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H Murer

Publications and source records attributed to H Murer.

At least 343 records · Page 19Linked to original sources
PubMed1983

Transport of L-cystine by rat renal brush border membrane vesicles.

Brush border membranes were isolated from rat renal cortex by a divalent cation precipitation method. L-35S-cystine uptake into the vesicles was measured by a rapid filtration method. Covalent incorporation of tracer into membrane proteins was observed after prolonged incubations. At short incubation periods (1 min) binding was small and allowed an analysis of transmembrane transport. To guarantee transport of L-cystine, the experiments were performed in the presence of the oxidant diamide. Sodium stimulated L-cystine uptake specifically. A potassium/valinomycin induced inside negative diffusion potential stimulated sodium dependent L-cystine transport. Thus, transport is potential sensitive in the presence of sodium. At low substrate and inhibitor concentrations, L-cystine transport was inhibited by L-lysine, L-ornithine and L-arginine but not by D-lysine in the presence and absence of sodium. At higher inhibitor concentration, the neutral amino acids L-phenylalanine and L-leucine also inhibited L-cystine uptake, but only the sodium dependent uptake. These inhibition experiments suggest that L-cystine is transported by the brush border membrane by a transport system for basic amino acids not necessarily requiring sodium. In addition, transport of L-cystine can also proceed via sodium dependent transport pathways for neutral amino acids. In the concentration range tested (up to 0.225 mmoles/l), no saturation of L-cystine transport was observed in the presence and absence of sodium.

Amino Acids↗
PubMed1983

Transport of L-cysteine by rat renal brush border membrane vesicles.

Brush border membranes were isolated from rat renal cortex by a divalent cation precipitation method. L-35S-cysteine uptake into the vesicles was measured by a rapid filtration method. Only minimal binding of the amino acid to the vesicles was observed. Sodium stimulates L-cysteine uptake specifically. Anion replacement experiments, experiments in the presence of potassium/valinomycin-induced diffusion potential as well as experiments with a potential-sensitive fluorescent dye document an electrogenic sodium-dependent uptake mechanism for L-cysteine. Tracer replacement experiments as well as the fluorescence experiments indicate a preferential transport of L-cysteine. Transport of L-cysteine is inhibited by L-alanine and L-phenylalanine but not by L-glutamic acid and the L-basic amino acids. Initial, linear influx kinetics provide evidence for the existence of two transport sites. The results suggest (a) sodium-dependent mechanism(s) for L-cysteine shared by other neutral amino acids.

Alanine↗
PubMed1983

Co- and counter-transport mechanisms in brush border membranes and basal-lateral membranes of intestine and kidney.

One way to obtain a deeper understanding of the complex function of the small intestinal and renal proximal tubular epithelium is to dissect it into single components and then, having defined the components under well-controlled conditions, try to describe the behaviour of the whole system on the basis of the properties of the single components. Brush border and basal-lateral membranes can be isolated by different methods, including free flow electrophoresis, differential and gradient centrifugation. Transport can be analysed in vesiculated membrane fractions by tracer techniques and spectrophotometric techniques. Different sodium-solute co-transport mechanisms were identified in the brush border membrane. Until now, studies with vesicles failed to document a sodium-chloride co-transport mechanism satisfactorily. On the other hand, a sodium/proton and a chloride/hydroxyl exchange mechanism were documented. These two exchange mechanisms could represent partial reactions of the postulated electroneutral sodium-chloride co-transport. In addition to different sodium-independent transport systems, the basal-lateral membrane contains an ATP-driven transport system for calcium as well as a sodium/calcium exchange mechanism. Studies with membrane vesicles isolated from animals which have been exposed to different dietary conditions or in which the parathyroid hormone or 1.25(OH)2VitD3 level has been altered show altered transport of calcium and inorganic phosphate. Thereby, it might be possible to identify the biochemical mechanisms involved in transport regulation.

Acid-Base Equilibrium↗
PubMed1982

Sulphate and phosphate transport in the renal proximal tubule.

