Search PubMed⌕ Search

Biomedical subjects

I T Johnson

Publications and source records attributed to I T Johnson.

At least 109 records · Page 6Linked to original sources
PubMed1984

The mechanism of action of guar gum in improving glucose tolerance in man.

Experiments were carried out in human volunteers to investigate the mechanism by which guar gum improves glucose tolerance. Guar reduced both plasma glucose and insulin responses to an oral glucose load, and delayed gastric emptying. However, there was no correlation between changes in individual blood glucose responses and changes in gastric emptying rates induced by guar. With a steady-state perfusion technique, glucose absorption was found to be significantly reduced during perfusion of the jejunum with solutions containing guar, but returned to control values during subsequent guar-free perfusions. Preperfusing the intestine with guar did not affect electrical measurements of unstirred layer thickness in the human jejunum in vivo. Experiments in vitro established that glucose diffusion out of a guar/glucose mixture was delayed under conditions of constant stirring. We conclude that guar improves glucose tolerance predominantly by reducing glucose absorption in the small intestine. It probably does this by inhibiting the effects of intestinal motility on fluid convection.

Adolescent↗
PubMed1984

Hexose transport and mucosal morphology in the small intestine of the zinc-deficient rat.

Immature male Wistar rats were given a zinc-depleted semi-synthetic diet (2 mg Zn/kg diet) for 4 weeks. Control groups received a similar diet containing 60 mg Zn/kg diet, either ad lib. or in amounts matched to the consumption of the Zn-deficient group. Rates of growth and food conversion efficiency were markedly lower in the Zn-deficient group compared to controls. The phloridzin-sensitive uptake of galactose and 3-O-methyl glucose by everted jejunal rings from each group was measured over a range of concentrations. Kinetic analysis revealed that the maximum transport rate (Vmax) for the uptake of both sugars was significantly higher in the Zn-deficient group than in the control groups. The villi, microdissected from samples of proximal jejunum, were significantly shorter and narrower across the base in the Zn-deficient group than those of the control groups, but were present in greater numbers per unit serosal area. The present study shows that Zn deficiency leads to significant morphological changes in rat small intestine, which are accompanied by an increase in the capacity of the tissue to take up sugars across the mucosal membrane.

3-O-Methylglucose↗
PubMed1984

Effect of dietary supplements of guar gum and cellulose on intestinal cell proliferation, enzyme levels and sugar transport in the rat.

Male Wistar rats (approximately 200 g) were given fibre-free semi-synthetic diets containing either sucrose (S) or a sucrose-starch mixture (SS) as the carbohydrate component, or a diet similar to SS containing 40 g guar gum/kg (G), or 100 g cellulose/kg (C). The animals remained healthy, and weight gain after 30 d was similar in all groups. The small intestines of the animals given diet G were significantly longer than those of the other groups, and showed signs of increased mitotic activity and mucosal growth. No significant differences in mucosal enzyme activity were detected between the two fibre-free control groups. Lactase (EC 3.2.1.23) and alkaline phosphatase (EC 3.1.3.1) activities were significantly lower than controls in group G, but were higher in group C. Kinetic analysis of 3-O-methyl glucose uptake by isolated intestine indicated that the maximum transport rate (Vmax) of tissue from group G tended to be lower than from the fibre-free group SS and group C. It is concluded that materials which are classed as dietary fibre but which differ markedly in their physical properties may also differ in the functional changes to which they give rise in the small intestine. These changes may be at least partially mediated by effects on mucosal cell proliferation.

3-O-Methylglucose↗
PubMed1983

The influence of guar gum on the movements of inulin, glucose and fluid in rat intestine during perfusion in vivo.

A two-stage perfusion technique was used to study the effect of guar gum on the inulin-accessible space and the uptake of water and glucose in rat intestine. Pre-perfusion of test loops with low concentrations of guar, dispersed in saline, modified the rate of equilibration of inulin with the mucosal fluid space during a subsequent perfusion. The glucose absorption rate in such loops was reduced at a concentration of 50 mM, but not at 100 or 150 mM glucose. Fluid absorption was inhibited by pre-treatment with guar gum at all glucose concentrations tested. These results suggest that guar forms a layer closely associated with the mucosal surface which modifies the viscosity of the immediate fluid compartment, so that its resistance to diffusion is increased by means of an unstirred layer effect.

