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G Burckhardt

Publications and source records attributed to G Burckhardt.

At least 19 recordsLinked to original sources
PubMed2000

Genomic structure and in vivo expression of the human organic anion transporter 1 (hOAT1) gene.

The human organic anion transporter 1 (hOAT1) plays a key role in the secretion of an array of potentially toxic organic anions including many clinically important drugs. Here we report on the genomic cloning of hOAT1. A human genomic library was used for screening of a PAC (P1 artificial chromosome) clone applying PCR techniques. Sequencing of several restriction subclones and of a PCR-generated clone revealed that the hOAT1 gene spans 8.2 kb and is composed of 10 exons divided by 9 introns. RT-PCR studies in a human kidney specimen led to the detection of two new splice variants, hOAT1-3 and hOAT1-4, showing a 132-bp in-frame deletion. Using fluorescence in situ hybridization (FISH) we mapped the hOAT1 gene as a single signal to chromosome 11q13.1-q13.2. Additionally, 600 bp of the 5' flanking region was analyzed, illustrating the probable transcription start site at nt -280, a NF-kappaB-site at nt -397 and several putative transcription factor binding sites.

Anion Transport Proteins↗
PubMed2000

Functional and molecular biological evidence of SGLT-1 in the ruminal epithelium of sheep.

Because of the effective catabolism of D-glucose to short-chain fatty acids by intraruminal microorganisms, the absorption of D-glucose from the rumen was thought to be of minor importance. However, clinical studies suggested that significant quantities of D-glucose are transported from the ruminal contents to the blood. We therefore tested the ruminal epithelium of sheep for the presence of Na(+)-glucose cotransporter 1 (SGLT-1) on both the functional and mRNA levels. In the absence of an electrochemical gradient, 3-O-methylglucose (3-OMG) was net absorbed across isolated ruminal epithelia mounted in Ussing chambers. The net transport of 3-OMG followed Michaelis-Menten kinetics and was sensitive to phlorizin or decreasing Na(+) concentrations. The mucosal addition of 10 mM D-glucose induced an immediate, phlorizin-sensitive increase in short-circuit current (I(sc)). I(sc) could also be increased by serosal addition of D-glucose or D-mannose, but electrogenic uptake of D-glucose or 3-OMG added on the mucosal side was still detectable after serosal stimulation of I(sc). RT-PCR using primers specific for the ovine intestinal SGLT-1 with subsequent TA cloning and sequencing revealed 100% identity between the cloned cDNA and mRNA fragment 187-621 of ovine intestinal SGLT-1. In conclusion, the ruminal epithelium has a high-affinity SGLT-1, which indicates that it maintains the capacity for D-glucose absorption.

3-O-Methylglucose↗
PubMed2000

Structure of renal organic anion and cation transporters.

Here we review the structural and functional properties of organic anion transporters (OAT1, OAT2, OAT3) and organic cation transporters (OCTN1, OCTN2, OCT1, OCT2, OCT3), some of which are involved in renal proximal tubular organic anion and cation secretion. These transporters share a predicted 12-transmembrane domain (TMD) structure with a large extracellular loop between TMD1 and TMD2, carrying potential N-glycosylation sites. Conserved amino acid motifs revealed a relationship to the sugar transporter family within the major facilitator superfamily. Following heterologous expression, most OATs transported the model anion p-aminohippurate (PAH). OAT1, but not OAT2, exhibited PAH-alpha-ketoglutarate exchange. OCT1-3 transported the model cations tetraethylammonium (TEA), N(1)-methylnicotinamide, and 1-methyl-4-phenylpyridinium. OCTNs exhibited transport of TEA and/or preferably the zwitterionic carnitine. Substrate substitution as well as cis-inhibition experiments demonstrated polyspecificity of the OATs, OCTs, and OCTN1. On the basis of comparison of the structurally closely related OATs and OCTs, it may be possible to delineate the binding sites for organic anions and cations in future experiments.

Amino Acid Sequence↗
PubMed1999

Expression cloning and characterization of a novel sodium-dicarboxylate cotransporter from winter flounder kidney.