Experiments performed on microperfused proximal tubules and brush-border membrane vesicles revealed that inorganic phosphate is actively reabsorbed in the proximal tubule involving a 2 Na+-HPO2-4 or H2PO-4 co-transport step in the brush-border membrane and a sodium-independent exit step in the basolateral cell membrane. Na+-phosphate co-transport is competitively inhibited by arsenate. The transtubular transport regulation is mirrored by the brush-border transport step: it is inhibited by parathyroid hormone intracellularly mediated by cyclic AMP. Transepithelial inorganic phosphate (Pi) transport and Na+-dependent Pi transport across the brush-border membrane correlates inversely with the Pi content of the diet. Intraluminal acidification as well as intracellular alkalinization led to a reduction of transepithelial Pi transport. Data from brush-border membrane vesicles indicate that high luminal H+ concentrations reduce the affinity for Na+ of the Na+-phosphate co-transport system, and that this mechanism might be responsible for the pH dependence of phosphate reabsorption. Contraluminal influx of Pi from the interstitium into the cell could be partly inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). It is not, however, changed when dicarboxylic acids are present or when the pH of the perfusate is reduced to pH 6. Sulphate is actively reabsorbed, involving electroneutral 2 Na+-SO2-4 co-transport through the brush-border membrane. This transport step is inhibited by thiosulphate and molybdate, but not by phosphate or tungstate. The transtubular active sulphate reabsorption is not pH dependent, but is diminished by the absence of bicarbonate. The transport of sulphate through the contraluminal cell side is inhibited by DIDS and diminished when the capillary perfusate contains no bicarbonate or chloride. The latter data indicate the presence of an anion exchange system in the contraluminal cell membrane like that in the erythrocyte membrane.

Animals↗
PubMed1982

Membrane transport in the proximal tubule and thick ascending limb of Henle's loop: mechanisms and their alterations.

Over the past few years, our knowledge on renal tubular transport mechanisms has increased considerably. Due to new technical developments, it is now possible to understand in part transepithelial transport and its pathological and pharmacological alterations at the level of the cell membranes. Different membrane transport mechanisms are discussed in this article, whereby sodium coupled solute transport in the proximal tubule and sodium chloride transport in the thick ascending limb of Henle's loop are taken as examples. It is indicated that an altered function of the kidney can often be equated with an alteration of the membrane transport.

Animals↗
PubMed1982

Asymmetry in the transport of lactate by basolateral and brush border membranes of rat kidney cortex.

The uptake of L(+)lactate into rat renal cortical brush border (BBV) and basolateral (BLV) membrane vesicles, isolated through differential centrifugation and free flow electrophoresis, were studied using a rapid filtration technique. In contrast to the lactate transport into the BBV, that into the BLV: 1) was found to proceed only towards equilibrium, 2) showed Na+ -independent coupling of the influx of L(+)lactate and the efflux of L(+) but not to the efflux of D(-)lactate, 3) was not inhibited by D(-)lactate, 2-thiolactate or 3-phenyl-lactate, but 4) was inhibited by 3-thiolactate and alpha-hydroxybutyrate and 5) was accelerated by changes in inwardly directed ionic gradients or by increases in cation conductance both of which led to increased intravesicular positivity. The latter changes had the opposite effect on the uptake of L(+)lactate by BBV. Thus, while the L(+)lactate transport system present in BBV showed the characteristics of Na-dependent electrogenic cotransport system, that in the BLV was consistent with a carrier mediated Na-dependent, facilitated diffusion system.

Animals↗
PubMed1982

Phosphate transport in brush border membrane vesicles isolated from renal cortex of young growing and adult rats. Comparison with whole kidney data.

Recent clearance studies have demonstrated that the maximal tubular reabsorption of inorganic phosphate (Pi) per ml of glomerular filtrate (max. TRPi/ml GF) of the whole kidney is markedly lower in adult than in young growing rats fed either normal (0.8 g %) or low (0.2 g %) phosphorus diet. In addition, in adult rats clearance studies indicate that enhancement of max. TRPi/ml GF is observed 21 days but not 8 days after starting the low (0.2%) phosphorus diet. In the present work we have studied in the same experimental condition the Na+-dependent Pi uptake in brush border membrane vesicles (BBMV) isolated from renal cortex of either young growing or adult rats. The results of this study indicate that under the low (0.2%) but not under the normal (0.8%) phosphorus diet the Na+-dependent Pi uptake by BBMV was significantly depressed in adult as compared to young growing rats. In adult rats the Pi transport response to Pi restriction monitored at the brush border membrane level was different from that observed by clearance studies in the whole kidney. Indeed, the Pi uptake by BBMV was already enhanced after 8 days of Pi restriction and it did not increase further when studied 21 days after starting the low (0.2%) phosphorus diet. These results suggest that the regulation of the overall transfer of Pi across the renal epithelium may involve other additional modulating factors than the Na+-dependent Pi transport system present in the luminal membrane of the proximal tubule.

