Rat Kidney Membranes Research Articles

Control of ENaC ubiquitination.

Ubiquitination influences the expression of the epithelial Na+ channel (ENaC). We assessed the mechanisms of selective ubiquitination of the mature, cleaved form of γENaC in both native rodent kidneys and Fisher rat thyroid (FRT) cells expressing the channel heterologously. In both models, singly cleaved and fully cleaved γENaCs were strongly ubiquitinated, implying that the second cleavage releasing an inhibitory peptide was not essential for the process. To see whether location of the protein in or near the apical membrane rather than cleavage per se influences ubiquitination, we studied mutants of γENaC in which cleavage sites are abolished. These subunits were ubiquitinated only when coexpressed with α- and βENaC, facilitating trafficking through the Golgi apparatus. To test whether reaching the apical surface is necessary we performed in situ surface biotinylation and measured ENaC ubiquitination in the apical membrane of rat kidney. Ubiquitination of cleaved γENaC was similar in whole kidney and surface fractions, implying that both apical and subapical channels could be modified. In FRT cells, inhibiting clathrin-mediated endocytosis with Dyngo-4a increased both total and ubiquitinated γENaC at the cell surface. Finally, we tested the idea that increased intracellular Na+ could stimulate ubiquitination. Administration of amiloride to block Na+ entry through the channels did not affect ubiquitination of γENaC in either FRT cells or the rat kidney. However, presumed large increases in cellular Na+ produced by monensin in FRT cells or acute Na+ repletion in rats increased ubiquitination and decreased overall ENaC expression.NEW & NOTEWORTHY We have explored the mechanisms underlying the ubiquitination of the γ subunit of epithelial Na+ channel (ENaC), a process believed to control channel internalization and degradation. We previously reported that the mature, cleaved form of the subunit is selectively ubiquitinated. Here we show that this specificity arises not from the cleavage state of the protein but from its location in the cell. We also show that under some conditions, increased intracellular Na+ can stimulate ENaC ubiquitination.

The Anxiolytic Etifoxine Binds to TSPO Ro5-4864 Binding Site with Long Residence Time Showing a High Neurosteroidogenic Activity

The low binding affinity of the approved anxiolytic drug etifoxine (Stresam) at the steroidogenic 18 kDa translocator protein (TSPO) has questioned the specific contribution of this protein in mediating the etifoxine neurosteroidogenic efficacy. Residence time (RT) at the binding site of the classical TSPO ligand PK11195 is emerging as a relevant neurosteroidogenic efficacy measure rather than the binding affinity. Here etifoxine was evaluated for (i) the in vitro neurosteroidogenic activity in comparison to poorly neurosteroidogenic reference TSPO ligands (PK11195 and Ro5-4864) and (ii) the affinity and RT at [3H]PK11195 and [3H]Ro5-4864 binding sites in rat kidney membranes. Etifoxine shows (i) high neurosteroidogenic efficacy and (ii) low affinity/short RT at the [3H]PK11195 site and low affinity/long RT at the [3H]Ro5-4864 site, at which etifoxine competitively bound. These findings suggest that the long RT of etifoxine at the Ro5-4864 binding site could account for its high neurosteroidogenic efficacy.

Fish Oil Supplementation Ameliorates Long Term Cisplatin Treatment Induced Toxicity and Oxidative Damage in Rat Kidney