A cDNA coding for a Na+-dicarboxylate cotransporter, fNaDC-3, from winter flounder (Pseudopleuronectes americanus) kidney was isolated by functional expression in Xenopus laevis oocytes. The fNaDC-3 cDNA is 2384 nucleotides long and encodes a protein of 601 amino acids with a calculated molecular mass of 66.4 kDa. Secondary structure analysis predicts at least eight membrane-spanning domains. Transport of succinate by fNaDC-3 was sodium-dependent, could be inhibited by lithium, and evoked an inward current. The apparent affinity constant (Km) of fNaDC-3 for succinate of 30 microM resembles that of Na+-dicarboxylate transport in the basolateral membrane of mammalian renal proximal tubules. The substrates specific for the basolateral transporter, 2,3-dimethylsuccinate and cis-aconitate, not only inhibited succinate uptake but also evoked inward currents, proving that they are transported by fNaDC-3. Succinate transport via fNaDC-3 decreased by lowering pH, as did citrate transport, although much more moderately. These characteristics suggest that fNaDC-3 is a new type of Na+-dicarboxylate transporter that most likely corresponds to the Na+-dicarboxylate cotransporter in the basolateral membrane of mammalian renal proximal tubules.

Amino Acid Sequence↗
PubMed1999

Adenosine inhibits the transfected Na+-H+ exchanger NHE3 in Xenopus laevis renal epithelial cells (A6/C1).

1. Adenosine influences the vectorial transport of Na+ and HCO3- across kidney epithelial cells. However, its action on effector proteins, such as the Na+-H+ exchanger NHE3, an epithelial brush border isoform of the Na+-H+ exchanger (NHE) gene family, is not yet defined. 2. The present study was conducted in Xenopus laevis distal nephron A6 epithelia which express both an apical adenosine receptor of the A1 type (coupled to protein kinase C (PKC)) and a basolateral receptor of the A2 type (coupled to protein kinase A (PKA)). The untransfected A6 cell line expresses a single NHE type (XNHE) which is restricted to the basolateral membrane and which is activated by PKA. 3. A6 cell lines were generated which express exogenous rat NHE3. Measurements of side-specific pHi recovery from acid loads in the presence of HOE694 (an inhibitor with differential potency towards individual NHE isoforms) detected an apical resistant Na+-H+ exchange only in transfected cell lines. The sensitivity of the basolateral NHE to HOE694 was unchanged, suggesting that exogenous NHE3 was restricted to the apical membrane. 4. Stimulation of the apical A1 receptor with N 6-cyclopentyladenosine (CPA) inhibited both apical NHE3 and basolateral XNHE. These effects were mimicked by the addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and partially prevented by the PKC inhibitor calphostin C which also blocked the effect of PMA. 5. Stimulation of the basolateral A2 receptor with CPA inhibited apical NHE3 and stimulated basolateral XNHE. These effects were mimicked by 8-bromo-cAMP and partially prevented by the PKA inhibitor H89 which entirely blocked the effect of 8-bromo-cAMP. 6. In conclusion, CPA inhibits rat NHE3 expressed apically in A6 epithelia via both the apical PKC-coupled A1 and the basolateral PKA-coupled A2 adenosine receptors.

Adenosine↗
PubMed1999

NH4+ conductance in Xenopus laevis oocytes.II. Effect Of hypoosmolality.

Superfusing Xenopus laevis oocytes with NH4Cl (10 mmol/l, pH 7.5) resulted in an inward current at a clamp potential of -70 mV. In paired experiments (n=22), the NH4Cl-induced peak current was -293+/-94 nA, under control conditions (osmolality: 240 mosmol/kg), and rose to -523+/-196 nA when osmolality was reduced to 144 mosmol/kg. In parallel with the rise in NH4Cl-induced inward current, membrane conductance at -70 mV doubled and the zero-current potential changed from +3.3+/-9.4 mV to -22.0+/-8.0 mV (n=22) in the presence of NH4Cl during exposure to a hypoosmolar solution. In the absence of NH4Cl, oocytes responded to hypoosmolality with a shift in zero-current potential to more negative values and an increased conductance which became partially sensitive to isosorbiddinitrate (ISDN), suggesting the activation of a volume-sensitive K+ channel. Membrane conductance in the presence of NH4Cl was decreased by ISDN to similar extents under isoosmolal and hypoosmolal conditions, indicating that NH4+ enters the oocytes through a volume-sensitive conductance separate from the ISDN-sensitive K+ channel.

Ammonium Chloride↗
PubMed1999

Chloride conductance in HT29 cells: investigations with apical membrane vesicles and RT-PCR.

