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Biomedical subjects

J R Bronk

Publications and source records attributed to J R Bronk.

At least 37 records · Page 2Linked to original sources
PubMed1986

D-galactose transport in rat intestinal brush border membrane vesicles studied with a molecular-sieve technique.

A technique using G-50 Sephadex molecular-sieve chromatography has been developed to study both the accumulation of transported solutes by rat intestinal brush border membrane vesicles and the wash-out of these solutes from pre-loaded vesicles. The extent to which D-galactose was taken up into the brush border membrane vesicles was dependent on the medium osmolarity and this dependence was significantly reduced (P less than 0.01) by pre-treating the vesicles with the non-ionic detergent Triton X-100. The transport of D-galactose into the brush border membrane vesicles was sodium dependent, electrogenic and phloridzin sensitive. It is, therefore, qualitatively similar to D-glucose transport into brush border membrane vesicles. The wash-out of D-galactose from pre-loaded brush border membrane vesicles was inhibited by phloridzin and accelerated by the presence of an outwardly directed sodium gradient. D-Galactose was released from the pre-loaded brush border membrane vesicles more slowly than D-glucose and this difference was consistent with the higher overshoot found for D-galactose during the transport of the sugars into the vesicles.

Animals↗
PubMed1986

The transport of uric acid across mouse small intestine in vitro.

The in vitro recirculation technique was used to study the uptake and transport of uric acid by the jejunum of mouse small intestine. Three components of the serosal secretions appeared to be endogenously derived nucleic acid derivatives; two of these were identified as uric acid and uracil. There was no detectable metabolism of uric acid by the intestine. Uric acid transported from the lumen appeared in the serosal fluid at a concentration higher than that in the lumen. The final serosal/luminal concentration ratio of about 1.18 for exogenous uric acid was found to be constant over the concentration range studied (0.01-0.1 mM). The presence of exogenous uric acid in the lumen did not affect the production of endogenous uric acid by the intestine and its release into the serosal secretions. Mucosal concentration of exogenous uric acid was below, but the total mucosal concentration (exogenous+endogenous) was above, that in the lumen. There was no evidence for the secretion of endogenous uric acid into the lumen. Oxypurinol significantly decreased the rate of serosal appearance of exogenous uric acid. Allopurinol did not affect the transport of exogenous uric acid from the lumen and there was negligible metabolism of allopurinol to oxypurinol by the tissue. Uracil did not affect the transport of exogenous uric acid from the lumen, or the serosal appearance of endogenous uric acid. Likewise uracil transport was unaffected by luminal uric acid.

Allopurinol↗
PubMed1983

Transport and metabolism of glucose by rat small intestine.

Glucose transport and metabolism by rat small intestine was investigated by using a preparation for the combined perfusion of the lumen and the vascular bed. 2. When 5 mM-glucose was present in the lumen, only 29% was transported unchanged to the vascular side. Lactate output into the vascular and luminal fluids accounted for a further 53% and 4% respectively of the glucose taken up from the lumen. 3. Glucose was readily taken up when added at 5 mM to the vascular compartment only. Vascular lactate output accounted for approx. 33% of the glucose uptake, and luminal lactate for 6%. 4. When 2 mM-glucose was added to the lumen with 5 mM-glucose in the vascular perfusate, there was no detectable net transport of glucose to the vascular side. However, of the glucose taken up from the lumen, 31% and 2% could be accounted for by increases in vascular and luminal lactate respectively. 5. When 10 mM-glucose was added to the lumen, with 5 mM-glucose in the vascular perfusate, 33% of the glucose disappearing from the lumen was transported to the vascular side. Extra lactate output to the vascular and lumen perfusates accounted for 50% and 9% respectively of the glucose uptake from the lumen. 6. These studies indicate that at low luminal glucose concentrations no sugar is transferred to the blood unchanged, and at sugar concentrations of 5--10 mM only 25--50% of the glucose leaving the lumen reaches the serosal side intact. Furthermore, the small intestine has a greater propensity to form lactate from luminal glucose than from vascular glucose.