Background: Cisplatin (CP) is an effective chemotherapeutic agent widely used against various malignancies. However, CP exerts several side effects through oxidative stress by generating reactive oxygen species (ROS). The major side effect of CP chemotherapy is dose dependent nephrotoxicity that limits its clinical usage. We have previously shown that fish oil (FO) supplementation ameliorated gentamicin induced nephrotoxicity as evident by increase in the activities of brush border membrane (BBM) and free radical scavenging enzymes in rat kidney. In the present study we aimed to investigate whether fish oil (FO) can prevent the long term CP-induced nephrotoxic effects. Methods: Rats were divided into four groups viz. control, CP, CPF and FO alone group. Rats in the groups CPF and FO were fed experimental diet prior to and simultaneously with or without CP treatment (3 mg/kg bwt i.p) respectively. Results: FO supplementation to CP treated significantly mitigated the CP induced increase in serum creatinine (Scr) and blood urea nitrogen (BUN) and decrease in the activities of brush border membrane (BBM) enzymes in renal cortical and medullary homogenates as well as in isolated BBM vesicles (BBMV). FO consumption also reduced CP induced alterations in the activities of carbohydrate metabolism enzymes and in the enzymatic and non-enzymatic antioxidant parameters. Conclusion: These results suggest that FO have potential to abrogate CP induced nephrotoxicity and can be a useful adjuvant in long term CP chemotherapy without any side effects. Moreover, the inherent anti-cancer properties of FO with CP treatment may exhibits synergistic beneficial effects.

Identification of the Sigma-2 Receptor: Distinct from the Progesterone Receptor Membrane Component 1 (PGRMC1).

σRs are unique intracellular chaperone proteins [8] initially thought to be opioid receptor subtypes [9]. They have been classified into two subtypes based on specific radioligand binding assays using [3H](+)pentazocine for σ1Rs and [3H]1,3-di-o-tolylguanidine ([3H]DTG, in the presence of dextrallorphan to mask the σ1R) for σ2Rs in rat liver and kidney membranes [10]. Currently, the more selective σ1R ligand (+)-pentazocine has replaced dextrallorphan to mask the σ1R [7,11-14]. The σ1R has already been cloned as a 25-29 kDa chaperone protein composed of 223 amino acids [4,8,15]. It is widely distributed throughout the body [16-20]. Upon binding with agonists or under cellular stress, σ1Rs translocate from their primary endoplasmic reticulum (ER) location to different subcellular compartments where they can regulate ion channels and G-protein-coupled-receptor (GPCR) signaling [8,21-24]. In vivo functional studies on σ1Rs suggest that they play a substantial role in various cellular functions. Drugs acting at this receptor have been studied for their potential therapeutic effects in cancer, human immunodeficiency virus (HIV) infection, various psychiatric disorders, and substance abuse [1,25].

Platelet-activating factor (PAF)-dependent transacetylase and its relationship with PAF acetylhydrolases.

Platelet-activating factor (PAF)-dependent transacetylase (TA) is an enzyme that transfers an acetyl group from PAF to acceptor lipids such as lysophospholipids and sphingosine. This enzyme is distributed in membrane and cytosol of the cells. We previously revealed that TA purified from rat kidney membrane showed an amino acid sequence similarity to that of bovine PAF-acetylhydrolase (AH) (II). In the present study, we purified TA from the rat kidney cytosol and analyzed its amino acid sequence. The amino acid sequence of the cytosolic TA is similar to that of bovine PAF-AH (II) and membrane TA. To clarify the relationship between TA and PAF-AH (II), we isolated cDNA of rat PAF-AH (II). The predicted amino acid sequence of rat PAF-AH (II) from isolated cDNA included all the sequences found in TAs purified from the membrane and cytosolic TAs. In addition, monoclonal antibody to recombinant PAF-AH (II) cross-reacted with both cytosolic and membrane TAs. Consistent with sequence identity, recombinant PAF-AH (II) showed TA activity, whereas recombinant PAF-AH Ib, which is a different subtype of intracellular PAF-AHs, did not possess TA activity. Analysis of a series of site-directed mutant PAF-AH (II) proteins showed that TA activity was decreased, whereas PAF-AH activity was not affected in C120S and G2A mutant proteins. Thus, Cys(120) and Gly(2) are implicated in the catalysis of TA reaction in this enzyme. Furthermore, the transfer of acetate from PAF to endogenous acceptor lipids was significantly increased in a time-dependent manner in CHO-K1 cells transfected with PAF-AH (II) gene. These results demonstrate that PAF-AH (II) can function, as a TA in intact cells, and PAF-AH (II) and TA are the same enzyme.

