 times greater in Bcrp1(-/-) mice than in wild-type mice. These data suggest that imatinib transport at the mouse BBB is limited by p-gp and probably by Bcrp1, and that CGP74588 transport is restricted by Bcrp1.     

Extracellular metabolism-dependent uptake of lysolipids through cultured monolayer of differentiated Caco-2 cells

Glycerophospholipids are known to be hydrolyzed in the intestinal lumen into free fatty acids and lysophospholipids that are then absorbed by the intestinal epithelial cells. A monolayer of enterocyte-differentiated Caco-2 cell is often used to assess the intestinal bioavailability of nutrients. In this study, we examined how differentiated Caco-2 cells process lysoglycerolipids such as lysophosphatidylcholine (LPC). Our findings were twofold. (1) Caco-2 cells secreted both a lysophospholipase A-like enzyme and a glycerophosphocholine-phosphodiesterase enzyme into the apical, but not basolateral, lumen, suggesting that food-derived LPC is converted to a free fatty acid, sn-glycerol-3-phosphate, and choline through two sequential enzymatic reactions in humans. The release of the latter enzyme was differentiation-dependent. (2) Fatty acid-releasing activities toward exogenous fluorescent LPC, lysophosphatidic acid and monoacylglycerol were shown to be higher on the apical membranes of Caco-2 cells than on the basolateral membranes. These results suggest that human intestinal epithelial cells metabolize lysoglycerolipids by two distinct mechanisms involving secreted or apical-selective expression of metabolic enzymes.     

In vivo relevance of Mrp2-mediated biliary excretion of the Amanita mushroom toxin demethylphalloin

To determine which efflux carriers are involved in hepatic phalloidin elimination, hepatobiliary [(3)H]-demethylphalloin (DMP) excretion was studied in normal Wistar rats and in Mrp2 deficient TR(-) Wistar rats as well as in normal wild-type FVB mice, Mdr1a,b(-/-) knockout mice, and Bcrp1(-/-) knockout mice by in situ bile duct/gallbladder cannulation. A subtoxic dose of 0.03 mg DMP/kg b.w. was used, which did not induce cholestasis in any tested animal. Excretion of DMP into bile was not altered in Mdr1a,b(-/-) mice or in Bcrp1(-/-) mice compared with wild-type FVB mice. Whereas 17.6% of the applied dose was excreted into bile of normal Wistar rats, hepatobiliary excretion decreased to 7.9% in TR(-) rats within 2 h after intravenous application. This decrease was not due to reduced cellular DMP uptake, as shown by normal expression of Oatp1b2 in livers of TR(-) rats and functional DMP uptake into isolated TR(-) rat hepatocytes. Tissue concentrations of phalloidin were also not altered in any of the transgenic mice. Interestingly, the decrease of biliary DMP excretion in the TR(-) rats was not followed by any increase of phalloidin accumulation in the liver but yielded a compensatory excretion of the toxin into urine, indicating that hepatocytes of TR(-) rats expelled phalloidin back into blood circulation.     

Comprehensive analysis of mechanism underlying hypouricemic effect of glucosyl hesperidin

Hyperuricemia is caused by hepatic overproduction of uric acid and/or underexcretion of urate from the kidneys and small intestine. Although increased intake of citrus fruits, a fructose-rich food, is associated with increased risk of gout in humans, hesperidin, a flavonoid naturally present in citrus fruits, reportedly reduces serum uric acid (SUA) levels by inhibiting xanthine oxidase (XOD) activity in rats. However, the effects of hesperidin on renal and intestinal urate excretion were previously unknown. In this study, we used glucosyl hesperidin (GH), which has greater bioavailability than hesperidin, to clarify comprehensive mechanisms underlying the hypouricemic effects of hesperidin in vivo. GH dose-dependently decreased SUA levels in mice with hyperuricemia induced by potassium oxonate and a fructose-rich diet, and inhibited XOD activity in the liver. GH decreased renal urate excretion without changes in kidney URAT1, ABCG2 or GLUT9 expressions, suggesting that reducing uric acid pool size by inhibiting XOD decreased renal urate excretion. We also found that GH had no effect on intestinal urate excretion or protein expression of ABCG2. Therefore, we concluded that GH exhibits a hypouricemic effect by inhibiting XOD activity in the liver without increasing renal or intestinal urate excretion. Of note, this is the first study to elucidate the effect of a flavonoid on intestinal urate excretion using a mice model, whose findings should prove useful in future food science research in the area of urate metabolism. Taking these findings together, GH may be useful for preventing hyperuricemia, especially in people with the overproduction type.     

The effects of severe zinc deficiency on intestinal amino acid losses in the rat

To determine whether intestinal amino acid losses might occur during zinc deficiency, labeled aminoisobutyric acid was given parenterally to zinc deficient rats and to appropriate zinc-sufficient controls. After 24 hours, the aminoisobutyric acid loss into the intestinal lumen was measured by in situ perfusion of isolated intestinal segments under conditions of either net water absorption or water secretion. Net amino acid losses were larger in the jejunum of the zinc deficient rats and losses were exacerbated during net water secretion in the jejunum and colon segments. The contribution of amino acid losses to fecal nitrogen, particularly during osmotic diarrhea, may be important in the growth retardation of zinc deficiency. Further, these alterations may indicate defective enterocyte transport functions during severe deficiency.     

In vivo relevance of Mrp2-mediated biliary excretion of the Amanita mushroom toxin demethylphalloin

To determine which efflux carriers are involved in hepatic phalloidin elimination, hepatobiliary [³H]-demethylphalloin (DMP) excretion was studied in normal Wistar rats and in Mrp2 deficient TR(−) Wistar rats as well as in normal wild-type FVB mice, Mdr1a,b(−/−) knockout mice, and Bcrp1(−/−) knockout mice by in situ bile duct/gallbladder cannulation. A subtoxic dose of 0.03 mg DMP/kg b.w. was used, which did not induce cholestasis in any tested animal. Excretion of DMP into bile was not altered in Mdr1a,b(−/−) mice or in Bcrp1(−/−) mice compared with wild-type FVB mice. Whereas 17.6% of the applied dose was excreted into bile of normal Wistar rats, hepatobiliary excretion decreased to 7.9% in TR(−) rats within 2 h after intravenous application. This decrease was not due to reduced cellular DMP uptake, as shown by normal expression of Oatp1b2 in livers of TR(−) rats and functional DMP uptake into isolated TR(−) rat hepatocytes. Tissue concentrations of phalloidin were also not altered in any of the transgenic mice. Interestingly, the decrease of biliary DMP excretion in the TR(−) rats was not followed by any increase of phalloidin accumulation in the liver but yielded a compensatory excretion of the toxin into urine, indicating that hepatocytes of TR(−) rats expelled phalloidin back into blood circulation.