Animals↗
PubMed1983

The influence of cooked kidney beans (Phaseolus vulgaris) on intestinal cell turnover and faecal nitrogen excretion in the rat.

Male Wistar rats were fed on semi-synthetic diets containing cooked white kidney beans (Phaseolus vulgaris) or equivalent levels of protein and carbohydrate. No change was observed in over-all nitrogen balance in animals fed on the bean diet, but there was a two- to three-fold increase in their faecal excretion, compared with control rats. This was compensated by a decrease in urinary-N excretion. Homogenized small intestinal mucosa, prepared from bean-fed animals, showed a 28% increase in protein content compared with control material. Measurements of 3H-labelled thymidine turnover indicated that mucosal cell exfoliation was increased by approximately 35% in the small intestines of bean-fed rats compared with controls. It is concluded that though a diet rich in cooked P. vulgaris leads to some increase in mucosal cell turnover in the small intestine of rats, the consequent increase in mucosal protein loss could not account for the increased faecal-N excretion seen in these animals.

Animals↗
PubMed1983

The influence of guar gum on intestinal cholesterol transport in the rat.

Everted sacs of rat proximal small intestine were used to determine the effect of guar gum (5 g/l) on the uptake of cholesterol (0.1 mM) from a solution of micelles. The uptake of cholesterol was found to be linear both in the presence and absence of guar gum. When guar was present throughout the whole of the incubation medium, the uptake of cholesterol was reduced to approximately 40% of control values. Sacs which had been pre-incubated in guar gum before exposure to cholesterol in a guar-free medium also showed a reduction in cholesterol uptake but this was less pronounced. A two-stage perfusion technique, previously described (Blackburn & Johnson, 1981), was used to determine the effect of a guar layer adsorbed to the mucosal surface on cholesterol absorption in vivo. Such a layer leads to a reduction of approximately 36%; it was concluded that guar slows the absorption of cholesterol from micelles by a mechanism, or mechanisms, involving an increased resistance to diffusion in the aqueous medium. Groups of rats were meal-fed for at least 30 d on semi-synthetic diets with or without the inclusion of guar gum (20 g/kg). Rates of intestinal absorption of cholesterol, glucose and fluid were then determined by the perfusion technique in vivo. There was no reduction in absorption in the test animals compared with the controls. It is proposed that guar gum is able to slow the intestinal transport of cholesterol from a suspension of pre-formed micelles, but only when both are present in the lumen together. No evidence was obtained to suggest that the consumption by rats of a diet containing guar gum, at a level similar to that used in human studies, leads to any adaptive reduction in their rates of cholesterol or glucose absorption.

Animals↗
PubMed1982

Influence of viscous incubation media on the resistance to diffusion of the intestinal unstirred water layer in vitro.

Everted sacs of rat intestine were incubated in media made viscous with Na-carboxymethylcellulose (CMC). This increased the resistance to diffusion of the mucosal unstirred water layer, as measured by the half-time for the establishment of osmotic streaming potentials. The increase ws not however linearly related to viscosity, but reached a maximum, equivalent to an apparent thickness of 500 mu, at a viscosity of about 100 cP (shear rate = 50 s-1). The inhibition of maltose hydrolysis by CMC conformed to theoretical expectations, and was characterised by an increased apparent Km, with no change in Vmax. This was not true however of galactose transport by everted sacs which showed a decline in maximum transport rate, with no apparent increase in Km. It appears that increased unstirred layer resistance may induce partial anoxia in the everted sac preparation and thus reduce the energy available for active transport. We conclude that the effect of enhanced viscosity is to reduce turbulent flow, and hence stirring, in the intervillous compartment. Such an effect may be brought about by viscous components of the diet and may influence rates of absorption and digestion in the intact intestine in vivo.