The influence of iron ions on ATP-hydrolases activity of cell membranes of rat colon smooth muscle and kidney

To elucidate the specific features of the ATP-hydrolases structural resistance in the membrane under the action of the prooxidants: Fe2' and hydrogen peroxide, and N-ethylmaleimide (NEM) the colonic smooth muscle (CSM) Na+, K(+)-ATPase activity was compared with activities of the corresponding Mg(2+)-ATP-hydrolase and ATPases from kidney medullar layer of rats. The inhibition study of the CSM Na+, K(+)-ATPase by divalent iron shows the decrease of the activity by 30% at 0.1 μM FeSO4 and in the range of 0.1-10 μM--to 45% of residual activity. When comparing with kidney enzyme (rep- resents exclusively α1-isozyme) the CSM Na+, K(+)-ATPase sensitivity to Fe2+ is reliably higher at its submicromolar concentration. CSM Mg2+-ATPase is much more resistant to iron ions effect, than kidney one. However for two tissues Mg2(+)-ATPase activity is always more resistant as compared with corresponding Na+, K(+)-ATPase activity. Against 1 mM EGTA Na+,K(+)-ATPase and Mg2(+)-ATPase activities of GMOK and kidneys are equally insensitive to effect of hydrogen peroxide in concentration up to 1 mM. But in the presence of 20 μM FeSO4 in the concentration range of 1 nM-1 mM of H2O2 the Na+, K(+)-ATPase is inhibited to greater extent, than Mg2+-ATPase activity. NEM sensitivity of the two ATP-hydrolase systems corresponds to prooxidant sensitivity that indicates the distinct importance of SH-groups for their functioning. It is concluded that Na+, K+-ATPase can serve as a marker of membrane sensitivity to oxidation, Mg(2+)-ATPase is resistant to oxidation and can be considered as criterion of the oxidation resistance when comparing membrane enzyme complexes, es- pecially in GMOK.

Binding of glycated ovocystatin to rat renal brush border membranes

Glycated proteins are considered as one of the factors involved in the pathogenesis of diabetic complications, including nephropathy. These proteins are formed endogenously under conditions of hyperglycemia, as well as being provided with food containing sugars, which was subjected to high temperature. Examples are egg products. One of the proteins found in eggs in a relatively high concentration is chicken cystatin (ovocystatin). It is now believed that some proteins can passage the intestinal epithelium by transcytosis directly into the bloodstream. Thus, glycated protein present in food can be an additional source of glycotoxins. The aim of this study was to compare the affinity of native and glycated cystatin to the brush border membranes of rat kidney. Kinetic analysis was performed with surface plasmon resonance technique using sensor chip L1. Dissociation constants for native and glycated cystatin (Kd ) were 2.76 μmol/L and 3.82 μmol/L, respectively. The results of our study indicate that glycation only slightly affects binding of cystatin to brush border membranes. This suggests that glycated cystatin and other glycated proteins may also be efficiently taken up in the kidney proximal tubule. The observation may be important for understanding the mechanisms involved in the development of diabetic nephropathy.

Dietary supplementation of flaxseed oil ameliorates the effect of cisplatin on rat kidney

Cisplatin (CP) is an effective chemotherapeutic agent widely used in the treatment of many malignant tumours. However, its therapeutic utility is limited by dose dependent nephrotoxicity. Several agents/strategies have attempted to prevent CP nephrotoxicity but were not found suitable for clinical practice. Dietary flaxseed oil (FXO), a rich source of omega-3 fatty acids has been shown to prevent/reduce the progression of certain types of cardiovascular and renal disorders. The protective effect of FXO on CP induced nephrotoxic and other deleterious effects was investigated. Rats were prefed experimental diets for 10days and then received a single dose of CP (6mg/kg body weight) intraperitoneally while still on diet. Serum/urine parameters, enzymes of brush border membrane (BBM), oxidative stress and carbohydrate metabolism in rat kidney were analyzed. CP nephrotoxicity was recorded by increased serum creatinine and blood urea nitrogen. CP decreased the activities of metabolic enzymes, antioxidant defense system and BBM enzymes. In contrast, FXO alone increased enzyme activities of carbohydrate metabolism and brush border membrane. FXO feeding to CP treated rats markedly enhanced resistance to CP-elicited deleterious effects. Dietary FXO supplementation ameliorated CP induced specific metabolic alterations and oxidative damage due to its intrinsic biochemical antioxidant properties.

