Anion Transport Inhibitor Research Articles

High Risk of Embryo-Fetal Toxicity: Placental Transfer of T-2 Toxin and Its Major Metabolite HT-2 Toxin in BeWo Cells

Though T-2 toxin is the most harmful mycotoxin to the fetuses, it remains unclear whether T-2 toxin and its major metabolite, HT-2 toxin, could pass the placenta into the fetus and which kind of placental transport is involved in the passage. To illustrate their placenta transfer mechanism, the uptake and efflux of T-2 and HT-2 toxins across apical membranes of placenta with BeWo cells as a model were studied at different temperatures, pHs, and in the presence of transporter inhibitors with a developed liquid chromatography-tandem mass spectrometry to determine the amount of toxins in both fetal and maternal sites. Higher unidirectional transport of T-2 toxin was observed in the apical-to-basolateral direction than basolateral-to-apical one, whereas HT-2 toxin exhibited similar transport rate from the 2 directions. The main ATP-binding cassette transporters had no effect on the efflux of 2 toxins. Initial uptake of T-2 toxin was sodium dependent and saturable, and the apical uptake was temperature dependent and enhanced under acidic condition. The apical uptake of T-2 toxin was inhibited by metabolic inhibitors and the organic anion and organic cation transporter inhibitors. These results suggested that an active transport mechanism was responsible for the uptake of T-2 toxin, whereas passive diffusion was the principal mechanism for HT-2 toxin transport in the placenta. Taken together, these data characterized the placental transfer of T-2 and HT-2 toxins. The present study offered new ways of reducing the risks of T-2 and HT-2 toxins to both mother and fetuses.

Folate Transport in Mouse Cumulus-Oocyte Complexes and Preimplantation Embryos1

Endogenous folate stores are required in preimplantation embryos of several species, but how folates are accumulated and whether they can be replenished has not been determined. Folates are generally taken up into cells by specific transporters, mainly the reduced folate carrier RFC1 (SLC19A1 protein) and the high-affinity folate receptors FOLR1 and FOLR2. Quantitative RT-PCR showed that Slc19a1 mRNA was expressed in mouse cumulus-oocyte complexes (COCs) and oocytes, whereas Folr1 showed expression only in preimplantation embryos, increasing from the 2-cell stage onward. The mRNAs encoding Folr2 and the intestinal folate transporter Slc46a1 were not detected. Methotrexate (MTX), an antifolate often used as a model substrate for folate transport, exhibited saturable transport in COCs and in preimplantation embryos starting at the 2-cell stage. However, folate transport characteristics differed between COCs and embryos. In COCs, transport of MTX and the reduced folate leucovorin was inhibited by the anion transport inhibitor SITS that blocks RFC1 but was insensitive to dynasore, a specific dynamin inhibitor that instead inhibits folate receptor-receptor mediated endocytosis, whereas the opposite was found in 2-cell embryos and blastocysts. The inhibitor profile and transport properties of MTX and leucovorin in COCs correspond to established transport characteristics of RFC1 (SLC19A1), whereas those in 2-cell embryos and blastocysts correspond with those of FOLR1, consistent with the mRNA expression patterns. Considerable folate was accumulated in COCs via RFC1, but the presence of cumulus cells did not enhance folate accumulation in the enclosed oocyte, indicating a lack of transfer from cumulus to oocyte.

The anthraquinone drug rhein potently interferes with organic anion transporter-mediated renal elimination

Rhein, a major metabolite of the prodrug diacerein and a major component of the medicinal herb Rheum sp., is used for its beneficial effects in a variety of clinical applications including the treatment of osteoarthritis and diabetic nephropathy. The physicochemical properties of rhein are consistent with those of known organic anion transporter (OAT) substrates and inhibitors. Therefore, the inhibitory effect of rhein on human (h) OAT1, hOAT3, hOAT4, and murine (m) Oat1 and mOat3 was examined in heterologous cell lines stably expressing each transporter in isolation. Rhein was shown to potently inhibit hOAT1 and hOAT3, with IC50 estimates in the low nanomolar range (IC50=77.1±5.5nM and 8.4±2.5nM, respectively), while poor affinity was observed for hOAT4 (IC50>100μM). Marked species differences were observed with hOAT1 and hOAT3 exhibiting 3- and 28-fold higher affinity for rhein as compared to their murine orthologs. The estimated drug–drug interaction (DDI) indices (≫0.1) indicated a very strong potential for clinically relevant, rhein perpetrated DDIs mediated by inhibition of hOAT1 (DDI index=5.0; 83% inhibition) and/or hOAT3 (DDI index=46; 98% inhibition) transport activity. These results suggested that rhein, from herbal medicines and/or prodrug conversion, may significantly impact the dosing, efficacy and toxicity (i.e., pharmacokinetics and pharmacodynamics) of co-administered hOAT1 and/or hOAT3 drug substrates.