Animals↗
PubMed1981

The effect of guar gum on the viscosity of the gastrointestinal contents and on glucose uptake from the perfused jejunum in the rat.

1. Male Wistar rats were meal-fed for at least 10 d a control semi-synthetic diet containing no guar gum, or one of three similar test diets containing 3, 10 or 20 g dry guar gum/kg. 2. Rats were killed 6 h after feeding, and contents of stomach, small and large intestine were collected separately. The apparent viscosities of stomach and small intestine contents from animals fed on diets containing 10 and 20 g guar gum/kg were increased relative to control animals, but large intestine contents were unchanged. 3. In the second part of this study, male Wistar rats were anaesthetized and two consecutive lengths of jejunum were perfused, initially with Ringer only (control) or Ringer plus 5 or 6 g guar gum/1 (test). Following this pre-perfusion, both segments were perfused with Ringer containing glucose (10 mM), [3H]glucose and [14C]inulin, and the rate of glucose absorption was determined. 4. The rate of glucose absorption was decreased relative to control values in segments pre-perfused with both 5 and 6 g guar gum/1 solution, but this reduction was significant only in the instance of the 6 g/l solution (P less than 0.001). 5. These results provide evidence to support previous assumptions that ingestion of guar gum will increase the apparent viscosity of the contents of the stomach and small intestine. We propose that a possible mechanism by which guar reduces post-prandial glycaemia is a reduction of glucose absorption from the small intestine, resulting from an increase in viscosity of the contents.

Animals↗
PubMed1981

Effect of gel-forming gums on the intestinal unstirred layer and sugar transport in vitro.

The effect of two gel-forming polysaccharide gums, guar gum and Na-carboxymethyl-cellulose (CMC), on glucose transport in vitro was investigated using everted sacs of rat jejunum. The gums were added to the mucosal bathing media to give apparent viscosities in the range of 1-110 Pascal seconds X 10(-3), mPa.s(cP). Serosal glucose transport fell steeply by about 60% as the viscosities of the mucosal media rose to 20mPa.s, and levelled off thereafter. A similar effect was observed in sacs preincubated with guar gum (15 minutes) and exposed to glucose in a subsequent guar-free incubation. Glucose transport with and without the addition of guar gum was found to be sensitive to mucosal stirring, so that, when shaken at 130 oscillations per minute, sacs exposed to guar gum (0.25 %, viscosity c.a. 16 mPa.s (cP) transported glucose at a similar rate to sacs incubated without guar at 80 oscillations per minute. By measuring the time course for the establishment of osmotic induced potentials, it was shown that incubation with guar or CMC led to an increase in the apparent thickness of the unstirred fluid layer overlying the mucosa (guar-free thickness = 317 +/- 15 mu, guar treated thickness = 468 +/- 25 mu). It is suggested that the presence of a polysaccharide gum in the fluid film surrounding the villi increases its viscosity, and thus gives rise to a thickening of the rate-limiting unstirred layer. If such an effect occurs in vivo, this could contribute to the diminished post-prandial glycaemia observed in human subjects fed guar gum.

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

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↗
PubMed1978

Acid microclimate in coeliac and Crohn's disease: a model for folate malabsorption.

The surface pH of human proximal jejunum was measured in biopsy samples and found to be more acid than the phosphate buffer in which they were incubated. The in vitro jejunal surface pH was 5.93 +/- 0.05 in control subjects and 6.19 +/- 0.09 in treated coeliac patients. A group of untreated coeliac patients with a surface pH of 6.56 +/- 0.14 had a significantly less acid surface pH compared to controls, as did a group of Crohn's patients with a surface pH of 6.21 +/- 0.04. These two groups with a significantly raised surface pH were subdivisible into 'high' and 'low'groups. Surface pH was found to remain low in the treated coeliac and control groups but became more acid over the incubation period reaching almost normal values in the Crohn's group and the untreated coelic initial surface pH. The raised surface pH in untreated coeliac disease and Crohn's disease would alter the amount of a weak acid available for non-ionic diffusion. Therefore the present results may help to explain the folate malabsorption known to occur in untreated coeliac disease and the frequently seen low serum folate levels in Crohn's disease.

