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J R Bronk

Publications and source records attributed to J R Bronk.

13 recordsLinked to original sources
PubMed1989

The transport and metabolism of the uridine mononucleotides by rat jejunum in vitro.

1. Both uridine 3'-monophosphate (3'-UMP) and uridine 5'-monophosphate (5'-UMP) when perfused through the lumen of isolated rat jejunum gave rise to uracil as the only transported pyrimidine appearing in the serosal medium; neither the nucleotide nor the nucleoside could be detected in the serosal fluid. 2. There was a low level of the nucleoside, uridine, in the luminal fluid after the nucleotide had passed through the jejunal segment. Luminal nucleoside appearance was more marked from the 3' form of the nucleotide. 3. The hydrolysis of the nucleotides to the nucleoside form occurred via a brush-border membrane enzyme, which had the same maximal velocity (Vmax) for the two nucleotides (699 +/- 35 and 747 +/- 10 nmol min-1 (mg protein)-1 for 3'-UMP and 5'-UMP, respectively) but a different Michaelis constant (Km) so that 3'-UMP (Km = 58 +/- 3 microM) hydrolysis is favoured over 5'-UMP hydrolysis (Km = 108 +/- microM) at lower concentrations. 4. At 0.05 mM, luminal 3'-UMP gave rise to a higher rate of serosal uracil appearance than luminal 5'-UMP, but at higher luminal concentrations (0.1-0.2 mM) the rate of serosal uracil appearance was the same from both nucleotides. 5. The transmural transport of uracil from the uridine mononucleotides is discussed with reference to the metabolism and compartmentalization of the small intestine responsible for the appearance of the free pyrimidine in the serosal fluid.

Animals↗
PubMed1989

The specificity of pyrimidine nucleoside transport and metabolism by rat jejunum in vitro.

1. 5-Methyluridine perfused through the lumen of isolated loops of rat jejunum gave rise to serosal thymine as the major species transported across the epithelial layer; no serosal 5-methyluridine was detected. 2. At 0.1 mM-luminal 5-methyluridine there was enhanced transmural transport of thymine (P less than 0.001) when compared with either 0.1 mM-thymine or 0.1 mM-thymidine as the luminal substrate for thymine transport. 3. At low luminal concentrations (0.025 mM) thymine was a significantly better substrate (P less than 0.001) for the transport of the free pyrimidine than either of the two nucleosides (thymidine or 5-methyluridine). 4. Luminal deoxyuridine gave rise to uracil as the major species appearing in the serosal secretion. High luminal concentrations of deoxyuridine (0.5 and 1.0 mM) gave rise to low levels of the nucleoside in the serosal fluid. 5. As a consequence of the specificity of the mucosal phosphorolysis the ribonucleosides are favoured over the deoxyribonucleosides as substrates for transmural pyrimidine transport.

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↗