Uric acid induces oxidative stress and growth inhibition by activating adenosine monophosphate-activated protein kinase and extracellular signal-regulated kinase signal pathways in pancreatic β cells

Hyperuricaemia is a disorder of purine metabolism, and is strongly associated with insulin resistance and abnormal glucose metabolism. As the producer of insulin, pancreatic β cells might be affected by elevated serum uric acid levels and contribute to the disregulated glucose metabolism. In this study, we investigated the effect of high uric acid on rat pancreatic β cell function. Under high uric acid condition, proliferation of pancreatic β cells was inhibited, production of reactive oxygen species increased, and glucose stimulated insulin secretion was also compromised. Further examination on signal transduction pathways revealed that uric acid-induced ROS is involved in the activation of adenosine monophosphate-activated protein kinase (AMPK), and extracellular signal-regulated kinase (ERK). Pharmacological inhibition of ERK activation rescued β cells from growth inhibition. More importantly, activation of ERK induced by uric acid is significantly diminished by AMPK inhibitor, indicating ERK as a downstream target of AMPK in response to high uric acid condition. We also investigated the transportation channel for uric acid into pancreatic β cells. While major urate transporter URAT1 is not expressed in β cells, organic anion transporter (OAT) inhibitor successfully blocked the activation of ERK by uric acid. Our data indicate that high uric acid levels induce oxidative damage and inhibit growth of rat pancreatic β cells by activating the AMPK and ERK signal pathways. Hyperuricemia may contribute to abnormal glucose metabolism by causing oxidative damage and function inhibition of pancreatic β cells.

DIDS inhibits Na-K-ATPase activity in porcine nonpigmented ciliary epithelial cells by a Src family kinase-dependent mechanism

The anion transport inhibitor DIDS is known to reduce aqueous humor secretion but questions remain about anion dependence of the effect. In some tissues, DIDS is reported to cause Na-K-ATPase inhibition. Here, we report on the ability of DIDS to inhibit Na-K-ATPase activity in nonpigmented ciliary epithelium (NPE) and investigate the underlying mechanism. Porcine NPE cells were cultured to confluence on permeable supports, treated with drugs added to both sides of the membrane, and then used for (86)Rb uptake measurements or homogenized to measure Na-K-ATPase activity or to detect protein phosphorylation. DIDS inhibited ouabain-sensitive (86)Rb uptake, activated Src family kinase (SFK), and caused a reduction of Na-K-ATPase activity. PP2, an SFK inhibitor, prevented the DIDS responses. In BCECF-loaded NPE, DIDS was found to reduce cytoplasmic pH (pHi). PP2-sensitive Na-K-ATPase activity inhibition, (86)Rb uptake suppression, and SFK activation were observed when a similar reduction of pHi was imposed by low-pH medium or an ammonium chloride withdrawal maneuver. PP2 and the ERK inhibitor U0126 prevented robust ERK1/2 activation in cells exposed to DIDS or subjected to pHi reduction, but U0126 did not prevent SFK activation or the Na-K-ATPase activity response. The evidence points to an inhibitory influence of DIDS on NPE Na-K-ATPase activity by a mechanism that hinges on SFK activation associated with a reduction of cytoplasmic pH.