Studies on the protective effect of dietary fish oil on cisplatin induced nephrotoxicity in rats

Cisplatin (CP) is a major antineoplastic drug for the treatment of solid tumors, however, dose dependent nephrotoxicity remains the major concern for its long term use. Several agents/strategies were attempted to prevent CP nephrotoxicity but were not found suitable for clinical practice. Dietary fish oil (FO) enriched in ω-3 fatty acids has been shown to prevent/reduce the progression of certain types of cancers, cardiovascular and renal disorders. The present study was undertaken to see whether FO can prevent CP-induced nephrotoxic and other deleterious effects. Rats were prefed experimental diets for 10days and then received a single dose of CP (6mg/kg body weight) intraperitoneally while still on diet. Serum/urine parameters, enzymes of carbohydrate metabolism, brush border membrane (BBM) and oxidative stress in rat kidney were analyzed. CP nephrotoxicity was recorded by increased serum creatinine and blood urea nitrogen. CP decreased the activities of metabolic enzymes, antioxidant defense system and BBM enzymes. In contrast, FO alone increased enzyme activities of carbohydrate metabolism and brush border membrane (BBM). FO feeding to CP treated rats markedly enhanced resistance to CP-elicited deleterious effects. Dietary FO supplementation ameliorated CP induced specific metabolic alterations and oxidative damage due to its intrinsic biochemical antioxidant properties.

Calcium-sensing Receptor Decreases Cell Surface Expression of the Inwardly Rectifying K+ Channel Kir4.1

The Ca(2+)-sensing receptor (CaR) regulates salt and water transport in the kidney as demonstrated by the association of gain of function CaR mutations with a Bartter syndrome-like, salt-wasting phenotype, but the precise mechanism for this effect is not fully established. We found previously that the CaR interacts with and inactivates an inwardly rectifying K(+) channel, Kir4.1, which is expressed in the distal nephron that contributes to the basolateral K(+) conductance, and in which loss of function mutations are associated with a complex phenotype that includes renal salt wasting. We now find that CaR inactivates Kir4.1 by reducing its cell surface expression. Mutant CaRs reduced Kir4.1 cell surface expression and current density in HEK-293 cells in proportion to their signaling activity. Mutant, activated Gα(q) reduced cell surface expression and current density of Kir4.1, and these effects were blocked by RGS4, a protein that blocks signaling via Gα(i) and Gα(q). Other α subunits had insignificant effects. Knockdown of caveolin-1 blocked the effect of Gα(q) on Kir4.1, whereas knockdown of the clathrin heavy chain had no effect. CaR had no comparable effect on the renal outer medullary K(+) channel, an apical membrane distal nephron K(+) channel that is internalized by clathrin-coated vesicles. Co-immunoprecipitation studies showed that the CaR and Kir4.1 physically associate with caveolin-1 in HEK cells and in kidney extracts. Thus, the CaR decreases cell surface expression of Kir4.1 channels via a mechanism that involves Gα(q) and caveolin. These results provide a novel molecular basis for the inhibition of renal NaCl transport by the CaR.