Vesicles enriched in a marker enzyme for apical membranes were isolated from HT29 cells. These vesicles contain an anion conductance with the selectivity gluconate approximately sulphate 4, 4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS)>glibenclamide. The Cl- conductance was insensitive to Ca2+ and to extravesicular cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP) and inosine 5'-triphosphate (ITP). Using the reverse transcription polymerase chain reaction (RT-PCR) technique and sequencing of the amplified products we detected messenger ribonucleic acid (mRNA) for the cystic fibrosis transmembrane conductance regulator (CFTR), the putative Cl- channel or Cl- channel regulator pICln, and the Cl- channels ClC-2, ClC-3, ClC-5 and ClC-6 in HT29 cells. The properties of the vesicles' Cl- conductance resemble those of the intermediate conductance outwardly rectifying Cl- channel and tentatively exclude contributions of CFTR, pICln and ClC-2. Whether ClC-3, ClC-5, ClC-6 are involved in Cl- conductance remains to be determined.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1999

Demonstration of a Na(+)-dicarboxylate cotransporter in bovine adrenocortical cells.

Our study found the uptake of [14C]succinate into bovine adrenocortical cells to be sodium-dependent, inhibited by lithium, and to have an apparent K(m) of 146 mumol/l. Succinate uptake was inhibited by glutarate, fumarate, alpha-ketoglutarate, and maleate but not by 2,3-dimethylsuccinate or cis-aconitate, specific inhibitors of the basolateral Na(+)-dicarboxylate transporter of renal proximal tubule cells. Succinate uptake was highest at pH 6.0 and decreased with increasing pH. Transport of succinate was not significantly inhibited by citrate at pH 7.4 whereas at pH 6.0 inhibition of succinate uptake by citrate was small but significant. The affinity of the adrenal dicarboxylate transporter towards succinate ranges in between the low affinity of the renal luminal dicarboxylate transporter and the high affinity of the respective basolateral transporter. The pH dependency of succinate uptake and the missing inhibition by citrate at pH 7.4 differ from both the luminal and from the basolateral dicarboxylate transporters in kidney, liver, intestine, and placenta. These functional characteristics provide evidence for the existence of a Na(+)-dicarboxylate cotransporter in adrenocortical cells which may supply cholesterol metabolism with reducing substrates.

Adrenal Cortex↗
PubMed1999

Triple helix formation inhibits DNA gyrase activity.

The goal of this work was to examine the effect of triple helix-forming oligonucleotides on a gyrase target region and on the activity of the enzyme. Using melting temperature measurements and gel mobility shift analysis, it was found that modified oligonucleotides can form a triple helix along the 29-nucleotide region of a 32-bp duplex representing part of the gyrase DNA-target sequence of the 162-bp fragment from pBR322. Triplex formation with this target region has been achieved at pH 7.5 by using a synthetic oligonucleotide in which cytosine was replaced by the C-nucleoside of 2-aminopyridine. The results of the enzymic experiments in vitro with the 162-bp fragment demonstrated that the cleavage reaction mediated by gyrase can be efficiently inhibited by the triplex-forming oligonucleotide modified with 2-aminopyridine. A possible inhibitory mechanism is discussed.

Base Sequence↗
PubMed1999

Demonstration of a probenecid-inhibitable anion exchanger involved in the release of cortisol and cAMP and in the uptake of p-aminohippurate in bovine adrenocortical cells.

Recently we provided evidence for the involvement of a probenecid-inhibitable anion exchanger in cortisol release from primary cultures of bovine adrenocortical cells. In the present study, we further characterized this exchange transporter. Adrenocorticotropic hormone stimulated 3H-p-aminohippurate (3H-PAH) uptake into as well as cortisol release from the cells about two- and tenfold, respectively. Probenecid inhibited both 3H-PAH uptake and cortisol release by about 55 and 63%. Preincubation of the cells with 1 mM PAH trans-stimulated 3H-PAH uptake by 30%, whereas cortisol release was inhibited by 30%. 3H-PAH uptake was cis-inhibited by 1 mM glutarate or by 1 mM cortisol in the medium, while cortisol release was trans-stimulated by glutarate. PAH in the incubation medium showed saturable cis-inhibition of 3H-PAH uptake. The release of cyclic adenosine monophosphate, a substrate of the renal PAH exchanger, was also inhibited by probenecid and trans-stimulated by glutarate. In summary, the trans-stimulation and cis-inhibition experiments support the concept of an anion exchanger involved in cortisol and cyclic adenosine monophosphate release from and PAH uptake into adrenocortical cells.