DIDS and the Janus-faced Na+-K+-activated ATPase. Focus on “DIDS inhibits Na-K-ATPase activity in porcine nonpigmented ciliary epithelial cells by a Src family kinase-dependent mechanism”

the maintenance of mammalian cell volume fundamentally rests on net cation extrusion through Na+-K+-activated ATPase, the Na+ pump. The publication by Shahidullah et al. ([11][1]) persuasively identifies the series of events by which a widely used inhibitor of anion transport, DIDS, can reduce Na+

The l‐kynurenine–probenecid combination reduces neuropathic pain in rats

l-Kynurenine has antinociceptive effects in acute and inflammatory pain. This study determined the effect of l-kynurenine and its metabolite (kynurenic acid) on rats subjected to neuropathic pain. L5/L6 spinal nerve ligation induced tactile allodynia as measured with von Frey filaments using the up-down method. High-performance liquid chromatography and Western blot analysis determined kynurenic acid levels and expression of kynurenine amino transferase II (KAT II), respectively. l-Kynurenine (50-200 mg/kg, i.p.) or probenecid (100 mg/kg, i.p.) did not affect allodynia in neuropathic rats. In contrast, l-kynurenine (50-200 mg/kg, i.p.) in combination with probenecid (100 mg/kg, i.p.), an inhibitor of organic anion transport, reversed allodynia. Furthermore, intrathecal kynurenic acid (1-30 μg) reversed allodynia. Probenecid (100 mg/kg, i.p.) supplementation enhanced the maximal antiallodynic effect of intrathecal kynurenic acid (10 μg). Only the combined administration of l-kynurenine (200 mg/kg)/probenecid (100 mg/kg) increased the kynurenic acid concentration in cerebrospinal fluid. KAT II is expressed in dorsal root ganglia and dorsal spinal cord. KAT II expression was unchanged by the spinal nerve ligation or l-kynurenine/probenecid combination. The kynurenine/probenecid combination did not affect motor activity. l-Kynurenine produces its antiallodynic effect in the central nervous system through kynurenic acid. This effect may result from blockade of N-methyl-d-aspartate receptors. KAT II is expressed in dorsal root ganglion and dorsal spinal cord. Combined l-kynurenine and probenecid therapy has the potential to reduce neuropathic pain in humans.

Uric acid-induced phenotypic transition of renal tubular cells as a novel mechanism of chronic kidney disease

Recent experimental and clinical studies suggest a causal role of uric acid in the development of chronic kidney disease. Most studies have focused on uric acid-induced endothelial dysfunction, oxidative stress, and inflammation in the kidney. The direct effects of uric acid on tubular cells have not been studied in detail, and whether uric acid can mediate phenotypic transition of renal tubular cells such as epithelial-to-mesenchymal transition (EMT) is not known. We therefore investigated whether uric acid could alter E-cadherin expression and EMT in the kidney of hyperuricemic rats and in cultured renal tubular cells (NRK cells). Experimental hyperuricemia was associated with evidence of EMT before the development of significant tubulointerstitial fibrosis at 4 wk, as shown by decreased E-cadherin expression and an increased α-smooth muscle actin (α-SMA). Allopurinol significantly inhibited uric acid-induced changes in E-cadherin and α-SMA with an amelioration of renal fibrosis at 6 wk. In cultured NRK cells, uric acid induced EMT, which was blocked by the organic anion transport inhibitor probenecid. Uric acid increased expression of transcriptional factors associated with decreased synthesis of E-cadherin (Snail and Slug). Uric acid also increased the degradation of E-cadherin via ubiquitination, which is of importance since downregulation of E-cadherin is considered to be a triggering mechanism for EMT. In conclusion, uric acid induces EMT of renal tubular cells decreasing E-cadherin synthesis via an activation of Snail and Slug as well as increasing the degradation of E-cadherin.

Calcium Mobilization Assay to Measure the Activity of Gq-coupled Receptors

Calcium mobilization assay is a cell-based second messenger assay to measure the calcium flux associated with Gq-protein coupled receptor activation or inhibition. The method utilizes a calcium sensitive fluorescent dye that is taken up into the cytoplasm of most cells. In some cell lines in which organic-anion transporters are particularly active (e.g. CHO, HeLa), addition of probenecid, an inhibitor of anion transport, is required for retention of this dye in the cells. The dye binds the calcium released from intracellular store and its fluorescence intensity increases. The change in the fluorescence intensity is directly correlated to the amount of intracellular calcium that is released into cytoplasm in response to ligand activation of the receptor of interest. This protocol can be applied to most mammalian cell lines expressing both endogenous and transiently/stably transfected receptors. The method is sensitive enough to be used for low-expressing systems or high throughput screening of target of interest. Note: The method does not differentiate the Ca2+ mobilization induced by Gqα from the Ca2+ mobilization induced by Gβγ.