A combination method of chemical with enzyme reactions for identification of membrane proteins

A simple method for effective analysis of various proteins has been developed, including membrane proteins, with LC–MS/MS, using CNBr and acetic acid cleavage in one reaction for the digestion of both the M/ and /D/ positions within the target proteins. This dual chemical reaction has been compared with traditional CNBr or an acid cleavage method using a rat kidney membrane fraction and it showed an advantage of the dual reaction with respect to a high number of peptides detected and a high protein recovery. Furthermore, when this dual chemical reaction was combined with trypsin digestion, the number of proteins surprisingly increased approximately 3.0 times more than in the cases with the trypsin digestion only. It was also 1.9 times more than in cases dealing with Tube-Gel trypsin digestion, which is one of the most efficient digestion methods. In addition, it was shown that this dual chemical reaction could be applied to an in-gel digestion. Using the combination of the chemical and enzyme reaction, 172 proteins including 95 membrane proteins were identified. This indicated that this method is one of the efficient systems in single MS/MS analysis. In particular, many membrane proteins identified in this study were detected by a new combination, but not by a traditional trypsin digestion method.

L-NAME-treatment alters ectonucleotidase activities in kidney membranes of rats

Aims To investigate the effect of Nω-Nitro- l-arginine methyl ester ( l-NAME) treatment, known to induce a sustained elevation of blood pressure, on ectonucleotidase activities in kidney membranes of rats. Main methods l-NAME (30 mg/kg/day) was administered to Wistar rats for 14 days in the drinking water. Enzyme activities were determined colorimetrically and their gene expression patterns were analyzed by semi-quantitative RT-PCR. The metabolism of ATP and the accumulation of adenosine were evaluated by HPLC in kidney membranes from control and hypertensive rats. PKC phosphorylation state was investigated by Western blot. Key findings We observed an increase in systolic blood pressure from 115 ± 12 mmHg (control group) to 152 ± 18 mmHg ( l-NAME-treated group). Furthermore, the hydrolysis of ATP, ADP, AMP, and p-Nph-5′TMP was also increased (17%, 35%, 27%, 20%, respectively) as was the gene expression of NTPDase2, NTPDase3 and NPP3 in kidneys of hypertensive animals. Phospho-PKC was increased in hypertensive rats. Significance The general increase in ATP hydrolysis and in ecto-5′-nucleotidase activity suggests a rise in renal adenosine levels and in renal autoregulatory responses in order to protect the kidney against the threat presented by hypertension.

Human B-type natriuretic peptide is not degraded by meprin A

B-type natriuretic peptide (BNP) combats cardiac stress by reducing blood pressure and ventricular fibrosis. Human BNP is inactivated by unknown cell surface proteases. N-terminal cleavage of mouse BNP by the renal protease meprin A was reported to increase inactivating degradation by a second protease named neprilysin. Since the sequence surrounding the meprin A cleavage site in BNP differs between species, we tested whether meprin A degrades human BNP. Using a recently developed proteolytic bioassay, the ability of various protease inhibitors to block the inactivation of BNP was measured. In rat kidney membranes, inhibitors of meprin A or neprilysin partially or completely blocked inactivation of rat BNP1–32 when added individually or in combination, respectively. In contrast, neither inhibitor alone or in combination prevented the inactivation of human BNP1–32 by human kidney membranes. Leupeptin, a serine protease inhibitor, totally blocked inactivation of human BNP by human membranes, substantially blocked the inactivation of rat BNP1–32 by human membranes, but had no effect on the inactivation of rat BNP1–32 by rat kidney membranes. Purified neprilysin reduced the bioactivity of rat BNP1–32 and human BNP. Digestion with both meprin and neprilysis caused the greatest reduction in rat BNP1–32 but had no effect on the bioactivity of human BNP1–32. We conclude that meprin A does not degrade BNP in humans and should not be considered a pharmacologic target of the natriuretic peptide system.