Animals↗
PubMed1982

Effects of feeding and lighting regimes on sugar transport in the rat small intestine.

1. The use of a controlled feeding regime which restricted rats to feeding during the 12 h dark period reduced the variability of sugar transport measurements made with the vascularly perfused intestine. 2. When the timing of the 12 h feeding or lighting periods was shifted by 12 h, pronounced changes in the rate of D-galactose and L-glucose transport were observed. 3. These variations were shown most clearly when the data were expressed as the ratio of the rate of appearance of the actively transported D-galactose to that of the poorly transported L-glucose in the vascular perfusate. This ratio was reduced in those animals studied during their feeding period. The lower ratios in these animals were attributable mainly to an increased rate of transport of L-glucose. 4. Significant reductions in D-galactose/L-glucose ratios were also found when the lighting period was shifted by 12 h, indicating that sugar transport is influenced by the lighting regime as well as by the feeding pattern. 5. An examination of the tissue sugar concentrations under the different feeding and lighting regimes revealed that the variations in sugar transport were associated with changes in the selectivity of the process responsible for the exit of sugars from the mucosa into the vascular bed. This phase of transepithelial transport appeared to be influenced mainly by the lighting cycle.

Animals↗
PubMed1980

Ion transport in liver mitochondria from normal and thyroxine-treated rats.

Liver mitochondria isolated from rats 24 h after a single subcutaneous injection of 8 mg thyroxine per kilogram body weight were compared with those isolated from control rats that received injections of isotonic saline at the same time. The mitochondria isolated from the thyroxine-treated rats show higher rates of energy-dependent K+ and phosphate accumulation than those from control animals. It was also found that mitochondria from the hormone-treated animals required a larger addition of Ca2+/mg mitochondrial protein in order to uncouple oxidative phosphorylation, and showed smaller tendency to swell in vitro under energizing conditions. The data obtained on ion movements support previous observations that the stimulation of the basal rate of mitochondrial respiration by thyroxine is associated with an increase in the transmembrane protonic electrochemical potential difference, and indicate the in vivo the hormone raises the intramitochondrial concentration of K+ and phosphate.

Animals↗
PubMed1980

Distribution of actively absorbed diffusible sugars in the jejunal epithelium of the rat.

Electron-microscopy autoradiography, using freeze-dried frozen sections of unfixed tissue, was used to study the distribution of actively transported materials in the jejunal epithelium of the rat in vitro. After a few minutes incubation, the grain density over the organelle-packed interiors of the apical cytoplasm of the columnar absorptive cells was significantly greater than that over the structureless peripheral cytoplasm. This difference in the relative specific activities of the 2 subcellular compartments increased during accumulation of labelled galactose, and decreased as preloaded galactose was washed out of the epithelium. A similar compartmentation was observed in vascularly perfused intestines exposed to labelled galactose from either the mucosal or the serosal sides. These observations suggest the presence of an intracellular mechanism controlling the location and concentration of transported substrates during intestinal absorption.

Animals↗
PubMed1979

Glycolysis measurements in red cells from drug-treated epileptic and anaemia patients.

Glycolytic activity was measured in red cells from epileptic patients treated with anticonvulsant drugs. These results are compared with measurements of red cell glycolytic activity from megaloblastic anaemia patients and normal controls. The differences obtained between epileptic patients and controls, as well as the differences between anaemia, epileptic patients and controls is discussed.

Adolescent↗
PubMed1979

The influence of thyroxine administered in vivo on the transmembrane protonic electrochemical potential difference in rat liver mitochondria.

When mitochondria from normal and thyroxine-treated rats were energized by incubation with succinate, phosphate and MgCl2, it was found that the hormone treatment increased the transmembrane protonic electrochemical potential difference by 16mV and the respiration rate by 46%. Other experiments show these changes to be associated with increases in the intramitochondrial K+ and phosphate concentrations.

Animals↗
PubMed1979

Electron microscope autoradiography of a diffusible intracellular constituent, using freeze-dried frozen sections of mammalian intestinal epithelium.