Sulphate and Chloride-Dependent Potassium Transport in Human Erythrocytes are Affected by Crude Venom from Nematocysts of the Jellyfish Pelagia noctiluca

Background: It has been reported that biologically active compounds extracted from Cnidaria venom may induce damage by oxidative stress. Erythrocytes are constantly exposed to oxidative stresses, which can contribute to sulphydril (SH-) group oxidation and cell membrane deformability accompanied with activation of K-Cl co-transport and inhibition of anion transport. In this regard, Band 3 protein is responsible for mediating the electroneutral exchange of chloride (Cl^-) for bicarbonate (HCO₃^-), particularly in erythrocytes, where it is the most abundant membrane protein. The aim of this study was to elucidate the effect of crude venom extracted from Pelagia noctiluca nematocysts on Band 3 -mediated anion transport in human erythrocytes. Methods: Erythrocytes were tested for SO₄^2- uptake, K⁺ efflux, glutathione (GSH) levels and concentration of SH- groups. Results: The rate constant of SO₄^2- uptake decreased progressively to 58% of control with increasing venom doses, and showed a 28% decrease after 2 mM NEM treatment. These effects can be explained by oxidative stress, which was reflected by decreased GSH levels in venom-treated erythrocytes. Hence, the decreased efficiency of anion transport may be due to changes in Band 3 structure caused by SH-group oxidation and reduced GSH concentration. In addition, an increased Cl^--dependent K⁺ efflux was observed in venom-treated erythrocytes. Conclusion: Our results suggest that crude venom from Pelagia noctiluca alters cell membrane transport in human erythrocytes.

Targeting Organic Anion Transporter 3 with Probenecid as a Novel Anti-Influenza A Virus Strategy

Influenza A virus infection is a major global health concern causing significant mortality, morbidity, and economic loss. Antiviral chemotherapeutics that target influenza A virus are available; however, rapid emergence of drug-resistant strains has been reported. Consequently, there is a burgeoning need to identify novel anti-influenza A drugs, particularly those that target host gene products required for virus replication, to reduce the likelihood of drug resistance. In this study, a small interfering RNA (siRNA) screen was performed to identify host druggable gene targets for anti-influenza A virus therapy. The host organic anion transporter-3 gene (OAT3), a member of the SLC22 family of transporters, was validated as being required to support influenza A virus replication. Probenecid, a prototypical uricosuric agent and chemical inhibitor of organic anion transporters known to target OAT3, was shown to be effective in limiting influenza A virus infection in vitro (50% inhibitory concentration [IC(50)] of 5.0 × 10(-5) to 5.0 × 10(-4) μM; P < 0.005) and in vivo (P < 0.05). Probenecid is widely used for treatment of gout and related hyperuricemic disorders, has been extensively studied for pharmacokinetics and safety, and represents an excellent candidate for drug repositioning as a novel anti-influenza A chemotherapeutic.

Potent Inhibitors of Human Organic Anion Transporters 1 and 3 from Clinical Drug Libraries: Discovery and Molecular Characterization

Transporter-mediated drug-drug interactions in the kidney dramatically influence the pharmacokinetics and other clinical effects of drugs. Human organic anion transporters 1 (hOAT1) and 3 (hOAT3) are the major transporters in the basolateral membrane of kidney proximal tubules, mediating the rate-limiting step in the elimination of a broad spectrum of drugs. In the present study, we screened two clinical drug libraries against hOAT1 and hOAT3. Of the 727 compounds screened, 92 compounds inhibited hOAT1 and 262 compounds inhibited hOAT3. When prioritized based on the peak unbound plasma concentrations of these compounds, three inhibitors for hOAT1 and seven inhibitors for hOAT3 were subsequently identified with high inhibitory potency (>95%). Computational analyses revealed that inhibitors and noninhibitors can be differentiated from each other on the basis of several physicochemical features, including number of hydrogen-bond donors, number of rotatable bonds, and topological polar surface area (TPSA) for hOAT1; and molecular weight, number of hydrogen-bond donors and acceptors, TPSA, partition coefficient (log P(7.4)), and polarizability for hOAT3. Pharmacophore modeling identified two common structural features associated with inhibitors for hOAT1 and hOAT3, viz., an anionic hydrogen-bond acceptor atom, and an aromatic center separated by ∼5.7 Å. Such model provides mechanistic insights for predicting new OAT inhibitors.