Adrenal Cortex↗
PubMed1998

DNA sequence recognition by bis-linked netropsin and distamycin derivatives.

We studied the interaction of cis-diammine Pt(II)-bridged bis-netropsin, cis-diammine Pt(II)-bridged bis-distamycin and oligomethylene-bridged bis-netropsin with synthetic DNA fragments containing pseudosymmetrical AT-rich nucleotide sequences and compared it with the interaction of the parent compounds netropsin and distamycin A. For fragments containing multiple blocks of (AIT)4 and (T/A)4 separated by zero, one, two and three GC-base pairs, DNase I footprinting and CD spectroscopy studies reveal that 5'-TTTTAAAA-3' is the strongest affinity binding site for cis-diammine Pt(II)-bridged bis-netropsin and bis-distamycin. They both bind less strongly to a DNA region containing the sequence 5'-AAAATTTT-3'. Netropsin, distamycin A and oligomethylene-bridged bis-netropsin exhibit far less sequence discrimination.

Base Sequence↗
PubMed1998

Binding of 2-azaanthraquinone derivatives to DNA and their interference with the activity of DNA topoisomerases in vitro.

We have investigated the binding ability to DNA of compounds belonging to the 2-azaanthraquinone-type structure and have examined the effect on the activity of DNA gyrase as well as on mammalian topoisomerases in vitro. Using different biophysical techniques it was found that one of these ligands, 9-((2-dimethylamino)ethyl)amino)-6-hydroxy-7-methoxy-5, 10-dihydroxybenzo[g]isoquinoline-5,10-dione (TPL-I), is an intercalating DNA binding agent, whereas the parent compound tolypocladin (TPL) and a derivative (TPL-II) showed almost no similar affinity to DNA. CD measurements demonstrated a significant and selective binding tendency of TPL-I to alternating purine/pyrimidine sequences with some preference for poly(dA-dT). poly(dA-dT). Tm values were increased of the ligand complex with the alternating AT-containing duplex polymer. The binding to various DNAs was characterized by CD and visible absorption spectral changes. From the latter, different binding constants of 6.2 x 10(5) and 1.5 x 10(5) M-1 were obtained for poly(dA-dT).poly(dA-dT) and poly(dA). poly(dT), respectively. Sedimentation measurements with supercoiled pBR322 plasmid DNA clearly indicated an intercalative binding mechanism associated with an unwinding angle of about 18 degrees. These results suggest that the intercalative binding of TPL-I is promoted by the 2-(dimethylamino)ethylamino group substituted on carbon 9 of the anthraquinone system. The cytotoxic compound TPL-I, but not TPL or TPL-II, effectively inhibited the DNA supercoiling reaction of DNA gyrase and the activity of mammalian topoisomerases I and II as measured by the relaxation assay. TPL-I affects the cleavage reaction of topoisomerases on a single site located in alternating purine-pyrimidine sequence regions. The inhibitory potency of TPL-I can be ascribed to a blocking of cleavage sites on the DNA substrate, which correlates with the sequence preference of the ligand.

Anthraquinones↗
PubMed1998

Characterization of the chloride conductance in porcine renal brush-border membrane vesicles.

The chloride conductance in brush-border membrane vesicles prepared from pig kidney cortex was investigated using a light-scattering assay, anion-diffusion-potential-dependent Na+-D-glucose cotransport and 36Cl- influx. K+-diffusion-potential-driven salt exit from, or entry into, the vesicles was slow in the presence of gluconate, SO42- and F-, intermediate with Cl- and Br-, and fast with I-, NO3-, and SCN-. Stimulation of Na+-D-glucose uptake followed a similar anion sequence. Conductive Cl- flux had a low activation energy and was inhibited by suphhydryl reagents, the stilbene disulphonates 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS) and 4, 4'-diisothiocyanato-2,2'-disulphonate (DIDS), and the arylaminobenzoates diphenylamine-2-carboxylic acid (DPC) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). Intravesicular Ca2+ and extravesicular nucleotides were without effect on conductive Cl- flux. These characteristics tentatively exclude some known Cl- channels and leave members of the ClC family as possible candidates responsible for the Cl- conductance in brush-border membranes.

