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Biomedical subjects

G Burckhardt

Publications and source records attributed to G Burckhardt.

102 records · Page 6Linked to original sources
PubMed1981

Sodium-coupled taurocholate transport in the proximal convolution of the rat kidney in vivo and in vitro.

Using the standing droplet technique in the renal proximal convolution and simultaneous microperfusion of the peritubular capillaries, the zero net flux transtubular concentration difference of taurocholate (DeltaC(TC-)) at 45 s was determined as a measure of active bile acid reabsorption in vivo. Starting with 0.1 mmol/liter taurocholate in both perfusates the control DeltaC(TC-) of 0.042 mmol/liter fell to 0.006 mmol/liter (P < 0.001) when the Na(+) concentration in the perfusates was reduced to zero. Removal of bicarbonate from the perfusates to alter pH had no influence on DeltaC(TC-). When glycocholate was added to the perfusates DeltaC(TC-) was decreased, while probenecid increased DeltaC(TC-). These observations were extended by studies performed with brush border membrane vesicles derived from renal cortex. The initial (20 s) uptake of 0.01 mmol/liter taurocholate in the presence of a Na(o) (+) > Na(i) (+) gradient was stimulated twofold compared with its uptake in the absence of a Na(+) gradient. Uptake of taurocholate was osmotically and temperature sensitive. Membranes preloaded with unlabeled glycocholate showed accelerated entry of labeled taurocholate (trans-stimulation) only in the presence of Na(+). Replacement of Na(+) in the media with K(+), Li(+), and choline(+) decreased initial taurocholate uptake by 49, 53, and 62%, respectively. Stimulation of taurocholate transport by cation gradient diffusion potentials was unlikely inasmuch as the addition of valinomycin under K(+) gradient conditions had no effect. A transmembrane pH gradient (pH(o) < pH(i)) did not influence initial uptake of taurocholate. Finally, in the presence of Na(+) taurocholate transport showed cis-inhibition with unlabeled bile acids and saturation kinetics with respect to increasing taurocholate concentrations. The micropuncture and vesicle data indicate that the net transport of taurocholate in the proximal tubule is the result of an electroneutral Na(+)-taurocholate cotransport across the brush border membrane.

Animals↗
PubMed1980

The influence of an uncoupler on amino acid accumulation in Ehrlich mouse ascites tumor cells.

In Ehrlich ascites tumor cells, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) acts at two different sites depending upon the concentration employed. (1) In non-glycolysing respiring cells, FCCP is seen to uncouple the mitochondria and thereby it inhibits the ATP-dependent (Na+, K+) pump. (2) In glycolysing cells, FCCP does not affect the electrogenic (Na+, K+) pump, but depolarizes the plasma membrane potential difference as visualized by the distribution of the lipid-soluble cation, tetraphenylphosphonium, and by an inhibition of the rheogenic, Na+-dependent uptake of alpha-aminoisobutyric acid. A depolarization by FCCP also occurs under conditions where a K+-diffusion potential is present and the pump is blocked by metabolic inhibition or by ouabain. Depolarization and FCCP-induced increase in H+ fluxes across the plasma membrane exhibit a similar FCCP-concentration dependency. The imposition of proton-concentration differences in the presence of FCCP inhibits (pHi > pHo) or stimulates (pHi < pHo) alpha-aminoisobutyric acid uptake and tetraphenylphosphonium accumulation. The experiments indicate that FCCP shifts the plasma membrane potential of Ehrlich cells, which are normally relatively impermeable for protons, towards an H+-diffusion potential.

Amino Acids↗
PubMed1980

The effects of potassium and membrane potential on sodium-dependent glutamic acid uptake.

The uptake of L-glutamic acid into brush-border membrane vesicles isolated from rat renal proximal tubules is NA+-dependent. In contrast to Na+-dependent uptake of D-glucose, pre-equilibration of the vesicles with K+ stimulates L-glutamic acid uptake. Imposition of a K+ gradient ([Ki+] > [Ko+]) further enhances Na+-dependent L-glutamic acid uptake, but leaves K+-dependent glucose transport unchanged. If K+ is present only at the outside of the vesicles, transport is inhibited. Intravesicular Rb+ and, to a lesser extent, Cs+ can replace intravesicular K+ to stimulate L-glutamic acid uptake. Changes in membrane potential incurred by the imposition of an H+-diffusion potential or anion replacement markedly affect Na+-dependent glutamic acid uptake only in the presence of K+. Experiments with a potential-sensitive cyanine dye also indicate that, in the presence of intravesicular K+ a charge movement is involved in Na+-dependent transport of L-glutamic acid. The data indicate that Na+-dependent L-glutamic acid transport can be additionally energized by a K+ gradient. Furthermore, intravesicular K+ render Na+-dependent L-glutamic acid transport sensitive to changes in the transmembrane electrical potential difference.

Animals↗
PubMed1977

A compact from of methylated DNA is solutions containing poly (ethylene glycol).