Characterization of the carrier-mediated transport of ketoprofen, a nonsteroidal anti-inflammatory drug, in rabbit corneal epithelium cells

Using rabbit corneal epithelial cells (RCECs), the transport of a nonsteroidal anti-inflammatory drug (NSAID) [(3)H]ketoprofen across the cornea was investigated with the aim of revealing the mechanism of uptake. [(3)H]Ketoprofen transport was evaluated by measuring the permeability across the RCECs layers. [(3)H]Ketoprofen uptake was time, temperature and pH dependent. Maximal uptake occurred from a solution with a pH of 5.25. Uptake was also reduced by metabolic inhibitors (sodium azide and dinitrophenol (DNP)) and proton-linked monocarboxylate transporter (MCT) inhibitors (carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and α-cyano-4-hydroxycinnamic acid (CHC)). [(3)H]Ketoprofen uptake was significantly inhibited by various monocarboxylates and other NSAIDs and by MCT and/or organic anion transporter (OAT) inhibitors probenecid and p-aminohippurate, but was unaffected by organic anion-transporting polypeptide (OATP) inhibitors bromosulfophthalein and taurocholate. The specific uptake of [(3)H]ketoprofen was saturable. Eadie-Hofstee plots indicated the involvement of high- and low-affinity components. The K(m) and V(max) values for the high- and low-affinity components of [(3)H]ketoprofen uptake were 0.56 and 24 mm, and 0.37 and 61 nmol/min/mg of protein, respectively. Benzoic acid, a substrate and inhibitor of MCTs, selectively inhibited low-affinity [(3)H]ketoprofen uptake. Conversely, indometacin inhibited high-affinity [(3)H]ketoprofen uptake. The results of this study suggest that the monocarboxylate transport system partly accounts for the low-affinity component of [(3)H]ketoprofen uptake, and that the carrier-mediated transport systems such as the OAT family, shared by NSAIDs account for the high-affinity component.

A uremic toxin, 3-carboxy-4-methyl-5-propyl-2-furanpropionate induces cell damage to proximal tubular cells via the generation of a radical intermediate

3-Carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF), a furan fatty acid uremic toxin (UT) and a substrate for organic ion transporters, contributes to the accumulation of CMPF in renal tubular cells. Although oxidative stress induced by UTs has been proposed as a mechanism of its toxicity in chronic kidney disease, little information is available regarding the redox property of CMPF and its relation to renal cell damage. The findings herein show that CMPF enhances the production of reactive oxygen species (ROS) in HK-2 cells in the presence of angiotensin II (A-II), an inducer of O2−. When iron is also present, CMPF and A-II induce the Fenton reaction, resulting in a further increase in ROS production. Such CMPF-induced oxidative stress increases TGF-β1secretion in HK-2 cells, and a positive correlation between CMPF-induced ROS production and the secretion of active TGF-β1 was observed. CMPF caused a reduction in cell viability which was negatively correlated with intracellular ROS production. These negative effects of CMPF in HK-2 cells were completely suppressed by probenecid, an inhibitor of organic anion transport. Interestingly, in vitro ROS assays indicate that CMPF directly interacts with superoxide anion radicals (O2−) and peroxy radicals (LOO) to produce CMPF radicals. The subsequent interaction of CMPF radicals with dissolved oxygen leads to the overproduction of O2−. Based on these findings, we conclude that CMPF, which accumulates in the renal cells, appears to play a prominent role as a pro-oxidant which subsequently leads to renal cellular damage via the overproduction of O2−.

Quantitative Structure–Activity Relationship Analysis of a Series of Human Renal Organic Anion Transporter Inhibitors

Organic anion transporters (OATs) have been proved to play important roles in the membrane transport of numerous potentially toxic xenobiotics, drugs, and endogenous metabolites. In general, OATs substrates can compete with one another for the transporter to mutually decrease renal secretion and thus delay the clearance and prolong the duration of action of each compound. Such interactions have the potential to bring about adverse outcomes for clinical cases. Therefore, it is very important to assess the molecular bioactivity to inhibit OATs during the development of new drugs and co-administration. In this work, the relationships between 45 chemicals and their corresponding hOAT1 and hOAT3 inhibitory activities were analyzed. The quantitative structure-activity relationship (QSAR) model was developed by genetic algorithm and multiple linear regression method. The predictive power of the proposed model was strictly evaluated, and the applicability domain was also defined. The proposed models were robust and satisfactory and could provide a feasible and effective tool for hOAT1 or hOAT3 inhibitor screening.