Animals↗
PubMed1998

Endosomes: another extra-mitochondrial location of type-1 porin/voltage-dependent anion-selective channels.

Endocytotic vesicles (EV) isolated from rat renal cortex were subjected to SDS-polyacrylamide gel electrophoresis and Western blotting. A monoclonal antibody against human type-1 porin (31 kDa) detected a strong band of 31 kDa. The same antibody has been used as the primary antibody in indirect immunocytochemistry. Light microscopy of cryostat sections of rat renal cortex showed a heavy staining of EV underneath the brush-border membrane. Electron microscopy was performed by "preembedding immunogold staining" of rat renal cortex, the sections of which showed an extensive labelling of EV with gold particles. These results demonstrate that the expression of type-1 porin is not restricted to outer mitochondrial membranes. The biological function of endosomal type-1 porin has as yet to be ascertained.

Animals↗
PubMed1998

S3226, a novel inhibitor of Na+/H+ exchanger subtype 3 in various cell types.

Inhibition of Na+/H+ exchange (NHE) subtypes has been investigated in a study of the mouse fibroblast L cell line (LAP1) transfected with human (h) NHE1, rabbit (rb) NHE2, rat (rt) or human (h) NHE3 as well as an opossum kidney cell line (OK) and porcine renal brush-border membrane vesicles (BBMV). S3226 ¿3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N-isopro pylidene-2-methyl-acrylamide dihydro-chloride¿ was the most potent and specific NHE3 inhibitor with an IC50 value of 0.02 micromol/l for the human isoform, whereas its IC50 value for hNHE1 and rbNHE2 was 3.6 and approximately = 80 micromol/l, respectively. In contrast, amiloride is a weak NHE3 inhibitor (IC50>100 micromol/l) with a higher affinity to hNHE1 and rbNHE2. Cariporide (4-isopropyl-3-methylsulphonyl-benzoyl-guanidine methane-sulphonate), which has an IC50 for NHE3 of approximately 1 mmol/l, is a highly selective NHE1 inhibitor (0.08 micromol/l). Therefore, S3226 is a novel tool with which to investigate the physiological and pathophysiological roles of NHE3 in animal models.

Amiloride↗
PubMed1997

Expression cloning and characterization of a renal organic anion transporter from winter flounder.

The cDNA coding for a renal p-aminohippurate (PAH) transporter from winter flounder (Pseudopleuronectes americanus), designated fROAT, was cloned by functional expression in Xenopus laevis oocytes. fROAT is approximately 2.8 kbp in length and encodes a protein of 562 amino acids, related to the rat renal organic anion transporter ROAT1/OAT1 and the organic cation transporters OCT1 and OCT2. In oocytes, fROAT mediated probenecid-sensitive PAH uptake, with a Km for PAH of about 20 microM, and inhibited by external glutarate (GA) (1 mM). The functional characteristics suggest that fROAT is the basolateral PAH/dicarboxylate exchanger of the flounder kidney.

Amino Acid Sequence↗
PubMed1997

NH4+ conductance in Xenopus laevis oocytes. I. Basic observations.

Current-clamp and voltage-clamp techniques were used to study the effects of NH4+ on the cell membrane conductance in Xenopus laevis oocytes. Superfusing the oocytes with NH4Cl resulted in a depolarization of the oocyte's cell membrane potential and, at a clamp potential of -70 mV, in an inward current. The magnitude of the inward current was proportional to the NH4Cl concentration in the extracellular solution and on membrane potential. The reversal potential, Erev , was -35.5 +/- 11.6 mV under control conditions and -3.1 +/- 11.0 mV (n = 19) in the presence of NH4Cl (10 mmol/l). Superfusion of the oocytes with nominally Ca2+-free solution affected the NH4Cl-evoked response only marginally. Replacement of extracellular Na+ by N-methyl-D-glucamine+ markedly reduced, but did not eliminate, the NH4Cl-sensitive current and shifted the reversal potential to more negative potentials. The NH4Cl-induced current was substantially inhibited by 0.1 mmol/l flufenamate, and was less affected by blockers of the endogenous K+ conductance, Ba2+ and isosorbiddinitrate (ISDN). The results are compatible with the activation of a conductance by NH4Cl for Na+ and NH4+. The mechanism by which NH4Cl activates the conductance remains unknown.

Animals↗