RhCG is the major putative ammonia transporter expressed in the human kidney, and RhBG is not expressed at detectable levels

Rhesus glycoprotein homologs RhAG, RhBG, and RhCG comprise a recently identified branch of the Mep/Amt ammonia transporter family. Animal studies have shown that RhBG and RhCG are present in the kidney distal tubules. Studies in mouse and rat tissue suggest a basolateral localization for RhBG in cells of the distal tubules including the α-intercalated cells (α-IC), but no localization of RhBG has been reported in human tissue. To date RhCG localization has been described as exclusively apical plasma membrane in mouse and rat kidney, or apical and basolateral in humans, and some mouse and rat tissue studies. We raised novel antibodies to RhBG and RhCG to examine their localization in the human kidney. Madin-Darby canine kidney (MDCKI) cell lines stably expressing human green fluorescent protein-tagged RhBG or RhCG and human tissue lysates were used to demonstrate the specificity of these antibodies for detecting RhBG and RhCG. Using immunoperoxidase staining and antigen liberation techniques, both apical and basolateral RhCG localization was observed in the majority of the cells of the distal convoluted tubule and IC of the connecting tubule and collecting duct. Confocal microscopic imaging of normal human kidney cryosections showed that RhCG staining was predominantly localized to the apical membrane in these cells with some basolateral and intracellular staining evident. A proportion of RhCG staining labeled kAE1-positive cells, confirming that RhCG is localized to the α-IC cells. Surprisingly, no RhBG protein was detectable in the human kidney by Western blot analysis of tissue lysates, or by immunohistochemistry or confocal microscopy of tissue sections. The same antibodies, however, could detect RhBG in rat tissue. We conclude that under normal conditions, RhCG is the major putative ammonia transporter expressed in the human kidney and RhBG is not expressed at detectable levels.

Studies on the protective effect of dietary fish oil on gentamicin-induced nephrotoxicity and oxidative damage in rat kidney

Gentamicin (GM)-induced nephrotoxicity limits its long-term clinical use. Several agents/strategies were attempted to prevent GM nephrotoxicity but were not found suitable for clinical practice. Dietary fish oil (FO) retard the progression of certain types of cancers, cardiovascular and renal disorders. We aimed to evaluate protective effect of FO on GM-induced renal proximal tubular damage. The rats were pre-fed experimental diets for 10 days and then received GM (80 mg/kg body weight/day) treatment for 10 days while still on diet. Serum/urine parameters, enzymes of carbohydrate metabolism, brush border membrane (BBM), oxidative stress and phosphate transport in rat kidney were analyzed. GM nephrotoxicity was recorded by increased serum creatinine and blood urea nitrogen. GM increased the activities of lactate and glucose-6-phosphate dehydrogenases whereas decreased malate, isocitrate dehydrogenases; glucose-6 and fructose-1,6-bisphosphatases; superoxide dismutase, catalase, glutathione peroxidase and BBM enzymes. In contrast, FO alone increased enzyme activities of carbohydrate metabolism, BBM and oxidative stress. FO feeding to GM treated rats markedly enhanced resistance to GM elicited deleterious effects and prevented GM-induced decrease in 32Pi uptake across BBM. Dietary FO supplementation ameliorated GM-induced specific metabolic alterations and oxidative damage due to its intrinsic biochemical/antioxidant properties.

DPA-714, a New Translocator Protein–Specific Ligand: Synthesis, Radiofluorination, and Pharmacologic Characterization