Multidrug resistant proteins mediate extracellular ATP release in Caco2 cell monolayers

Epithelial ATP release is an essential component of extracellular purinergic secretory regulation; ATP transporting channels, pannexin (Panx) hemichannels and multidrug resistant proteins (MDR/MRP) have been implicated. The robust intestinal ecto‐ATPase activity present in Caco‐2 cells impairs measurements of medium ATP. We thus transfected c‐Myc‐tagged GPI‐anchored plasma membrane luciferase (pmeLuc) to Caco2 cell monolayer to monitor ATP release at the cell surface. Confluent Caco‐2 cell monolayers were transfected with pmeLuc, confirmed by immunolocalization. Although transfection efficacy into Caco2 cell is ~ 10 %, much less than in HEK293 ~ 80 %, luminescence was measurable over the background (~ 3000 in Caco‐2, ~ 30,000 in HEK293). Luminescence was stable over 40 min, suggesting constitutive ATP release. Among the pharmacological reagents reported as inhibitors for ATP release, MK571, a specific MDR/MRP inhibitor dose‐dependently inhibited ATP release. Stilbene anion transport inhibitors or Panx inhibitors had no effect. Vesicles expressing P‐glycoprotein (Mdr1) and MRP4, but not MRP2 accumulated ATP inside. MDR/MRP mediates ATP release in intestinal epithelia, a novel mechanism for transmembrane ATP transport. Regulation of ATP release may have important implications for the physiological functioning of intestinal and other secretory epithelia. NIH‐NIDDK R01 DK54221 &amp; R21 DK081199

Contribution of the Extracellular cAMP-Adenosine Pathway to Dual Coupling of β2-Adrenoceptors to Gsand GiProteins in Mouse Skeletal Muscle

β(2)-Adrenoceptor (β(2)-AR) agonists increase skeletal muscle contractile force via activation of G(s) protein/adenylyl cyclases (AC) and increased generation of cAMP. Herein, we evaluated the possible dual coupling of β(2)-AR to G(s) and G(i) proteins and the influence of the β(2)-AR/G(s)-G(i)/cAMP signaling cascade on skeletal muscle contraction. Assuming that the increment of intracellular cAMP is followed by cAMP efflux and extracellular generation of adenosine, the contribution of the extracellular cAMP-adenosine pathway on the β(2)-AR inotropic response was also addressed. The effects of clenbuterol/fenoterol (β(2)-AR agonists), forskolin (AC activator), cAMP/8-bromo-cAMP, and adenosine were evaluated on isometric contractility of mouse diaphragm muscle induced by supramaximal direct electrical stimulation (0.1 Hz, 2 ms duration). Clenbuterol/fenoterol (10-1000 μM), 1 μM forskolin, and 20 μM rolipram induced transient positive inotropic effects that peaked 30 min after stimulation onset, declining to 10 to 20% of peak levels in 30 min. The late descending phase of the β(2)-AR agonist inotropic effect was mimicked by either cAMP or adenosine and abolished by preincubation of diaphragm with pertussis toxin (PTX) (G(i) signaling inhibitor) or the organic anion transporter inhibitor probenecid, indicating a delayed coupling of β(2)-AR to G(i) protein which depends on cAMP efflux. Remarkably, the PTX-sensitive β(2)-AR inotropic effect was inhibited by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and ecto-5'-phosphodiesterase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt, indicating that β(2)-AR coupling to G(i) is indirect and dependent on A(1) receptor activation. The involvement of the extracellular cAMP-adenosine pathway in β(2)-AR signaling would provide a negative feedback loop that may limit stimulatory G protein-coupled receptor positive inotropism and potential deleterious effects of excessive contractile response.