Age Factors↗
PubMed1982

Ca++-transport across basal-lateral plasma membranes from rat small intestinal epithelial cells.

Basal-lateral plasma membrane vesicles were isolated from rat duodenum and jejunum by a Percoll gradient centrifugation technique. Ca-uptake into and Ca-release from the vesicles was studied by a rapid filtration method. In the absence of Na (K-medium) at a Ca concentration of 0.05 mmol/liter and pH 7.4, addition of 5 mM MgATP stimulated Ca-uptake up to 10-fold as compared to a control without ATP. Since the Ca-ionophore A23187 (2 microgram/ml) prevented the accumulation of Ca above the equilibrium uptake and rapidly released Ca accumulated by the vesicles in the presence of ATP, it is concluded that the ATP-dependent uptake of Ca involves accumulation of Ca inside the vesicles. The ATP-driven Ca-transport comigrates with the (Na +K)-ATPase and dissociates from the marker enzymes for mitochondrial inner membrane, endoplasmic reticulum and brush border membrane. It is not inhibited by 1 microgram/ml oliomycin or 0.1 mmol/liter ruthenium red. Replacing K by Na inhibits ATP-dependent Ca-uptake by 60%. Efflux of Ca from passively preloaded vesicles is strongly temperature sensitive and enhanced by A23187. An inwardly directed Na-gradient stimulates Ca-efflux as compared to a K-gradient. Addition of gramicidin reduces the Na-stimulation of Ca-efflux, indicating direct coupling of Na and Ca fluxes across basal-lateral membranes. The results suggest that basal lateral membranes possess two distinct mechanisms for Ca-transport: a) ATP-driven Ca-transport and b) Na/Ca-exchange.

Animals↗
PubMed1982

Calcium uptake in isolated hepatic plasma-membrane vesicles.

A liver plasma-membrane fraction capable of Ca2+ uptake was isolated. The fraction exhibited high Na+, K+-ATPase, low glucose-6 phosphatase activity, and transported alanine in a Na+-dependent fashion. The uptake of Ca2+ was ATP-dependent; UTP, GTP, or CTP did not substitute for ATP. The presence of oxalate did not significantly alter the rate of uptake. The pH optimum of the reaction was basic (no uptake was visible at pH 6.8). These properties are at variance with those of the endoplasmic reticulum Ca2+ uptake system, which is oxalate-dependent, and has an acid pH optimum. The ATP-dependent Ca2+ uptake has a Km(Ca2+) of 1.4 X 10(-8) M and a Vmax of transport of 30 nmol X mg protein-1 X min-1. No conclusive results were obtained on the calmodulin-sensitivity of the process: addition of calmodulin to the vesicles did not stimulate uptake, and the anti-calmodulin drug trifluoperazine had no inhibitory effect. However, another anti-calmodulin drug (R24571) had a limited, but statistically significant, inhibitory action. A partial release of the accumulated Ca2+ from the vesicles could be induced by the addition of Na+, and incubation of the vesicles in a high Na+ medium (as compared to high K+ medium) resulted in lower (about 25%) calcium uptake. Partial release of the accumulated Ca2+ could be induced also by the addition of H+. The releasing effect of H+, taken together with the absence of Ca2+ uptake at acid pH, suggests the possibility of a H+/Ca2+ exchange.

Animals↗
PubMed1982

Regulation of Na+-Pi cotransport by 1,25-dihydroxyvitamin D3 in rabbit duodenal brush-border membrane.

Brush-border membrane vesicles were isolated from rabbit duodenum by a Mg2+ precipitation method, and phosphate transport was analyzed by a rapid filtration technique. Uptake of inorganic phosphate (Pi) was stimulated by an inwardly directed sodium gradient, indicating the operation of a Na-Pi cotransport system in brush-border membrane vesicles. Treatment of the animals with ethane-1-hydroxy-1,1-diphosphonate (EHDP), which is known to decrease the circulating levels of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], reduced within 3 days the sodium-dependent Pi transport in the brush-border vesicles. Injections of 1,25(OH)2D3 into rabbits increased within 9 h the sodium-dependent Pi transport in membranes from EHDP-treated animals as well as in untreated ones. The Na-D-glucose cotransport system appeared to be unaffected by these maneuvers. These results suggest that the Na-Pi cotransport system is an important site of regulation of intestinal transepithelial Pi transport by 1,25(OH)2)D3.

Animals↗