The translocator protein (18 kDa) (TSPO), formerly known as the peripheral benzodiazepine receptor, is dramatically upregulated under pathologic conditions. Activated microglia are the main cell type expressing the TSPO at sites of central nervous system pathology. Radioligands for the TSPO can therefore measure active disease in the brain. This article details the synthesis, radiofluorination, and pharmacologic evaluation of a new TSPO-specific pyrazolopyrimidine, DPA-714. The affinity of DPA-714 for the TSPO was measured in rat kidney membranes with (3)H-PK11195. The in vitro functional activity of DPA-714 was measured in a steroidogenic assay in which the ability of DPA-714 to increase pregnenolone synthesis was measured with rat C6 glioma cells. The radiofluorination of DPA-714 was achieved by nucleophilic (18)F-fluoride displacement of the tosylate precursor. (18)F-DPA-714 was assessed in rats harboring unilateral quinolinic acid (QA) lesions. In addition, pretreatment experiments were performed with PK11195 (5 mg/kg), DPA-714 (1 mg/kg), and DPA-713 (1 mg/kg). The in vivo binding and biodistribution of (18)F-DPA-714 were determined in a baboon with PET. Experiments involving presaturation with PK11195 (1.5 mg/kg) and displacement with DPA-714 (1 mg/kg) were conducted to evaluate the specificity of radioligand binding. In vitro binding studies revealed that DPA-714 displayed a high affinity for the TSPO (dissociation constant, 7.0 nM). DPA-714 stimulated pregnenolone synthesis at levels 80% above the baseline. (18)F-DPA-714 was prepared at a 16% radiochemical yield and a specific activity of 270 GBq/mumol. In rats harboring unilateral QA lesions, an 8-fold-higher level of uptake of (18)F-DPA-714 was observed in the ipsilateral striatum than in the contralateral striatum. Uptake in the ipsilateral striatum was shown to be selective because it was inhibited to the level in the contralateral striatum in the presence of PK11195, nonlabeled DPA-714, or DPA-713. PET studies demonstrated rapid penetration and good retention of (18)F-DPA-714 in the baboon brain. Pretreatment with PK11195 effectively inhibited the uptake of (18)F-DPA-714 in the whole brain, indicating its selective binding to the TSPO. The injection of nonlabeled DPA-714 20 min after the injection of (18)F-DPA-714 resulted in radioligand washout, demonstrating the reversibility of (18)F-DPA-714 binding. (18)F-DPA-714 is a specific radioligand for the TSPO, displaying promising in vivo properties and thus warranting further investigation.

The Contribution of Organic Anion Transporters OAT1 and OAT3 to the Renal Uptake of Rosuvastatin

Rosuvastatin is a potent inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase and has been shown to be highly effective in reducing low-density lipoprotein cholesterol. Clinical trials have demonstrated that renal excretion and, in particular, tubular secretion, plays a role in rosuvastatin clearance. The aim of this study was to determine the involvement of the basolateral organic anion transporters, OAT1 and OAT3, in the renal uptake of rosuvastatin. Expression of human (h) OAT3 in Xenopus oocytes significantly increased the uptake of rosuvastatin above control levels (K(m) = 7.4 microM). In contrast hOAT1 did not mediate rosuvastatin uptake. Furthermore, hOAT3-mediated estrone-3-sulfate uptake could be inhibited, with a rank order of potency, by atorvastatin, rosuvastatin, simvastatin, and pravastatin, whereas hOAT1-mediated PAH uptake was only significantly inhibited by simvastatin. To estimate the contribution of hOAT3 to the overall renal uptake of rosuvastatin, a series of experiments were conducted using rat kidney slices. Rosuvastatin uptake in rat renal slices was abolished in the presence of the rat (r) Oat3-specific inhibitor benzylpenicillin, suggesting that rOat3 is responsible for the majority of rosuvastatin uptake across the basolateral membrane in rat kidney. From these findings, we can suggest that hOAT3 contributes to the renal uptake of rosuvastatin in humans.

Transport of γ-Hydroxybutyrate in Rat Kidney Membrane Vesicles: Role of Monocarboxylate Transporters