Rubidium fluxes in Xenopus laevis (Amphibia) erythrocytes induced by swelling and relationship with taurine pathway

Rubidium (86Rb+) fluxes were used to probe for K+ uptake in Xenopus erythrocytes in vitro following exposure to various ambient osmolalities. Rb+ uptake is composed of two major components, one which is ouabain-sensitive and linearly related to changes in medium osmolarity, and a smaller (1/10 of total) that was ouabain-insensitive. The latter component that was activated when medium osmolarity was diluted 40% to 166 mOsm/l increased 2.5 times. The ouabain-insensitive K+ uptake was reduced to one-third on replacement of Cl– by other anions and was not influenced by DIDS, an anion transport inhibitor. Cl– uptake that was probed from SO4 2– uptake was tripled on medium dilution and was almost completely inhibited by DIDS (4, 4 diisothiocyanatostilbene-2, 2-disulfonic acid). The β-adrenergic stimulating agonist, isoproterenol, had no effect on these processes. Parallel measurements of Rb+ and taurine uptake in relation to medium osmolarity show different kinetics. Parallel measurements of 86Rb+ and taurine fluxes were made on erythrocytes from Bufo viridis. Based on the dissimilarity of responses on inhibitors and the different kinetics, it is concluded that the two solutes that are engaged in cellular volume regulation are transported separately.

A role for the organic anion transporter OAT3 in renal creatinine secretion in mice

Tubular secretion of the organic cation, creatinine, limits its value as a marker of glomerular filtration rate (GFR) but the molecular determinants of this pathway are unclear. The organic anion transporters, OAT1 and OAT3, are expressed on the basolateral membrane of the proximal tubule and transport organic anions but also neutral compounds and cations. Here, we demonstrate specific uptake of creatinine into mouse mOat1- and mOat3-microinjected Xenopus laevis oocytes at a concentration of 10 μM (i.e., similar to physiological plasma levels), which was inhibited by both probenecid and cimetidine, prototypical competitive inhibitors of organic anion and cation transporters, respectively. Renal creatinine clearance was consistently greater than inulin clearance (as a measure of GFR) in wild-type (WT) mice but not in mice lacking OAT1 (Oat1-/-) and OAT3 (Oat3-/-). WT mice presented renal creatinine net secretion (0.23 ± 0.03 μg/min) which represented 45 ± 6% of total renal creatinine excretion. Mean values for renal creatinine net secretion and renal creatinine secretion fraction were not different from zero in Oat1-/- (-0.03 ± 0.10 μg/min; -3 ± 18%) and Oat3-/- (0.01 ± 0.06 μg/min; -6 ± 19%), with greater variability in Oat1-/-. Expression of OAT3 protein in the renal membranes of Oat1-/- mice was reduced to ∼6% of WT levels, and that of OAT1 in Oat3-/- mice to ∼60%, possibly as a consequence of the genes for Oat1 and Oat3 having adjacent chromosomal locations. Plasma creatinine concentrations of Oat3-/- were elevated in clearance studies under anesthesia but not following brief isoflurane anesthesia, indicating that the former condition enhanced the quantitative contribution of OAT3 for renal creatinine secretion. The results are consistent with a contribution of OAT3 and possibly OAT1 to renal creatinine secretion in mice.

Analysis of the Mobilities of Band 3 Populations Associated with Ankyrin Protein and Junctional Complexes in Intact Murine Erythrocytes

Current models of the erythrocyte membrane depict three populations of band 3: (i) a population tethered to spectrin via ankyrin, (ii) a fraction attached to the spectrin-actin junctional complex via adducin, and (iii) a freely diffusing population. Because many studies of band 3 diffusion also distinguish three populations of the polypeptide, it has been speculated that the three populations envisioned in membrane models correspond to the three fractions observed in diffusion analyses. To test this hypothesis, we characterized band 3 diffusion by single-particle tracking in wild-type and ankyrin- and adducin-deficient erythrocytes. We report that ∼40% of total band 3 in wild-type murine erythrocytes is attached to ankyrin, whereas ∼33% is immobilized by adducin, and ∼27% is not attached to any cytoskeletal anchor. More detailed analyses reveal that mobilities of individual ankyrin- and adducin-tethered band 3 molecules are heterogeneous, varying by nearly 2 orders of magnitude and that there is considerable overlap in diffusion coefficients for adducin and ankyrin-tethered populations. Taken together, the data suggest that although the ankyrin- and adducin-immobilized band 3 can be monitored separately, significant heterogeneity still exists within each population, suggesting that structural and compositional properties likely vary considerably within each band 3 complex.