Some peculiarities of compactization of double-stranded DNA molecules containing methylated nitrogen bases have been studied in water-salt solutions of PEG. It is shown that the methylation of N7-atoms of guanyl residues in original DNA molecules does not prevent the formation of DNA compact particles, but results in a decrease of the amplitude of the negative band in the CD spectrum of compact particles. The influence of N7-guanine methylation on the shape of the CD spectrum being the greater, the lower is the concentration of PEG. The dependence of the negative band amplitude in the CD spectrum on the content of methylated guanyl residues is practically the same for low-molecular weight DNA's from different sources. The observed decrease in the negative band amplitude is interpreted as a result of alterration of guanyl residue orientation relative to the helix axis which leads to diminished optical activity of the "microcrystalline" domains of compact particles. The evidence obtained suggests that changes in the secondary structure of DNA lead to considerable difference between CD spectra of compact particles of methlated DNA and psi-form of DNA. (The changes in the CD spectrum of the DNA compact particles occur also as a result of methylation of C5-atoms of cytosine residues). It is suggested that the negative band in the CD spectrum can be used a criterion for detection of negligible alterations in the DNA secondary structure.

Animals↗
PubMed1976

Conformation and reactivity of DNA in the complex with proteins. III. Helix-coil transition and conformational studies of model complexes of DNA's with poly-L-histidine.

Differences in the interaction of poly-L-histidine with DNA of various base composition have been demonstrated using melting and CD measurements. The two types of complexes formed with DNA at pH values below the pK of 5.9 and in the region of pH 6.5 are very different in their CD spectral properties. The binding effects with highly protonated poly-L-histidine are AT-dependent as reflected by large negative CD spectra indicating the formation of psi-DNA as a condensed state of the double helix. GC-rich DNA may, however, also form psi-DNA structures with poly-L-histidine under certain conditions. At pH 6.5 complex formation with the weakly protonated polypeptide is GC-dependent. From the results it is concluded that protonated poly-L-histidine interacts more specifically at AT base pairs, prabably along the small groove while the weakly protonated poly-L-histidine tends to interact preferentially with GC regions which seems to occur rather in the large groove.

Binding Sites↗
PubMedDate not supplied

Molecular recognition between oligopeptides and nucleic acids: DNA binding selectivity of a series of 1,2,4-triazole-containing lexitropsins.

The synthesis of a series of 1,2,4-triazole-containing oligopeptide lexitropsins related to the natural antitumor antibiotic distamycin is described. The binding properties of these new agents to both native DNAs and synthetic polydeoxyribonucleotides were determined by UV absorption and circular dichroism studies. The DNA binding and sequence selectivity of these agents have been determined by complementary strand MPE footprinting on two restriction fragments of pBR322 DNA. The lexitropsins bearing one 1,2,4-triazole moiety bind to AT sequences like distamycin but also tolerate GC sites. However, 3, which contains two contiguous 1,2,4-triazole moieties, appears not to recognize AT stretches and demands two consecutive GC bases in the middle of a four-base recognition site. The triazole lexitropsins avoid ATTT (4264-4267) and AAAA (4273-4276) to which distamycin binds strongly to the EcoRI/Hind III fragment. Determination of pKa values indicates the 1,2,4-triazole units are protonated under physiological conditions. The avoidance of AT stretches and the recognition of GC base sites by 3 suggest molecular recognition in these cases is determined, not by electrostatic effects, but rather by invoking a hydrogen bond between the triazole 4-nitrogen and G-2NH2 in the minor groove.

Base Sequence↗
PubMedDate not supplied

Effect of cholate on H(+)-ATPase and other proteins of dog renal brush-border membrane.

A short treatment of dog renal brush-border membrane vesicles (BBMV) with sodium cholate, followed by dialysis of the detergent, reorients the polarity of H(+)-ATPase in the membrane and exposes its ATP binding sites to the extravesicular space, as previously shown with pig BBMV. In cholate-pretreated vesicles, the H(+)-ATPase remains fully active, but is inserted under the reversed polarity in sealed vesicles. A large spontaneous N-ethylmaleimide-sensitive ATPase activity is thus observed, as well as a steep intravesicular acidification upon external ATP addition, two findings absent in native vesicles. The ability of nitrate plus ATP to dissociate the hydrolytic subunits ot the proton pump in cholate-pretreated vesicles, but not in native vesicles, demonstrates that most of the ATP binding subunits are accessible to ATP following cholate treatment. The sensitivity of the cytoplasmic domain of the H(+)-ATP activity to trypsin also confirms the reorientation of the enzyme in cholate-pretreated vesicles. The H(+)-ATPase and alkaline phosphatase remain largely associated with the membranes after the treatment with cholate, but gamma-glutamyltranspeptidase, aminopeptidase N, and neutral endopeptidase are largely solubilized. Upon dialysis of cholate, all these enzymes are in part reinserted in the membrane according to their original polarity. The reorientation process is however specific for the H(+)-ATPase. Cholate treatment does not increase the formation of inside-out vesicles. Thus the treatment with cholate really reorients the polarity of the H(+)-ATPase in vesicles and allows for study of the proton pumping capacity of vacuolar H(+)-ATPase of proximal tubules.

Adenosine Triphosphate↗