Intoxication with gamma-hydroxybutyrate (GHB) is associated with coma, seizure, and death; treatment of overdoses is symptomatic. Previous studies in our laboratory have demonstrated that L-lactate and pyruvate treatment can increase the renal clearance of GHB and increase its elimination in rats, suggesting that GHB may undergo renal reabsorption mediated by monocarboxylic acid transporters (MCTs). The goals of this study were to characterize the renal transport of GHB in rats and to determine the role of MCT in its renal transport. Brush-border membrane (BBM) and basolateral membrane (BLM) vesicles were isolated from rat kidney cortex, and the uptake of L-lactate and GHB was characterized. L-Lactate and GHB undergo both pH- and sodium-dependent transport in BBM vesicles and pH-dependent transport in BLM vesicles. A simple Michaelis-Menten equation best described the pH-dependent uptake of GHB in BBM (Km, 8.0 +/- 1.8 mM; Vmax, 838 +/- 45 pmol/mg/s) and in BLM vesicles (Km, 10.5 +/- 2.6 mM; Vmax, 806 +/- 253 pmol/mg/s). mRNA of MCT1 and MCT2 was determined in rat kidney cortex using reverse transcriptase-polymerase chain reaction; using Western blot, the protein expression of MCT1 was present mainly in BLM vesicles, with weak expression in BBM vesicles, whereas that of MCT2 was exclusively in BLM vesicles. Studies with rat MCT1 gene-transfected MDA-MB231 cells demonstrated that GHB was a substrate of MCT1. The data suggest that rat MCT1 may represent an important transporter for GHB in renal tubule cells. This investigation provides evidence for the importance of MCTs in the reabsorption of the monocarboxylic acids l-lactate and GHB in the kidney.

Investigation of Exenatide Elimination and Its In Vivo and In Vitro Degradation

Exenatide is a 39 amino acid incretin mimetic for the treatment of type 2 diabetes, with glucoregulatory activity similar to glucagon-like peptide-1 (GLP-1). Exenatide is a poor substrate for the major route of GLP-1 degradation by dipeptidyl peptidase-IV, and displays enhanced pharmacokinetics and in vivo potency in rats relative to GLP-1. The kidney appears to be the major route of exenatide elimination in the rat. We further investigated the putative sites of exenatide degradation and excretion, and identified primary degradants. Plasma exenatide concentrations were elevated and sustained in renal-ligated rats, when compared to sham-operated controls. By contrast, exenatide elimination and degradation was not affected in rat models of hepatic dysfunction. In vitro, four primary cleavage sites after amino acids (AA)-15, -21, -22 and -34 were identified when exenatide was degraded by mouse kidney membranes. The primary cleavage sites of exenatide degradation by rat kidney membranes were after AA-14, -15, -21, and -22. In rabbit, monkey, and human, the primary cleavage sites were after AA-21 and -22. Exenatide was almost completely degraded into peptide fragments <3 AA by the kidney membranes of the species tested. The rates of exenatide degradation by rabbit, monkey and human kidney membranes in vitro were at least 15-fold slower than mouse and rat membranes. Exenatide (1-14), (1-15), (1-22), and (23-39) were not active as either agonists or antagonists to exenatide in vitro. Exenatide (15-39) and (16-39) had moderate-to-weak antagonist activity compared with the known antagonist, exenatide (9-39). In conclusion, the kidney appears to be the primary route of elimination and degradation of exenatide.

Inhibition of Angiotensin Converting Enzyme Activity by Flavanol-Rich Foods

Angiotensin converting enzyme (ACE) activity was evaluated in the presence of flavanol-rich foods, i.e., wines, chocolates, and teas, and of purified flavonoids. All foods assayed inhibited ACE activity, red wines being more effective than white wine, and green tea more effective than black tea. The inhibition of ACE activity was associated with both phenolic and flavanol content in the foods. When isolated polyphenols were assayed, procyanidins (dimer and hexamer) and epigallocatechin significantly inhibited enzyme activity; similar concentrations of (+)-catechin, (-)-epicatechin, gallic acid, chlorogenic acid, caffeic acid, quercetin, kaempferol, and resveratrol were ineffective. When ACE activity was assayed in rat kidney membranes in the presence of chocolate extracts or purified procyanidins, it was observed that the inhibition depended on the chocolate content of flavanols and the number of flavanol units constituting the procyanidin. These experiments demonstrate that flavanols either isolated or present in foods could inhibit ACE activity. The occurrence of such inhibition in vivo needs to be determined, although is supported by the association between the consumption of flavanol-rich foods and reductions in blood pressure observed in several experimental models.