Autoradiograms were prepared from freeze-dried frozen sections of rat intestinal epithelium, previously incubated in vitro with 3H-galactose. Although the sections showed evidence of disruption by ice-crystal growth during freezing, they retained sufficient recognizable ultrastructure to facilitate the identification of several subcellular compartments. Statistical analysis of the grain distribution indicated that a significant non-uniform distribution of the labelled galactose occurs in the absorptive cells during transport, and is maintained in the sections throughout the various stages of autoradiography.

Animals↗
PubMed1979

Sugar transfer from the lumen of the rat small intestine to the vascular bed.

1. A modification of the vascular perfusion technique was used to investigate sugar transfer from the lumen of the rat small intestine to the vascular perfusion technique was used to investigate sugar transfer from the lumen of the rat small intestine to the vascular bed. 2. With this method the transfer of both D-[3H]galactose and L-[14C]glucose were followed in the same experiments. The results indicate that when equimolar concentrations of the two sugars are perfused through the lumen there is a preferential transfer of galactose from the mucosal epithelium to the vascular bed as well as from lumen to the epithelial layer. 3. At high rates of galactose transfer from the lumen to the vascular bed, accumulation of the sugar within the mucosal epithelium is minimal unless the vascular flow is stopped. 4. Phlorizin inhibits galactose transfer from the lumen to the vascular bed, but the inhibition of sugar transfer appears to be restricted to the luminal face of the epithelial cells since the selectivity of the basolateral exit process is unaffected. 5. Replacement of Na+ by K+ in the luminal perfusate reduces the rate of galactose transfer by a factor of 40, but does not completely abolish the preferential transfer of galactose. The presence of luminal Na+ also stimulates the exit of galactose from the mucosal epithelium to the vascular bed. 6. The presence of D-glucose in the vascular perfusate produces a two- to threefold increase in the rate of sugar transfer from the lumen to the vascular bed. This effect is not observed with vascular galactose or luminal glucose.

Animals↗
PubMed1978

Ion movements during energy-linked mitochondrial structural changes.

The structure of isolated rat liver mitochondria has been observed in the electron microscope following incubation of the mitochondria in vitro under a variety of conditions. The results show that ultrastructural changes are only associated with the energization and deenergization of isolated mitochondria if the composition of the incubation medium permits ion movements in or out of the matrix. The mechanism of energy coupling does not appear to depend on these major mitochondrial structural changes. The addition of low levels of valinomycin greatly increases the rate at which the matrix compartment swells and shrinks on energization and deenergization even at low K+ concentrations.

Adenosine Diphosphate↗
PubMed1978

A study of rapid mitochondrial structural changes in vitro by spray-freeze-etching.

The spray-freeze-etching technique has been used to study energy-linked mitochondrial structural changes in rat liver mitochondria incubated in vitro. The technique involves spraying the suspension of mitochondria into liquid propane at -190 degrees C, and does not require the use of cryoprotectants or chemical fixatives. The results confirmed that freshly isolated mitochondria have a condensed matrix and that this expands at the expense of the outer compartment to give the orthodox configuration when the mitochondria are incubated in a K+ medium in the presence of substrate and phosphate. Addition of adenosine diphosphate (ADP) caused a rapid shrinkage of the matrix compartment, and the time-course and extent of this shrinkage has been measured quantitatively by coupling a rapid sampling device to the spray-freezing apparatus. These data show that for orthodox mitochondria the onset of phosphorylation is accompanied by a reduction of 30% in the matrix volume in 20's, and there is no evidence that the decrease in matrical volume affects the phosphorylation efficiency. These results suggest that natural ionophores in the mitochondrial inner membrane make it permeable enough to permit a rapid readjustment of matrix volume after the addition of ADP, and that the associated ion movement does not cause uncoupling of oxidative phosphorylation.

Adenosine Diphosphate↗
PubMed1976

Evidence for carrier-mediated uptake and efflux of sugars at the serosal side of the rat intestinal mucosa in vitro.