Adolescent↗
PubMed1977

Ultrastructural tracer studies on the permeability of the Malpighian tubule of the pill millipede, Glomeris marginate (Villers).

The electron-dense tracers ferritin, and iron-dextran, and the protein horseradish peroxidase, have been used to investigate the ultrastructural basis of permeability in the upper and lower segments of the Malpighian tubules of Glomeris marginata. All these materials were able to cross the basal lamina and enter the tubule lumen of the upper segment, and it was established that horseradish peroxidase was able to enter the channels which interrupt the apical junctions. In the upper segment, ferritin, iron-dextran, and horseradish peroxidase are taken up and accumulated within intracellular vesicles. In the lower segment ferritin and iron-dextran enter the cells but become generally distributed over the cytoplasm, as well as entering membrane-bounded vacoules. The behaviour of horseradish peroxidase could not be assessed by owing to the presence of endogenous peroxidase activity in the cells. After fixation by direct application of glutaraldehyde to the undissected tubules, the extracellular spaces contained large numbers of membrane-bounded vesicles. The significance of these observations is discussed in relation to the physiological activities of the tubules.

Animals↗
PubMed1977

Ultrastructural studies on the Malpighian tubule of the pill millipede, Glomeris marginata (villers).

Ultrastructural studies on the Malpighian tubules of Glomeris marginata (Villers) reveal considerable morphological differences between the upper, fluid secreting, segment, and the lower segment which is at present of unknown function. Previous reports have shown that the upper tubule has a high permeability to compounds of high molecular weight. This may be accounted for by the fact that the epithelium shows very extensive intercellular spaces which are linked directly to junctions apparently specialised to provide a low resistance extracellular pathway between the haemocoel and the tubule lumen. Histochemical studies on the localisation of phosphatase enzymes reveal intracellular vesicles with acid phosphatase activity. The basal labyrinth of the lower tubule exhibits considerable alkaline phosphatase activity which is apparently identical in location to the enzyme revealed by two different ATPase localisation techniques.

Acid Phosphatase↗
PubMed1976

Alternative methods of animal sacrifice: the effect on intestinal function in vitro.

The total fluid uptake of everted sacs of rat jejunum was compared in animals killed by stunning and decapitation, or anaesthetised with ether. Fluid transport was significantly higher in the tissue prepared from stunned and decapitated animals. It is suggested that etherization may have adverse effects on the physiological viability of subsequently isolated tissues.

Animals↗
PubMed1976

Aspects of intestinal folate transport in the rat.

1. In vitro preparations of rat jejunum were used to study the metabolic dependence, the structural specificity, and the pH sensitivity of the folic acid absorption process. 2. The presence of 2:4 dinitrophenol, and the absence of oxygen, in the muscosal bathing medium both led to a decrease in the tissue accumulation and serosal transfer of folic acid by everted sacs. 3. 10-formylfolic acid, present in the mucosal medium at a molar ratio of 10:1 with labelled folic acid, appears to compete for tissue accumulation sites but has no significant effect on serosal transfer. 4. The efflux of folic acid from pre-loaded jejunal sacs is stimulated by the presence in the mucosal medium of the folic acid and related compounds. Pteroyl-L-glutamic acid, 10-formylfolic acid and methotrexate elicit a significant increase in the efflux rate; pteroyl-D-glutamic acid was significantly less effective and pteroic acid had no effect. 5. The uptake of folic acid by isolated jejunal cells prepared by enzymic disaggregation of the mucosa, was found to be influenced by the pH of the bathing medium, uptake being enhanced at a pH of between 5 and 6. 6. It is concluded that the effect of metabolic inhibitors and acid pH conditions on the uptake of folic acid in vitro is consistent with a passive absorption mechanism, influenced in intact preparations and in vivo by the jejunal pH microclimate. However the occurrence of competitive inhibition and stimulated efflux may indicate the existence of a structurally specific accumulation process at some site, or sites, within the mucosa.

Animals↗