A modification of the everted sac technique is described which allows several sacs to be prepared rapidly and simultaneously from the same segment of rat intestine. 2. A method has been developed for comparing the transport of two sugars by measuring changes in the ratios of their concentrations as they pass across the intestinal wall. 3. With this method significant differences were observed between the D-[3H]galactose and L-[14C]glucose ratios in the mucosal epithelium, the serosal tissue and the serosal compartment. These results indicate that both the efflux of galactose from the serosal side of the mucosal epithelium and the uptake of the sugar into the mucosa are carrier-mediated processes. 4. The mediated efflux of galactose at the serosal side of the epithelial layer is inhibited by the presence of phlorizin on the mucosal side and to some extent by any reduction in the mucosal Na+ concentration. Both of these treatments inhibited galactose uptake at the brush border. Serosal efflux of the sugar appeared to be saturated at high concentrations of D-galactose. 5. Pre-treatment of the sacs with mercuric chloride considerably reduced D-galactose uptake from the luminal side, but did not affect its efflux relative to L-glucose at the serosal side of the mucosal epithelium. 6. Carrier-mediated sugar uptake into the mucosal epithelium from the serosal side was also examined. The role of the bidirectional, carrier-mediated sugar transport processes at the serosal pole of the mucosal epithelial cell in transintestinal transport is discussed.

Animals↗
PubMed1975

Lactate formation by rat small intestine in vitro.

The formation of lactic acid by mucosal slices, rings and muscle from rat jejunum has been studied for periods of up to 8 min. Lactate output by mucosal slices incubated in the absence of glucose was characterised by two phases: a rapid, initial phase of release lasting about 1 min, followed by a much slower phase extending over the remainder of the incubation period. Glucose addition at 30 s initiated a second rapid phase of lactate release into the medium which was again followed by a slower rate of lactate output up to 8 min. The time course of lactate output suggested that there was a negative Pasteur effect in mucosal slices, which could not be reversed by the addition of ADP or glucose 6-phosphate. By contrast, the rate of lactate formation by rings and muscle from rat jejunum increased steadily over the incubation period, indicating a positive Pasteur effect. When Na+ in the incubating medium were replaced by K+, lactate formation by mucosal slices and rings was considerably reduced. Measurements of tissue lactate content before and during incubation revealed that about three-quarters of the lactate released by mucosal slices during the first 30 s of incubation was present initially in the tissue. After the first 30 s the tissue lactate remained constant both in the presence and absence of glucose so that the lactate released into the incubation medium is equivalent to the lactate formed by the slices. The role of the various tissue components of the small intestine in lactate formation is discussed in relation to sites of glucose entry.

Adenosine Diphosphate↗
PubMed1974

The absorption of a mixture of amino acids by rat small intestine.

1. The absorption of a mixture of amino acids by the small intestine has been studied in vitro with mucosal slices from rat jejunum. The mixture contained eighteen amino acids and was used at a concentration of 9.5 mM in the incubation medium. The uptake of fifteen of the amino acids was followed in each sample with the aid of an amino acid analyser.2. The endogenous amino acid content of the slices decreased during preparation and further substantial losses occurred when the slices were preincubated for 30 sec before the addition of the amino acid mixture to the incubation medium.3. When the slices were incubated with the amino acid mixture for 4.5 min all of the amino acid studied were accumulated to give concentration ratios of approximately 2.0.4. The amino acids were taken up rapidly by the tissue so that on average 82% of the amino acids absorbed in 4.5 min had already accumulated after one min of incubation.5. The data for individual amino acids revealed no obvious competition between the various amino acids, and the extent to which the various amino acids were accumulated by the tissue was proportional to their concentrations in the mixture.6. When the Na(+) in the incubation medium was replaced by Li(+), the slices accumulated all of the amino acids studied to concentrations significantly higher than those in the incubation medium. However, the concentration ratios obtained with the Li(+) medium were all lower than those obtained with the Na(+) medium and averaged slightly more than 1.5.7. When the Na(+) in the incubation medium was replaced by K(+) there was little amino acid accumulation, although concentration ratios significantly greater than one were achieved for aspartic acid, glycine, histidine and methionine.

Amino Acids↗